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Vacuum heat treatment of carburized steel parts

Performance requirements for carburized steel

Carburized steel usually refers to the type of steel used after carburizing, quenching and low-temperature tempering. It is generally low-carbon high-quality carbon structural steel and alloy steel. The stress conditions of parts such as automobile and tractor transmission gears are very complex, and the tooth roots are subjected to alternating bending stress; during the meshing process, the tooth surfaces contact each other in line and slide, during which there is contact fatigue and wear; the clutch during driving The teeth are also subject to greater impact when engaging or braking suddenly. The main failure modes of gears are tooth surface wear and spalling and tooth fracture. Therefore, gear steel is required to have high bending fatigue strength and contact fatigue strength, high wear resistance, and the core structure should also have high toughness. to prevent gear breakage.

Vacuum heat treatment characteristics of carburized steel

For carburized steel parts, pre-vacuum heat treatment before machining is usually performed in two steps. First, the steel parts are heated above the Aa line for normalization, and then vacuum annealing (for pearlitic steel) and high-temperature vacuum tempering (for martensitic steel) are performed respectively according to the different hardenability of the alloy steel. The purpose of normalizing is to refine the grains, reduce the degree of banding in the structure and adjust the hardness to facilitate mechanical processing. Steel after normalizing has equiaxed grains. For pearlitic type steel, one-time annealing at about 800°C is usually used instead of normalizing, which can achieve the same effect, that is, to refine the grains and improve the cutting performance; for martensitic type steel, it must be normalized before normalizing. High-temperature vacuum tempering is performed at a temperature below Ac1 to obtain a tempered sorbite structure, which can reduce the hardness of martensitic steel from 380 to 550HBS to 207 to 240HBS, allowing for smooth cutting.

The vacuum heat treatment process (final vacuum heat treatment) of alloy carburized steel after carburization is generally vacuum quenching directly after carburization and then tempered at low temperature. Parts using this process usually only require high surface hardness and wear resistance, but do not have high requirements on the performance of the matrix. If in addition to high surface hardness and high wear resistance, there are higher requirements for the matrix performance, air cooling (i.e. normalizing treatment) is performed after carburizing to refine the structure, and then vacuum is carried out according to the surface composition after carburizing. Quenched and tempered at low temperature. This process is mainly used for steel types that are prone to overheating after carburization, such as 20Cr, 20Mn2, etc. When the requirements for the surface and matrix properties of the parts are very strict, vacuum quenching must be performed twice after carburizing and air cooling. The first time is heating and quenching according to the matrix composition of the steel, and the heating temperature is relatively high (about 870°C). The purpose is to refine the core structure and eliminate the reticular cementite in the surface carburized layer; the second time is according to the composition of the high carbon steel. The component is (surface) quenched in order to obtain a fine martensite plus granular carbide structure on the surface to meet the requirements of high surface performance. Finally, low temperature tempering is performed to eliminate stress, stabilize the structure and stabilize the size. This type of vacuum heat treatment process is mainly used for vacuum heat treatment of aircraft engine gears.

Vacuum heat treatment application of carburized steel

Now, taking 20CrMnTi steel as an example, we will analyze the arrangement of the vacuum heat treatment process on the processing route when making automobile transmission gears.

The processing route of automobile transmission gears made of 20CrMnTi steel is: forging-normalizing-tooth processing-partial copper plating-vacuum carburizing-precooling quenching, low-temperature tempering-shot peening-precision grinding.

The gear blank is normalized after forging in order to reduce forging stress, refine grains, uniform chemical composition, and improve cutting performance. The hardness after normalizing is 170~210HBS, and the cutting performance is good.

The carburizing temperature of 20CrMnTi steel is about 920°C, and it is directly oil-quenched after carburizing and pre-cooling to 840~860°C. The purpose of precooling is to reduce the amount of retained austenite during quenching and prevent deformation after quenching. After precooling and quenching, the surface layer is fine needle-like martensite + retained austenite + carbide, and the core structure is low carbon martensite. After vacuum quenching and low-temperature tempering at 200°C, the surface layer has high hardness (58~60HRC) and wear resistance, and the core has high strength and sufficient impact toughness.

Vacuum quenching and vacuum tempering are followed by shot peening to further improve the fatigue strength of the surface.