In the heat treatment process, induction furnace sometimes adopts secondary quenching process, that is, the first time quenching with higher temperature, and then the lower temperature quenching and tempering. The purpose of this is mainly for the quenching treatment of carburizing steel. Because the parts after carburizing often have mesh cementite in the high carbon area of the surface, two methods are commonly used to eliminate high temperature normalizing and high temperature quenching. High temperature normalizing is usually used in small effective size parts; for the parts with large effective size, the medium frequency furnace normalizing often appears slow cold speed, which can not eliminate the mesh carburizing body. Therefore, it is necessary to use rapid quenching cooling to eliminate the net cementite. But if electric furnace quenching is not enough, the phenomenon of coarse grain will appear, because it is necessary to quench again to eliminate the coarse martensite needle after high temperature quenching. The second quenching is the normal quenching, which aims to refine the structure and obtain normal martensite or crystallographic martensite.
When using secondary quenching process, many people worry about crack easily, however, in practice, it is seldom used in practice. Because the first quenching is usually direct quenching after carburization, the temperature is higher and the stability of austenite is strong, so there are more retained austenite after induction furnace quenching. At the same time, there are many dislocation phenomena in high temperature quenching. The microstructure is coarse but plastic is better, so there are few cracks. Therefore, in the second quenching, as long as the heating temperature is appropriate and the holding time is not too long, the grain can be refined and hidden martensite may appear, and good mechanical properties can be obtained. So some manufacturers may have secondary quenching in order to refine the product structure.
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