According to different hardenability or strength grade, alloy carburizing steel can be divided into three types.
1) Low hardenability alloy carburizing steel
That is, low-strength carburizing steel (tensile strength ≤ 800MPa), such as 15Cr, 20Cr, 15mn2, 20Mn2, etc. After carburizing, quenching and low temperature tempering, the core strength of this kind of steel is low and the matching of strength and toughness is poor. It is mainly used to manufacture wear-resistant parts with small stress and low strength requirements, such as camshaft, piston pin, slider, pinion, etc.
This kind of steel is easy to grow when carburizing, especially manganese steel. If the performance requirements are high, this kind of steel often adopts the secondary quenching method after carburizing, that is, normalizing treatment is carried out first after carburizing to eliminate the overheated structure formed during carburizing, and then reheating and quenching.
2) Medium hardenability alloy carburizing steel
That is, medium strength carburizing steel (tensile strength = 800-1200mpa), such as 20CrMnTi, 12CrNi3A, 20CrMnMo, 20mnvb, etc. The total amount of alloying elements in this kind of steel is about 4%. The hardenability and mechanical properties (tensile strength = 1000-1200mpa) can be improved more effectively by adding CR and Mn into the steel. It is generally used to manufacture heavy load medium and small wear-resistant parts and medium load gears with large module. For example, gearbox and rear axle gear, gear shaft, cross pin head, spline shaft sleeve, valve seat, cam plate, etc.
Because this kind of steel contains Ti, V, Mo, the tendency of austenite grain growth is small during carburizing, so it can be directly quenched by self carburizing temperature precooling to about 870 ° C, and the parts have better mechanical properties after low temperature tempering.
3) High hardenability alloy carburizing steel
High strength carburizing steel (tensile strength > 1200Mpa), such as 12cr2ni4, 18Cr2Ni4WA, etc. The total amount of alloying elements in this kind of steel is less than or equal to 7.5%. Because there are more Cr and Ni elements in this kind of steel, the hardenability of the steel can be greatly improved, especially the addition of more Ni, which can improve the strength and toughness of the steel. This kind of steel can be used as important large parts to bear heavy load and strong wear, such as driving traction gear of diesel locomotive, crankshaft, connecting rod and precision bolt of cylinder head, etc.
Because of the high content of alloy elements, the C curve moves to the right, so the martensite structure can be obtained by cooling in the air; in addition, the martensite transformation temperature also drops sharply, so that a large amount of retained austenite will be retained in the carburized surface after quenching. In order to reduce the amount of retained austenite after quenching, the carbide can be spheroidized by high temperature tempering before quenching or cold treatment after quenching.
