1) Mainly to improve the mechanical properties of mild steel and improve machinability,
2) Grain refinement, elimination of structural defects, preparation for subsequent heat treatment, etc.2. Carburizing and hardening (carbonitriding)——
Carburizing is generally used to increase the wear resistance of the surface. As the concentration of carbon increases, the wear resistance also increases. The normalized structure of high carbon steel obtained without quenching after carburization can obtain a carbide-pearlite mesh structure under the condition of sufficient carbon concentration and sufficiently slow cooling rate. This structure cannot provide wear resistance.
Carburization is the penetration of activated carbon atoms into the surface of the workpiece to increase the carbon content in the surface layer of the workpiece. After carburizing, the structure is a pearlite structure, and carbon atoms must rely on martensitic phase transformation to achieve a hardening effect, so it must be quenched to turn the structure into martensite. The workpiece without carburization is subjected to quenching to obtain low-carbon martensite and quenching after carburizing to obtain high-carbon martensite, while the hardness is increased.3. Depth of the carburizing layer and depth of the hardening layer
The carburized layer is detected using a metallographic microscope, which is the vertical distance from the surface of the carburized workpiece to a specified value of carbon content (typically 0.4% C). The hardened layer is detected using a Vickers hardness tester. national standard from surface detection to 550HV1 distance.
Empirical data: hardened layer depth = (0.18~0.26)*M
The carburized layer is deeper than the hardened one4. Surface hardness and core hardness
The material of carburizing billet is usually low carbon steel or low carbon alloy steel (less than 0.25% carbon content). After carburization, the chemical composition of the steel surface can be close to that of high carbon steel. After carburizing, the workpiece must be quenched to obtain high surface hardness, high wear resistance and fatigue strength, and maintain the strength and toughness of the low carbon steel in the core after quenching, so that the workpiece can withstand impact loads.
Surface hardness: 58~62HRC
Heart hardness: 30~40HRC2. Medium carbon steel and medium carbon alloy steel (45 steel, 40Cr, 42CrMo) 1. Preheat
Annealing——2. Surface hardening (induction hardening)
1) High-frequency quenching is mainly used for surface hardening of industrial metal parts. It is a metal heat treatment method in which a certain induced current is generated on the surface of the workpiece, which quickly heats up the surface of the part, and then quickly quenches it. Induction heating surface hardening has the advantages of good qualitysurface properties, low brittleness, hardened surface, which is not easy to oxidize and decarburize, and has little deformation.
2) The difference between conventional hardening and induction hardening
Normal tempering: tough inside and out. Induction Hardening: The surface is hard, but the core is not hard, and the part is hard.
3) The greater the depth of the hardened layer, the lower the required frequency, and vice versa.3. Hardening and tempering (hardening + high temperature tempering)
1) Quenching and tempering refers to the method of double heat treatment of quenching and high temperature tempering, the purpose of which is to give the workpiece good comprehensive mechanical properties. High temperature tempering refers to tempering between 500-650°C.
2) Quenching and tempering can greatly change the characteristics and material of the steel, its strength, ductility and toughness are good, and it also has good comprehensive mechanical properties
3) Quenched and tempered steel generally refers to medium carbon steel with a carbon content of 0.3-0.6%.
4) Hardness after quenching and tempering: 28~35HRC4. Nitriding
1) Nitriding refers to a chemical-thermal treatment process in which nitrogen atoms penetrate the surface of a workpiece in a specific environment at a specific temperature. Nitrided products have excellent wear resistance, fatigue resistance, corrosion resistance and high temperature resistance.
2) Often there is gas nitriding and ion nitriding
3) Nitriding layer depth: white bright layer 0.08~0.15mm, nitriding layer depth 0.2~0.3mm.
4) Surface hardness: 600~800HV