Carburizing of steel

Carburizing is the process of putting steel in carburizing medium, heating to single-phase austenite area, holding for a certain period of time, making carbon atoms penetrate into the steel surface.

(I) purpose and use of carburizing

There are many parts with complex stress when they are working, such as variable speed gears, piston pins, friction plates and shafts of automobiles and tractors. The virtue requirements for these parts are hard and wear-resistant on the surface, good strength and toughness of the core, and high fatigue strength. These properties, only material selection and heating, are difficult to meet. High carbon steel quenching, low carbon steel quenching, induction heating surface quenching Fire is not ideal. So carburizing is put forward.

(II) carburizing method

1. Solid carburizing

The parts are placed in the carburizing box, filled with solid carburizing agent around, sealed with cover and refractory mud, and then put into the heating furnace, heated to 900-950 ° C, and discharged after holding for a certain time, the surface carburizing workpiece can be obtained.

2. Gas carburizing

Place the workpiece in a sealed heating furnace, introduce carburizing agent, and heat it to the carburizing temperature of 900-950 ° C.

(III) structure and heat treatment after carburizing

1. Structure after carburizing

After carburizing, the carbon content of the surface layer of the workpiece is 0.85 ~ 1.05%, and the carbon content from the surface layer to the core decreases gradually, and the core is the carbon content of the original low carbon steel.

2. Heat treatment after carburizing

After carburizing, the workpiece must be heat treated. Only in this way can the carburizing layer function. There are three common heat treatment methods after carburizing:

(1) direct quenching method

The carburizing temperature of the workpiece is 900 ~ 950 ° C. The purpose of carburizing + pre cooling + quenching + low return pre cooling is to reduce the quenching deformation and cracking, and to precipitate some carbides on the surface, reduce residual a, and improve the surface hardness. The pre cooling temperature is slightly higher than Ar3, so as to avoid ferrite precipitation.

(2) one time quenching method

After carburizing, slow cooling + reheating to quenching temperature for quenching + low return

(3) secondary quenching method

Carburizing + slow cooling + first quenching (or normalizing) (refine the core structure, eliminate the net Cementite on the surface, and heat to (850-900 ℃);

Carburizing + slow cooling + second quenching (or normalizing) (to improve the microstructure and properties of carbonized layer at 750-800 ° C).