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Introduction to Vacuum Carburizing Technology

The development of carburizing technology mainly focuses on developing new energy-saving and environmentally friendly processes; improving carburizing quality, making the control of the carburizing process more precise; shortening the carburizing time, etc.

Vacuum carburizing

Vacuum carburizing refers to a carburizing process performed under conditions below 0.1MPa. The carburizing temperature is mostly 980-1100℃. During carburizing, propane, acetylene, methane, etc. are directly introduced into the furnace for carburizing. Vacuum carburizing is actually carburizing performed under pressure below atmospheric pressure.

During vacuum carburizing, pulse gas supply is often used. The basic process is as follows: gas supply carburizing → gas stop, vacuum → diffusion → gas supply carburizing again. After several cycles, the carburizing process is completed. Pulse gas supply is conducive to obtaining a uniform carburized layer for workpieces with small holes or non-through holes. After carburizing, the workpiece is quickly moved to a cooling room with nitrogen for pre-cooling and direct quenching (oil quenching or high-pressure gas quenching). If the carburizing temperature is high and the grains are coarsened, the temperature is lowered to about 600℃, and then reheated under vacuum conditions to refine the grains. After heating, quenching and cooling are carried out in a cooling chamber with nitrogen protection.

Advantages of vacuum carburizing

The advantages of vacuum carburizing are: the carburizing time is greatly shortened (1/2 to 1/3 of ordinary gas carburizing), no decarburization, and internal oxidation can be avoided; propane, acetylene, methane, etc. are directly introduced into the furnace for carburizing, saving gas preparation equipment; gas consumption is only a fraction of ordinary carburizing; energy saving, environmental protection, and clean workpiece surface; good carburizing effect can be achieved for parts with deep holes, narrow slits, etc., which are not well carburized by ordinary gas, and stainless steel.

The main reason for the fast carburizing speed of vacuum carburizing is that the atomic diffusion rate is greatly accelerated at high temperature; the oxides on the surface of the workpiece are decomposed under vacuum, and the residual grease and dirt evaporate, so that the surface of the workpiece is in a fresh, clean and good activation state, which improves the absorption capacity of the workpiece surface for active carbon atoms.

Vacuum carburizing currently relies mainly on pulse gas supply, adjusting furnace pressure and reasonably arranging carburizing and diffusion time to roughly control the carbon potential, but it is difficult to accurately control the carbon potential of furnace gas.