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.

Fig. 10 compares the loss of rupture strength of various steels exposed to pressurized hydrogen at 540°C:

  • the unalloyed carbon steel has obviously suffered the damage described above, loosing more tha 50 % of it’s original strength after less tha 50 hours exposure,
  • additions of 0.5% Mo or 1%Cr-0.5%Mo show a slight improvement, but are not adequate under the given conditions, whereas
  • the alloy content of 2.25%Cr plus 1% Mo provide protection to the extent, that after 500 hours exposure the original rupture strength of the steel has not deteriorated at all.
Effect of composition and exposure time on the strength of steels exposed to hydrogen

Fig. 10: Effect of composition and exposure time on the strength of steels exposed to hydrogen at 63 bar at 540°C. The strength of samples exposed in argon is taken as 100%. (After Nelson5)

The positive effect of Cr plus Mo in this context used to be described as carbide forming and is now referred to as elements lowering grain boundary energy. In any event with suitable selection of Mo and Cr contents the steel will be resistant to hydrogen attack in respect to decarburization, fissuring and loss of strength (4; page 65ff).