Durability Engineering( Hydrogen Embrittlement)

Hydrogen embrittlement occurs when hydrogen is absorbed by metals that are vulnerable to the effect, causing them to lose flexibility and become weaker under stress. Cracking and fracture can occur in embrittled materials at stresses below the metal’s yield strength if the stress is applied continuously.

The Embrittling Procedure

Hydrogen atoms can enter metals and spread throughout the structure via grain diffusion. This can happen both at normal temperatures and higher ones. Little bubbles occur at the metal grain boundaries as the absorbed hydrogen mixes.

The air bubbles exert a force on the metal grains, compressing them. When the pressure is high enough, the metal loses its ductility and begins to break at stresses well below its yield strength. Intergranular cracking occurs when a metal fractures between its individual grains.

Example

Hydrogen embrittlement is demonstrated via a broken fastener made of zinc-plated steel. About three days after being put in place, the fastener broke. An installed fastener experiences tensile pressures along its length.

The fracture surface, as seen through a scanning electron microscope, is depicted on the right. The fracture surface’s rough texture suggests that it opened up along the metal’s grain boundaries. During the zinc electroplating process, the steel become brittle.

• Hydrogen embrittlement causes broken fasteners
• Fracture surface as seen by scanning electron microscopy
• Crevice between granules
• Image of the broken surface at a higher magnification.
• Hydrogen embrittlement prerequisites
• Hydrogen embrittlement becomes a problem only if three conditions are met.

• A weak substance
• The presence of hydrogen in one’s surrounding
• Tension that lasts for a while
• In rare circumstances, the overall tensile stress on a component can be high enough due to residual stress that cracking occurs.

Hydrogen embrittlement is a problem for high-strength steels. It includes steel with a hardness of at least 30 HRC or a tensile strength of above 140 ksi (1,000 MPa).

Absorption of hydrogen can happen during production, assembly, or even when using a component. Electroplating, acid pickling, and phosphating are just a few examples of processes that expose components to hydrogen. Sulfide stress cracking, corrosion, and chemical reactions involving acids and other substances can all expose metal to hydrogen during use.

Hydrogen embrittlement prevention

Hydrogen can cause carbon and low-alloy steel components to become brittle, although there are ways to prevent this. 

Those things are:

• Metals treated with hydrogen should be baked following treatment.
• Use steels of lower strength
• Tension relief: residual and applied

The steel’s strength determines the baking temperature and duration. Hydrogen escapes from the metal in a slow, steady stream as it bakes. After being exposed to hydrogen, the bake process must be completed within a few hours. If too much time passes between being exposed to hydrogen and being baked, the bake will be ineffective. There are baking procedures that conform to ASTM standards that can be used.

When hydrogen absorption happens in-service, it may be desirable to use lower strength steels, reduce residual stress, and reduce applied stress. A less harsh quench can be used during the strengthening heat treatment if the right alloy is chosen. Increasing the cross-section of a component decreases the amount of stress being applied to it.

Discernment and Management

The risks of hydrogen embrittlement and the ways to avoid it are both well-known. Selection and control, as was addressed in a recent post, are crucial to trustworthiness.

If hydrogen embrittlement is a concern during component design, testing should be performed to ensure that the component is not embrittled after manufacturing or will not become embrittled during usage. In order to avoid hydrogen embrittlement, it is crucial to ensure that high-strength steel is surface finished correctly.

Video 01: What is hydrogen embrittlement and what can be done to prevent it?