FprCEN ISO/TR 20491

Fasteners - Fundamentals of hydrogen embrittlement in steel fasteners (ISO/TR 20491:2019)

This document presents the latest knowledge related to hydrogen embrittlement, translated into know-how in a manner that is complete yet simple, and directly applicable to steel fasteners.

Mechanische Verbindungselemente - Grundlagen der Wasserstoffversprödung in Verbindungselementen aus Stahl (ISO/TR 20491:2019)

Fixations - Principes de la fragilisation par l'hydrogène pour les fixations en acier (ISO/TR 20491:2019)

Vezni elementi - Osnove o vodikovi krhkosti v jeklenih pritrdilnih elementih (ISO/TR 20491:2019)

General Information

• Status: Not Published
• ICS: 21.060.01 - Fasteners in general
• Technical Committee: CEN/TC 185 - Threaded and nonthreaded mechanical fasteners and accessories
• Current Stage: 5060 - Closure of Vote - Formal Approval
• Due Date: 28-Oct-2021
• Completion Date: 28-Oct-2021
• Ref Project: kSIST-TP FprCEN ISO/TR 20491:2021 - Fasteners - Fundamentals of hydrogen embrittlement in steel fasteners (ISO/TR 20491:2019)

Standards Content (sample)

SLOVENSKI STANDARD
kSIST-TP FprCEN ISO/TR 20491:2021
01-oktober-2021
Vezni elementi - Osnove o vodikovi krhkosti v jeklenih pritrdilnih elementih
(ISO/TR 20491:2019)
Fasteners - Fundamentals of hydrogen embrittlement in steel fasteners (ISO/TR
20491:2019)
Mechanische Verbindungselemente - Grundlagen der Wasserstoffversprödung in
Verbindungselementen aus Stahl (ISO/TR 20491:2019)

Fixations - Principes de la fragilisation par l'hydrogène pour les fixations en acier

(ISO/TR 20491:2019)
Ta slovenski standard je istoveten z: FprCEN ISO/TR 20491
ICS:
21.060.01 Vezni elementi na splošno Fasteners in general
kSIST-TP FprCEN ISO/TR 20491:2021 en,fr,de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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kSIST-TP FprCEN ISO/TR 20491:2021
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kSIST-TP FprCEN ISO/TR 20491:2021
TECHNICAL ISO/TR
REPORT 20491
First edition
2019-02
Fasteners — Fundamentals of
hydrogen embrittlement in steel
fasteners
Fixations — Principes de la fragilisation par l'hydrogène pour les
fixations en acier
Reference number
ISO/TR 20491:2019(E)
ISO 2019
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kSIST-TP FprCEN ISO/TR 20491:2021
ISO/TR 20491:2019(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2019

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Fax: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved
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Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Symbols and abbreviated terms ........................................................................................................................................................... 4

5 General description of hydrogen embrittlement ................................................................................................................ 4

6 Hydrogen damage mechanism ............................................................................................................................................................... 4

7 Fracture morphology ....................................................................................................................................................................................... 5

8 Conditions at the tip of a crack .............................................................................................................................................................. 7

9 Conditions for hydrogen embrittlement failure .................................................................................................................. 7

9.1 Root cause and triggers for hydrogen embrittlement failure ......................................................................... 7

9.2 Material susceptibility ...................................................................................................................................................................... 8

9.2.1 General...................................................................................................................................................................................... 8

9.2.2 Defects and other conditions causing abnormal material susceptibility .....................10

9.2.3 Methodology for measuring HE threshold stress .............................................................................10

9.3 Tensile stress .........................................................................................................................................................................................11

9.4 Atomic hydrogen ................................................................................................................................................................................12

9.4.1 Sources of hydrogen ........................................................................................................................................... ........12

9.4.2 Internal hydrogen ........................................................................................................................................................12

9.4.3 Environmental hydrogen .......................................................................................................................................13

10 Case-hardened fasteners ..........................................................................................................................................................................13

11 Hot dip galvanizing and thermal up-quenching ...............................................................................................................15

12 Stress relief prior to electroplating ...............................................................................................................................................16

13 Fasteners thread rolled after heat treatment......................................................................................................................16

14 Hydrogen embrittlement test methods .....................................................................................................................................17

15 Baking ...........................................................................................................................................................................................................................17

Bibliography .............................................................................................................................................................................................................................19

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Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso

.org/iso/foreword .html.

This document was prepared by Technical Committee ISO/TC 2 Fasteners, Subcommittee SC 14, Surface

coatings.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/members .html.
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Introduction

High strength mechanical steel fasteners are broadly characterized by tensile strengths (R ) above

1 000 MPa and are often used in critical applications such as in bridges, engines, aircraft, where a

fastener failure can have catastrophic consequences. Preventing failures and managing the risk of

hydrogen embrittlement (HE) is a fundamental consideration implicating the entire fastener supply

chain, including: the steel mill, the fastener manufacturer, the coater, the application engineer, the joint

designer, all the way to the end user. Hydrogen embrittlement has been studied for decades, yet the

complex nature of HE phenomena and the many variables make the occurrence of fastener failures

unpredictable. Researches are typically conducted under simplified and/or idealized conditions that

cannot be effectively translated into know-how prescribed in fastener industry standards and practices.

Circumstances are further complicated by specifications or standards that are sometimes inadequate

and/or unnecessarily alarmist. Inconsistencies and even contradictions in fastener industry standards

have led to much confusion and many preventable fastener failures. The fact that HE is very often

mistakenly determined to be the root cause of failure as opposed to a mechanism of failure reflects the

confusion.
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kSIST-TP FprCEN ISO/TR 20491:2021
TECHNICAL REPORT ISO/TR 20491:2019(E)
Fasteners — Fundamentals of hydrogen embrittlement in
steel fasteners
1 Scope

This document presents the latest knowledge related to hydrogen embrittlement, translated into know-

how in a manner that is complete yet simple, and directly applicable to steel fasteners.

2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
hardness

resistance of a metal to plastic deformation, usually by indentation or penetration by a solid object (at

the surface or in the core)
3.2
work hardening

increase of mechanical strength and hardness (3.1) when a metal is plastically deformed at ambient

temperature (by rolling, drawing, stretching, sinking, heading, extrusion, etc.) also resulting in a

decrease of ductility
3.3
heat treatment

process cycle (controlled heating, soaking and cooling) of a solid metal or alloy product, to obtain a

controlled and homogeneous transformation of the material structure and/or to achieve desired

physical or mechanical properties

Note 1 to entry: Quenching and tempering, annealing, case-hardening and stress relief are examples of heat

treatment for fasteners.
3.4
quenching and tempering

heat treatment (3.3) process of quench hardening comprising austenitizing and fast cooling, under

conditions such that the austenite transforms more or less completely into martensite (and possibly

into bainite), followed by a reheat to a specific temperature for a controlled period, then cooling, in

order to achieve the required level of physical or mechanical properties
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3.5
case-hardening

thermochemical treatment process consisting of carburizing or carbonitriding followed by quenching

which induces an increase of hardness (3.1) in the surface of the fastener steel

Note 1 to entry: This process is used for tapping screws, thread forming screws, self-drilling screws, etc.

3.6
stress relief

heat treatment (3.3) process by which fasteners are heated to a predetermined and controlled

temperature followed by a slow cooling, for the purpose of reducing residual stresses induced by work

hardening (3.2)
3.7
baking

process of heating fasteners for a specified duration at a given temperature in order to minimize the

risk of internal hydrogen embrittlement (3.15)

[SOURCE: ISO 1891-2:2014, 3.4.11, modified — "time" was replaced with "duration"]

3.8
crack
beginning of fracture (3.10) without complete separation
[SOURCE: ASTM F2078-15, modified — "line" was replaced with "beginning"]
3.9
failure

loss of the ability of a fastener to perform a specified function, which in some cases can lead to complete

fracture (3.10)
3.10
fracture

break occurring when the plastic deformation in a fastener increases locally above its resistance limit,

resulting in the separation of the fastener into two or more pieces, during testing or in service

3.11
fracture morphology
structure and aspect of the fractured surface
3.12
ductile

exhibiting a large amount of plastic deformation before fracture (3.10) with a resulting non-flat fracture

surface showing fibrous ductile dimple morphology that is typically dull or matte

3.13
brittle

exhibiting little or no plastic deformation before fracture (3.10) with a resulting flat fracture surface

showing brittle morphology that is typically shiny

Note 1 to entry: Brittle fracture along cleavage planes is known as transgranular fracture.

Note 2 to entry: Brittle fracture by separation at prior austenite grain boundaries is known as intergranular

fracture.
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3.14
hydrogen embrittlement

permanent loss of ductility in a metal or alloy caused by atomic hydrogen in combination with load

[1]

induced and/or residual tensile stress that can lead to brittle (3.13) fracture (3.10) after certain time

Note 1 to entry: In the context of describing hydrogen embrittlement of high strength steel fasteners, the term

“hydrogen” refers to atomic hydrogen and not molecular H gas.
[SOURCE: ISO 1891-2:2014, 3.4.9, modified — Note 1 to entry has been added.]
3.15
internal hydrogen embrittlement
IHE

embrittlement caused by residual hydrogen from manufacturing processes, resulting in delayed brittle

failure (3.9) of fasteners under load induced and/or residual tensile stress
[SOURCE: ISO 1891-2:2014, 3.4.10]
3.16
environmental hydrogen embrittlement
EHE

embrittlement caused by hydrogen absorbed as atomic hydrogen from a service environment,

resulting in delayed brittle failure (3.9) of fasteners under tensile stress (i.e. load induced and/or

residual tensile stress)
[SOURCE: ISO 1891-2:2014, 3.4.13]
3.17
hydrogen embrittlement threshold stress

critical stress below which hydrogen embrittlement (3.14) does not occur, which represents the degree

of susceptibility of a steel for a given quantity of available hydrogen
3.18
stress corrosion cracking
SCC

category of environmental hydrogen embrittlement (3.16) where failure (3.9) occurs during service by

cracking under the combined action of corrosion generated hydrogen and load induced tensile stress

[SOURCE: ISO 1891-2:2014, 3.4.14]
3.19
hydrogen diffusion

propagation of hydrogen and interaction with metallurgical features within the steel microstructure

(microcracks, dislocations, precipitates, inclusions, grain boundaries, etc.) which constitute areas of

traps into the fastener material: non-reversible traps (characterized by high bonding energies and low

probability of hydrogen being released) and reversible traps (characterized by low bonding energies

and hydrogen being released more readily)
3.20
hydrogen effusion

outward migration of hydrogen from the fastener material, occurring naturally at ambient temperature

due to concentration gradient or as the result of a thermal driving force [e.g. baking (3.7)]

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4 Symbols and abbreviated terms
EHE environmental hydrogen embrittlement
HAC hydrogen assisted cracking
HE hydrogen embrittlement
HELP hydrogen enhanced local plasticity
HIC hydrogen induced cracking
IHE internal hydrogen embrittlement
SCC stress corrosion cracking
5 General description of hydrogen embrittlement

Generally, hydrogen embrittlement is classified under two broad categories based on the source of

hydrogen: internal hydrogen embrittlement (IHE) and environmental hydrogen embrittlement (EHE).

IHE is caused by residual hydrogen from steelmaking and/or from processing steps such as pickling

and electroplating. EHE is caused by hydrogen introduced into the metal from external sources while it

is under stress, such as in-service fastener.

The term “stress corrosion cracking” (SCC) is used in relation to EHE that occurs when hydrogen is

produced as a by-product of surface corrosion and is absorbed by the steel fastener. Cathodic hydrogen

absorption is a subset of SCC. Cathodic hydrogen absorption occurs in the presence of metallic coatings

such as zinc or cadmium that are designed to sacrificially corrode to protect a steel fastener from

rusting. If the underlying steel becomes exposed, a reduction process on the exposed steel surface

simultaneously results in the evolution of hydrogen in quantities that are significantly greater than in

the case of uncoated steel.

The terms “de-embrittlement” and “re-embrittlement” are also used in the aerospace field but are

technically incorrect because embrittlement is not reversible. De-embrittlement is misused to describe

the effect of baking, and re-embrittlement is misused to describe the effect of hydrogen absorption

during service or by use of maintenance cleaning fluids.
6 Hydrogen damage mechanism

High strength steel is broadly defined as having a tensile strength (R ) above 1 000 MPa. When high

strength steel is tensile stressed, as is the case with a high strength fastener that is under tensile

load from tightening, the stress causes atomic hydrogen within the steel to diffuse (i.e. move) to the

location of greatest stress (e.g. at the first engaged thread or at the fillet radius under the head of a

bolt). As increasingly higher concentrations of hydrogen collect at this location, steel that is normally

ductile gradually becomes brittle. Eventually, the concentration of stress and hydrogen in one location

causes a hydrogen assisted (brittle) microcrack. The brittle microcrack continues to grow as hydrogen

moves to follow the tip of the propagating crack, until the fastener is overloaded and finally fractures.

This phenomenon is often called hydrogen assisted cracking (HAC) [or hydrogen induced cracking

(HIC)]. The hydrogen damage mechanism as described causes the fastener to fail at stresses that are

[1][2]

significantly lower than the basic strength of the fastener as determined by a standard tensile test .

Theoretical models that describe hydrogen damage mechanisms under idealized conditions have been

[2]

proposed since the 1960s . In the case of high strength steel, these models are based primarily on two

[3] [4][5][6]

complementary theories of decohesion and hydrogen enhanced local plasticity (HELP) . Given the

[7]

complexity of HE phenomena, hydrogen damage models continue to evolve and be refined . An in-

depth review of the theories of hydrogen damage is outside the scope of this technical report. However,

detailed information is given in the references listed in the Bibliography.
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Hydrogen "traps" refer to metallurgical features within the steel microstructure such as grain

[8]

boundaries, dislocations, precipitates, inclusions, etc., to which hydrogen atoms can become bonded .

Hydrogen thus “trapped” is no longer free to diffuse (i.e. move) to areas of high stress where it can

participate in the mechanism of HAC. Traps are typically classified as reversible or non-reversible based

on their bonding energies. Reversible traps are characterized by low bonding energies: in other words,

hydrogen is more easily released from the trap. Non-reversible traps are characterized by high bonding

energies: in other words, hydrogen requires a great deal of energy (e.g. from heat or stress field) to be

released from the trap. Non-trapped hydrogen which is free to move in the metal lattice is called mobile

[9][10][11]
hydrogen; it is also known as interstitial or diffusible hydrogen .
7 Fracture morphology

With quenched and tempered high strength steel fasteners, the fracture surface resulting from

hydrogen assisted cracking (HAC) is typically characterized by brittle intergranular morphology which

is caused by a crack growth path that follows the grain boundaries (see Figure 1). The morphology of

a fracture surface varies based on the susceptibility of the material and the degree of embrittlement.

Clearly defined grain facets (i.e. sharp and angular features) and/or a high proportion of brittle versus

[12]

ductile features are indicative of high degree of embrittlement . Figure 1 illustrates a fracture surface

that is 100 % intergranular with very well-defined grain facets. Less susceptible materials can present

fracture surfaces that contain a mix of intergranular and cleavage (i.e. trans-granular) morphologies.

With a tensile loaded fastener, a brittle hydrogen assisted crack typically grows up to a point where

the reduced cross section of the fastener can no longer withstand the applied load. At this point, the

fastener fractures rapidly (i.e. fast fracture). A normal fracture morphology corresponding to fast

fracture is ductile, characterized by ductile dimples. Figure 2 illustrates a fracture surface where the

brittle hydrogen assisted crack propagation ended (i.e. final crack tip) prior to final ductile fast fracture

of the fastener.

Other forms of embrittlement failure are caused by phenomena not related to the presence of hydrogen

such as temper embrittlement, quench embrittlement, quench crack, etc., that must be distinguished

from hydrogen embrittlement failures. These other types of embrittlement can exhibit similar

intergranular fracture surfaces but are principally distinguished from hydrogen embrittlement by the

fact that they are not time dependent.
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Figure 1 — Fracture surface showing 100 % well defined brittle intergranular morphology —

Cr-Mo alloy steel (AISI 4135), quenched and tempered to 530 HV, zinc electroplated

Figure 2 — Fracture surface showing both brittle intergranular morphology resulting from

HAC and ductile dimple morphology indicative of final fracture — Cr-Mo alloy steel (AISI 4135)

at 530 HV, zinc electroplated
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8 Conditions at the tip of a crack

A microcrack can be initiated in a loaded fastener by several mechanisms that are not necessarily

related to HAC (e.g. fatigue, overloading, grain boundary weakening by phosphorous segregation).

However, once a crack is initiated by any mechanism including HAC, the conditions at the tip of the

[13]

crack, notably the concentration of stress, are often much more severe than initial conditions . The

crack can propagate readily by a single or a combination of mechanisms that seek to reduce the stress

at the tip of the crack. If it happens that a sufficient quantity of hydrogen is available to interact with

the crack tip, then the propagation of the crack can be facilitated by HAC (see Figure 3). For example,

even in low susceptibility materials, an existing crack under static or cyclic load exposed to a corrosive

[14][15]
environment can propagate in part by stress corrosion cracking .
Key
1 atomic hydrogen
2 propagating crack

Figure 3 — An existing sharp crack surrounded by atomic hydrogen that can interact with the

crack tip to cause hydrogen assisted crack propagation

In the case where HAC is the mechanism of an initial microcrack, the time to failure is significantly

shortened as available hydrogen continues to interact with and follow the tip of the propagating crack.

In such a scenario, HAC is the primary failure mechanism. A failure investigation needs to distinguish

the scenario where HAC is the mechanism of an initial microcrack from a scenario where the mechanism

of the initial crack is not related to HAC. The fracture surface presented by the latter scenario can

nevertheless exhibit intergranular features if hydrogen becomes available to interact with the crack

tip; in this case, HAC must be considered only as a secondary fracture mechanism.

9 Conditions for hydrogen embrittlement failure
9.1 Root cause and triggers for hydrogen embrittlement failure

Three elemental conditions must be present concurrently to cause hydrogen embrittlement failure (see

Figure 4):
— material condition that is susceptible to hydrogen damage,

— tensile stress (typically from an externally applied load or residual stress), and

— atomic hydrogen.

If all three of these elements are present in sufficient and overlapping quantities, and given time,

hydrogen damage results in crack initiation and growth until the occurrence of fracture. Time to failure

can vary, depending on the severity of the conditions and the source of hydrogen. Stress and hydrogen

are considered triggers, whereas material susceptibility is the fundamental requirement for HE to occur

[16]
and is therefore associated with the root cause .
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Figure 4 — Confluence of the three necessary conditions
for delayed hydrogen embrittlement (HE) failure to occur
9.2 Material susceptibility
9.2.1 General

Susceptibility of a material to hydrogen damage (i.e. material susceptibility) is a function of the

material condition, which is comprehensively described by the metallurgical structure and mechanical

properties of a material such as steel. Examining material susceptibility is the fundamental basis for

understanding hydrogen embrittlement phenomena.

Given that hydrogen embrittlement causes loss of ductility and, consequently, loss of strength, the

foundation for studying and quantifying susceptibility of a material to hydrogen damage begins with

mechanical testing. This testing measures the behaviour of the material under increasing stress, first

without, and then with the addition of absorbed hydrogen. A detailed description of such a methodology

is given in 9.2.2.

Material strength (i.e. tensile strength and/or hardness) has a first order effect on HE susceptibility

of steel. As strength increases, steel becomes harder, less ductile, less tough and more susceptible

to hydrogen damage. The susceptibility of steel fasteners increases significantly when the specified

[17]

hardness is above 390 HV . This increase in susceptibility is characterized by a ductile-brittle

transition, whereby the material rapidly loses its ductility. The ductile-brittle transition can occur over

[17]
a narrow range of increasing hardness . See Figure 5.
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Key
X hardness (HV)
Y normal scatter range - percent notch fracture strength (NFS )
Not susceptible.

Ductile-brittle transition (transition begins as hardness is increased above 390 HV).

Susceptible [high probability of failure by hydrogen embrittlement (HE)].
Acceptance threshold for fasteners.
Figure 5 — Sc
...