Effect of Molybdenum on the Properties of Stainless Steel

Like tungsten, molybdenum is a refractory rare metal with a melting point of 2620℃, strong interatomic bonding, small expansion coefficient, high electrical conductivity and good thermal conductivity. The effects of molybdenum on the performance of stainless steel are as follows:

Molybdenum has no significant oxidation effect on stainless steel. Therefore, when chromium-nickel stainless steel maintains a single austenite structure and no intermetallic precipitation, the addition of molybdenum has little effect on its room temperature mechanical properties. However, with the increase of molybdenum content, the high temperature strength of steel is improved, such as durability, creep and other properties are greatly improved. Therefore, molybdenum-containing stainless steel is also often used at high temperatures.

The addition of molybdenum increases the high temperature deformation resistance of steel. In addition, there is often a small amount of ferrite in steel. Therefore, the hot processing property of molybdenum-containing stainless steel is worse than that of non-molybdenum-containing steel, and the higher the molybdenum content, the worse the hot processing performance. In addition, one million κ phases are easily precipitated in molybdenum-containing stainless steel, which will significantly deteriorate the plasticity and toughness of the steel. Therefore, in the production, equipment manufacturing and application of molybdenum-containing stainless steel, attention should be paid to prevent the formation of intermetallic phases in steel.

The main role of molybdenum in stainless steel is to improve the steel’s corrosion resistance to reducing media and its resistance to pitting and crevice corrosion. The following are the effects of molybdenum on the corrosion resistance of chromium-nickel stainless steel in media such as nitric acid, sulfuric acid, acetic acid, phosphoric acid and urea. Except in the oxidizing medium HNO3, the role of molybdenum is beneficial. Therefore, stainless steel containing molybdenum generally does not need to be resistant to nitric acid corrosion.

Although the reason why molybdenum acts as an alloying element on the resistance of stainless steel to reducing media, surface pitting corrosion and crevice corrosion is not yet fully understood, a large number of experiments have pointed out that the corrosion resistance of molybdenum is only effective when the steel contains a high amount of chromium. Molybdenum mainly strengthens the corrosion resistance of chromium in steel. At the same time, the corrosion inhibition effect of molybdenum after forming acid salts has also been confirmed by experiments. In terms of stress corrosion resistance in high-concentration chloride solutions, although molybdenum as an alloying element has a strong effect on the resistance of stainless steel to reducing media, pitting corrosion and crevice corrosion, it is not yet fully understood.

In terms of stress corrosion resistance in high-concentration chloride solutions, although some experiments have pointed out. Chromium-nickel-molybdenum stainless steel containing molybdenum has a higher resistance to pitting corrosion, so in practical applications, it often has better chloride stress corrosion resistance than steel without molybdenum.