Causes and hazards: if the carburizing is heated sharply, the temperature is too high, or if the solid carburizing is carried out with new carburizing agent, or if there are too many strong carburizing agents, the carburizing concentration will be too high. As the carbon concentration is too high, massive and coarse carbides or network carbides appear on the workpiece surface. Because of this kind of hard and brittle structure, the toughness of carburized layer decreases sharply. High carbon martensite is formed during quenching, and grinding cracks are easy to appear during grinding.
Prevention methods:
① It can’t be heated rapidly. It is necessary to adopt proper heating temperature to prevent the grain from growing well. If the grains are coarse during carburizing, the grains should be refined by normalizing or twice quenching after carburizing.
② The uniformity of furnace temperature shall be strictly controlled and the fluctuation shall not be too large. Special attention shall be paid to the solid Carburization in the reverberatory furnace.
The material of carburized parts is generally low carbon steel or low carbon alloy steel (carbon content is less than 0.25%). After carburizing, quenching is necessary to give full play to the beneficial effect of carburizing. After carburizing and quenching, the surface microstructure of workpiece is mainly martensite with high hardness plus residual austenite and a small amount of carbide. The core microstructure is low carbon martensite with good toughness or non martensite, but ferrite should be avoided. Generally, the depth of carburized layer ranges from 0.8 mm to 1.2 mm, and the depth of carburized layer can reach 2 mm or deeper. The surface hardness is hrc58-63, and the core hardness is hrc30-42. After carburizing and quenching, the surface of the workpiece produces compressive internal stress, which is beneficial to improve the fatigue strength of the workpiece. Therefore, carburizing is widely used to improve the strength, impact toughness and wear resistance of parts, so as to extend the service life of parts.
Low carbon concentration and prevention methods
Causes and hazards: large temperature fluctuation or too little catalyst will cause insufficient carbon concentration on the surface. The ideal carbon concentration is 0.9-1.0%, lower than 0.8% C, the parts are easy to wear. As the carbon concentration is almost not too low in the actual production, even in the later stage, it is difficult to deal with it. Freezing treatment can improve the hardness.