Molybdenum in the steel pipe can improve the hardenability and thermal strength. Prevent tempering brittleness, increase remanence and coercivity, and resist corrosion in certain media.
In tempered steel, molybdenum can make the larger section of the parts quenched deep, quenched through, improve the tempering resistance or tempering stability of the steel, so that the parts can be tempered at higher temperatures, so as to more effectively eliminate (or reduce) residual stress, improve plasticity.
In addition to the above functions in carburized steel, molybdenum can also reduce the tendency of carbide to form a continuous network on the grain boundary in the carburizing layer, reduce the residual austenite in the carburizing layer, and relatively increase the wear resistance of the surface layer.
In the forging die steel, molybdenum can also maintain the steel has a relatively stable hardness, increase the resistance to deformation, cracking and wear.
In stainless acid-resistant steel pipes, molybdenum can further improve the corrosion resistance of organic acids (such as formic acid, acetic acid, oxalic acid, etc.) and hydrogen peroxide, sulfuric acid, sulfite, sulfate, acid dyes, bleaching powder liquid. In particular, the addition of molybdenum prevents the point corrosion tendency caused by the presence of chloride ions.
(1) The effect of microstructure and heat treatment of steel pipes
A. Molybdenum can be solidly dissolved in ferrite, austenite and carbide in steel, and it is an element that reduces the austenitic phase region.
B, when the steel content is low, the composite cementite is formed with iron and carbon; When the content is higher, the special carbide of steel can be formed.
C, molybdenum to improve the hardenability of steel, its role is stronger than chromium. And a little less than manganese.
D, molybdenum improve the tempering stability of steel, as a single alloying element, increase the tempering brittleness of steel; When co-existing with chromium, manganese, etc., molybdenum reduces or inhibits the temper brittleness caused by other elements.
(2) The effect on the mechanical properties of the steel pipe
A. Molybdenum has A solid solution strengthening effect on ferrite. At the same time, it also improves the stability of the carbide, thus improving the strength of the steel.
B, molybdenum to improve the ductility and toughness of steel and wear resistance play a favorable role.
C, due to molybdenum, the softening and recovery temperature and recrystallization temperature after deformation strengthening are increased, and the creep resistance of ferrite is strongly increased, and the accumulation of cementite is effectively inhibited at 450-600℃. Promote the precipitation of special carbides, and thus become the most effective alloying element to improve the thermal strength of steel.
(3) The effect on the physical, chemical and technological properties of the steel pipe
A, in the magnetic steel containing 1.5% carbon, 2%-3% molybdenum improves the residual magnetic induction and coercivity.
B, in reducing acid and strong oxidizing salt solution can passivate the steel surface. Therefore, molybdenum can generally improve the corrosion resistance of steel and prevent pitting corrosion of steel in chloride solution.
C, molybdenum content is higher (>3%) to deteriorate the oxidation resistance of steel.
D, the steel containing no more than 8% molybdenum can still be forged and rolled, but when the content is higher, the deformation resistance of the steel to hot processing is increased.
(4) Application in steel
A. It has been widely used in tempered and carburized structural steel, spring steel, bearing steel, tool steel, stainless acid-resistant steel, heat-resistant steel and magnetic steel.
B, chromium molybdenum steel in many cases can replace chromium nickel steel to manufacture important parts.
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