Low-carbon steel and low-carbon alloy steel (20 steel, 20Cr, 20CrMo, 20CrMnTi)
1. Preheat treatment – normalizing
- Mainly to improve the mechanical properties of low carbon steel, improve machinability,
- Refining grains, eliminating structural defects, making organizational preparations for subsequent heat treatment, etc.
2. Carburizing and quenching (carbonitriding)
Carburizing is mainly to improve the wear resistance of the surface, and as the carbon concentration increases, the wear resistance is also good.
The normalized structure of high carbon steel without quenching after carburizing can obtain network carbide + pearlite structure when the carbon concentration is sufficient and the cooling rate is slow enough. This kind of organization can’t play the role of wear resistance.
3. Depth of carburized layer and depth of hardened layer
The carburized layer is detected by a metallographic microscope, which is the vertical distance from the surface of the carburized workpiece to the specified value of carbon (generally 0.4%C). The hardened layer is detected by a Vickers hardness tester. The national standard is detected from the surface to 550HV1 distance.
Empirical data: depth of hardened layer = (0.18~0.26)*M
The carburized layer is deeper than the hardened layer
4. Surface hardness and core hardness
The material of the carburizing workpiece is generally low-carbon steel or low-carbon alloy steel (the carbon content is less than 0.25%). After carburizing, the chemical composition of the steel surface points can be close to high-carbon steel. After carburizing, the workpiece must be quenched to obtain high surface hardness, high wear resistance, and high strength. And maintain the strength and toughness of low-carbon steel after quenching in the core, so that the workpiece can withstand impact loads. Surface hardness: 58~62HRC Heart hardness: 30~40HRC.
Medium carbon steel and medium carbon alloy steel (45 steel, 40Cr, 42CrMo)
1. Preheat treatment – normalizing
2. Surface hardening (induction hardening)
- High-frequency quenching is mostly used for surface quenching of industrial metal parts. It is a metal heat treatment method that generates a certain induced current on the surface of the workpiece, rapidly heats the surface of the part, and then rapidly quenches it. Induction heating surface quenching has the advantages of good surface quality, low brittleness, hardened surface is not easy to oxidize and decarburize, and has small deformation.
- The difference between ordinary quenching and induction hardening:
Ordinary quenching: hard inside and out. Induction hardening: the surface is hard but the core is not hard, and the part is hard.
- The greater the depth of the hardened layer, the lower the required frequency, and vice versa.
3. Quenching and tempering
- Quenching and tempering treatment refers to the double heat treatment method of quenching and high temperature tempering, the purpose of which is to make the workpiece have good comprehensive mechanical properties. High temperature tempering refers to tempering between 500-650°C.
- Quenching and tempering can greatly adjust the performance and material of steel, and its strength, plasticity and toughness are all good, and it has good comprehensive mechanical properties
- Quenched and tempered steel generally refers to medium carbon steel with a carbon content of 0.3-0.6%.
- Quenched and tempered hardness: 28~35HRC
4. Nitriding treatment
- Nitriding treatment refers to a chemical heat treatment process in which nitrogen atoms penetrate into the surface of the workpiece in a certain medium at a certain temperature. Nitrided products have excellent wear resistance, fatigue resistance, corrosion resistance and high temperature resistance.
- There are often gas nitriding and ion nitriding
- Depth of nitriding layer: 0.08~0.15mm for white layer, 0.2~0.3mm for nitriding layer.
- Surface hardness: 600~800HV
