What is the purpose of molybdenum in steel?

The role of molybdenum in steel is mainly reflected in its influence on the microstructure, mechanical properties, physical and chemical properties, and processability of steel. ‌‌

Firstly, molybdenum can be solid dissolved in ferrite, austenite, and carbides in steel, reducing the austenite phase zone and forming composite carbides with carbon and iron. When the molybdenum content is high, special molybdenum carbides can be formed. Molybdenum can also improve the hardenability of steel, which is stronger than chromium but slightly inferior to manganese. In addition, molybdenum can improve the tempering stability of steel, but it increases tempering brittleness in the presence of a single alloying element, while coexisting with other elements such as chromium and manganese can reduce or suppress tempering brittleness.

In terms of mechanical properties, molybdenum has a solid solution strengthening effect on ferrite, improving the stability of carbides and thus enhancing the strength of steel. It also helps improve the ductility, toughness, and wear resistance of steel. Molybdenum can also improve the creep resistance of ferrite, effectively suppress the aggregation of cementite at 450-600 ℃, and promote the precipitation of special carbides. Therefore, it is one of the most effective alloying elements for improving the thermal strength of steel.

In terms of physical, chemical, and process properties, steel with a mass fraction of molybdenum not exceeding 8% can still be forged and rolled, but higher content will increase the deformation resistance of hot working. Molybdenum can improve the corrosion resistance of steel in certain media and prevent pitting corrosion, but its content will deteriorate the oxidation resistance when it exceeds 3%. In addition, molybdenum can enhance the remanence and coercivity of magnetic steel.

Molybdenum is widely used in steel, including quenched and tempered structural steel, spring steel, bearing steel, tool steel, stainless acid resistant steel, heat-resistant steel, and magnetic steel. For example, chromium molybdenum steel is commonly used to manufacture valves and pressure vessels that are resistant to high temperatures and pressures.

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