Molybdenum: A Key Element in Steel Making

Elemental molybdenum is a silvery-white metal, hard and tough. Density 10, 2 g/cm3. Melting point 2610°C. Boiling point 5560°C°C. Chemical valence +2, +4 and +6, stable valence of +6. Molybdenum double has a solid solution strengthening effect on ferrite, but also improves the stability of carbides, and therefore has a favorable effect on the strength of steel. Molybdenum in steel, due to the formation of special carbides, can improve the resistance to hydrogen erosion at high temperatures and pressures. In addition, the addition of molybdenum to steel also passivates the steel surface, but the effect is not as significant as that of chromium. Molybdenum can generally improve the corrosion resistance of steel.

I. Molybdenum on the microstructure of steel and the impact of heat treatment

(1) molybdenum in steel can be solid solution in ferrite, austenite and carbide, it is to narrow the austenite phase area of the element.

(2) When the molybdenum content is low, with iron and carbon to form a composite carburite, the content is higher to form a special carbide of molybdenum.  

(3) molybdenum to improve the hardenability of steel, its role is stronger than chromium, and slightly inferior to manganese.

(4) Molybdenum improves the tempering stability of steel. As a single alloying element, increase the tempering brittleness of steel; with chromium, manganese and other coexistence, molybdenum and reduce or inhibit the tempering brittleness caused by other elements.

II. The effect of molybdenum on the mechanical properties of steel  

(1) molybdenum has a solid solution strengthening effect on ferrite, but also improve the stability of carbide, thereby improving the strength of steel.  

(2) to improve the ductility, toughness and wear resistance of steel play a favorable role.  

(3) Because molybdenum makes the softening temperature after deformation strengthening and recrystallization temperature increase, and greatly improve the creep resistance of ferrite, can effectively inhibit the aggregation of carburite at 450 ~ 600 ℃, and promote the precipitation of special carbides, thus becoming the most effective alloying element to improve the thermal strength of steel.

III. The effect of molybdenum on the physical, chemical and technological properties of steel

(1) the mass fraction of molybdenum is greater than 3%, so that the steel oxidation resistance deterioration.  

(2) the mass fraction of molybdenum does not exceed 8% of the steel can still be forged, rolled, but the content is higher, the steel deformation resistance to hot work increased.  

3) Molybdenum can improve the corrosion resistance of steel and prevent pitting of steel in chloride solutions.  

(4) in the carbon mass fraction of 1.5% of the magnet steel, molybdenum mass fraction of 2%-3% can improve the residual magnetic susceptibility and coercivity.  

IV. The application of molybdenum in steel

(1) chromium-molybdenum steel in many cases can replace chromium-nickel steel to manufacture important parts, commonly used in the manufacture of some high temperature and high pressure resistant valves and pressure vessels, as shown in the figure.

a) High pressure valves

b) Pressure vessels

2) It is widely used in quenched and carburized structural steel, spring steel, bearing steel, tool steel, stainless and acid-resistant steel, heat-resistant steel and magnetic steel.