Metallurgy of Mo in stainless steel
Molybdenum adds corrosion resistance and high temperature strength
Molybdenum primarily increases the corrosion resistance of stainless steels (see Grades and Properties). Molybdenum containing grades of stainless steels are generally more corrosion resistant than molybdenum-free grades. They are used in applications that are more corrosive, such as chemical processing plants or in marine applications. There are many grades of stainless steels with different molybdenum (and chromium, nickel, nitrogen, etc.) contents. The best grade for a given application is selected based on the corrosivity of the service environment.
As a large atom, molybdenum increases the elevated temperature strength of stainless steels through solid solution hardening. This effect is used in heat exchangers and other elevated temperature equipment such as in automotive exhaust systems.
Molybdenum is a ferrite former
To discuss the influence of molybdenum on the metallurgy of stainless steels it is useful to look at the metallurgy of stainless steels in general. Based on their microstructure, stainless steels are divided into the following families:
- precipitation hardenable
The division based on microstructure is useful because the members within one family tend to have similar physical and mechanical properties. However, the properties for one family can be very different from the properties for another family. For example, austenitic stainless steels are non-magnetic, while ferritic and duplex stainless steels are magnetic.