12XH3A / 12CrNi3 Low Alloy Carburizing Material Heat Treatment Process

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Through upgrading of heat treatment technology for low-alloy carburizing material such as 12CrNi3, i.e. The traditional gas carburizing treat mentand routine carbonitriding (cyaniding) heat treatment are replaced with accurate controllable rare-earth cyaniding heat treatment. The heat treatment technological time is shortened, resulting in enhanced gear strength as well as improved dry friction performance and anti- sliding wear ability of cyanided layer. Theoretical analytical data and practical experience are provided for production of high performance gears using 12CrNi3 material.

Key words: gears using low-alloy carburizing material such as 12CrNi3,carburization,carbonitriding (cyaniding), rare-earth cyaniding, atmosphere control

The hardenability of 12CrNi3 advanced carburized carbon steel is higher, annealing is more difficult, generally adopt tempering treatment, make the hardness reduced to <35HRC, to meet the cutting process.As a result of non-carburized surface without protection treatment (such as plating copper, brush anti-seepage paint, etc.), so the need for high temperature tempering after carburization, reduce the hardness, easy to cut off the carburized layer of non-carburized surface.The processing of the material into gear requires complex chemical heat treatment, so that the core hardness reaches 31-41 =HRC, and the surface hardness is ≥60HRC.

Heat treatment is the key process of gear quality control, the quality of heat treatment directly affects the gear tooth surface engagement and service life.Traditional gear heat treatment process is gas carburizing process, a small part of the use of carbon – nitrogen co – carburizing (cyanidation) process.In the past, due to the installation level and control level of heat treatment equipment, the cyanidation process often failed to obtain good permeation layer microstructure.In this paper, computer control is used to control such factors as the depth of permeation layer, the carbon content of surface hardness permeation layer, the morphology distribution of tissue carbide and residual austenite, and the hardness gradient.On this basis, the author also studied the rare earths cyanide process, the heat treatment process time of the gear, also makes the carbonitriding layer structure is improved, the permeability of rare earth after cyanide layer of tissue microstructure distribution evenly gently, make gear cyanide layer of dry friction performance and resistance to sliding wear obviously promoted, significantly improve the performance of the gear.

Carburizing temperature /°C quenching temperature /°C first and second cooling medium tempering temperature /°C cooling medium 900℃ ~ 950℃,860℃,780℃ oil 200℃ water, air.

The carburizing temperature of alloy steel is generally 900~ 920 °C after the initial grinding and before the fine grinding.Surface carburizing treatment is to carbon steel in the medium high carbon potential of the environment, by having the high activity of spread to the interior of the steel, carbon atoms form – thickness and high carbon content of carburizing layer, after quenching and tempering, the workpiece surface layer of high carbon steel, while the core due to the concentration of carbon content to keep the original carbon low alloy steel.The hardness of alloy steel is mainly related to its carbon content, so through carburizing treatment and subsequent heat treatment, the workpiece can obtain the performance of external hardness and internal toughness.The role of carburizing treatment is to improve the surface wear resistance, while maintaining the core has a high impact resistance, that is strong toughness.

Gas carburizing heat treatment process
12CrNi3 steel gear gas carburizing process is as follows: 900~ 920 °C carburizing →650~ 700 °C high temperature tempering →780 ~800 °C oil quenching -→-70~-40.C cold treatment – tempering at 150 ~200 °C.

Production test results
Hardness of carburized layer ≥58 HRC.The carbide diffusion layer of carburized layer is relatively shallow, about 0.15mm. Hardenability ≤50 mm, thick core structure.When the 12CrNi3 steel is first quenched and heated at 860 °C, the carbide melts less, the matrix w(C)=0.10% ~0.17%, w(Cr)=0.6% ~ 0.9%, the carbide fuses less sulfur and phosphorus, and nickel is the main element.

The hardness of the permeation layer, the dispersion of the surface hardness and the structure of the core part have not reached the effect of using the product, the hardness of the permeation layer is not uniform, and the hardness of the core part is on the high side.