Surface quenching of metal materials

In actual production, many machine parts work under alternating loads such as torsion and bending, as well as impact loads. The surface layer is subjected to friction, alternating or pulsating contact stresses, and sometimes impact. For example, transmission shaft, transmission gear, etc. The surface of these parts bears higher stress than the core, so it requires higher strength, hardness and wear resistance within the limited depth range of the working surface, while the core requires sufficient plasticity and wear resistance. Have the resilience to withstand a certain amount of pressure. Impact load. Based on this requirement and the quenching and hardening laws of metal materials, the surface quenching process was developed.

Surface quenching is one of the important means to strengthen the surface of metal materials. Any metal material that can increase its strength and hardness through quenching can be strengthened through surface quenching.
The workpiece after surface quenching treatment can achieve the effect of "hard surface but tough core", that is, not only the surface has high hardness, strength and wear resistance, but also matches the core structure obtained by the preliminary heat treatment of the workpiece. workpiece and has good toughness and fatigue strength. Therefore, surface quenching is widely used in industrial production.

Surface quenching is a heat treatment process that uses rapid heating to heat the workpiece above the phase transformation point within a limited depth range on the surface, and then rapidly cools it to obtain martensite only within a certain depth range on the surface of the workpiece to achieve the purpose of strengthening the surface of the workpiece.
Gears, cams, crankshafts and various shaft parts work under alternating loads such as torsion and bending, and are subject to friction and impact. Their surfaces experience higher stresses than their cores. The purpose of surface quenching is to obtain the martensite structure within a certain depth range of the workpiece surface, while the core remains surface quenched (quenching and tempering or normalizing state), thereby obtaining the required higher hardness and wear resistance of the part surface. properties, while the core maintains a certain strength, sufficient plasticity and toughness, that is, the surface is hard and the core is tough.
Just in order to quickly reach the austenitizing temperature within the limited depth range of the workpiece surface, while the core temperature is still very low, extremely high thermal energy density must be provided to the workpiece surface (generally the thermal energy density needs to be ≥102W/cm2) to make it The surface is rapidly heated to the austenitizing temperature, and the heat on the surface cools down first before it can be transferred to the core, keeping the core temperature at a lower temperature.

There is no phase change in this heart part. There are many ways to meet this rapid heating requirement. Depending on the heat source, steel surface quenching mainly includes induction heating surface quenching, laser heating surface quenching, flame heating surface quenching, etc. In addition, there are electron beam heating, electrical contact heating, electrolyte heating, etc. Surface quenching uses a variety of heating methods, such as heating, plasma beam and iso-infrared focused heating.

Since the above heating methods each have their own characteristics and limitations, they are all applied under certain conditions. The most commonly used are induction heating surface quenching and flame heating surface quenching. Laser beam heating and electron beam heating are currently new high-energy-density heating and quenching methods. Since they have some advantages that other methods do not have, they have gained some applications.

Surface quenching is widely used in mechanical parts made of medium-carbon quenched and tempered steel or ductile iron with a carbon content of 0.4% to 0.5%. Since medium carbon quenched and tempered steel is surface quenched after quenching and tempering or normalizing pretreatment, it can not only maintain high comprehensive mechanical properties in the core, but also make the surface have high hardness (>50HRC) and wear resistance. Performance, such as machine tool spindles, gears, diesel engine crankshafts, camshafts, etc. In principle, there are gray cast iron, ductile iron, malleable cast iron, alloy cast iron, etc. The matrix is equivalent to medium carbon steel with pearlite and ferrite as the matrix, and can be surface quenched. However, ductile iron has the best process performance and has high comprehensive mechanical properties after surface quenching, so it is the most widely used.
After surface quenching of high carbon steel, although the surface hardness and wear resistance are improved, the plasticity and toughness of the core are low. Therefore, surface hardening of high carbon steel is mainly used for tools that can withstand smaller impacts and alternating loads. Measuring tools and highly cooled rollers.
The strengthening effect after surface quenching of low carbon steel is not significant, so it is rarely used.