Api Tr 21tr1.pdf

Materials Selection for Bolting

API TECHNICAL REPORT 21TR1 FIRST EDITION, AUGUST 2019 ADDENDUM 1, APRIL 2020

Special Notes API publications necessarily address problems of a general nature. With respect to particular circumstances, local, state, and federal laws and regulations should be reviewed. Neither API nor any of API's employees, subcontractors, consultants, committees, or other assignees make any warranty or representation, either express or implied, with respect to the accuracy, completeness, or usefulness of the information contained herein, or assume any liability or responsibility for any use, or the results of such use, of any information or process disclosed in this publication. Neither API nor any of API’s employees, subcontractors, consultants, or other assignees represent that use of this publication would not infringe upon privately owned rights. Classified areas may vary depending on the location, conditions, equipment, and substances involved in any given situation. Users of this technical report should consult with the appropriate authorities having jurisdiction. Users of this technical report should not relay exclusively on the information contained in this document. Sound business, scientific, engineering, and safety judgment should be used in employing the information contained herein. API publications may be used by anyone desiring to do so. Every effort has been made by the Institute to assure the accuracy and reliability of the data contained in them; however, the Institute makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any authorities having jurisdiction with which this publication may conflict. API publications are published to facilitate the broad availability of proven, sound engineering and operating practices. These publications are not intended to obviate the need for applying sound engineering judgment regarding when and where these publications should be used. The formulation and publication of API publications is not intended in any way to inhibit anyone from using any other practices. Any manufacturer marking equipment or materials in conformance with the marking requirements of an API standard is solely responsible for complying with all the applicable requirements of that standard. API does not represent, warrant, or guarantee that such products do in fact conform to the applicable API standard.

All rights reserved. No part of this work may be reproduced, translated, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher. Contact the Publisher, API Publishing Services, 200 Massachusetts Avenue, NW, Suite 1100, Washington, DC 20001-5571. Copyright © 2019 American Petroleum Institute ii

Foreword Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for the manufacture, sale, or use of any method, apparatus, or product covered by letters patent. Neither should anything contained in the publication be construed as insuring anyone against liability for infringement of letters patent. The verbal forms used to express the provisions in this document are as follows. Shall: As used in a standard, “shall” denotes a minimum requirement to conform to the standard. Should: As used in a standard, “should” denotes a recommendation or that which is advised but not required to conform to the standard. May: As used in a standard, “may” denotes a course of action permissible within the limits of a standard. Can: As used in a standard, “can” denotes a statement of possibility or capability. This document was produced under API standardization procedures that ensure appropriate notification and participation in the developmental process and is designated as an API standard. Questions concerning the interpretation of the content of this publication or comments and questions concerning the procedures under which this publication was developed should be directed in writing to the Director of Standards, American Petroleum Institute, 200 Massachusetts Avenue, Suite 1100, Washington, DC 20001. Requests for permission to reproduce or translate all or any part of the material published herein should also be addressed to the director. Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least every five years. A one-time extension of up to two years may be added to this review cycle. Status of the publication can be ascertained from the API Standards Department, telephone (202) 682-8000. A catalog of API publications and materials is published annually by API, 200 Massachusetts Avenue, Suite 1100, Washington, DC 20001. Suggested revisions are invited and should be submitted to the Standards Department, API, 200 Massachusetts Avenue, Suite 1100, Washington, DC 20001, [email protected].

iii

Contents Page

1 Scope................................................................................................................................................................ 1 2

Normative References....................................................................................................................................... 1

3 Terms, Definitions, Acronyms, Abbreviations, and Symbols.............................................................................. 1 3.1 Terms and Definitions........................................................................................................................................ 1 3.2 Acronyms, Abbreviations, and Symbols............................................................................................................ 4 General Design Information.............................................................................................................................. 5 4 4.1 General.............................................................................................................................................................. 5 4.2 Applicability....................................................................................................................................................... 6 4.3 Risk Assessment............................................................................................................................................... 6 4.4 Bolting Threats and Barriers.............................................................................................................................. 8 4.5 Manufacturing Processes................................................................................................................................ 14 4.6 Protective Coatings......................................................................................................................................... 22 Annex A (informative) Testing of API 20E Fastener Materials for Susceptibility to Hydrogen Embrittlement Under Cathodic Protection in Simulated Seawater .................................... 26 Bibliography ........................................................................................................................................................... 32

Figures 1 2 3 4 5 A.1 A.2

Types of Fasteners............................................................................................................................................ 5 Example of Offshore Bolting Corrosion........................................................................................................... 12 Overview of Production Processes................................................................................................................. 15 Ferrite (light) and Pearlite (dark) Banding....................................................................................................... 16 Thread Types................................................................................................................................................... 20 Notch Fracture Strength vs. Cathodic Potential (SCE) for 4140–140 and 4140–160..................................... 30 Permeation Transient Curves of All Four Strength Levels............................................................................... 30

Tables 1 2 3 A.1 A.2 A.3 A.4 A.5

Applicable API Standards.................................................................................................................................. 6 Carbon and Low-alloy Steel Bolting Selection Guide and Limitations............................................................. 23 CRA Bolt Selection Guide and Limitations...................................................................................................... 24 Product Analysis in Weight Percent of 4140 Bar............................................................................................. 26 Manufacturer Reported Mechanical Properties of Heat-Treated Bar.............................................................. 26 Laboratory Reported Mechanical Properties of Heat-Treated Bar.................................................................. 27 Rockwell C Hardness Testing.......................................................................................................................... 28 Notch Fracture Strength.................................................................................................................................. 29

v

Introduction Fasteners are manufactured to a variety of standards covering dimensions, tolerances, materials, mechanical properties, testing procedures, coating, and other manufacturing processes. This document is intended to provide guidance for the proper selection of materials and manufacturing processes for the oil and gas industry where materials selection and key manufacturing processes are critical barriers to the failure of fasteners. Understanding the failure modes and their associated barriers is critical to the proper selection of fasteners for the specific environmental conditions where they will be installed. “Bolting” and “bolt” are terms used in this document to collectively describe fasteners, including screws, nuts, bolts, washers, and studs. The use of the terms “bolt” or “bolting” includes all of the fasteners listed above, unless otherwise specifically noted herein.

vii

Materials Selection for Bolting 1 Scope This document provides guidance for the selection of materials and manufacturing processes for low-alloy steel bolting manufactured in accordance with API Specification 20E and nickel-based and stainless alloys manufactured in accordance with API Specification 20F. Table 2 and Table 3 are provided as guidance for the material selection of fasteners.

2 Normative References There are no referenced documents that are indispensable for the application of this document.

3 Terms, Definitions, Acronyms, Abbreviations, and Symbols 3.1 Terms and Definitions 3.1.1 aging A thermal cycle that usually follows solution annealing in precipitation hardening materials. NOTE Aging can be performed at different temperatures and times to strengthen precipitation hardening materials, such as some stainless steel grades and nickel-based alloys.

3.1.2 annealing A thermal cycle involving heating and holding material at or above its solutionizing temperature, and then cooling at a slow rate, for such purposes as reducing hardness, improving machinability, facilitating cold working, producing a desired microstructure, or obtaining desired mechanical or other properties. 3.1.3 austenitize, quench, and temper A heat treatment process commonly associated with steels that strengthens by martensitic transformation, then restores toughness. NOTE The process consists of heating the material to its solutionizing temperature and holding, followed by rapid cooling (commonly in water, polymer, or oil media). When martensitic structure is obtained, the material is very strong, but extremely brittle. The tempering process reduces stresses and changes the microstructure to tempered martensite, which gives a very desirable combination of high strength and toughness.

3.1.4 banding The microstructural manifestation of segregated alloy elements. 3.1.5 barrier coating A coating that is not anodic to the base metal in the intended service environment and that provides protection by isolating the base metal from the environment. 3.1.6 bolt A type of fastener with a head on one end of a shank or body and a thread on the other end designed for insertion through holes in assembly parts; it is mated with a tapped nut (see Figure 1).

1

2

API Technical Report 21TR1

NOTE Tension is normally induced in the bolt to compress the assembly by rotating the nut. This may also be done by rotation of the bolt head.

3.1.7 case hardening One or more processes of hardening steel in which the outer portion, or case, is made substantially harder than the inner portion, or core. NOTE Most of the processes involve either enriching the surface layer with carbon, nitrogen, or boron, usually followed by quenching and tempering, or the selective hardening of the surface layer by means of flame or induction hardening.

3.1.8 cold working The process of deforming metal to a desired shape at a temperature below its recrystallization temperature. 3.1.9 cooling rates The time it takes steel to solidify from the molten state to solid state. 3.1.10 corrosion (general mass loss) Temperature plus the presence of water on the steel surfaces (aqueous phase) and oxygen to drive the reaction producing iron oxide (Fe2O3 or Fe3O4). 3.1.11 creep The tendency of metal to deform slowly and permanently under the influence of mechanical stresses. It can occur as a result of continuous exposure to high levels of stress that are still below the yield strength of the material. NOTE Creep is pronounced in materials that are subjected to heat and stress for extended periods. Creep generally increases as the materials near their melting point.

3.1.12 dies Tools used in manufacturing to shape by cutting or pressing the raw material. 3.1.13 fatigue Progressive cracking or crack growth that occurs at some existing defect in the metal at a point of high stress (such as a notch), and grows in length over time with subsequent loading and unloading. 3.1.14 galling A severe form of adhesion wear that results in the transfer of metal from one part to the mating part. 3.1.15 hardness A unit of measure related to the resistance to plastic deformation from indentation. 3.1.16 hot working The process of deforming metal to a desired shape after the metal has been heated above its recrystallization temperature.

Materials Selection for Bolting

3

3.1.17 hydrogen embrittlement HE A permanent loss of ductility in a metal caused by hydrogen with or without stress. 3.1.18 immersion zone A region where a component or assembly would remain continuously submerged in seawater. 3.1.19 marine atmosphere Located above the water level and above tidal and wave action. 3.1.20 marine fouling The accumulation of aquatic organisms, such as microorganisms, plants, and animals, on surfaces and structures immersed in or exposed to the aquatic environment [1]. 3.1.21 mud zone The region at the bottom of a body of marine saltwater that includes a sediment layer and some of the sublayers. 3.1.22 normalizing A thermal cycle involving heating to and holding at a solutionizing temperature, then cooling in air, to produce material uniformity resulting in improved machinability, strength, and toughness. NOTE

This process is often used in combination with quench and temper.

3.1.23 proof load The point to which a material may be loaded without evidence of permanent deformation. 3.1.24 reduction ratio The change in cross-section of material from its initial stock (example: ingot or billet) size to its final size after being hot worked or cold worked. 3.1.25 sacrificial coating A coating that is anodic to the base metal in the intended service environment and that will corrode preferentially to the base metal, thereby protecting the base metal even if it is exposed. 3.1.26 screw A headed and threaded bolt used without a nut that is inserted into an internally tapped hole; tension is induced by the rotation of the screw head (see Figure 1). 3.1.27 shear The transverse rupture caused by a pushing or pulling force 90 degrees from the axis of the bolt. 3.1.28 solution annealing A process of heating the metal to a temperature that allows for alloying elements to be in solution (solutionizing temperature).

4

API Technical Report 21TR1

NOTE This process is generally followed by controlled cooling; for alloys, which have tendency to form intermetallic phases, it is common to quench after solution annealing to prevent or minimize their formation.

3.1.29 splash zone Part of a structure that is intermittently exposed to air and immersed in the sea due to wave action. 3.1.30 stress concentration Local surface region that results in an area of increased tensile stress, such as a notch or a fillet. 3.1.31 stress relaxation The decrease of stress with time in a material that has been strained to a constant amount. 3.1.32 stress relieving A thermal cycle involving heating to a suitable temperature, usually below the tempering temperature, holding long enough to reduce residual stresses from either cold deformation or thermal treatment, and then cooling slowly enough to minimize the development of new residual stresses. 3.1.33 stud A fastener with no head that is threaded at both ends with an unthreaded shank in between (double-end or tapend), or is threaded from end to end (continuous or full-thread), and is secured into a tapped hole; the other end is used with a nut to create tension (see Figure 1). 3.1.34 stud bolt A fastener with both ends secured with a nut (double-end and continuous studs) (see Figure 1). 3.1.35 tensile strength The maximum tensile stress that a material is capable of sustaining. 3.1.36 transition temperature The transition from ductile to brittle behavior with change in temperature. 3.1.37 tidal zone A region where a component or assembly is exposed to air or seawater environment, due to the rise and fall of the water level attributed to tidal changes. 3.1.38 torque The twisting load applied to the bolt. 3.1.39 yield strength The stress at which a material exhibits a specified deviation from the proportionality of stress to strain. NOTE

The deviation is expressed in terms of strain, and in the offset method, usually a strain of 0.2 percent is specified.

3.2 Acronyms, Abbreviations, and Symbols Cermet ceramic-metal

Materials Selection for Bolting

CP

cathodic protection

CRA

corrosion-resistant alloy

CSCC

chloride stress corrosion cracking

EAC

environmentally assisted cracking

EMS

electromagnetic stirring

HIC

hydrogen-induced cracking

HPHT

high pressure, high temperature

HRC

Rockwell C hardness

HSC

hydrogen stress cracking

LME

liquid metal embrittlement

PH

precipitation hardening

PREN

pitting-resistance equivalent number

SCC

stress corrosion cracking

SSC

sulfide stress cracking

wt weight

Figure 1—Types of Fasteners

4 General Design Information 4.1 General During the bolting selection process, one should consider: a) applicable design codes; b) design load case; c) environment; and d) material selection.

5

6

API Technical Report 21TR1

4.2 Applicability This document may be useful for material selection in applications such as, but not limited to, those described in the documents listed in Table 1. Table 1—Applicable API Standards Standard

Title

API 6A

Wellhead and Tree Equipment

API 6D

Specification for Pipeline and Piping Valves

API 16A

Specification for Drill-Through Equipment

API 16C

Choke and Kill Equipment

API 16D

Specification for Control Systems for Drilling Well Control Equipment and Control Systems for Diverter Equipment

API 16F

Specification for Marine Drilling Riser Equipment

API 16Q

Recommended Practice for Design, Selection, Operation, and Maintenance of Marine Drilling Riser Systems

API 17D

Design and Operation of Subsea Production Systems—Subsea Wellhead and Tree Equipment

API 17TR8

High-pressure High-temperature Design Guidelines

API 6DSS

Specification for Subsea Pipeline Valves

4.3 Risk Assessment 4.3.1 General The risk associated with the use of bolting in the oil and gas industry depends largely on the functional categories and environments in which the bolting is intended to be used. 4.3.2 Functional Categories Some examples of functional categories are: — pressure-containing: part exposed to wellbore fluids whose failure to function as intended would result in a release of wellbore fluid to the environment; — pressure-retaining: part not exposed to wellbore fluids whose failure to function as intended will result in a release of wellbore fluid to the environment; — pressure-controlling: parts intended to control or regulate the movement of wellbore fluids; — fatigue sensitive: susceptible to fatigue loads; — structural: primary and non-primary load path. Bolting generally falls into the pressure-retaining category; however, they also may be classified as fatiguesensitive, primary load path, closure bolting, and external loading from environment (vortex-induced vibration). Specifications may have different functional categories. These functional categories should be considered in assessing the criticality of components. Based on this assessment, fasteners may be assigned bolting specification levels (BSL) in accordance with API 20E or API 20F specifications. In addition to the functional categories, service environments can compound the criticality of fastener components. The primary service environments in the oil and gas industry are surface, splash zone, and subsea.

Materials Selection for Bolting

7

Subsea environments can be categorized further into more specific environments that encompass different hazards and risks. 4.3.3 Environmental Considerations 4.3.3.1 General The environment in which the bolting operates can have a profound effect on the performance of the selected bolting. 4.3.3.2 Offshore Environments The following are examples of offshore environments: a) Marine atmosphere: Corrosion is a function of the amount of salt or seawater mist that collects on the metal’s surface in a marine atmosphere. Corrosion may be particularly severe in crevices or low spots where mist or runoff can collect. The concentration of salts in these areas may be much higher than in seawater due to evaporation. Oxygen is readily available to accelerate corrosion. Rainfall may increase or reduce the rate of corrosion depending on circumstances. Heavy rainfall may reduce corrosive attack by rinsing off salt residues on metal surfaces. Rainfall may also accelerate corrosion by providing the moisture necessary for salts on the metal’s surface to stay in an aqueous solution. b) Splash zone: For low-alloy steels, the splash zone is the most corrosive. The seawater is well oxygenated. The impingement of the waves may mechanically remove corrosion products from the steel’s surface, thereby exposing fresh metal to further attack and erosion. Metals such as stainless steels and nickel-based alloys that rely on a thin chromium oxide surface layer for their corrosion resistance fare much better in the splash zone than low-alloy steels because the well-aerated seawater immediately supplies oxygen to repair any damage done to the oxide film. There is little or no marine fouling in the splash zone. c) Tidal zone: Steel corrosion is often relatively low in the tidal zone because of the formation of oxygen concentration cells. Metal surfaces in the tidal zone are exposed to well-aerated seawater, while adjacent metal surfaces that are submerged are exposed to less oxygen (especially if covered with marine growth). This establishes an oxygen concentration cell, with the submerged metal being anodic to the metal in the tidal zone. The submerged metal just below the tidal zone will corrode, but in doing so suppresses corrosion of the metal in the tidal zone (the cathode of the cell). Small, isolated panels of steel in the tidal zone are rapidly attacked by the highly oxygenated seawater. Marine fouling may be present in the tidal zone. d) Immersion zone: The rate of corrosion in the immersion zone is primarily dependent on the rate at which oxygen can diffuse through rust, other surface deposits, marine growth, etc., to reach and react with the metal’s surface. Water near the surface of the ocean is usually saturated with oxygen. The oxygen content at other depths varies by region. In the Pacific Ocean, the oxygen content at great depths is much lower than at the surface, while in the Atlantic Ocean there is little difference between the two. Marine fouling may be heavy in shallow depths. Marine fouling can produce varied corrosion effects. It may form an effective barrier to the environment under some circumstances or cause accelerated corrosion through oxygen concentration cells in others. At depths below 300 feet where sunlight does not penetrate, there is no plant fouling and greatly reduced animal fouling. e) Mud zone: There are several competing processes that make the rate of corrosion in the mud zone difficult to predict. Oxygen concentration cells may establish themselves at the mud-line water interface. Anaerobic bacteria in or on the mud may cause corrosion. The reduction in the corrosion rate of steel well below the mud line is attributable to the lower oxygen content and to the fact that any protective films that form on the steel’s surface are protected in turn by the surrounding mud. f)

High pressure, high temperature: With the continued exploration of high-pressure, high-temperature (HPHT) environments, there are additional standards, specifications, and technical recommendations that need to be considered to ensure proper design and qualification of fasteners, such as API 17TR8 and API PER15K.

8

API Technical Report 21TR1

4.3.3.3 Land Environments The following are examples of land environments: a) Desert or arid: The desert environment is the most benign to bolting as humidity is low. The common threats in an arid environment are extreme temperatures and thermal gradients, potentially abrasive action from dust storms, and powdery deposits. b) Tropical and subtropical: The effects of a tropical or subtropical environment are like those caused by a marine atmosphere, except for the presence of corrosive salts. In this environment, bolting buried underground may experience accelerated corrosion from salts and minerals attacking the metal surface in combination with water and humidity. c) Arctic: Extremely low temperatures are the greatest threat to bolting in this environment. Extreme temperature drops can cause certain alloys to fall below the nil ductile transformation temperature or ductile-to-brittle transition temperature. 4.4 Bolting Threats and Barriers 4.4.1 General The threats to bolting integrity that can lead to bolting failure should be understood. The application of appropriate barriers to mitigate those threats are an important part of the design considerations, materials selection, manufacturing processes, and installation of bolts to ensure satisfactory service life in critical bolting installations. In corrosive environments involving sour service, or that are outside of typical operating pressures and temperatures, additional design considerations may be required. The following section discusses some of the most common threats to bolt integrity and the barriers employed to prevent in-service bolting failures. Studs and bolts should be equipped with heavy hex nuts with minimum hardness equal to the mating stud and bolt. 4.4.2 Over-torqueing and Under-torqueing Torque occurs upon installation or in service due to an externally applied load. Over-torqueing and undertorqueing can be prevented by employing proper torqueing procedures. The sequence for torqueing bolts should always be taken into consideration. Some coatings can have an effect on the desired torque values, so this should be taken into consideration when writing torqueing procedures. Torqueing is not a direct measure of the load; it is a measure of the frictional resistance. There are several influences on torque that make it an unreliable indicator of tension, including thread class, thread fit, thread finish, lubrication, coating, cleanliness, contaminants, mechanical upsets, tears, nicks, dings, etc. Calibration of torqueing equipment should be an integral part of proper torqueing procedures. An alternative method of measuring load may be necessary. In the field, this is most easily accomplished through the measurement of the bolt in a relaxed, unloaded state, then again after tightening. As the tightening process stretches the bolt, measuring this stretch directly correlates how much load is introduced. Over-torqueing can lead to tensile overload and under-torqueing can contribute to fatigue-related failures. There are codes and industry standards that deal with torque values and torqueing sequences. For example, ASME PCC-1 describes the installation procedure for bolting; additionally, API Specification 6A has an informative annex with recommended flange bolt torque. API 17D, Annex G also provides guidance on installation procedures and torque values. 4.4.3 Shearing (Overload or ductile failure) Fasteners for the applications addressed here are not generally designed to be loaded in shear; special consideration should be taken during design to ensure the fastener is not loaded in shear. Shear loads may not come from operational loads, but could be a result of handling, transportation, or installation. If shear cannot be avoided, the fastener should be made from a material with a composition and processing capable of achieving the minimum strength requirements throughout the full cross-section.

Materials Selection for Bolting

9

4.4.4 Tensile Overload Tensile overload can be avoided by the proper use of safety factors during design, as well as by ensuring material processing results consistently meet or exceed the minimum tensile properties required by the design through testing frequency and appropriate supply chain management. 4.4.5 Stress Concentration Notches may cause intensification or concentration in the stress field. When shaping the head of a bolt either by forming or machining in the manufacturing process, local notches can be introduced. Common measures, such as larger radii or proper tooling, can assist in the reduction of stress concentration. 4.4.6 Fatigue Loading and unloading cycles cause cracks to grow to a critical length and results in complete failure of the bolt. There are many known variables that can cause fatigue failures in fasteners. The most common barriers against fatigue are: a) conservative design and allowable stress loads (% of minimum yield or % of minimum tensile strength); should be one of the primary ways to mitigate fatigue; b) special consideration given to reduce the effects of loads from vibration; some equipment may be affected by vortex-induced vibrations, which should to be considered during design; c) metal processing practices, which can greatly improve or degrade fatigue resistance of materials: 1) Modern melting practices can increase fatigue life through improved material cleanliness. 2) Heat treatment should also be considered, as it can affect fatigue life. 3) Forming (rolling) threads, rather than cutting threads, can induce compressive surface stresses, which can result in better fatigue resistance of the fastener. The same can be said for other types of surface treatments, provided they result in the same type of compressive stresses. 4) Special consideration is to be given to preload of bolted joint design to avoid fatigue related failures. NOTE Certain codes, such as BS 7608 or DNVGL-RP-C203, provide S-N curves for bolts.

d) surrounding environments, as some materials can suffer a significant loss in fatigue life (or fatigue resistance) depending on the environment they are operating in (e.g. subsea, land equipment, sour service, etc.). This is called corrosion fatigue. 4.4.7 Hydrogen Embrittlement Externally applied or internal residual stress may expedite the embrittlement process. Depending on different mechanisms for hydrogen intake and load case, hydrogen embrittlement can be referred to as hydrogen stress cracking (HSC), hydrogen-induced cracking (HIC), and many other nomenclatures. To address the problem of hydrogen embrittlement, emphasis is placed on controlling the amount of residual hydrogen in steel, controlling the amount of hydrogen pickup in processing, developing alloys with improved resistance to hydrogen embrittlement, developing low- or no-embrittlement plating or coating processes, and restricting the amount of in situ hydrogen introduced during the service life. There is a relationship between the strength and hardness of metals and their resistance to hydrogen embrittlement. For low-alloy steels and some martensitic stainless steels, laboratory testing and field experience indicates the threshold between a low-alloy steel that is susceptible to hydrogen embrittlement in seawater with cathodic charging and one that is not is ~34 HRC. This threshold is approximate, however, as the quality of microstructure of the low-alloy steel can influence the susceptibility to hydrogen embrittlement. Good metal-

10

API Technical Report 21TR1

processing practices that minimize banding and nonmetallic inclusions are essential to make these materials less susceptible to hydrogen embrittlement. Some coating applications produce hydrogen around the material as the coating is being deposited (such as in electro or electroless plating). Other applications that require phosphating prior to coating to create an anchor pattern can also generate hydrogen in the base metal, which may be detrimental to the base metal for steel with specified minimum tensile strengths of 125 ksi or higher. The effects of process-induced hydrogen can be mitigated by proper heat treatment that immediately follows the pickling, plating, and/or phosphating processes. API Specification 20E or ASTM B850 are sources that provide bake-out procedures and times. Hydrogen bakeout should be performed immediately after the plating and/or phosphating processes to prevent hydrogen embrittlement on high-strength, low-alloy steels, and martensitic stainless steels. NOTE There is currently discussion concerning the actual materials threshold strength for susceptibility to hydrogen embrittlement. Studies and tests are underway to better define such limits. The results of these studies may result in a change to the required strength values.

Cathodic protection (CP) in seawater can generate hydrogen around the base metal being protected, which could be detrimental. Typically, hydrogen ions will combine at the surface of the metal and will depart the system as hydrogen gas. If the CP is improperly applied, the hydrogen ions may persist at the steel surface and can be absorbed into the base metal, causing hydrogen embrittlement. Hydrogen generation can also occur around steel from cathodic protection by coatings made of metals such as zinc, aluminum, or zinc-nickel. More noble metals, such as nickel, nickel-cadmium, or nickel-cobalt, are sometimes considered because they do not charge hydrogen into the steel bolting materials or, from a practical standpoint, corrode. When dealing with coatings more noble than the base metal of the bolting, it is important to consider corrosion of the steel due to porosity or damage of the coating. In this case, the bolting material is anodic (sacrificial) to the coating and will selectively corrode. The severity and speed of this type of pitting corrosion may vary proportionally to the surface area of steel around the discontinuity, and whether the system surrounding the bolting is cathodically protected. Furthermore, if CP is not properly applied, the selective degradation of the substrate could accelerate significantly. In HPHT designs, which need increased capacity over existing stainless steel (austenitic) or duplex stainlesssteel bolts, higher-strength corrosion-resistant alloy (CRA) materials, such as precipitation hardening (PH) nickel-based alloys, may be used. The oil and gas industry has also had HSC or HE of bolts manufactured from PH nickel-based alloys in subsea applications due to CP exposure or galvanic coupling. 4.4.8 Liquid Metal Embrittlement Liquid metal embrittlement (LME) occurs when liquid metals cause the ductile metals to lose tensile ductility or cause a brittle fracture. Liquid metals cause decohesion along grain boundaries with the assistance of external tensile stress or internal stress, i.e. residual stresses as a consequence of manufacturing. This is commonly associated with improper hot dip galvanizing technique. This is also a specific issue associated with cadmium coatings, which have low liquid metal embrittlement temperature. This issue can be mitigated by proper process control and by ensuring that LME-susceptible materials are used at temperatures lower than their LME threshold. 4.4.9 Embrittlement from Intermetallic Phases (Sigma, Delta, Sensitizing) Embrittlement from intermetallic phases often occurs during heat treatment; however, post-processing of base metal can also generate enough heat to cause a phase transformation of materials, which can lead to detrimental nonmetallic phases. The manufacturer should address proper heating and cooling rates to avoid the formation of deleterious nonmetallic phases along the grain boundaries. Post-processing, which can result in detrimental nonmetallic phases, should be avoided. 4.4.10 Galling This failure mechanism is often seen in stainless steels, particularly in austenitic stainless steels; however, other fasteners, such as aluminum, titanium, low-alloy steel, or nickel-based alloy may also be susceptible.

Materials Selection for Bolting

11

Galling can be prevented by the appropriate selection of coatings and materials, such as pairing dissimilar alloys, dissimilar crystal structure, with special consideration not to create large galvanic potential between the two as that could also lead to galling. The use of coatings on fasteners is often the solution to keep galling from occurring. A hardness differential between mating components can also improve resistance to galling. A related failure mechanism is fretting, where severe adhesion wear occurs without the transfer of materials between the fastener parts. 4.4.11 Thread Seizing (General Corrosion) Thread seizing often occurs due to general corrosion while the threads are engaged under high load. Although this does not often result in breaking of a connection, it can be considered a common failure because fasteners are likely to be destroyed or rendered no longer fit for service should they ever come off. Coatings are a good solution to this problem. Durability of the coating should be suitable for the intended service and expected life of the fastener being used. 4.4.12 Stress Relaxation Increases in temperature above 500 °F (260 °C) provide enough thermal activity in the material at the atomic level to allow the material to elongate to accommodate the strain; this results in the stress decreasing. Stress relaxation is used to remove residual stresses and reduce the risk of environmental cracking, such as stress corrosion cracking or hydrogen embrittlement. Controlling the grain size and being aware of environmental considerations can reduce this effect. 4.4.13 Creep When bolt material starts to creep, the bolt extension increases without any increase in the bolt load. This reduces the bolt stress and hence, the load retained in the bolt. Because creep is defined as continued extension under constant stress, the effect seen in a bolt is not consistent with the definition of creep. Other terms, such as high-temperature stress relaxation or creep relaxation, are used, but the cause is the same. 4.4.14 Corrosion 4.4.14.1 General The discussion of corrosion in this section will be limited to some of the most common types of corrosion in the oil and gas industry and its effects on low alloy steel, stainless steel and CRA bolting. This discussion will have a higher focus on external environment than internal environments due to the complexity of environments that can be found in drilling and production environments. a) Low-alloy steel considerations (if Ni + Cr + Mo + Cu < 5 wt %). For low-alloy steel, bolting corrosion occurs predominantly as general corrosion (See Figure 2). For the most part, general corrosion, sometimes called “general mass loss corrosion,” occurs in moist locations, such as offshore topsides, or in coastal areas with marine environments. Also, subsea environments lead to general corrosion if the bolting is not protected by the cathodic protection system or by use of barrier coatings.

12

API Technical Report 21TR1

Figure 2—Example of Offshore Bolting Corrosion b) CRA and stainless considerations: More aggressive corrosion environments may require corrosion-resistant alloys, such as stainless, duplex/super duplex, copper alloys, nickel-chrome-molybdenum alloys, cobalt, or super alloys with mostly nickel making up the composition. Crevice corrosion and pitting is more prevalent in CRA materials. Making the right choice of CRA or stainless material will help mitigate the problem of these types of corrosion. 4.4.14.2 Sulfide Stress Cracking (SSC) Sulfide stress cracking (SSC) can occur when low-alloy steel bolts are in the presence of a sour service environment under tensile load, tensile components, or even in the presence of internal stresses (without exterior load). For low-alloy steel bolts that are not directly exposed to production or process fluids, a sour environment can be produced due to production-fluid leakage to areas under insulation. Selecting material in compliance with the requirements of NACE MR0175/ISO 15156 can eliminate the risk of SSC. Bolting material manufactured from CRAs when used for external applications can also be exposed to a sour environment when production fluid leaks to areas under insulation. As a result, bolting material when used in the aforementioned situation falls under the requirements of NACE MR0175/ISO 15156–3 to prevent SSC. 4.4.14.3 Environmentally Assisted Cracking (EAC) Environmentally assisted cracking refers to a general group of cracking threats that occurs in bolts made from CRA under the conditions of stress, an environment, and a material that is susceptible to cracking due to its properties, such as microstructure, hardness, composition, and processing history. EAC in low-alloy steel bolting is not considered in this document since low-alloy steels do not normally exhibit this mode of failure in low-pH environments. 4.4.14.4 Stress Corrosion Cracking (SCC) Stress corrosion cracking (SCC) may occur in bolts made from CRAs under stress and exposed to a corrosive environment at elevated temperatures. The stress can be internal (such as residual stress) or external. Failure of an otherwise ductile part occurs at a stress much lower than its yield strength, originating at surface imperfections (typically pits or cracks) created by the corrosive environment. The three factors needed for SCC to occur are a susceptible microstructure, a corrosive environment, and tensile stress. To prevent SCC, one of the three contributing factors needs to be eliminated. 4.4.14.5 Chloride Stress Corrosion Cracking (CSCC) Bolts made from stainless steels are mostly susceptible to this type of cracking mechanism. This occurs when the stainless steel bolting under stress is exposed to a chloride-containing environment, such as seawater at elevated temperatures. The concentration of chloride and temperature to promote CSCC depends on the grade of stainless steel. A more obvious means of prevention is selection of CRA bolting materials other than stainless steels that are fit for purpose. This includes bolts manufactured from nickel-based alloys.

Materials Selection for Bolting

13

4.4.14.6 Localized Corrosion Bolts manufactured from stainless steels or even higher-grade CRA material can be susceptible to localized corrosion (pitting or crevice) if they are not protected by CP. This degradation mechanism can become more prominent in environments with high oxygen and halide anion concentration (such as chloride [Cl]) at elevated temperatures. Corrosion pits are very localized and difficult to protect as it can form a pinhole where coating was not properly applied or has broken down over the service life. Crevice corrosion is equally difficult to protect, as it occurs in small crevices between the bolting/flange and bolting/nut areas. Pitting and crevice corrosion can eventually lead to SCC or CSCC of the bolting due to the imposed stresses. To prevent pitting/crevice corrosion, bolting material selection for the intended service is extremely important. For seawater applications, industry standards recommend CRA bolting material with a high pitting-resistance equivalent number (PREN). The PREN is based on the proportions of elements such as chromium (Cr), molybdenum (Mo), tungsten (W), and nitrogen (N) in the chemical composition of the alloy. The higher the PREN number, the higher the resistance of the CRA material to pitting and localized corrosion. Alloys with PREN greater than 40 have demonstrated adequate resistance to localized corrosion. Pitting corrosion is a form of localized corrosion that requires only one metal in the presence of an electrolyte to set up an attack system. This type of corrosion begins with the creation of a differential in the concentration of oxygen at the metal surface, which produces an extremely localized battery effect. Pitting corrosion is more difficult to predict or protect, as it can form in very small pinholes where coating was not properly applied or has broken down over the service life. Bolting can potentially fail when severe pitting occurs. To prevent pitting, proper material selection for the intended service is paramount; in seawater applications, austenitic stainless steels and stainless steels with a PREN greater than 40 have demonstrated to have good pitting resistance (among other materials). Additionally, coatings may be used as a barrier to enhance the pitting resistance of materials otherwise known to be susceptible to pitting in a given environment. It is important to note that some coatings may not produce a perfect barrier or may not be durable, or may fall off or degrade. In some cases, these conditions can result in more localized and more severe pitting. 4.4.14.7 Corrosion Barriers for Steels General corrosion is a uniform loss of mass in the metal when it reacts with the surrounding environment. With regards to this document, general corrosion is mostly associated with low-alloy steel bolting when exposed to an environment without protective barriers. Corrosion barriers to corrosion threats are surface barriers that limit or stop the bolting steel surface from being wetted by water or exposed to oxygen. Barriers take the form of: — organic coating; — inorganic metal coatings; — inhibitors/oils. Another approach or mitigation barrier for bolting submerged in water or seawater is cathodic protection, which again acts on the steel surface to shift the surface environment to a condition that prevents corrosion. Organic coatings reduce or mitigate the exposure of bolting to oxygen or water. These coatings include epoxies or polytetrafluoroethylene (PTFE)-based coating systems. Application discontinuities or degradation of the paints or coating over time allows the ingress of oxygen or water to the steel substrate. This happens through coating holidays (discontinuities) or voids through misapplication, cracks in the coatings, degradation of the coating by ultraviolent exposure, aging resulting in flaking off or cracking of the coating, and mechanical damage in-service, such as make-up or break-out of bolting and handling.

14

API Technical Report 21TR1

Plating with inorganic metal coatings is a popular approach to mitigating general or crevice corrosion. Plating applications used in the industry can be chrome, zinc, aluminum, cadmium, nickel-zinc, nickel-cobalt and others. Cadmium is still being used; however, due to the hazardous nature in the plating process and exposure to the environment, it is being phased out. Chrome plating is used in limited applications due to its propensity for cracking and flaking. The use of chrome plating is generally associated with galling prevention rather than general or crevice corrosion. Zinc, aluminum, and nickel-zinc provide a barrier as a secondary mitigation. The primary barrier is one of cathodic protection that shut down the surface corrosion reaction by shifting the potential (voltage) to a more negative value. This shifting of potential is caused by a galvanic couple between the steel and the zinc, aluminum, or nickel-zinc. These coatings work fairly well; however, the plating is consumed with time requiring more plating to be sacrificed to last years versus months in water (seawater). These coatings are typically applied at 0.20–0.30 mils and do not require overtapping of nuts. Alternate performance plating corrosion barriers that mitigate corrosion include high-phosphorous, electroless nickel plating, nickel-cobalt plating, and other non-sacrificial barriers. As with other plating processes, these barriers do not generally require oversizing of the nut to allow make-up. Often, bake-out is required if hardness of a low-alloy steel bolting system exceeds 39 HRC (see ASTM B633 and ASTM F1941). However, when API 20E BSL3 is specified, all low-alloy steel bolting that is electroplated requires baking. Some of these plated coatings resist deformation and mechanical damage. Finally, the corrosion resistance of non-sacrificial plating allows easy break-out and mitigates seizing after extended service of the bolting. Inhibitors/oils have shown fair protection of bolting by acting as a barrier to water on the steel surface for shortterm storage prior to use. However, they work best in mild service conditions, such as indoor, low-humidity, and non-submersed onshore conditions. For any coating, it is important to recognize that the thickness of the coating should be controlled such that threads can stay within dimensional tolerance and nuts are not oversized. The consequence of requiring thicker plating may lead to thickness buildup and adhesion issues, and/or thread interference. To solve this issue of interference, manufacturers have oversized the nuts for bolting, which would lead to lower bolting strength and load-carry ability. Oversizing of nuts is not allowed in API Specification 20E, 2nd edition. 4.5 Manufacturing Processes 4.5.1 General This section details some common manufacturing processes that may affect the performance properties of bolting. The quality of the steel and how the bolts are manufactured contributes to different properties within the bolts. Figure 3 provides an overview of the primary production processes discussed in this document.

Materials Selection for Bolting

15

Figure 3—Overview of Production Processes 4.5.2 Steelmaking 4.5.2.1 General The following steelmaking processes/properties impact the performance of bolting. It is up to the discretion of the bolt manufacturer to determine the appropriate thresholds/limits of each of these factors depending on their application. 4.5.2.2 Reduction Ratio An increase in reduction ratio aids in the homogenization of the microstructure and closes any porosity that may present itself during solidification. Typically based on the starting size of the steelmaking process, ingot cast material tends to have a higher reduction ratio than continuous cast material. The more reduction there is in steel, the smaller the banding wavelength will be. The reduction ratio in round bar is typically calculated by dividing the starting cross-sectional area by the finishing cross-sectional area. Large bars are typically press forged for center soundness. Minimum 4:1 reduction ratio is commonly specified, as it produces a fully wrought structure when proper tooling, forging sequence, and forging temperatures are employed. 4.5.2.3 Chemistry Variation Depending on the steelmaking process, the chemistry may vary throughout the product. Whether it is continuous cast and certain alloying elements coagulate at the end of the pour, or if it is ingot cast and there is a difference between the top of the ingots and the bottom of the ingots, manufacturing processes (such as stirring) ensure the chemistry variation is limited throughout.

16

API Technical Report 21TR1

To measure chemistry variation, chemical analysis can be performed at different parts of the heat (ingot top and bottom, beginning of heat, end of heat, etc.). Various specifications address this issue, including a magnetic stepdown test; for example, SAE AMS2300. 4.5.2.4 Cooling Rates In terms of banding, if there is no time for carbon diffusion, a martensitic microstructure will result and there will be minimal microstructural banding. Slow cooling allows carbon diffusion during transformation (transegregation), producing non-martensite microstructural bands. 4.5.2.5 Microstructure/Macrostructure, Grain Size A targeted microstructure of materials can greatly improve the properties of the end product. This can be achieved by establishing controls in melting, hot working, and heat treatment. 4.5.2.6 Banding All steel contains some degree of microsegregation (i.e. banding) and will manifest in the microstructure (see Figure 4). During hot rolling, the as-cast dendritic structure is broken down and elongated parallel to the rolling direction. This forms the basis for the visible aligned banding morphology. During cooling after rolling (or heat treatment), alloy interactions modify the local transformation temperature in the microsegregation bands. These affinity differences will determine the final transformation product formed in each microsegregated band. The initial solidification dendrite structure is driven by the cooling rate and is a function of the continuous casting/ ingot process (superheating, section size, speed, stirring, specific equipment unit).

Figure 4—Ferrite (light) and Pearlite (dark) Banding Hot rolling breaks up the initial dendritic structure and aligns segregated regions into pancake/needle-like regions parallel to the bar. Besides the microstructural bands observed as a result of the solidification dendrite structure, there also exists a banding phenomenon known as “white banding.” These white bands are formed during the continuous-casting

Materials Selection for Bolting

17

process by the electromagnetic stirring (EMS) operation. EMS aids in the proper distribution of alloying elements within a continuous-casting process, but also forms the white bands that consist of a zone of negative segregation where there is solid-liquid interface during stirring and a zone of positive segregation as a result of the cessation of stirring. The positive segregation section of the white bands tends to be solute-rich at the end of the stirring operation. The most deleterious feature of the white bands is their cosmetic appearance, but the change from negative to positive segregation (with a transition region in between) does exist [2]. It is recommended to optimize the EMS stirring process to minimize the amount of white banding, while also ensuring that alloying elements are distributed homogeneously through the material. While the microstructural manifestation of banding can be masked by heat treatment, banding cannot be eliminated or even significantly reduced by thermal treatment. A homogenous microstructure will lead toward more consistent properties, so a minimized banded structure with as little variation in hardness between bands is desirable. In low-alloy steel, common substitutional alloy elements (Cr, Mn, Ni, Mo, Si, Cu) diffuse slowly, so to achieve a homogenized microstructure, high temperatures and long durations, followed by grain refinement heat treat cycles, is required. For example, if there is no time for C to diffuse, the microstructure will consist fully of martensite with minimal microstructural banding appearance. Slow cooling allows for C diffusion during transformation (transegregation), producing non-martensite microstructural bands. However, the cooling rates required to reduce/eliminate banding are too great to be commercially viable. Not all banding is considered deleterious; however, under certain conditions, it could cause hydrogen-induced cracking, as indicated in NACE MR0175/ISO 15156. Trap sites capable of causing HIC are commonly found in steels with high impurity levels that have a high density of planar inclusions and/or regions of anomalous microstructure (e.g. banding; see 3.1.4) produced by segregation of impurity and alloying elements in the steel. Tests for banding include those detailed in ASTM A534 and ASTM E1268. 4.5.2.7 Inclusions Inclusions are nonmetallic compounds that are usually formed during deoxidation, refining, teeming, and/or solidifying processes in steelmaking. Inclusions are categorized as either indigenous or exogenous. Indigenous oxide inclusions originate in the molten steel resulting from deoxidation or reoxidation reactions. This usually involves reactions between dissolved oxygen and deoxidizing elements, such as aluminum and silicon. Other types of indigenous inclusions include sulfides, and nitride and carbonitride precipitates. These types of inclusions form by solidification-driven segregation. When the solubility product of an inclusion compound is exceeded, the compound will form as an inclusion. Common examples of these types of inclusions are manganese sulfides (MnS) and titanium carbonitrides (TiCN). Additions of calcium or rare earth elements are made during refining and used to modify sulfide shape. Inclusions are most detrimental in the transverse direction. Exogenous inclusions form from the entrainment of nonmetallic materials that are in contact with the liquid steel. Exogenous inclusion sources can include slags and refractory material. These types of inclusions are usually trapped by turbulent conditions at the interface during refining and/or casting. Exogenous inclusions are typically large, irregularly shaped, and infrequently spaced in steel. While the size of exogenous inclusions can be particularly detrimental to steel component performance, the probability of these inclusions causing component failure is low due to their low frequency of occurrence as a result of controlled manufacturing processes. Qualification: The cleanliness of steel may be rated on both macroscopic and microscopic levels. Macrocleanliness may be determined by macroscopic rating methods such as high-resolution ultrasonic inspection, macroetch, or magnetic particle inspection to qualify a heat to either SAE AMS 2301 or SAE AMS 2304. Many microscopic rating methods exist for characterizing and measuring micro-cleanliness. One of the most common methods is ASTM E 45–05 Method A (worst field analysis). In this method, inclusions are categorized into the following groups: Type A-sulfides, Type B-alumina, Type C-silicates, and Type D-globular oxides. Brief descriptions of each type follow:

18

API Technical Report 21TR1

— Type A: Elongated inclusions, usually sulfides, with aspect ratios 2:1. Typically blue/gray in appearance with rounded ends. — Type B: Aligned cluster (at least three individuals) inclusions of angular low aspect ratio (<2:1). The clusters are aligned in the rolling direction. Typically dark/black aluminum oxide particles, but may also be sulfides, etc. — Type C: Elongated and continuous inclusions, typically with high aspect ratios (2:1) and sharp ends. Often dark/black silicates. — Type D: Isolated (not part of a stringer) low-aspect-ratio (<2:1) sulfide and oxide inclusions. Rated as number per field. Once the inclusions are classified by type, they are classified by their thickness as either thin or heavy. Inclusions whose width is less than 2 um (0.00008 in.) are not rated. A severity rating is determined for inclusion types A, B, and C by adding together the total inclusion length per field. Type D inclusions are counted instead of measured. The length measurements or counts and the severity level table in the ASTM standard are used to generate the severity number for each inclusion type. NOTE Other acceptable methods include, but are not limited to, ultrasonic examination and magnetic particle inspection to SAE AMS2301 and AMS2304.

4.5.3 Casting 4.5.3.1 General Subsequent to the steelmaking process, manufacturing operations will also affect the performance of bolting, as well as dictate what type of bolting is being produced. The casting process can be performed either through ingots or a continuous process. The bottom pour ingotmaking process involves filling an ingot mold with molten steel from the bottom up. Continuous casting is when cast steel is drawn through the bottom of a mold as it solidifies. 4.5.3.2 Ingot Versus Continuous Cast The wire, rod, and bar used for bolting or stud manufacturing are usually made from raw materials produced either by continuous casting or ingot casting. For continuous casting, the reduction ratio could be very small due to the starting cast size and the final product size. As a result, cast microstructure, porosity, banding, and/or other discontinuities may exist that degrade the bolting mechanical properties and increase susceptibility to HSC. By contrast, higher reduction ratio is often obtained in bolting manufactured from ingot casting materials, mainly due to the larger starting cast and the subsequent drawing (hot working) process. Therefore, the resulting materials have desirable wrought microstructure and significantly reduced detrimental features, such as porosity, segregation, and banded structure. The bar stocks (usually large dimensions) for bolting manufacturing can also be made by forging. In this process, a hot ingot is progressively pressed and shaped in a rotating manner until the desirable dimension is reached. Although materials produced by forging exhibit wrought microstructure, the grain flow may not be uniformly linear. Therefore, bolting manufactured by such a process may or may not have superior properties than the drawn ingot casting material. As a result, it was recommended in API Specification 20E, 2nd edition and API Specification 20F, 1st edition that the preferred material processing for bolt manufacturing for fatigue-sensitive subsea applications should be either from ingot casting or forging. Bolts manufactured by continuous casting have been used and still continue to be used. For fatigue-sensitive applications, the following conditions should be considered:

Materials Selection for Bolting

19

— Chemistry of the cast bloom or billet should have the total weight percentage of tramp elements below 0.05 % and calcium should be 0.005 % max. Boron should not be added intentionally and the max boron should not exceed 0.0005 %. — Steel microcleanliness should be performed as required by API Specification 20E for BSL3. — For continuous casting, the starting cast size should be large enough such that the final product size has experienced adequate drawing or hot working that has resulted in a desirable wrought microstructure. A high work ratio is desirable to ensure the bolt is fully worked. A work ratio of 20:1 minimum is recommended. — The wrought microstructure should contain no detrimental features, such as porosity, segregation, and an excessively banded structure. This should be validated during first article qualification. Extent of banding should be evaluated per API Specification 20E. One major difference in banded microstructures made from continuous cast steel vs. ingot cast is that there is a more frequent occurrence of white banding; this is a result from stirring processes common to continuous cast. — Microhardness should be performed in the dark and light areas of the microstructure (if apparent) and the hardness should not vary by more than 5 Knoop points (whether bands are apparent or not). Bolts manufactured by forging should have a wrought microstructure free from nonlinear grain flow that can result in inferior mechanical properties and HSC resistance. This should be validated during first article qualification. 4.5.4 Heat Treating Heat treatment covers various techniques that may be used to develop certain end product characteristics. Common procedures for fasteners include annealing, normalizing, “austenitize, quench and temper”, solution annealing, and aging. Other types of heat treatment may be case hardening, boronizing, and carbon restoration. Bolting made from precipitation-hardened nickel base alloys has been known to fail in service due in part to microstructural features that occur when heat treating to achieve high strength. It is recommended when using precipitation-hardened nickel alloy bolting to select material within the strength limits (as well as all other aspects) of API Standard 6ACRA, unless the application is such that deleterious phases such as delta phase will be of no consequence such as in high-temperature applications. 4.5.5 Production of Bolts 4.5.5.1 Starting Stock—Hot and Cold Working The choice of hot or cold working is dependent on the application, size of the bolting, and mechanical requirements. Hot working serves best where higher reductions are required. Cold working serves best when higher strength is required. Special consideration should be taken for sour service environments. See NACE MR0175/ISO 15156 for more information. 4.5.5.2 Cut Versus Rolled Threads Threads of a mechanical fastener (regardless of whether it is a headed bolt, rod, or bent bolt) can be produced by either cutting or rolling. The differences, advantages, and disadvantages of each method are described in this section. Cut threading is a process by which steel is cut away or physically removed from a round bar to form the threads. The size of the round bar must be equal to or greater than the finished size. Roll threading is a process by which steel is extruded to form the threaded portion of a fastener instead of being removed, as in cut threading. In this process, a bolt is manufactured from a reduced-diameter round bar. Rolled threads require dies to produce threading. Multiple dies would be required to produce different thread patterns. Multiple thread patterns can be achieved by the same machine when cut threading.

20

API Technical Report 21TR1

Figure 5—Thread Types Hardness and the susceptibility to fatigue and EAC growth are subjects that require balance in the design, material selection, and manufacturing processes specified to minimize the potential of failure for bolts that are susceptible to these failure mechanisms. It has long been a practice to use rolled threads for fatigue resistance; however, rolled threads can be counterproductive to resistance to hydrogen cracking in some service environments, particularly in seawater with CP applied. Information regarding the effects of rolled threads and cathodic protection can be found in SPE 178431. The severe cold work in the threads imparted by the rolling of the threads may produce hydrogen trap sites. The residual internal hydrogen poses a threat to cracking at the location of the highest stress and hardness, increasing the potential for hydrogen cracking in rolled threads. Written procedures and adherence to the postplating bake out requirements, along with maximum hardness levels, are necessary for minimizing the likelihood of HIC failures in bolts. When threads are rolled, API Specification 20E requires parts be subsequently stressrelieved at a temperature within 50 °F (28 °C) of, but not exceeding, the final tempering temperature. Because of the recognized benefit of rolled threads on fatigue resistance of bolts, many industry standards either explicitly or implicitly encourage the application of rolled threads. Many existing standards only require hardness measurements in the bolt core. The required hardness measurement in the bolt core is sufficient to define the mechanical properties of bolting, but does not consider the existence of increased hardness of the rolled threaded area and magnified risk of HE during subsea service and CP. However, stress relieving after a threadrolling operation reduces susceptibility to hydrogen embrittlement. Post-rolled thread heat treatment procedures should be qualified by bolting manufacturers to reduce the average near surface thread area hardness to 336 HV10 (34 HRC) for low-alloy steels. For low-alloy steel bolting that may be exposed to sour environment due to joint leakage beneath thermal insulation, refer to the requirements of NACE MR0175/ISO 15156–2. Threads produced using a machine-cut process are acceptable. Threads produced by roll threading are acceptable in low-alloy steel bolting that otherwise conform with heat treatment, composition, and hardness requirements of NACE MR0175/ISO 15156–2. 4.5.6 Chemical Composition and Hardenability of Low-alloy Steel Bolting For low-alloy steel bolting, it is important to note the mechanical properties may vary through the thickness of the material. Lack of hardenability sometimes causes this variation. This can be due to lean chemical composition or slow quench rate. B7 and L7 are chromium-molybdenum low-alloy steels used for bolting. Use of these bolting grades should be limited to 2.5 in. in diameter, as the leanest forms of their composition may result in lower mechanical properties through the thickness. For low-alloy steel bolting with a diameter greater than 2.5 in., it is recommended to use a different composition such as L43, which is based on AISI 4300 series chromium-nickel-molybdenum alloys. If the desire is to maintain the use of B7 or L7 composition, it is important to modify the chemical composition to produce an ideal diameter (Di) or a critical diameter (Dc) capable of achieving the desired properties throughout the thickness. The modification of chemical composition can be achieved through the use of multiplying factors for alloying elements and by taking the grain size and severity of quench into account. See ASTM A255 for guidance on how to calculate Di and Dc.

Materials Selection for Bolting

21

4.5.7 Mechanical Properties and Testing 4.5.7.1 General This section describes the different mechanical properties that may be applied to bolting manufacture. Final mechanical property requirements are determined by the user and confirmed by destructive testing. 4.5.7.2 Tensile Test Tensile properties should be selected such that bolting can take the intended loads of the design. Design codes will often set safety factors on bolting that must be taken into consideration when selecting minimum yield strength and tensile strength. Percent elongation and percent reduction of area should vary from one grade to another based on composition and strength. It is recommended to follow recognized industry standards to set minimum requirements. The testing is generally conducted in accordance with ASTM A370 or ASTM E8. When strength requirements are moderate, low-alloy steel can be tempered to ranges such as those specified in ASTM A193, B7M, or ASTM A320, L7M. Other low-strength applications may be satisfied by the use of austenitic stainless-steel bolting as per ASTM A193. High-strength fasteners can be made from low-alloy steels with medium or high carbon content, or by using precipitation-hardening materials. Suitable heat treatment processes may be used to achieve desired properties. The use of high-strength bolting in the oil and gas industry is generally limited by effects of environment. Strength limitations on low-alloy steel, stainless and nickel-based bolting are given in API Specification 20E and API Specification 20F among other codes. 4.5.7.3 Impact Testing At a minimum, impact testing should be performed on closure bolting and bolting used in the main load path on primary load carrying members. The testing is generally conducted in accordance with ASTM A370 or ASTM E23. The test temperature should reflect service temperature or lower. Impact testing has proven to be a good measure of the quality and toughness performance of low-alloy steels and can be used as a quality assurance tool to ensure material can be used in low-temperature applications. Many codes have acceptance criteria for absorbed impact energy when testing bolting. ASTM A320, L7 and L43, require impact testing and have a good track record of performance in low temperatures in oil and gas applications. ASTM A193 does not require impact testing unless specified as supplemental requirements. 4.5.7.4 Hardness Testing Hardness is usually measured by the Brinell, Rockwell, or Vickers indentation-hardness test methods. The testing is generally conducted in accordance with ASTM E10, ASTM E18, or ASTM E384 (respectively). Establishing hardness limits can help mitigate failure in different environments. For fasteners in direct contact with sour service environments, NACE MR0175/ISO 15156 has limited the use of low-alloy steel bolting to B7M, L7M, and ASTM A194 2HM, which are limited in ASTM to 22 HRC maximum. Other standards such as API 20E have limited the hardness of low-alloy steel fasteners to 34 HRC to mitigate the effects of hydrogen embrittlement. Hardness is often used as a quality control check to make sure the material is in the required tensile range. Microhardness is used to evaluate microstructural properties that macrohardness is too large to accurately measure. For example, the hardness variation from banding cannot be detected using Rockwell or Brinell testing.

22

API Technical Report 21TR1

4.6 Protective Coatings 4.6.1 General Coatings are applied to bolting to protect it from corrosive environments or to achieve/enhance desired properties, such as lubricity for consistent torqueing. There are different types of coatings that are used depending on the type of protection and duration of protection required. 4.6.2 Types of Protection The following are types of protective coatings: — sacrificial coating; — barrier coating. 4.6.3 Duration of Protection The following are types of protection durations: — Short-term protection: Typically, thin electrodeposited coatings provide enhanced shelf life, cosmetics, and limited corrosion protection. Short-term protection is generally used for storage purposes. — Long-term protection: Protection generated by sacrificial, barrier, or coating systems intended to last for an extended period (typically greater than four months). — Typical coatings and platings used in the oil and gas industry are: — phosphate coating; — polytetrafluoroethylene (PTFE)-based coating; — ceramic-metal (cermet) coating; — zinc plating; — nickel-cobalt plating; — cadmium plating; — zinc-nickel plating. 4.6.4 Types of Coating and Plating Metallic coatings are applied to fasteners for corrosion resistance, decorative purposes, and extended shelflife. Metallic coatings are primarily sacrificial in nature. Common electrodeposited metallic coatings for use on fasteners include zinc, cadmium, nickel-cobalt, and zinc-nickel. Corrosion resistance is dependent on the corrosion rate of the given coating system and the thickness of the coating. Hot dipped galvanizing is also a fastener coating, but is used less because of interference issues that arise due to inconsistent coating thickness and tight tolerances. Cermet coatings, typically aluminum filled, are used extensively as base coats, but require topcoats (typically fluoropolymer) for lubricity required for uniform torque. Cermet coatings could cause thread interference, and this should be considered (reference API Specification 20E for more guidance). The electroplating process varies with regard to types and desired results. Most plating processes generate hydrogen. Post-baking after plating is used to diffuse internal hydrogen. If the post-bake temperature is not high

Materials Selection for Bolting

23

enough, the hydrogen does not diffuse. If the post-bake temperature is too high, the plating can be compromised. If plating temperature exceeds the optimized operating limits of the bath or if improper temperature control during post-bake occurs, it can lead to premature bolt failures due to hydrogen embrittlement and loss of corrosion protection. The following precautions should be noted: a) The plating material is usually the controlling factor for maximum service temperature. b) Hydrogen embrittlement is possible with most common methods of plating, unless special manufacturing procedures are used. For example, ASTM B850 and ASTM F519. c) The use of dissimilar materials can create galvanic corrosion issues. d) Improper installation may contribute to stress relaxation and fatigue issues. Nonmetallic coatings for fasteners typically are fluoropolymers offering a combination of corrosion resistance, wear resistance, and lubricity. They can be applied over phosphate-etched substrate where lubricity is required, or as a topcoat to zinc, cadmium, or zinc-nickel. In some instances, nonmetallic coatings can introduce clearance issues due to the thickness of the coating system. Some examples of nonmetallic coatings are: — Corrosion protection: Fluoropolymer coatings offer corrosion resistance where the film is not damaged or breached. However, they can be subject to extensive damage during installation (mechanical or UV exposure), thus exposing the substrate to corrosion. — Lubricity: Some fluoropolymer fastener coatings are used primarily as a dry film lubricant, offering consistent torque values. Typically, no additional lubricants, greases, or otherwise are required with fluoropolymer coatings due to the PTFE content. Marine epoxy topcoats can be applied to fasteners after assembly for additional protection (specifically for offshore environments), but are not considered fastener class coatings due to limited corrosion resistance and thicknesses that may impair fit and function. Table 2—Carbon and Low-alloy Steel Bolting Selection Guide and Limitations B7

L7

105 minimum iield, 35 105 minimum yield, 35 HRC HRC max (34 HRC per API Max (34 HRC per API 20E); 20E); not impact tested impact tested Generally suitable for land and subsea with CP applications where low temperature is not anticipated on the design.

Generally suitable for land and subsea with CP applications.

B7M/L7M

L43

Minimum yield strength is 80,000 psi, but requirement varies with diameter; 235 HBN max

105 minimum yield, 35 HRC max (34 HRC per API 20E); impact tested

Generally suitable for land and subsea with CP applications.

Generally suitable for land and subsea with CP applications.

24

API Technical Report 21TR1

Table 2—Carbon and Low-alloy Steel Bolting Selection Guide and Limitations (Continued) B7

L7

B7M/L7M

L43

Can be used as closure bolting, including in sour service applications where bolting is not directly exposed to the environment and hydrogen has means of escaping to the environment (see NACE MR0175/ISO 15156). Since impact testing is not required, L7 is generally a better option for this application.

Can be used as closure bolting, including in sour service applications where bolting is not directly exposed to the environment and hydrogen has means of escaping to the environment (see NACE MR0175/ISO 15156).

Bolting to these grades is suitable for sour service environments

Can be used as closure bolting, excluding sour service applications.

Can be used in primary load path components for designs where low temperature is not an issue, but it is considered better suited for structural bolting not in the primary load path due to lack of impact test requirements.

Can be used in primary load path components.

L7M and B7M may both be used in primary load path; however, L7M is impact tested for low-temperature service.

Can be used in primary load path components.

For most applications, B7 will require a barrier coating against the environment both for shelf life and service environments.

For most applications, L7 will require a barrier coating against the environment both for shelf life and service environments.

For most applications, these grades will require a barrier coating against the environment both for shelf life and service environments.

For most applications L43 will require a barrier coating against the environment both for shelf life and service environments.

B7 bolting is not recommended when the design requires the fasteners to exceed 2.5 in. in diameter. It is generally recommended to use L43, or to specify a B7 composition with a minimum ideal diameter for the size of the fastener.

L7 bolting is not recommended when the design requires the fasteners to exceed 2.5 in. in diameter. It is generally recommended to use L43, or to specify an L7 composition with a minimum ideal diameter for the size of the fastener.

The properties of these grades may degrade in larger diameter due to alloy content. For larger diameter bolting, adjusting the ideal diameter (by composition) may be necessary.

Due to its alloy composition, L43 is very well suited for lowtemperature applications and for bolting in which diameter exceeds 2.5 in.

Special measures are required to minimize the risk of hydrogen embrittlement when exposed to environments that produce hydrogen ions, such as plating, phosphating, or some chemical cleaning operations; these can be alleviated by hydrogen bake-out.

Special measures are required to minimize the risk of hydrogen embrittlement when exposed to environments that produce hydrogen ions, such as plating, phosphating, or some chemical cleaning operations; these can be alleviated by hydrogen bakeout.

Special measures are required to minimize the risk of hydrogen embrittlement when exposed to environments that produce hydrogen ions, such as plating, phosphating, or some chemical cleaning operations; these can be alleviated by hydrogen bake-out.

NOTE Other low-alloy steel bolting, such as ASTM A540 grades B22 and B23, are explicitly allowed in API Specification 20E and may be used where higher hardenability is required.

Materials Selection for Bolting

25

Table 3—CRA Bolt Selection Guide and Limitations ASTM A453 Gr 660D

Age Hardened Nickel-based Alloys (API 20F)

105Ksi minimum yield, 24–35 HRC hardness range

Minimum Yield will vary for different alloys. It ranges from 110K to 160K minimum yield in API 6ACRA

Generally suitable for land and subsea with CP applications.

Generally suitable for land and subsea with CP applications, but mostly used in aggressive environments where high strength and resistance to general corrosion is required. Suitable for sour service applications provided there is compliance with NACE MR0175/ ISO 15156 (including specified service limitations)

Can be used in primary load path components.

Can be used in primary load path components.

Gr 660 bolting should be made from material made in accordance with API Standard 6ACRA.

Age hardened nickel-based alloy bolting should be made from material made in accordance with API Standard 6ACRA and API Specification 20F.

NOTE The bolting selection guide tables are not inclusive of all available bolting and are only to be used as guidelines. These tables are not intended to supersede requirements from applicable specifications and or construction codes.

Annex A (informative) Testing of API 20E Fastener Materials for Susceptibility to Hydrogen Embrittlement Under Cathodic Protection in Simulated Seawater A.1 Scope This study was completed in response to recommendations issued by the API Multi-Segment Task Group on Bolting Failures (February 29, 2016). The task group concluded if identification of maximum strength and hardness limits for subsea service were not resolvable through existing industry or ASTM standards, additional work should include requirements for maximum hardness on bolting material (TGR-1). A workgroup was established to develop a program to generate data to provide guidance to affected API product subcommittees. Low-alloy steel fastener materials manufactured in accordance with API 20E BSL-3 were submitted for testing. This Annex is a summary of the results. It should be noted this program did not evaluate the performance of lower API 20E specification levels or other industry standard bolting materials.

A.2 Materials Test materials were manufactured in accordance with API 20E BSL-3 by a licensed manufacturer. The steel was produced by electric arc furnace with secondary refinement into a primary melt ingot. The ingot was forged into intermediate billet and hot rolled into a 1.625-in. diameter bar with a final reduction ratio of 377:1. The certified product chemistry, shown in Table A.1, was within limits for AISI 4140. Samples were extracted and heat treated to four different target yield strengths and hardnesses: 120 ksi/30 HRC, 130 ksi/32 HRC, 140 ksi/34 HRC, and 160 ksi/38 HRC. The heat-treating process included normalizing at 1650 °F for 2.5 hours, with air cooling down to 1600 °F, then austenitizing for 2.5 hours. This was followed with an oil quench to 110 °F and final tempering for 2.5 hours at grade-specific temperatures, ending with a water quench to room temperature. Tempering temperatures and mechanical properties reported on the material certificates are shown in Table A.2. Grain size for the heat was determined to be 8 in accordance with ASTM E112-13. Table A.1—Product Analysis in Weight Percent of 4140 Bar Carbon

Manganese

Phosphorus

Sulfur

Silicon

Molybdenum

Chromium

Boron

0.41

0.97

0.011

0.03

0.23

0.20

1.03

0.0004

Table A.2—Manufacturer Reported Mechanical Properties of Heat-Treated Bar Tempering Temperature, F

Grade

0.2 % Yield Strength, ksi

UTS, ksi

Total Elongation, %

Reduction in Area, %

Hardness, HRC

1155

4140–120

123.1

141.2

20

61.1

30

1075

4140–130

134.6

153.9

18.2

60.9

34

1008

4140–140

143.7

157.7

17.9

59.4

34

925

4140–160

177.7

191.0

14.5

50.5

38

26

Materials Selection for Bolting

27

A.3 Testing A.3.1 General To determine the relative sensitivity of the different strength materials to cracking when exposed to simulated seawater and cathodic potentials, incremental step loading was used to determine cracking threshold stress. Testing was conducted in accordance with ASTM F519-13 and F1624-12(2018) standards for evaluating hydrogen embrittlement in service environments. Note the standard deviation is included in cases where it was calculated and reported by the testing laboratory.

A.3.2

Mechanical Properties

Additional tensile and Charpy V-notch impact testing was conducted on supplied materials. The results are summarized in Table A.3. Table A.3—Laboratory Reported Mechanical Properties of Heat-Treated Bar Grade

0.2% Yield Strength, ksi

UTS, ksi

% Elongation

% Area Reduction

Energy, J @ −20C

Energy, J @ RT

4140–120

112.1 ± 10.3

145 ± 2.5

22.0± 1.0

65 ± 3

117.9

130.6

4140–130

128.9 ± 4.2

157.1 ± 0.5

19.3± 0.6

58 ± 1

102.7

112.9

4140–140

144.8 ± 6.2

171.6 ± 0.1

16.3± 0.6

56 ± 0

47.2

77.2

4140–160

163.1 ± 9.0

188.1 ± 1.4

16.0± 1.0

55 ± 0

27.2

51.6

A.3.3

Hardness Testing

A key objective of this study was to validate the current maximum hardness limit of 34 HRC in API 20E. It was noted the manufacturer reported both the 130 grade and the 140 grade materials had the same reported hardness of 34 HRC. All hardness testing conducted by the manufacturer was on the surface of the forged and heated bar. To more fully characterize the hardness, additional hardness testing was conducted on the extracted notch fracture specimens by two labs, one conducting the fracture testing and the other an independent thirdparty lab. The results are shown in Table A.4 and demonstrate the need for additional testing to resolve hardness differences between the grades.

28

API Technical Report 21TR1

Table A.4—Rockwell C Hardness Testing Grade

4140– 120

4140– 130

4140– 140

4140– 160

A.3.4

Measurement

Bolting Manufacturer

Laboratory 1

Laboratory 2

No. of tests

12

32

30

Min.

28.0

27.5

30.9

Max.

31.2

31.8

32.0

Average

29.4

30.3

31.4

Std. dev.

1.0

1.0

0.3

No. of tests

9

32

25

Min.

30.6

30.5

32.8

Max.

34.6

33.6

34.5

Average

32.9

32.6

33.7

Std. dev.

1.1

0.8

0.4

No. of tests

9

36

30

Min.

33.2

33.1

36.7

Max.

36.1

37.1

37.6

Average

34.9

36.2

37.2

Std. dev.

0.9

0.8

0.3

No. of tests

12

36

25

Min.

37.0

37.7

41.2

Max.

40.2

41.8

42.6

Average

39.0

41.7

41.7

Std. dev.

1.1

0.9

0.3

Hydrogen Cracking Threshold Curves

A.3.4.1 General Four-point bend (ASTM F519-13, Type 1e) specimens were extracted from each of the grades and tested to establish baseline performance and maximum load limits in air. Samples were then exposed to simulated seawater and cathodic protection and incrementally step loaded in accordance with ASTM F1624-12(2018) to determine threshold stress necessary to propagate cracking. Tests were conducted at −0.80 V, −1.0 V, and –1.2 V Saturated Calomel Electrode (SCE) at ambient temperature. As noted in DNV RP B401, subsea galvanic CP systems are designed to produce potentials between −0.90 V and −1.050 V (Ag/AgCl) for the life of the system. Potentials more positive than −0.80 are underprotected; systems with potentials more negative than −1.15 V are considered overprotected and generally are only possible with impressed or stray currents. Two test samples of each grade were tested at each potential level. The stress level required to propagate cracking was recorded and divided by the baseline air values to produce the percent notch fracture strength (%NFS) shown in Table A.5.

Materials Selection for Bolting

29

ASTM standards were reviewed for overall %NFS acceptance criteria. ASTM 519-13 and F1624-12 provide guidance for in-service conditions, but note that establishing criteria requires additional testing for use-specific applications. ASTM F2660-13 provides criteria for fasteners; however, it has to be scaled for fastener configuration and size, and cannot be broadly applied. Instead of specific %NFS criteria, the performance of the materials was evaluated for transitions that indicate a shift from ductile to brittle crack propagation behavior. _ ISL ​ × 100​ ​%NFS = ​ FF  

where %NFS

the percent notch fracture strength at the given applied potential;

ISL

the fracture strength at imposed cathodic potential;

FF

fast fracture strength: the baseline fracture load measured by fast fracture of a test specimen at a loading rate of 100 lbs/min. Table A.5—Notch Fracture Strength

Grade 4140–120

−0.80 V (SCE) Underprotected

−1.0 V (SCE) Nominally Protected

−1.2V (SCE) Overprotected

Fast Fracture, lbf

%NFS

Crack Propagation

%NFS

Crack Propagation

%NFS

Crack Propagation

159.4 ± 0.5

94.1 ± 2.8

No

89.8 ± 1.6

No

92.7 ± 3.6

No

4140–130

168 ± 1.1

95.1 ± 1.3

No

89.1 ± 1.5

No

88.1 ± 0.1

No

4140–140

181.5 ± 0.2

98.3 ± 0.4

No

77.6 ± 1.2

Yes

70.8 ± 4.2

Yes

4140–160

200.9 ± 1.0

97.0 ± 3.4

No

77.5 ±11.1

Yes

59.2 ± 4.0

Yes

A.3.4.2

Grades 4140-120 and 4140-130

The AISI 4140 120- and 130-grade samples were loaded until localized yielding was detected. While crack initiation was observed, no propagation (threshold stress for cracking) was observed at any tested potential level. This indicates there is no ductile-to-brittle transition under the tested conditions, and therefore, no curve was plotted. Post-test evaluation of the fracture surfaces using optical microscopy confirmed no detectable propagation was observed.

A.3.4.3

Grades 4140-140 and 4140-160

Both the AISI 4140 140- and 160-grade samples initiated and propagated fractures with a clearly defined ductileto-brittle transition behavior. Both strength levels showed a start to brittle transition at −0.8 V (SCE) imposed cathodic potential, as seen in Figure A.1. Note that the 0.0 V points represent the fast fracture value in air and were included to represent the upper limit. The 160-grade material exhibited a lower shelf behavior at the highest potential tested, −1.2 V. Based on this data, the 140-grade sample is considered marginally susceptible to hydrogen, while the 160-grade sample is highly susceptible.

30

API Technical Report 21TR1

Figure A.1—Notch Fracture Strength vs. Cathodic Potential (SCE) for 4140–140 and 4140–160

A.3.5

Electrochemical Permeation Measurements

Thinned samples of the different tempering conditions were sectioned and polished for electrochemical permeation measurements using the Devanthan-Stachurski method. The effective diffusion coefficient, subsurface concentration, and steady-state flux were found to be in the same order of magnitude irrespective of the hardness of the material, as seen in Figure A.2. This is due to the crystal structures of the matrix of all samples that are the same, which is body-centered cubic (BCT) with space group: I4/mmm. While there were some differences in the dislocation density, the coarseness of the cementite, and the tetragonality of the tempered martensite, these were not enough to cause a significant change in hydrogen permeation rates.

Figure A.2—Permeation Transient Curves of All Four Strength Levels

Materials Selection for Bolting

31

A.4 Conclusions The following conclusions have been determined: — Yield and tensile strength directly affect susceptibility to hydrogen-induced cracking. Susceptibility increases with increasing strength. Actual yield strength and tensile strength values did not overlap and were distinctive for all grades. — AISI 4140 Grade 120 with maximum actual yield strength of 123 ksi (848 MPa) and a hardness range of 29.5 HRC to 32.0 HRC exhibited no ductile-to-brittle transition. — AISI 4140 Grade 130 with a maximum actual yield strength of 135 ksi (931 MPa) and a hardness range of 30.5 HRC to 34.6 HRC exhibited no ductile-to-brittle transition. This supports the current hardness limit of 34 HRC in API 20E and the addition of a maximum 135 ksi (931 MPa) yield limit to prevent hydrogen cracking. — AISI 4140 Grade 140 with a maximum actual yield strength of 145 ksi (1000 MPa) and a hardness range of 33.2 HRC to 37.6 HRC exhibited a ductile-to-brittle transition starting at −0.8 V (SCE), with a lower shelf indicated at −1.0 V (SCE). — AISI 4140 Grade 160 with a maximum actual yield strength of 177 ksi (1220 MPa) and a hardness range of 37.0 HRC to 42.6 HRC exhibited a ductile-to-brittle transition starting at −0.8 V (SCE), with a lower shelf indicated at −1.2 V (SCE). — The four grades exhibited the same diffusivities to hydrogen permeation due to similarities in their microstructure. — Overprotective potentials (more negative than −1.0 V) significantly decreased the notch fracture strength in AISI 4140 Grade 140 and Grade 160 materials.

Bibliography [1] MEPC1 62/24, Addendum1, Annex 26, Report of the Marine Environment Protection Committee on its SixtySecond Session [2] Bridge, M.R. & Rogers, G.D. MTB (1984) 15: 581. https://doi.org/10.1007/BF02657390 [3] Kulesza, B. S., Shick, J. E., Fletcher, F. B., & McDowell, E. E. (2016, June 14). Hydrogen Induced Cracking of ASTM A516 Pressure Vessel Plate Steels and the Apparent Effect of Microscopic Banding. NACE International2 [4] API Technical Report 1PER15K-1, Protocol for Verification and Validation of High-pressure High-temperature Equipment [5] API Standard 6ACRA, Age-hardened Nickel-based Alloys for Oil and Gas Drilling and Production Equipment [6] API Specification 17D, 2nd Edition, Design and Operation of Subsea Production Systems-Subsea Wellhead and Tree Equipment [7] API Specification 20E, Alloy and Carbon Steel Bolting for Use in the Petroleum and Natural Gas Industries [8] API Specification 20F, Corrosion Resistant Bolting for Use in the Petroleum and Natural Gas Industries [9] ASME PCC-1,3Guidelines for Pressure Boundary Bolted Flange Joint Assembly [10] ASTM A193,4 Standard Specification for Alloy-Steel and Stainless Steel Bolting for High Temperature or High Pressure Service and Other Special Purpose Applications [11] ASTM A255, Standard Test Methods for Determining Hardenability of Steel [12] ASTM A320, Standard Specification for Alloy-Steel and Stainless Steel Bolting for Low-Temperature Service [13] ASTM A370, Standard Test Methods and Definitions for Mechanical Testing of Steel Products [14] ASTM A534, Standard Specification for Carburizing Steels for Anti-Friction Bearings [15] ASTM B633-19, Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel [16] ASTM B850, Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement [17] ASTM E8, Standard Test Methods for Tension Testing of Metallic Materials [18] ASTM E10, Standard Test Method for Brinell Hardness of Metallic Materials [19] ASTM E18-15, Standard Test Methods for Rockwell Hardness of Metallic Materials [20] ASTM E23, Standard Test Methods for Notched Bar Impact Testing of Metallic Materials 1 2 3 4

International Maritime Organization, 4, Albert Embankment, London, SE1 7SR, United Kingdom, www.imo.org. NACE International, 15805 Park Ten Place, Houston, Texas, 77084, www.nace.org. American Society of Mechanical Engineers, Two Park Avenue, New York, New York 10016-5990, www.asme.org. ASTM International, 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, Pennsylvania, 19428-2959, www.astm.org.

32

Materials Selection for Bolting

33

[21] ASTM E45-05, Standard Test Methods for Determining the Inclusion Content of Steel [22] ASTM E384, Standard Test Method for Microindentation Hardness of Materials [23] ASTM E1268, Standard Practice for Assessing the Degree of Banding or Orientation of Microstructures [24] ASTM F519-13, Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/ Coating Processes and Service Environments [25] ASTM F1624-12 (2018), Standard Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique [26] ASTM F1941, Standard Specification for Electrodeposited Coatings on Mechanical Fasteners, Inch and Metric [27] ASTM F2660-13, Standard Test Method for Qualifying Coatings for Use on A490 Structural Bolts Relative to Environmental Hydrogen Embrittlement [28] BS 7608,5 Guide to Fatigue Design and Assessment of Steel Structures [29] DNVGL-RP-B401, Cathodic protection design [30] DNVGL-RP-C203,6 Fatigue design of offshore steel structures [31] NACE MR0175/ISO 15156-2, Petroleum and natural gas industries – Materials for use in H2S-containing environments in oil and gas production – Part 2: Cracking-resistant carbon and low-alloy steels, and the use of cast irons [32] NACE MR0175/ISO 15156-3, Petroleum and natural gas industries – Materials for use in H2S-containing environments in oil and gas production – Part 3: Cracking-resistant CRAs (corrosion-resistant alloys) and other alloys [33] SAE AMS2300,7 Steel Cleanliness, Premium Aircraft-Quality Magnetic Particle Inspection Procedure [34] SAE AMS2301, Steel Cleanliness, Aircraft Quality Magnetic particle Inspection Procedure [35] SAE AMS2304, Special Aircraft-Quality Steel Cleanliness Magnetic Particle Inspection Procedure [36] SPE 178431, On the Contradiction of Applying Rolled Threads to Bolting Exposed to Hydrogen-Bearing Environments, B.D. Craig

5 6 7

British Standards Institution, 12950 Worldgate Drive, Suite 800, Herndon, Virginia 20170, www.bsigroup.com DNV GL, Veritasveien 1, 1363 Oslo, Norway, www.dnvgl.com SAE International, 400 Commonwealth Drive, Warrendale, Pennsylvania, 15096, www.sae.org.

200 Massachusetts Avenue, NW Suite 1100 Washington, DC 20001-5571 USA 202-682-8000 Additional copies are available online at www.api.org/pubs Phone Orders: 1-800-854-7179 (Toll-free in the U.S. and Canada) 303-397-7956 (Local and International) Fax Orders: 303-397-2740 Information about API publications, programs and services is available on the web at www.api.org. Product No. G21TR101