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Study on carburizing process of 20CrMnTi Steel

20CrMnTi steel is a good performance of carburized steel, which has high hardenability, good wear resistance, small grain size, good carburizing quenching performance, low temperature impact toughness, strong normalizing after cutting performance, low price and other advantages, is widely used in the manufacture of compressive wear resistance gear shaft, gear ring, heavy or medium load mechanical parts.
20CrMnTi steel is mainly used in gear manufacturing, and its flexibility, wear resistance, impact resistance, bending resistance and other mechanical properties are emphasized in the production process. In the process of use, the gear that plays the role of transmitting power will appear various failure forms such as tooth surface contact fatigue and tooth root fracture due to impact and strain and other external forces. Chemical heat treatment can effectively improve the wear resistance, heat fatigue, fatigue strength, corrosion resistance, oxidation resistance and other properties of the specimen surface. Therefore, it is necessary to conduct chemical heat treatment for 20CrMnTi steel in the production process. Carburizing process is the most commonly used heat treatment process of 20CrMnTi steel. Carburizing is one of the earliest and most widely used chemical heat treatment methods. It has the advantages of strong applicability, high economic benefit and easy to be applied in combination with the actual needs under the existing production conditions. Carburizing technology can greatly improve the working performance of 20CrMnTi steel and prolong its working life.

1. Gas carburizing

Gas carburizing has the advantages of controllable carbon potential, high production efficiency, good working conditions and direct quenching, and it is the most widely used.
The high carburizing experiment of diffuse carbides on 20CrMnTi steel was carried out in a well type gas carburizing furnace. The furnace pressure was 147~196 Pa, the atmospheric CO2 was controlled between 0.15%~0.25%, the carbon potential in the strong cementing period was 1.20%C, the carbon potential in the diffusion period was 0.80%~0.90%C, and the carburizing temperature was 930 ℃. The diffusion time was 40 h after intense infiltration for 5 h. It is found that 20CrMnTi steel with high carburizing content can obtain more than 1.25 % of carbon dispersed carbide surface layer, high contact fatigue strength, good wear resistance, and the process has the advantages of simple operation, fast carburizing speed.

2 Gas carbonitriding

Single gas carburizing sometimes appears the surface hardness is not enough, wear resistance is low, researchers have developed gas carburizing technology to make up for these deficiencies. In the comparison test of carburizing and carburizing of 20CrMnTi gear, it is found that in the process of methanol gas carburizing, the appropriate amount of kerosene and ammonia gas can effectively improve the carbon potential in the furnace, shorten the recovery time of furnace gas, and shorten the subsequent co-cementation time. This method can effectively reduce the deformation of the gear, improve the mechanical properties, and its surface has higher hardness and wear resistance. The gear transmission will produce vibration and sound, and then produce large noise, so reducing the noise is also a problem to be considered in the production of 20CrMnTi steel. In the well carburizing furnace, after 4 h of rare earth carburizing for 20CrMnTi, the average penetration depth is 0.8~0.9 mm, the carbide at the tooth tip decreases by 2 stages, and the surface surface appears fine dispersed granular carbide, and the surface hardness is 62~64 HRC. Using rare earth carburizing at 860℃x 7 h, it can save 30% electricity and reduce gear transmission noise under the premise of reaching the specified middle limit of infiltration layer.