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:

  • austenitic
  • ferritic
  • martensitic
  • duplex
  • 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.

The difference between the families is fundamental on an atomic level. The arrangement of atoms in the ferrite crystal is different from the one in the austenite crystal:

In the ferritic stainless steel, the iron and chromium atoms are arranged on the corners of a cube and in the center of that cube. In the austenitic stainless steels the atoms, here iron, chromium and nickel, are arranged on the corners of the cube and in the center of each of the faces of the cube.

This seemingly small difference profoundly affects the properties of these steels.

Ferrite / Austenite figure

Fig 1: The ferritic stainless steel on the left has a body centered cubic (bcc) crystal structure. By adding nickel to this stainless steel the structure changes from bcc to face centered cubic (fcc), which is called austenitic.

Select properties of austenitic and ferritic stainless steels
Toughness Very high Moderate
Ductility Very high Moderate
Weldability Good Limited
Thermal expansion High Moderate
Stress corrosion cracking resistance Low Very high
Magnetic properties Non-magnetic Ferro magnetic

Because of their good mechanical properties and the ease of fabrication, austenitic stainless steels are much more widely used than ferritic stainless steels. About 75% of all stainless steel used worldwide is austenitic and about 25% is ferritic. The other families, martensitic, duplex and precipitation hardenable stainless steels each represent less than 1% of the total market.

Besides nickel there are other elements that tend to make the structure austenitic. These elements are called austenite formers. Alloying elements that tend to make the structure ferritic are called ferrite formers.

Ferrite and austenite formers
Ferrite formersAustenite formers
Iron Nickel
Chromium Nitrogen
Molybdenum Carbon
Silicon Manganese

Molybdenum is a ferrite former. That means that when molybdenum is added to improve the corrosion resistance of an austenitic stainless steel, there has to be an austenite former such as nickel or nitrogen added in order to keep the structure austenitic.

Duplex stainless steels have a mixture of austenitic and ferritic grains in their microstructure; hence they have a “duplex” structure. This effect is achieved by adding less nickel than would be necessary for making a fully austenitic stainless steel.

Molybdenum is mainly used for added corrosion resistance in austenitic and duplex stainless steels. In austenitic stainless steels between two and seven percent are added, in duplex stainless steels, between three and five percent. The addition of one or two percent molybdenum to ferritic stainless steels also significantly increases the corrosion resistance and the elevated temperature strength of these stainless steels.

Fig 2: Adding 8% nickel to a ferritic chromium stainless steel makes an austenitic chromium-nickel stainless steel, for example Type 304 stainless steel. If less nickel is added to a chromium steel, about four or five percent, a duplex structure, a mixture of austenite and ferrite, is created as in 2205 duplex stainless steel.