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.

Temper brittleness

Temper embrittlement may occur when steels are slowly cooled after tempering through the temperature range between 450 and 550°C. This is due to the segregation of impurities such as phosphorus, arsenic, antimony and tin on the grain boundaries. The molybdenum atom is very large relative to other alloying elements and impurities. It effectively impedes the migration of those elements and thereby provides resistance to temper embrittlement.

Fig 9 shows the ductile-to-brittle transition temperature for two steels. This temperature is an indication for the lower limit of the service temperature without the risk of brittle failure.

If the steels are water quenched after tempering, both steels one without molybdenum and one with 0.15% molybdenum, have essentially the same ductile to brittle behaviour (transition at approx. -50°C). However, if the steels are slowly cooled in the furnace after tempering the picture changes. The transition to brittle fracture has shifted to +25°C for the Cr-steel, while it remained at -50°C for the Cr-Mo steel. The slow cooling rate has not embrittled the molybdenum containing steel, it is, therefore, less susceptible to temper embrittlement.