Metallurgy of Mo in alloy steel & iron
Hardened steel must be tempered, i.e. reheated for two reasons:
- Different cooling rates between edge and core of components result in internal stresses, which must be relieved.
- High hardness martensite and bainite must be softened to avoid brittle cracking.
The improvement of ductility is inevitably accompanied by softening, i.e. a loss of strength. This is very pronounced with carbon steel. An important function of alloying elements is to delay temper softening.
Through its capability of forming carbides, molybdenum, carefully combined with chromium and vanadium, is very efficient in delaying the loss of strength during tempering while improving fracture toughness. The resulting structure, tempered martensite, is very strong with an acceptable level of toughness.
Fig 8 shows the effect of molybdenum on the hardness after tempering of a 0.35% carbon steel. It significantly delays softening of the steel. At sufficiently high Mo contents the hardness curve may even increase with increasing tempering temperature. This is known as secondary hardening.
The effect of secondary hardening on tempering is an important function of molybdenum in high speed steels and in some tool- and die steels.