Hydrogen embrittlement susceptible material

While the conventional slow strain-rate test offers many benefits, it does suffer from a tendency to overstate the susceptibility of materials to hydrogen embrittlement. Thus structural steels of modest strength will fail even under conditions giving relatively low rates of hydrogen entry. This is... 

The martensitic alloys contain 12 to 20 percent chromium with controlled amounts of carbon and other additives. Type 410 is a typical member of this group. These alloys can be hardened by heat treatment, which can increase tensile strength from 550 to 1380 MPa (80,000 to 200,000 Ibf/in ). Corrosion resistance is inferior to that of austenitic stainless steels, and martensitic steels are generally used in mildly corrosive environments (atmospheric, freshwater, and organic exposures). In the hardened condition, these materials are very susceptible to hydrogen embrittlement. 

N = equivalent number of full displacement cycles during the expected service life of the piping system.5 N shall be increased by a factor of 10 for all materials that are susceptible to hydrogen embrittlement (carbon and low alloy steels) when the system design temperature is within the hydrogen embrittlement range [up to 150°C (300°F)]. 

A-2.8.1 Pressure and Temperature. An important general trend is that structural metals become more susceptible to hydrogen embrittlement as hydrogen gas pressure increases. An example of this trend is the measured threshold stress intensity factor, Km, as a function of gas pressure for low-alloy steels. Kjh decreases as gas pressure increases. Increasing hydrogen gas pressure enhances the concentration of dissolved hydrogen in materials, which promotes hydrogen embrittlement. 

It has been reported that hydrogen embrittlement is a form of stress corrosion cracking (SCC). Three basic elements are needed to induce SCC the first element is a susceptible material, the second element is environment, and the third element is stress (applied or residual). For hydrogen embrittlement to occur, the susceptible material is normally higher strength carbon or low-alloy steels, the environment must contain atomic hydrogen, and the stress can be either service stress and/or residual stress from fabrication. If any of the three elements are eliminated, HE cracking is prevented. 

Steels become more susceptible to hydrogen embrittlement as the materials are exposed to higher gas pressures. Thermodynamic equilibrium between hydrogen gas and dissolved atomic hydrogen is expressed by the general form of Sievert s law ... 

Having confirmed the susceptibility of the material to direct embrittlement by hydrogen, it then becomes worthwhile considering the contributions made by hydrogen embrittlement to the loss in ductility in other environments, particularly where aggressive solutions are involved that may cause significant amounts of corrosion. 

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