Heat treatment defects

Heat treatment can improve the processing performance of steel to improve its performance, significantly improve the mechanical properties of steel, and extend its life.

Dynamic load and friction in a variety of work under conditions of gears, crankshafts and other parts, they require the surface with high hardness, high wear resistance, while the heart requires a sufficient ductility and toughness. But there are some common defects of heat treatment.

Workpiece in the heating process, the heating of the surrounding medium from the surface of the steel chemical reaction, oxidation and decarburization of steel will seriously affect the quality of the workpiece hardening.

Since the formation of iron oxide, the size of the workpiece decreases, reducing the surface roughness, but also seriously affect the cooling rate of quenching, resulting in the workpiece surface and insufficient hardness soft spots. Since diffusion of carbon faster, so the steel decarburization rate is always greater than the rate of oxidation. Beneath the oxide layer of the steel, there is always a certain thickness is usually decarburization, the decarburization of the steel surface of the carbon content decreased, resulting in surface hardness after quenching steel insufficient, fatigue strength is decreased, and the quenching of the steel susceptible to surface cracks.

When austenitic steel for heating, such as heating temperature is too high or the heating time is too long, it will cause coarsening of austenite grain growth, the formation of martensite is roughened, a phenomenon called overheating. Hardly prevent overheating of the workpiece quenching cracks. Because there are a lot of martensite generated microcracks, this martensitic quenching cracks will develop cracks.

In the case of the higher heating temperature, the austenite grain coarsening further and produce grain boundary oxidation, can also cause severe grain boundary melting phenomenon called burnt. Produced a burning piece, its performance dramatically. Overheating defective parts, can be organized to conduct a detailed normalizing or annealing, quenching and then re-normal specification. Had burned defective parts due to inability to save but only scrapped. In addition, quenching is due to quenching stress cracks in the workpiece surface when the tensile stress exceeds the cooling caused by the breaking strength of steel, this crack in the workpiece Soon after entering the cooling medium, the temperature dropped to the Ms point (about 250 degrees) The following are generated.

This is because the workpiece quenched from the austenitizing temperature to the Ms point in the process, because the plastic martensitic transformation drastically reduced, while the stress increases rapidly, it is easy to form cracks.