1 Scope
This document provides a brief but complete update on the knowledge of hydrogen embrittlement from a specialist technical point of view. This document applies to steel fasteners.
2 Normative references
There are no normative references in this document.
3 Terminology and definitions
The following terms and definitions apply to this document.
3.1
Hardness
The resistance of a metal to plastic deformation, usually expressed by indentation or penetration of a solid (surface or core).
3.2 work hardening
Work hardening
When a metal is deformed plastically at room temperature (by rolling, drawing, stretching, rolling, head and extrusion, etc.), the strength and hardness (3.1) increase, while the ductility decreases.
4 Abbreviations
The following abbreviations apply to this document.
5 General description of hydrogen embrittlement
In general, hydrogen embrittlement can be divided into two main categories according to the source of hydrogen: endogenous hydrogen embrittlement (IHE) and environmental hydrogen embrittlement (EHE). Endogenous hydrogen embrittlement is caused by hydrogen remaining inside the material during processes such as steelmaking and/or processing (pickling and electroplating). Environmental hydrogen embrittlement is caused by hydrogen from external sources under stress, e.g. fasteners in service.
Stress corrosion cracking (SCC) is associated with environmental hydrogen embrittlement (EHE) which occurs when hydrogen is absorbed by steel fasteners as a by-product of surface corrosion. Cathodic hydrogen absorption is part of Stress Corrosion Cracking (SCC). Cathodic hydrogen absorption occurs when a metal coating such as zinc or cadmium is used as a sacrificial anode to protect steel fasteners from rusting. If the steel substrate is exposed to an ambient medium, the exposed steel substrate surface will, in conjunction with the reduction reaction, also lead to hydrogen absorption, resulting in a significantly higher hydrogen content than uncoated steel fasteners.
The terms "debrittlement" and "re-embrittlement" are also used in the aerospace sector, but this is technically incorrect as embrittlement is irreversible. Debrittlement is incorrectly used to describe the effects of baking and re-embrittlement is incorrectly used to describe the effects of hydrogen absorption in a service environment or when using maintenance cleaning fluids.
6 Mechanisms of hydrogen damage
High strength steels are broadly defined as having a tensile strength (Rm) of more than 1000 MPa. When high strength steels are subjected to tensile stresses, such as when high strength fasteners are subjected to the tensile loads generated by tightening, the stresses cause hydrogen atoms in the steel to diffuse (migrate) to the point of maximum stress (e.g. the first snap screw thread or the lower arc of the bolt head). As hydrogen accumulates at these locations, the normally ductile steel becomes progressively more brittle. Eventually, stress concentrations and hydrogen build-up at certain locations will lead to hydrogen-promoted (brittle) microcracking. The hydrogen migrates as the tip of the open crack expands, causing brittle microcracking to continue to grow until the fastener is overloaded and eventually fractured. This phenomenon is commonly referred to as hydrogen-accelerated cracking (HAC) [or hydrogen cracking
7 Fracture shape
8 Crack tip conditions
In loaded fasteners, cracking can occur through several mechanisms unrelated to HAC (e.g. fatigue, overloading, weakening of grain boundaries due to phosphorus segregation). However, once any mechanism, including HAC, has initiated crack initiation, the conditions at the crack tip, particularly the stress concentration, are often much more severe than the initial conditions [13]. Cracks can easily be extended by a combination of one or more mechanisms to release the stress concentration at the crack tip. Hydrogen-accelerated cracking (HAC) can promote crack extension if a sufficient amount of hydrogen interacts with the crack tip (see Figure 3). For example, even in materials with low susceptibility, cracks exposed to corrosive environments and subjected to static or cyclic loading may extend partly through stress corrosion cracking.
9 Hydrogen embrittlement failure conditions
9.1 Root causes and predisposing factors for hydrogen embrittlement failure
Three conditions must be present simultaneously to cause hydrogen embrittlement failure (Figure 4):
The material is sensitive to hydrogen damage;
tensile stress (usually from externally applied loads or residual stresses);
and hydrogen atoms.
If there is a sufficient overlap between these three conditions, hydrogen damage will lead to crack initiation and girdling over a given period of time until fracture occurs. The time at which failure occurs may vary with the severity of the various conditions and the source of hydrogen. Stress knives and hydrogen blankets are considered to be the triggering
factors, while the sensitivity of the material to hydrogen damage is the basic condition for the occurrence of hydrogen embrittlement (HE) and is the root cause.
9.2 Material susceptibility
9.2.1 General principles
The susceptibility of a material to hydrogen damage is a function of the material conditions, including the metallographic and mechanical properties of the material. The study of material susceptibility to hydrogen damage is fundamental to the understanding of the phenomenon of hydrogen embrittlement.
Considering that hydrogen embrittlement leads to a loss of ductility and hence strength, the basis for studying and quantifying the susceptibility of materials to hydrogen damage starts with mechanical testing. The test measures the properties of the material under increasing stress under non-hydrogen absorbing and hydrogen absorbing conditions. The test method is described in detail in 9.2.2.
The strength of the material (tensile strength and/or hardness) is the primary factor affecting the susceptibility of the steel to hydrogen embrittlement. As the strength increases, the hardness of the steel increases, the plasticity and toughness decrease and the steel becomes more sensitive to hydrogen damage. When the hardness of steel fasteners is higher than 390 HV [17], the susceptibility to hydrogen embrittlement increases significantly. This increase in susceptibility is characterised by a ductile-brittle transition, i.e. a rapid loss of material ductility. A relatively small increase in hardness may lead to a ductile-brittle transition17], see Figure 5.
9.3 Tensile stresses
The normal conditions of use of fasteners are subject to stresses generated by the load. Fasteners such as bolts and screws are mainly subjected to tensile stresses and varying degrees of torsional stresses during tightening. In some cases, fasteners may be subjected to shear loads, particularly in unthreaded rod sections. In some rare but important cases, fasteners may also be subjected to unexpected bending loads. Fasteners subjected to tensile loads will fail in hydrogen embrittlement if the tensile stress exceeds the material's hydrogen embrittlement threshold stress over a certain period of time. The threshold stress of hydrogen embrittlement is the critical stress at which hydrogen embrittlement does not occur. As described in 9.2.3, the hydrogen embrittlement threshold stress is a measure of the susceptibility of a material to a certain amount of hydrogen content. The time at which failure occurs depends on the magnitude of the threshold stress being exceeded. The time to failure decreases as the stress increases.
Stress in a bolt or screw is a function of the load conditions in the connection. These load conditions are a combination of the fastener connection design (i.e. working load) and the installation preload. Typically, the installation preload of a bolt is between 50% and 100% of the yield strength. For performance class 10.9 fasteners, there is no significant sensitivity to endogenous hydrogen embrittlement (IHE) and the stress loading is below the material's hydrogen embrittlement threshold stress. However, if the hardness of these fasteners is higher than the specified limit, or if other defects such as poor organisation or low toughness (see 9.2.2) are present, abnormal hydrogen embrittlement threshold stresses below the normal installation preload may occur. Under these conditions, even with the same hydrogen content and normal installation preload, the probability of exceeding the material's hydrogen embrittlement threshold stress increases significantly, thereby increasing the risk of hydrogen promoted cracking (HAC).
Note: As with all failure mechanisms, hydrogen-promoted cracking usually originates at the site of maximum stress concentration:
For screws and bolts, the areas of maximum stress concentration are the under-head fillet, the end of the thread and the bottom of the first screw thread.
In the case of nuts, the load is distributed over the internal threads so that the actual maximum stress does not exceed the material's hydrogen embrittlement threshold stress. Therefore, hydrogen embrittlement of the nut
failure, although theoretically possible, is very rare in practice.
For non-flat washers, large tensile stresses will occur when the washer is compressed. Therefore, it is not uncommon for plated high hardness spring washers to fail hydrogen promoted
is not uncommon, unless the washer is adequately baked.
Improper fastener design, poor manufacturing processes, excessive pickling or corrosive environments can lead to unexpected geometric irregularities such as sharp corners, non-rounded transitions, surface defects or pits. Particular attention should be paid to the small radius of the thread root and to thread folding, which are areas of high stress concentration. These irregularities can often lead to unexpected crack initiation and thus deteriorate the stress state, especially in materials sensitive to hydrogen embrittlement.
9.4 Atomic hydrogen
9.4.1 Sources of hydrogen
There are two possible sources of hydrogen: internal and environmental.
10 Surface-hardened fasteners
In the case of case-hardened screws, the risk is increased due to the fact that the surface is intentionally hardened to meet the functional requirements of self-drilling, self-extrusion and/or self-tapping. Depending on the purpose of the application, there are many different types of these screws. They are used to join wood, steel, galvanised steel, aluminium or a combination of these materials. As a result, they have a core hardness of 250 HV to 450 HV (25 HRC to 45 HRC) and a surface hardness of up to 600 HV (55 HRC). The combination of high surface hardness and core hardness makes the surface-hardened screw material very sensitive to both internal and external hydrogen embrittlement. Surface-hardened screws are sometimes galvanised, zinc alloyed or coated with a zinc-rich organic coating, but more commonly they are electro-galvanised. The presence of hydrogen provides sufficient preconditions for the development of hydrogen embrittlement. Possible sources of hydrogen include:
11 Hot-dip galvanising - the effects of thermal shock
In the 1970s, the failure of hot-dip galvanised high-strength fasteners due to stress corrosion cracking (SCC) prompted the ASTM Fastener Committee to prohibit hot-dip galvanising of [ASTM A49036] high-strength bolted structural bolts used primarily in North America.
In Europe and elsewhere, it is still standard practice to hot dip galvanise bolts in performance class 10.9 according to ISO 898-1. As an additional precaution, material and process-specific guidelines and requirements are established in ISO 10684 to avoid or minimise the risk of hydrogen embrittlement. Despite never having been in contact with acid prior to hot-dip galvanising and not having been subjected to environmental corrosion, hot-dip galvanised fasteners still fail shortly after installation.
12 Stress relief prior to plating
Stress relieving prior to plating is not necessary or suitable for hardened and tempered fasteners. This is because tempering is effective in removing residual stresses.
On the other hand, after quenching and tempering the fastener, the residual tensile stresses resulting from work hardening prior to plating can lead to the development of hydrogen-promoted microcracking. Hydrogen-promoted cracking will only occur if three conditions for hydrogen embrittlement are met: i.e. the material is hydrogen embrittlement sensitive, there is sufficient hydrogen and the residual stresses from work hardening exceed the hydrogen embrittlement threshold stress of the steel. In such cases, it is beneficial to carry out a stress relief treatment prior to plating as a preventive measure.
13 Thread rolling after heat treatment of fasteners
Fasteners after heat treatment (i.e., ordinary year tempering) rolled to m pattern has several results: cold shaking pressure and mountain this produces residual star stress to improve
Fatigue performance; studies have shown that fasteners heat treated after rolling threads also in addition to the second in n phenomenon is explained by: in the root region of the threads, by reducing the valence lattice space and increasing the density of dislocations ie. (formation traps) from the face leads to an increase in trap sites and a decrease in hydrogen migration. Rolling of threads after heat treatment can be used as a strategy to limit the risk of hydrogen embrittlement failure.
14 Hydrogen embrittlement test methods
Due to the time dependence of hydrogen embrittlement test methods used to detect or measure the effects of degradation of mechanical properties due to the action of hydrogen need to include the time variable.
15 Baking
The potentially damaging effects of hydrogen absorbed during surface cleaning or plating can often be prevented by baking the fastener after processing. Baking is a lower temperature heat treatment that allows hydrogen to escape from the part, or to migrate to a trap location where it cannot move. As the amount of hydrogen movement/diffusion decreases, the time to failure and threshold stresses increase. However, it cannot be assumed that baking will completely eliminate endogenous hydrogen embrittlement in all cases.
Bibliography
1 Scope
2 Normative references
3 Terminology and definitions
4 Abbreviations
5 General description of hydrogen embrittlement
6 Mechanisms of hydrogen damage
7 Fracture shape
8 Crack tip conditions
9 Hydrogen embrittlement failure conditions
10 Surface-hardened fasteners
11 Hot-dip galvanising - the effects of thermal shock
12 Stress relief prior to plating
13 Thread rolling after heat treatment of fasteners
14 Hydrogen embrittlement test methods
15 Baking
Bibliography
