Brief analysis of hardness requirements and testing methods for quenched workpieces
The hardness of the quenched workpiece affects the quenching effect. The quenched workpiece is generally tested by Rockwell hardness tester for HRC hardness. Hardened thin hard steel and surface hardened workpieces can be used to test the hardness of HRA. For quenched steel sheets with a thickness of less than 0.8 mm, shallow surface hardened workpieces and quenched steel rods with a diameter of less than 5 mm, the surface hardness tester may be used to test the HRN hardness. When welding medium carbon steel and some alloy steels, quenching may occur in the heat-affected zone and become hard, which is easy to form cold cracks, which is to be prevented during the welding process.
Since the metal is hard and brittle after quenching, the residual surface stress will cause cold cracking, and tempering can be used as one of the means to eliminate cold cracks without affecting the hardness. Quenching is suitable for parts with small thickness and diameter. For oversized parts, the quenching depth is not enough, and carburizing has the same problem. In this case, alloys such as chromium should be added to the steel to increase the strength.
Quenching is one of the basic means of strengthening steel materials. Martensite in steel is a hard phase of zui in iron-based solid solution structure, so steel parts can be quenched to obtain high hardness and high strength. However, the martensite is very brittle, and after quenching, there is a large quenching internal stress inside the steel, so it is not suitable for direct application and must be tempered. Microhardness value of iron-based solid solution in steel.
The quenching process is widely used in the modern machinery manufacturing industry. Important parts in machinery, especially those used in automobiles, airplanes, and rockets, are almost quenched. In order to meet the technical requirements of various parts, various quenching processes have been developed. For example, according to the parts to be treated, there are integral, local quenching and surface quenching; whether the phase transformation is complete according to heating, complete quenching and incomplete quenching (for hypoeutectic steel, the method is also called subcritical quenching); The contents of the phase change include fractional quenching, austempering and underspeed quenching.