Metallurgy of Mo in alloy steel & iron
Hydrogen embrittlement and sulphide stress cracking
As outlined above, the strength levels obtained in quenched and tempered steels are based mainly on the high strength of martensite, a microstructure characterized by a high density of dislocations and high internal stresses.
Unfortunately, exactly these conditions enhance the diffusion of hydrogen into steel and cause hydrogen embrittlement. Tempering reduces the internal stresses and the dislocation density of martensite, hence reduces hydrogen diffusion. However, the strength can be lowered to insufficient levels. Molybdenum is efficient in mittigating this effect in two ways: through solid solution strengthening and the formation of complex carbides together with other elements such as chromium and niobium.
In cases, where hydrogen sulphide is the source of the hydrogen the phenomenon is called sulphide stress cracking (SCC). The capability of molybdenum to provide resistance to sulphide stress cracking has been the key to the development of a broad range of steel grades used for Oil Country Tubular Goods and in chemical and petrochemical plants.
High temperature hydrogen attack
Carbon steel has severe limitations at conditions of hydrogen attack above about 200 C as are common in processes such as petroleum destilling and catalytic reforming. The hydrogen diffusing into the steel combines with carbon present to form methane and other products. The result is first decarburization and subsequently fissuring due to high gas pressure at localized sites.