Low pressure vacuum carburizing process of Cr17Ni2 stainless steel

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Materials

The Cr17Ni2 stainless steel parts are used as connecting rods, and the furnace sample is a ring sample with an outer diameter of q24 mm, an inner diameter of 7 mm, and a thickness of 6 mm. The furnace sample is the test sample. After carburizing, the surface hardness is required to be ≥660 HV, the core hardness is 35~45 HRC, and the depth of the infiltration layer is 0.35~0.60mm.

The process curve is shown in the figure. The carburizing temperature is 980 °C. In the low-pressure carburizing stage, acetylene is introduced, and after a period of time under the pressure of 700~800 Pa, the heating chamber is pumped to a high degree of vacuum to enter the diffusion of carbon atoms into the sample. stage, complete a pulse process. This cycle is repeated, and the time of the vacuum diffusion stage of each pulse process is gradually extended until the carburization is finally completed.

Cr17Ni2 stainless steel vacuum carburizing needs to be quenched twice. The first quenching adopts the method of direct nitrogen cooling after carburizing, the gas pressure is bar, and then the sample is reheated at 980°C and 1000°C for the second oil quenching. After two quenchings, one is tempered with ice-cold treatment, and the other is directly tempered without ice-cold treatment. The purpose is to compare the effects of quenching temperature and ice-cold treatment on the microstructure and hardness of Cr17Ni2 stainless steel. The tempering process is 300°C×2h.

Hardness of Cr17Ni2 Steel after Vacuum Carburizing and Quenching

Carburized layer depth

Quenching temperature and ice cold treatment

It can be seen that the secondary quenching temperature has no obvious effect on the gradient distribution of the infiltrating layer, the microstructure of the infiltrating layer, the surface hardness and the core hardness of the sample, and the quenching temperature can be selected between 980℃ and 1000°C. The surface hardness of the samples after ice-cooling treatment is 1.0~2.5 HRC higher than that of the samples without ice-cooling treatment under the same process conditions, which shows that the surface of Cr17Ni2 steel contains a certain amount of retained austenite after quenching, so ice-cooling is adopted. Processing technology is very necessary.

The specimens treated by low pressure vacuum carburizing were first tempered at 650°C at high temperature, then quenched in oil and then quickly transferred to -80°C ice-cooling machine for 2 hours, and then tempered at 150°C for 2 hours.

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