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16MnCr5 Carburizing Heat Treatment Process

For a 16MnCr5 part with a thickness of 4mm, the surface hardness after carburizing and quenching is required to be greater than 700 HV3, but in the actual production process, the heat-treated surface hardness is less than 700 HV3. How to reasonably select the carburizing temperature, carburizing potential, quenching temperature, and quenching carbon potential, to meet the requirements by improving the amount of retained austenite on the surface after heat treatment.

1. Chemical Composition of Experimental Materials

After carburizing and quenching, the surface hardness is more than 700HV3, and only 10% of retained austenite is allowed in the surface structure.

C	Si	Mn	P	S	Cr
0.14-0.19	0.15-0.4	1.0-1.3	<0.035	<0.035	0.8-1.1

Tab.1 Chemical composition of 16MnCr5 (wt,%)

2. Verification of Surface Texture

After heat treatment, the surface hardness is low. According to PFMER analysis, the possible reason is that there is more retained austenite on the surface after heat treatment. After testing the metallographic structure, there is about 20% of retained austenite on the surface, as shown in Figure 1. In order to verify the authenticity of the retained austenite in the surface structure after heat treatment, it is verified by cold treatment and high temperature tempering treatment. The cold treatment transforms the retained austenite into martensite below 0°C; after carburizing, the parts are subjected to long-term high temperature tempering at about 680°C to decompose the austenite into pearlite, and then low temperature (750～780°C) ) Quenching.

Fig 1 Surface metallographic structure after heat treatment

3. Cryogenic Treatment

Carry out the cryogenic treatment first, keep it at -180℃ for 120 min; then carry out low temperature (200℃) heat preservation for 120 min and tempering in a multi-purpose furnace. After heat treatment, the hardness and metallographic structure were re-examined, and the hardness was above 770HV3, and there was no retained austenite in the surface structure.

4. High Temperature Tempering Treatment

The annealing is carried out in a pit furnace, the temperature is kept at 650°C for 120 min, and then the temperature is lowered with the furnace, and then it is cooled to about 250°C and then air-cooled. After heat treatment, the hardness and metallographic structure were re-examined, and the hardness was above 730HV3, and there was no retained austenite in the surface structure. According to the above analysis, due to the excessive amount of retained austenite in the surface structure, the surface hardness of the parts is low.

5. Analysis of the Causes of Excess Retained Austenite

The main reasons for the excessive amount of retained austenite are: ① Too much alloying elements in the steel. Elements such as Cr and Mn dissolve into the austenite to improve the stability of the austenite and increase the amount of retained austenite after quenching. ②The mass fraction of carburized carbon is too high. Too high carbon potential of carburizing gas and high carburizing temperature increase the amount of carbon dissolved in austenite and increase the amount of retained austenite after quenching. Since the chemical composition of raw materials cannot be changed, the surface structure can only be changed by reasonable selection of carburizing temperature, quenching temperature, and furnace carbon potential, reducing the amount of retained austenite, and making the surface hardness meet the technical requirements.

6. Heat treatment process test

6.1 Selection of carburizing temperature and quenching temperature

After checking the manual, it is found that quenching I (core): (850～880)℃, quenching II (surface): (810～840)℃ oil cooling; therefore, the heat treatment process test is selected at 840～860℃.

Selection of carburizing carbon potential and quenching carbon potential After heat treatment, the hardness of the part surface is the highest when the carbon content of the part is 0.7%. Considering that the carbon potential of the furnace gas is 0.2% to 0.4% higher than the carbon content of the part surface [5]. It is finally determined by DOE test that the carburizing temperature is 860°C and the carbon potential is 1.0%; the quenching temperature is 840°C and the carbon potential is 0.6%. After heat treatment, the surface hardness and metallographic structure are tested. As a result, the surface hardness is >700HV3, and there is about 5% of retained austenite in the surface structure, which meets the technical requirements of the product.

7. Conclusion

(1) When the heat treatment surface hardness of the parts is unqualified, the metallographic analysis of the structure shall be fully utilized, and the rationality of the organization judgment shall be verified through actual tests.

(2) Reasonably select the carburizing heat treatment process parameters to ensure that the metallographic structure meets the technical requirements, so that the hardness index is qualified.