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Gas carburization techniques

The purpose of gas carburizing: Gas carburizing is to increase the carbon content and a certain carbon concentration gradient on the surface of steel parts. The chemical heat treatment process in which steel parts are heated and kept in a carburizing medium to allow carbon atoms to penetrate into the surface of steel parts is called Carburizing and carburizing steel generally use ordinary carbon steel, high-quality carbon structural steel and low-carbon alloy structural steel. Q235 steel can also be used.

Preparation before starting the furnace:
(1) There should be no defects such as rust, dirt, cracks, and scars on the surface of the carburized workpiece.
(2) The parts of the workpiece surface that do not need carburization can be plated with copper or coated with anti-carburization paint to prevent carburization. The carburized layer can also be removed by cutting the parts that do not need carburization after carburization. The thickness of the copper plating layer should generally be greater than 0.03mm; the thickness of the anti-carburization coating should generally be greater than 0.3mm, and the coating is required to be dense.
(3) When drip-type carburizing is used, the carburizing agent is usually methanol (to form a carrier gas); kerosene, acetone, or ethyl acetate (to form an enriched gas). If conditions permit, other methods of controlled atmosphere carburization can also be used. carbon. In order to obtain high-quality carburization and reduce carbon black, it is best to use aviation kerosene dripping.
(4) Prepare the sample and intermediate sample. The sample should be taken from the same batch of parts, and no rust or oil stain is allowed on the surface of the sample. The intermediate sample is generally a Φ10mm×10 steel test bar.

Carburizing operation:
① Check the equipment before starting the furnace
② Carburize the furnace tank according to the shutdown time and the condition of the furnace tank, and in accordance with the provisions of the process documents. The carburizing time of the furnace tank is generally 6-12 hours for new furnace tanks and about 2-4 hours for old furnace tanks.
③Installing furnace:
a. Put workpieces with the same material, the same technical requirements for carburizing layers, and the same heat treatment method after carburizing into the same furnace for production. The specimen is placed in a representative position of the basket. The loading capacity and charging height of each furnace should be less than the maximum loading capacity and charging height specified by the equipment.
b. In order to ensure smooth circulation of the carburizing atmosphere in the furnace and make the carburizing layer uniform, a gap of more than 5mm in length and width should be left between the workpieces.
c. When the baskets are loaded into the furnace, they should be placed vertically. Each layer of baskets should be neatly arranged without any gaps. At the same time, the sample rod should be suspended in the middle.
d. After the workpiece is put into the furnace, cover the furnace cover tightly and no air leakage is allowed. After dripping the carburizing agent, the pressure in the furnace should be maintained at 196-490Pa. Ignite the exhaust gas with a flame height of 200-300mm.
④ Gas carburizing
a. Gas carburizing process curve: According to the requirements of the material and the microstructure of the carburized layer, the carburizing process curve is specified by the relevant process documents, generally for low-alloy steel workpieces that require strict control of carbides.

The description of the gas carburizing process curve is as follows:
Air driving: Its purpose is to drive away the air in the furnace and restore the air in the furnace to the carbon potential atmosphere specified by the process.
Insulation: Its purpose is to make the temperature of the workpiece in the furnace uniform. The holding time is generally 40min to 1h.
Carburizing temperature and carburizing time: The carburizing temperature is generally 900~940°C. The carburizing time should be determined according to the required depth of the carburizing layer. It is related to factors such as the carburizing temperature, the carbon potential of the furnace atmosphere, and the characteristics of the workpiece.
Selection and application of carburizing agent: When using drip carburizing, the carburizing agent can be selected according to the conditions of the production plant. Ordinary kerosene is used as the carburizing agent, so there is more carbon black in the furnace. Choose aviation kerosene, which is less likely to produce carbon black. Methanol is used as the carrier gas and kerosene is used as the enriched gas, so there is less carbon black in the furnace. Using methanol as carrier gas, acetone or ethyl acetate as enrichment gas, and less carbon black in the furnace, the carbon potential of the atmosphere can be controlled. The amount of carburizing agent is adjusted according to factors such as workpiece loading and surface area.