Both molybdenum and chromium are elements that form and stabilize ferrite and expand the ferrite phase region, and the ability of molybdenum to form ferrite is comparable to that of chromium. Molybdenum also promotes the precipitation of intermetallic phases in austenitic stainless steel, such as sigma phase, kappa phase, and Laves equivalent, which will adversely affect the corrosion resistance and mechanical properties of the steel, especially resulting in a decrease in ductility and toughness. In order to keep austenitic stainless steel with a single austenite structure, with the increase of molybdenum content in the steel, the content of austenite forming elements (nickel, nitrogen and manganese, etc.) should also be increased accordingly, so as to maintain the ferrite and the ferrite in the steel. A balance between austenite-forming elements.
The oxidation effect of molybdenum on austenitic stainless steel is not significant. Therefore, when the chromium-nickel austenitic stainless steel maintains a single austenitic structure and no intermetallic precipitation, the addition of molybdenum has little effect on its mechanical properties at room temperature. With the increase of molybdenum content, the high temperature strength of steel is improved, such as durability, creep and other properties are greatly improved, so molybdenum-containing stainless steel is often used at high temperature. However, the addition of molybdenum increases the high temperature deformation resistance of steel, plus There is often a small amount of delta ferrite in the steel, so the machinability of molybdenum-containing stainless steel is worse than that of molybdenum-free steel, and the higher the molybdenum content, the worse the hot workability. In addition, molybdenum-containing austenitic stainless steel is prone to κ ( σ) phase precipitation, which will significantly deteriorate the plasticity and toughness of the steel, so during the production, equipment manufacturing and application of molybdenum-containing austenitic stainless steel, care should be taken to prevent the formation of intermetallic phases in the steel.
Although molybdenum acts as an alloying element for austenitic stainless steel, the reasons for pitting corrosion and crevice corrosion are not completely clear, but a large number of experiments have pointed out that the corrosion resistance of molybdenum is only equivalent to that of steel containing a relatively high amount of chromium. Molybdenum is mainly used to strengthen the corrosion resistance of chromium in steel. At the same time, the corrosion inhibition effect of molybdenum after forming acid salt has also been confirmed by experiments. In terms of stress corrosion resistance of highly concentrated chloride solution, although molybdenum As an alloying element, the reasons for the resistance of austenitic stainless steel to reducing medium, pitting corrosion and crevice corrosion are not completely clear, but a large number of experiments have pointed out that the effect of molybdenum is only effective when the steel contains a relatively high amount of chromium. It is mainly to strengthen the corrosion resistance of chromium in steel. At the same time, the buffering effect of molybdenum after forming molybdate has also been confirmed by experiments. In terms of stress corrosion resistance to high concentration of chloride deposition, although this experiment refers to the same. Molybdenum below 3# is harmful to the stress corrosion resistance of austenitic stainless steel, but since common chromium-nickel austenitic stainless steel is mostly used in aqueous medium containing trace chloride and saturated oxygen, its stress corrosion is originated from pitting corrosion Therefore, molybdenum-containing chromium-nickel-molybdenum austenitic stainless steels often have better chloride stress corrosion resistance than molybdenum-free steels in practical applications due to their higher pitting corrosion resistance.
