What is carburizing?
Carburizing is a kind of surface heat treatment technology, which points to the heat treatment method of quenching after carburizing atoms on the surface of steel parts. Through the infiltration of carbon, the wear resistance, durability, toughness and other properties of steel components can be significantly improved. The steel used to make carburized parts is called carburized steel. The main heat treatment process of carburized steel is generally quenching and low temperature tempering after carburizing. After treatment, the core of the part is a low-carbon martensite structure with sufficient strength and toughness, and the surface layer is hard and wear-resistant tempered martensite and a certain amount of fine carbide structure.
Some structural parts work under strong impact and wear conditions, such as gearbox gears on automobiles and tractors, cams and piston pins on internal combustion engines.
According to the working conditions, these parts are required to have high surface hardness and wear resistance, while the core is required to have higher strength and appropriate toughness, that is, the performance of the workpiece “hard surface and tough inside” is required.
In order to take into account the above dual properties, low-carbon steel can be achieved by carburizing quenching and low-temperature tempering. At this time, the core of the part is a low-carbon steel quenched structure to ensure high toughness and sufficient strength, and the surface layer (at a certain depth ) Has high carbon content (0.85%～1.05%), high hardness (HRC>60) after quenching, and good wear resistance can be obtained.
Composition characteristics of carburized steel
The carbon content of carburized steel is generally very low (between 0.15% and 0.25%) and belongs to low-carbon steel. This carbon content ensures that the core of carburized parts has good toughness and plasticity.
In order to increase the strength of the core of the steel, a certain amount of alloying elements, such as Cr, Ni, Mn, Mo, W, Ti, B, etc. can be added to the steel.
Among them, the main role of Cr, Mn, Ni and other alloying elements is to increase the hardenability of steel, so that the surface layer and core structure can be strengthened after quenching and low temperature tempering.
In addition, a small amount of carbide forming elements such as Mo, W, and Ti can form stable alloy carbides, which can refine grains and prevent steel parts from overheating during carburization.
A small amount of B (0.001%～0.004%) can strongly increase the hardenability of alloy carburized steel.
Classification of carburizing steel
According to different hardenability or strength grades, alloy carburized steels are divided into three categories.
1) Low hardenability alloy carburized steel
That is, low-strength carburized steel (tensile strength≤800MPa), such as 15Cr, 20Cr, 15Mn2, 20Mn2, etc. This type of steel has low hardenability, the core strength is low after carburizing, quenching and low temperature tempering, and the strength and toughness are poorly matched. It is mainly used to manufacture wear-resistant parts with low stress and low strength requirements, such as camshafts, piston pins, sliders, pinions, etc. of diesel engines.
The core grains are easy to grow when this kind of steel is carburized, especially manganese steel. If the performance requirements are high, this type of steel often adopts the secondary quenching method after carburizing, that is, normalizing treatment is performed after carburizing to eliminate the overheated structure formed during carburizing, and then reheating and quenching.
2) Medium hardenability alloy carburized steel
That is, medium-strength carburized steel (tensile strength=800～1200MPa), such as 20CrMnTi, 12CrNi3A, 20CrMnMo, 20MnVB, etc. This type of steel contains about 4% of the total alloying elements. Because the two elements of Cr and Mn are added to the steel, it can more effectively improve the hardenability and mechanical properties (tensile strength=1000～1200MPa). Generally used to manufacture heavy-duty medium and small wear-resistant parts and medium-load gears with larger modulus. Such as gearboxes and rear axle gears of automobiles and tractors, gear shafts, cross pin heads, spline bushings, valve seats, cam discs, etc.
Because this kind of steel contains Ti, V, Mo, the tendency of austenite grains to grow during carburizing is small, so it can be quenched directly from the carburizing temperature to about 870°C, and after low temperature tempering, the parts have Good mechanical properties.
3) High hardenability alloy carburized steel
That is, high-strength carburized steel (tensile strength> 1200MPa), such as 12Cr2Ni4, 18Cr2Ni4WA and so on. This type of steel contains a total of alloying elements ≤7.5%. Because it contains more Cr and Ni elements, it can greatly improve the hardenability of the steel. In particular, the addition of more Ni can improve the strength while making the steel good Of resilience. This type of steel can be used as important large parts that bear heavy loads and strong wear, such as the active traction gear of diesel locomotives, diesel engine crankshafts, connecting rods and precision bolts on cylinder heads.
Due to the higher alloying elements, the C curve shifts to the right, so the martensite structure can be obtained even when cooled in air; in addition, the martensite transformation temperature also drops sharply, so that the carburized surface layer will remain after quenching A large amount of retained austenite. In order to reduce the amount of retained austenite after quenching, high temperature tempering can be used before quenching to spheroidize the carbides or cold treatment after quenching.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)