Role of molybdenum, tungsten and cobalt in stainless steel
(1) The role of molybdenum
Molybdenum is a ferritic forming element and has the same capacity as chromium.
After adding molybdenum into the steel, the oxidation film containing molybdenum is relatively dense and can prevent the penetration of chloride ion (C1-), so it can prevent pitting corrosion. For example, in high chromium ferritic stainless steel and Cr-MnN steel, the addition of 1%~3% molybdenum can improve the corrosion resistance of the steel in organic acids, and can resist the corrosion of boiling acetic acid at any concentration.
In martensitic chromium stainless steel, molybdenum can not only improve the corrosion resistance, but also improve the hardness and strength of steel, and enhance the secondary hardening effect of steel. This effect is very beneficial to stainless steel molds and cutting tools, generally in martensitic chromium stainless steel Mo content < 1%. The formation of δ ferrite has some adverse effects when molybdenum content is too high.
The corrosion resistance of 18-8 chrome-nickel austenitic stainless steel in dilute sulfuric acid can be improved by adding 1.5%~4% mo. In addition, the addition of molybdenum to 18-8 steel can also improve the corrosion resistance of steel in organic acids (such as acetic acid, oxalic acid, formic acid, etc.), hydrogen peroxide, sulfate, acid dye, bleaching powder and other media. Therefore, molybdenum-containing 18-8 chromium nickel steel is widely used in the above industry. However, 18-8Mo steel is not suitable for application in nitric acid, for example, 18-8 steel containing 2%~3%Mo, after solution treatment at 1050℃, the corrosion rate in boiling 65% nitric acid is twice that of steel without molybdenum.
Molybdenum can expand the passivation range in the reduction medium, improve the passivation film strength and corrosion resistance, H2SO4, HCl, H3PO4 and some organic acids, pitting corrosion and crevice corrosion resistance. Molybdenum can also improve the ability to stress corrosion fracture against chloride, especially in the case of halides or chloride ions. The strength of austenitic stainless steel and the tempering resistance of martensitic stainless steel can be improved by solution strengthening. In addition, molybdenum can improve the hardenability, high temperature strength and creep resistance of steel.
In addition, Mo is often added to stainless steel in order to improve the corrosion resistance of stainless steel in non-oxidizing media. In this way, the range of stainless steel passivation media is expanded, so that it has good corrosion resistance in hot sulfuric acid, dilute hydrochloric acid, phosphoric acid and organic acid.
(2) The role of tungsten
Tungsten has a strong tendency to form carbides. Like molybdenum, the formed carbides are softened during tempering. Tungsten can also improve the creep strength of stainless steel. The effects on hardenability, temper stability, mechanical properties and thermal strength of steel are similar to those of molybdenum.
