Chromium can increase the hardenability of steel and has a secondary hardening effect. It can improve the hardness and wear resistance of high carbon steel without making the steel brittle; When the content exceeds 12%. The steel has good high temperature oxidation resistance and corrosion resistance to oxidizing medium. It also increases the thermal strength of steel, and chromium is the main alloying element of stainless acid-resistant steel and heat-resistant steel.
Chromium can improve the strength and hardness of carbon steel in rolling condition. Reduce elongation and section shrinkage. When the chromium content exceeds 15%, the strength and hardness will decrease, and the elongation and section shrinkage will increase accordingly. The parts containing chromium steel are easy to obtain higher surface machining quality by grinding.
The main function of chromium in tempered structural steel is to improve hardenability. The steel has better comprehensive mechanical properties after quenching and tempering, and chromium carbides can be formed in carburized steel, so as to improve the wear resistance of the material surface.
Spring steels containing chromium are not easy to decarburize during heat treatment. Chromium can improve the wear resistance, hardness and hardenability of tool steel. Good tempering stability. In electrothermal alloys, chromium can improve the oxidation resistance, resistance and strength of the alloy.
(1) The effect of microstructure and heat treatment of steel
A. Chromium and iron form A continuous solid solution, reducing the austenite phase region. Chromium and carbon form a variety of carbides, and the affinity with carbon is greater than iron and manganese and lower than tungsten, molybdenum and so on. Chromium and iron can form an intermetallic compound σ phase (FeCr).
B, chromium reduces the concentration of carbon in pearlite and the limiting solubility of carbon in austenite.
C, slow down the decomposition rate of austenite, significantly improve the hardenability of steel. However, it also increases the tempering brittleness tendency of steel.
(2) Effect on mechanical properties of steel Pipe
A, improve the strength and hardness of steel. When other alloying elements are added, the effect is more significant.
B, significantly improve the brittle transition temperature of steel.
C. In Fe-Cr alloys with high chromium content, if there is σ precipitation, the impact toughness decreases sharply.
(3) The effect on the physical, chemical and technological properties of steel
A, improve the wear resistance of steel, grinding, easy to obtain a higher surface finish.
B, reduce the conductivity of steel, reduce the resistance temperature coefficient.
C, improve the coercive force and residual magnetic induction of steel. Widely used in the manufacture of permanent magnetic steel.
D, chromium promotes the formation of passivation film on the surface of the steel, when there is a certain content of the inscription, significantly improve the corrosion resistance of the steel (especially nitric acid). If chromium carbide precipitates, the corrosion resistance of the steel decreases.
E, improve the oxidation resistance of steel.
F, chromium steel is easy to form dendritic segregation, reducing the plasticity of steel.
G, because the thermal conductivity of the steel is reduced by chromium, the heat should be slowly heated during hot processing, and the cold should be slow after forging and rolling.
(4) Application in steel pipes
A. Chromium is mainly used in alloy structural steel to improve hardenability, and chromium carbides can be formed on the carburized surface to improve wear resistance.
B, the use of chromium and coalloying elements in spring steel to provide comprehensive properties.
C. Bearing steel mainly uses the special carbide of chromium to contribute to wear resistance and the advantages of high surface light selling after grinding.
D, tool steel and high-speed steel mainly use chromium to improve wear resistance, and has a certain degree of tempering stability and toughness.
E, stainless steel, heat-resistant steel chromium is often combined with manganese, nitrogen, nickel, etc., when it is necessary to form austenitic steel, stable ferritic chromium and stable austenitic manganese, nickel must have a certain proportion, such as Cr18Ni9.
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