Hydrogen absorption in steel lead to embrittlement by forming gas molecules or brittle phases. Why we are more concerned about high strength steel than low strength steel?
Logically speaking, high strength steels are more demanding, Its quality will be down graded by damages. Low graded steel is not that demanding, it is already low in quality.
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As far as I know, there are two aspects here:
1. high strength steels allow for larger elastic deformations, so the driving force for Hydrogen to go to a region of high tensile stress is larger (in a very simple picture, you can imagine thatit is energetically favourable for the hydrogen to go to a region of lower electron density). Therefore, the danger of hydrogen embrittlement at a stress concentration like a crack tip is larger.
2. High-strength steels usually have less of a "plastic reserve", i.e. the percentual difference between yield strength and ultimate strength is not so large and the strain at failure is also usually smaller. (Note that this is not true for all high-strength steels)
When people mention high strength steel, it traditioanlly means the steel contains martensite...Based on some study, martensite phase is the best phase for hydrogen to diffuse but the worst phase for hydrogen to dissolve. So whenever there is martensite in the material, it can work as the fast diffusion channel from environment to the material inside, so the hydrogen atoms takes the defect site of the material, when the condition is proper, atoms transform to molecules. Hydrogen molecules increases the internal pressure at the defect sites, so it causes the internal crack when the internal pressure increases over the cracking limit of material.
The factors responsible for this type of failure include having a susceptible material, an environment conductive to attack and the presence of stress (internal or applied). Once two of these three factors are present, failure is inevitable. Hydrogen embritlement is also known as hydrogen induced cracking or hydrogen atack. Materials that are most vulnerable include high-strength steels, titanium and aluminum alloys. Hydrogen embritlement mechanisms involve the ingress of hydrogen into the metal, reducing its ductility and load bearing capacity. Stress below the yield stress of the susceptible material then causes subsequent cracking and catastrophic britle failures . See like reference Herring, D.H. and Richard D. Sisson
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