(1) The role of molybdenum

Molybdenum is a ferrite-forming element, and its capacity is equivalent to that of chromium.

After adding molybdenum to the steel, since the oxide film containing molybdenum is denser, it can prevent the penetration of chloride ions (C1-), so it can prevent pitting corrosion. For example, in high chromium ferritic stainless steel and Cr-MnN steel, adding 1% to 3% molybdenum can improve the corrosion resistance of the steel in organic acids, and can resist the corrosion of any concentration of boiling acetic acid.

In martensitic chromium stainless steel, molybdenum can not only improve its corrosion resistance, but also increase the hardness and strength of the steel, and enhance the secondary hardening effect of the steel. This effect is very beneficial to stainless steel molds and cutting tools. Generally, the Mo content in martensitic chromium stainless steel is less than 1%. If the content of molybdenum is too high, it will promote the formation of δ ferrite and have some adverse effects.

In 18-8 chromium nickel austenitic stainless steel, adding 1.5% to 4% molybdenum can improve the corrosion resistance of the steel in dilute sulfuric acid. In addition, adding 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 dyes, bleaching powder and other media. Therefore, 18-8 chromium-nickel steel containing molybdenum is widely used in the above-mentioned industries. But 18-8Mo steel should not be used 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 faster than that of steel without molybdenum. To be twice as big.

Molybdenum can expand the range of passivation in the reducing medium, improve the strength and corrosion resistance of the passivation film, resist H2SO4, HCl, H3PO4 and some organic acids, and resist pitting and crevice corrosion. Molybdenum can also improve the ability of chloride stress corrosion cracking, especially in the case of halide or chloride ions. Using the method of solid solution strengthening, molybdenum can improve the strength of austenitic stainless steel and the tempering resistance of martensitic stainless steel. In addition, molybdenum can also improve the hardenability, high temperature strength and creep resistance of steel.

In addition, in order to improve the corrosion resistance of stainless steel in non-oxidizing media, Mo is often added to stainless steel. In this way, the range of passivation media for stainless steel is expanded, so that it has better corrosion resistance in hot sulfuric acid, dilute hydrochloric acid, phosphoric acid and organic acids.

(2) The role of tungsten

Tungsten carbides have a strong tendency to form. Like molybdenum, the carbides formed can resist softening during tempering. Tungsten can also increase the creep strength of stainless steel. The effects on hardenability, tempering stability, mechanical properties and thermal strength of steel are similar to those of molybdenum.

(3) The role of the drill

Cobalt has a solid solution strengthening effect, imparts thermal hardness to steel, and improves the high-temperature performance, oxidation resistance and corrosion resistance of steel. Important alloying element. Due to the high price of cobalt, not many common stainless steels add cobalt as an alloying element. The commonly used stainless steel is 90Cr17MoVCo (Co content is 1.2%~1.8%). The purpose of adding cobalt is not to improve the timeliness, but to increase the hardness, because this stainless steel is mainly used to make slicing mechanical knives, scissors and surgical knives. The precipitation hardening steel 12Cr14Co13Mo5V used in the past has been replaced by PCR steel (06Cr17Ni4Cu3Nb) due to its high cost.