Hydrogen Embrittlement is the permanent loss of ductility in a metal caused by the diffusion of hydrogen into the material in combination with applied or residual tensile stress. It can result in sudden material failure under tensile loading. Engineers should be aware of the risks of hydrogen embrittlement when specifying designs with high strength components.
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This page is divided into the following sections:
- ISO 898-1:2013: Mechanical properties of fasteners made of carbon steel and alloy steel – Part 1: Bolts, screws and studs with specified property classes – Coarse thread and fine pitch thread
Hydrogen Embrittlement (HE) is defined as “…the permanent loss of ductility in a metal or alloy caused by the diffusion of hydrogen into the material in combination with applied or residual tensile stress…”. HE can occur in high strength steels titanium and certain stainless steels. The process can be described as the introduction of atomic hydrogen (H1) into a material causing a reduction in ductility. The introduction of hydrogen can occur either during the production process, where the capability of the manufacturer becomes a factor, or as a result of atmospheric/environmental effects in service. The varying mechanisms by which hydrogen is introduced into the material gives rise to two distinct types of HE: Internal Hydrogen Embrittlement (IHE) and Environmental Hydrogen Embrittlement (EHE). For ease of reference, any brittle failure of a material or fastener due to one of the distinct forms of HE can be referred to by the catch-all term Hydrogen Induced Cracking (HIC).
2. The Failure Mechanism
Any hydrogen introduced at the surface of the steel diffuses over time into the tiny flaws and inclusions between the grain boundaries of the material and areas of greatest stress. Once this has occurred, the load carrying capacity of the material can be reduced by the initiation of cracks local to the concentrated pockets of hydrogen gathered around defects or flaws in the material. As the combined concentration of stress and hydrogen builds up at these concentrated locations, the normally ductile material becomes brittle, causing the material to fail by brittle fracture at stresses significantly below the nominal tensile strength.
As with all failure mechanisms, hydrogen induced cracking is normally initiated at the points of greatest concentration of stress. In fasteners, the failure location is often at the first engaged thread or at the fillet radius under the bolt head.
3. The Causes
Failure due to hydrogen embrittlement requires the simultaneous existence of three factors:
- Atomic hydrogen must have been introduced to the material through some mechanism.
- The material must be susceptible to hydrogen damage.
- The material must be subject to a high tensile stress.
Each of these factors is briefly explained below.
3.1. Hydrogen Introduction
The means by which hydrogen is introduced to the material varies between the two general types of hydrogen embrittlement, IHE and EHE.
3.1.1. Internal Hydrogen Embrittlement
The hydrogen is generally introduced during manufacturing through certain coating and surface preparation processes such as electroplating, pickling and other related surface cleaning processes. Theses finishing processes are the final manufacturing step, and coating materials can act as a barrier to hydrogen effusion. In other words, the coating prevents the natural tendency of hydrogen to effuse out of the steel at room temperature.
Warning: It is extremely important to note that secondary heat additive processes such as welding and brazing may also introduce hydrogen into the heat affected zone. Therefore it is strongly advised to avoid welding operations involving high strength steel items or fasteners, if possible. Manufacturer’s of high strength, proprietary, Commercial-Off-The Shelf (COTS) items such as high strength tension connections will often invalidate any warranty if a user welds to the item. Tight controls and additional treatments are required to ensure that hydrogen embrittlement does not occur during welding and this is heavily dependent on the capability of the fabricator and the quality controls put into place.
3.1.2. External Hydrogen Embrittlement
Environmental hydrogen is introduced as a result of corrosion. For example, galvanic corrosion of a sacrificial, cathodically protective coating such as zinc galvanising, zinc-nickel plating or cadmium plating will generate hydrogen. This hydrogen may then be absorbed by surface areas of a fastener which have been exposed when the coating is damaged, cracked, porous or partially consumed by corrosion. These conditions can lead to a specific form of Stress Corrosion Cracking (SCC).
3.2. Material Susceptibility
Tensile strength and material hardness have a significant effect on the susceptibility to HE. As strength increases, steel becomes harder, less ductile and more brittle. The susceptibility of fasteners to HE increase considerably at hardness values in excess of 39 HRC (380 HV). In fact, ISO 898-1 specifically identifies the risk of SCC in Grade 12.9 fasteners, who have a specified hardness range of 39-44 HRC (385-435 HV).
3.3. Tensile 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 the applied design loads and tightening/preload conditions of a fastener assembly. Generally, maximum bolt preloads are in the order of 50% to 70 % of Ultimate Tensile Strength (UTS). The recommended preload for any given joint is dependent on the joint application and the design standard you are using.
4. My Bolt Snapped. What happened?
In cases of hydrogen embrittlement initiated fracture of fasteners, the time to failure is a key factor in identifying the source of the problem. Failure due to internal hydrogen embrittlement normally occurs between 1 and 24 hours after tightening. If the failure occurs following a significant period after tightening/assembly, then EHE or SCC may be the cause. Typically, EHE failures occur after weeks or even years, as hydrogen is absorbed during corrosion processes. It is important to differentiate between the two hydrogen-related failures as HE can reasonably be considered the fault of the manufacturer and SCC the fault of the designer or the inspection regime.