Research on Vacuum Low Pressure Carburizing Process

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Although traditional atmosphere carburizing is currently widely used, it exposes many problems: oxidation within the workpiece; non-martensitic structure is difficult to avoid; large exhaust emissions; long carburizing cycle; workpiece is easy to oxidize and decarburize, etc.

Compared with the traditional atmosphere carburizing method, vacuum carburizing has no oxidation in the grain boundary, bright surface, smaller distortion, energy saving and environmental protection, and can achieve uniform carburization of small holes, blind holes and other parts. In addition, vacuum carburizing can obtain a good carburized layer for parts such as stainless steel, silicon-containing steel and other parts where ordinary gas carburizing is not effective or even difficult to carburize. Acetylene (C₂H₂) is now used as the carburizing medium, which to a large extent solves the problem of carbon black and tar pollution caused by propane, and injects new vitality into the development and application of vacuum carburizing.

Vacuum carburizing, also called low-pressure carburizing, is a non-equilibrium strong carburizing-diffusion carburizing process, that is, the parts are heated in a vacuum and gas carburized in a negative pressure carburizing atmosphere. It consists of decomposition, It consists of three processes: absorption and diffusion. It has been applied and developed in industry.

The general process of vacuum carburizing is: cleaning parts → loading parts and entering the furnace → vacuuming → heating and soaking → carburizing and diffusion → quenching heat treatment.

After the parts are put into the furnace, the parts are evacuated to vacuum conditions (or ≤10Pa, basically reaching non-oxidation conditions) for heating, temperature raising, preheating and soaking. Under vacuum, the oxides and grease contaminants on the surface of the workpiece can be removed, and the surface activation of the workpiece is beneficial to carburization. When the workpiece reaches the carburizing temperature and is uniform, carburizing gas (methane, propane or acetylene, etc.) is introduced for carburizing. Generally, the air pressure during carburizing is 300~2000Pa (400~800Pa is commonly used), and then diffuses to remove the carburizing gas (or fill it with N₂ to maintain the furnace pressure unchanged), so that the furnace reaches a working vacuum, and then carburizes and diffuses. In this way, pulse carburizing and diffusion are carried out alternately several times until the required carburized layer depth is reached. After carburizing is completed, the temperature is lowered to the quenching temperature and kept warm, and the air pressure is adjusted for oil quenching or high-pressure gas quenching.

Vacuum carburizing methods include one-stage, pulse and swing (pulse + one stage). For parts with carburizing requirements on the inner surfaces of slits and blind holes, pulse and swing carburizing should be used.

The process parameters that affect vacuum carburizing include: carburizing temperature (generally 920~1050℃, commonly used 920~980℃), carburizing time, expansion ratio, furnace pressure and gas flow.

Compared with traditional gas carburizing, the advantages of low-pressure vacuum carburizing are as follows:

1.The amount of carbon on the surface of the carburized layer and the carburizing depth can be controlled simply and accurately.

2.Uniform carburizing effect

3.The operation time can be shortened, and the carburizing time is about 1/2 to 1/3 of ordinary carburizing.

4.After carburization, the parts will still maintain a glossy state, without intergranular oxidation or decarburization, and the original color of the metal will be kept silver-gray and bright, which can save cleaning and shot blasting processes.

5.Compared with ordinary carburizing, the control range of vacuum carburizing gas quenching is small, the size changes are small, and the distribution is concentrated.

6.No fire curtain, no oil smoke, clean working environment, it is a safe and environmentally friendly heat treatment equipment

7.Can realize continuous, automatic and intelligent production

Applications:

1.Parts with holes, such as needle valve body injector

2.Gears

3.Thin-walled parts or parts with complex shapes

Learn More:

Research on vacuum annealing of titanium alloys

Characteristics of vacuum carburizing technology

Effect of vacuum heat treatment on microstructure of alloy pipes