CEN ISO/TR 20491:2021

SLOVENSKI STANDARD
01-februar-2022
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: CEN ISO/TR 20491:2021
ICS:
21.060.01 Vezni elementi na splošno Fasteners in general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

CEN ISO/TR 20491
TECHNICAL REPORT
RAPPORT TECHNIQUE
December 2021
TECHNISCHER BERICHT
ICS 21.060.01
English Version
Fasteners - Fundamentals of hydrogen embrittlement in
steel fasteners (ISO/TR 20491:2019)
Fixations - Principes de la fragilisation par l'hydrogène Mechanische Verbindungselemente - Grundlagen der
pour les fixations en acier (ISO/TR 20491:2019) Wasserstoffversprödung in Verbindungselementen aus
Stahl (ISO/TR 20491:2019)
This Technical Report was approved by CEN on 29 November 2021. It has been drawn up by the Technical Committee CEN/TC
185.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
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United Kingdom.
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© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN ISO/TR 20491:2021 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
The text of ISO/TR 20491:2019 has been prepared by Technical Committee ISO/TC 2 "Fasteners” of the
International Organization for Standardization (ISO) and has been taken over as
held by BSI.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
Endorsement notice
The text of ISO/TR 20491:2019 has been approved by CEN as CEN ISO/TR 20491:2021 without any
modification.
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
ISO/TR 20491:2019(E)
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
ii © ISO 2019 – All rights reserved

ISO/TR 20491:2019(E)
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
ISO/TR 20491:2019(E)
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.
iv © ISO 2019 – All rights reserved

ISO/TR 20491:2019(E)
Introduction
High strength mechanical steel fasteners are broadly characterized by tensile strengths (R ) above
m
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.
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
QT
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
ISO/TR 20491:2019(E)
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.
2 © ISO 2019 – All rights reserved

ISO/TR 20491:2019(E)
3.14
hydrogen embrittlement
HE
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)]
ISO/TR 20491:2019(E)
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
m
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.
4 © ISO 2019 – All rights reserved

ISO/TR 20491:2019(E)
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.
ISO/TR 20491:2019(E)
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
6 © ISO 2019 – All rights reserved

ISO/TR 20491:2019(E)
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 .
ISO/TR 20491:2019(E)
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.
8 © ISO 2019 – All rights reserved

ISO/TR 20491:2019(E)
Key
X hardness (HV)
Y normal scatter range - percent notch fracture strength (NFS )
%
a
Not susceptible.
b
Ductile-brittle transition (transition begins as hardness is increased above 390 HV).
c
Susceptible [high probability of failure by hydrogen embrittlement (HE)].
d
Acceptance threshold for fasteners.
Figure 5 — Scatter range of a model HE threshold stress curve for zinc electroplated notched
[36]
square bars tested in air under four-point bending load
Up to hardness of 390 HV (left part of Figure 5), steel does not exhibit any loss of fracture strength: in
other words, it is not embrittled.
Above 390 HV (right part of Figure 5), a ductile brittle transition occurs as hardness is increased. The
start of the ductile-brittle transition is dependent on the microstructural characteristics of the specific
[36][12]
steel alloy and the concentration of available hydrogen .
Steel fasteners with a specified hardness up to 390 HV, such as fasteners of property class 10.9 in
[6]
accordance with ISO 898-1 , have no significant susceptibility to hydrogen embrittlement failure.
In other words, these steel fasteners can tolerate the presence of hydrogen without any delayed
degradation of their mechanical strength. This assertion assumes that the fasteners are produced
by using appropriately selected steel, well-controlled steel making and fastener manufacturing
[12][17]
processes .
[18]
To minimize the risk of internal hydrogen embrittlement (IHE), ISO 4042 and ASTM F1941/
[19]
F1941M , which are the recommended standards for electroplated fasteners, classify susceptible
fasteners requiring mandatory baking as those having minimum specified hardness above 390 HV. The
mandatory baking limit of 390 HV is based on both scientific research (see Figure 5) and longstanding
fastener industry practice. These standard specifications also require appropriate process control
measures and test methods as additional tools for minimizing the risk of IHE.
NOTE Some coating specifications have defined hardness limits for mandatory baking that are lower than
390 HV. However, these lower limits are not supported by data and were originally adopted as a matter of
precaution.
ISO/TR 20491:2019(E)
[20]
To minimize the risk of environmental hydrogen embrittlement (EHE), ISO 898-1:2013, Table 2
contains a cautionary footnote warning about the risk of stress corrosion cracking for property class
12.9 fasteners for which the specified hardness range is 385 HV to 435 HV.
The scatter range shown in Figure 5 is caused by second order effects related to alloy composition
and microstructure of quenched and tempered steel that affect hydrogen transport and trapping.
Therefore, above 390 HV, for a given concentration of hydrogen, the critical hardness value above which
the ductile-brittle transition begins can vary. Hardness alone is not enough to predict these second
order effects. Measuring hardness, essentially quantifying local plasticity, is a quick and useful test to
estimate strength. Hardness is achieved by the combined effects of composition and heat treatment
that is specific to each steel alloy. In a tempered martensite structure, the same hardness can be
achieved by different combinations of composition and heat treatment, resulting in slightly different
microstructures, each characterized by slightly different stress-strain curves and slightly different
hydrogen transport and trapping characteristics. The scatter range depicted in Figure 5 represents
the normal range of susceptibility (i.e. lowest to highest susceptibility) as was determined from HE
[12]
threshold stress measurements of 10 different steel alloys at 4 hardness levels .
9.2.2 Defects and other conditions causing abnormal material susceptibility
Beyond normal variations of metallurgical structure described above, non-homogeneity of the
microstructure resulting from poorly controlled heat treatment and high occurrence of non-metallic
[11][21][22]
inclusions can cause an unpredictable, but measurable, increase in HE susceptibility of steel .
Heat treatment is the single most consequential process to achieve the required metallurgical structure
and physical properties of fasteners. Not surprisingly, the root cause of HE failures with fasteners that
are not normally considered susceptible is often linked to improper heat treatment. Consequences
of improper heat treatment include higher than expected hardness, unintended carburization and/
or incomplete martensite transformation. Therefore, it is imperative that the heat treatment process
produces fasteners that satisfy the explicit and implicit requirements specified in material standards,
such as adequate though-hardening, homogeneity of micro-structure and non-carburization.
Poorly controlled and non-homogeneous microstructures are typically characterized by low toughness.
[20]
Consequently, measurement of impact strength (e.g. in accordance with ISO 898-1) can be a useful
test to detect fasteners with aberrant microstructures.
Finally, when raw material is phosphate coated prior to cold forming, phosphorous diffusion at the
surface of the finished fastener can occur during austenitizing, producing a phosphorous enriched
white layer (δ-ferrite) and phosphorous segregation that can weaken the grain boundaries. With high
strength fasteners, this phenomenon can result in brittle intergranular cracking at the surface of the
fastener. The propensity to brittle intergranular cracking increases with increasing hardness. Although
brittle intergranular cracking by this mechanism happens in absence of hydrogen, once a crack has been
initiated, it can propagate with hydrogen assistance as described in Clause 8. Phosphorous diffusion
is mitigated by washing the fasteners before quenching and tempering to remove phosphate located
on the surface (i.e. de-phosphating). De-phosphating of property class 12.9 fasteners is mandatory in
[23][24][25]
ISO 898-1 .
9.2.3 Methodology for measuring HE threshold stress
The susceptibility of a material to hydrogen damage is characterized (i.e. measured) by its hydrogen
embrittlement threshold stress. The chart shown in Figure 5 is based on measured HE threshold stress
values that are expressed as a ratio of the baseline strength of the material. This ratio is known as
percent notch fracture strength (NFS ). The more a sample is hydrogen embrittled, the lower its HE
%
threshold stress, and the lower resulting NFS from the baseline strength of the sample (represented
%
[26][27]
by 100 NFS ) .
%
The data represented in Figure 5 were developed by testing material specimens shaped as single edge
notch square bars, illustrated in Figure 6. Dimensional specifications for the test specimens are given
[28]
in ASTM F519 (type 1e geometry) . The radius at the root of the notch is intended to simulate the
[12]
thread of a fastener . However, the square bar geometry is tailored for loading the specimen in
10 © ISO 2019 – All rights reserved

ISO/TR 20491:2019(E)
4-point bending, which generates higher stress than a test performed in tension using a notch round
bar specimen. Figure 6 also shows a simplified schematic of the bending load frame.
In brief, the loading method consists of incrementally increasing the load applied to the specimen.
This mode of loading is known as incremental step loading (ISL) and represents a modified form slow
strain rate loading (SSRL). The test methodology is designed to measure the HE cracking threshold of
the material, which is a measure of material susceptibility. The addition of hydrogen in the sample is
achieved by prior charging, such as exposure to an electroplating process (IHE), or during the test in
an environmental chamber where the sample is immersed in a 3,5 % mass fraction NaCl solution, and
where a potentiostat is used to impose the cathodic potential. The imposed potential and the resulting
[26]
current density control the quantity of hydrogen being introduced into the specimen .
The bending mode of applying load makes the test very severe and offers the benefit of increased
sensitivity for measuring the effect of changing variables. For the given notch radius, the applied
[29]
concentration of stress in bending is greater than stress generated in tension by a factor of 1,65 .
Given the increased severity of the test method, results must be translated to determine if a fastener
under normal service condition (i.e. loaded in tension), and exposed to similar hydrogen conditions,
will suffer hydrogen embrittlement. The acceptance threshold for fasteners (illustrated as the dashed-
line in Figure 5) is defined as the threshold above which a fastener under equal conditions (i.e. same
material and hydrogen concentration), but loaded in tension instead of being loaded in bending, will not
suffer hydrogen embrittlement. To determine the acceptance threshold for fasteners which are used in
tension, 100 NFS in bending is multiplied by a factor of 1/1,65 to convert the bending stress condition
%
to an equivalent tensile stress condition. By this conversion, 60 NFS in bending corresponds to the “no
%
[29]
risk” acceptance threshold for fasteners made of the same material .
Figure 6 — Dimensional specifications of ASTM F519 (Type 1e) single notch bend square bar
and schematic of loading frame showing the bending motion being applied to a test specimen
9.3 Tensile stress
Load induced stress is a normal service condition for fasteners. Tensile loaded fasteners such as bolts
and screws are primarily subject to tensile stress and a varying amount of torsional stress during
tightening. In some cases, fasteners can be subject to shear loads, typically in the unthreaded shank.
In some rare but critical cases, fasteners can also be subject to unintended bending loads. Given time,
ISO/TR 20491:2019(E)
tensile stress in the fastener can result in HE failure provided it exceeds the HE threshold stress of the
material. Hydrogen embrittlement threshold stress is defined as the critical stress below which HE does
NOT occur. As was described in 9.2.3, HE threshold stress is a measure of the degree of susceptibility
of a material for a given quantity of available hydrogen. Time to failure is dependent on the amount by
which the HE threshold stress is exceeded. Time to failure decreases with increasing stress.
The applied stress in a bolt or screw is a function of the loading conditions in the joint. These loading
conditions are a combination of joint design (i.e. service loads) and installation preload of the fastener.
Usually, bolts are installed to preloads ranging from 50 % to 100 % of the yield strength. For property
class 10.9 fasteners which have no significant susceptibility to IHE, this amount of loading is below the
HE threshold stress of the material. However, if these fasteners have hardness above the specified limit
or other defects such as poor microstructure and low toughness (see 9.2.2), they can exhibit abnormally
low HE threshold stress which is below the stress resulting from normal installation preload. Under
these conditions, given the same concentration of hydrogen and normal installation preload, the
probability of exceeding the HE threshold stress of the material becomes significantly greater, thus
increasing the risk of hydrogen assisted cracking (HAC).
NOTE As with all failure mechanisms, HAC is normally initiated at the points of greatest concentration
of stress:
— in the case of bolts and screws, this corresponds to the fillet radius under the head, the thread runout, or the
root of the first engaged thread;
— in the case of nuts, the distribution of load in internal threads makes it significantly less likely that the HE
threshold stress can be exceeded; consequently, HE failure of a nut, although possible, is extremely rare;
— in the case of non-flat washers, a significant tensile stress amount is present as the washer is compressed;
it is not unusual for electroplated high-hardness elastic washers to fail due to HAC, unless they are
adequately baked.
Unintended geometrical irregularities such as angles, sharp radii, unintended surface discontinuities
or pits can arise from poor fastener design, poor manufacturing, ove
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