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Vacuum quenching of KMN steel structure

KMN steel is a low-alloy high-strength steel. Due to its excellent weldability and good comprehensive mechanical properties, it is often used to manufacture parts that work at high temperatures in industrial sectors such as boilers, steam turbines, power machinery, industrial furnaces, and petrochemicals. Since such parts are used in a working environment of high temperature, high pressure and in contact with hydrogen, their mechanical properties, especially impact toughness, are very demanding.

In the fan industry, KMN steel is often used to manufacture the core components of centrifugal compressors, the impeller. The impeller is usually heat treated after the runner is milled and finished. The traditional heat treatment method is to use a pre-evacuated gas shielded furnace to heat it, then oil-cooled quenching and high temperature tempering. This method can obtain better comprehensive mechanical properties, but the runner is oxidized. Decarburization is serious, affecting the surface quality of the flow channel and reducing the efficiency of the impeller. Using a vacuum furnace to heat air-cooled quenching and high-temperature tempering can obtain mechanical properties close to oil quenching, and at the same time, the flow channel has almost no oxidation and decarburization, and the surface quality is excellent. Therefore, in this paper, vacuum heating and gas quenching, pre-evacuated gas shielding furnace heating, oil quenching, and high temperature tempering were performed on KMN steel forgings in this paper, and the effects of different quenching methods on the microstructure and mechanical properties of KMN steel were tested.

1 Test materials and methods

The KMN alloy steel used in the test adopts vacuum refining and electroslag remelting, and the ingot is formed after forging with a forging ratio of not less than 5. Vacuum air quenching of forgings is heated in a vacuum furnace, followed by gas quenching and high temperature tempering. Oil quenching is heated by pre-evacuated gas protection furnace, quenched by graded quenching oil, and tempered at high temperature respectively. Metallographic examination, room temperature tensile test and Charpy impact test were carried out on the samples after heat treatment. The metallographic samples were ground and mechanically polished and then etched with nitric acid alcohol solution, and the microstructure was observed with a Neophot 21 horizontal metallographic microscope.

2 Vacuum heat treatment process parameters

The research shows that the Vickers hardness of 12Cr2Mo1 steel increases gradually with the increase of normalizing temperature between 850 and 950 ℃; after 950 ℃, the Vickers hardness does not continue to increase. Combined with the actual production experience and considering the loss of furnace temperature, the oil quenching temperature of KMN steel used in this test is 970 ℃.

Since the cooling rate of vacuum air quenching is lower than that of oil quenching, the temperature of vacuum air quenching is higher than that of oil quenching from this point of view; in addition, since there is no quenching process in vacuum air quenching, there is no temperature loss. From this point of view, vacuum The gas quenching temperature should be lower than the oil quenching temperature. Based on the above reasons, the gas quenching temperature of KMN steel used in this test is 970 °C, which is the same as oil quenching, and the cooling rate is 5 bar. The tempering temperature after oil quenching and gas quenching is selected as 580℃.

3 Test results and analysis

Two groups of mechanical property samples of vacuum air quenching, oil quenching and high temperature tempering were taken respectively for testing. Under the same tempering temperature, the mechanical properties of KMN steel after 5 bar nitrogen cooling and oil cooling are slightly different. The microstructure after quenching and tempering in different media is consistent, mainly tempered sorbite.

When KMN steel is heat treated, in order to obtain a higher cooling rate so that the martensite can be more fully transformed, the oil quenching process is generally adopted. It can be seen that the cooling rate in the heat treatment process is very important to whether the final mechanical properties of KMN steel can meet the design and use requirements of the workpiece.

Therefore, whether vacuum air quenching can obtain excellent comprehensive mechanical properties, the cooling rate of air quenching is very important. by the above force

From the test results of chemical properties and microstructure, it can be seen that the mechanical properties of 5 bar nitrogen cooling and quenching are slightly lower than that of oil quenching, and the microstructures of the two quenching processes are basically the same, and there is no obvious change.

4 Conclusion

1) The mechanical properties of KMN steel forgings after vacuum heating 970℃ quenching (5 bar nitrogen cooling) and 580℃ high temperature tempering treatment and pre-evacuated gas protection furnace heating 970℃ oil quenching and 580℃ high temperature tempering treatment The mechanical properties are slightly lower than that of , about 8% lower, and different quenching methods have little effect on the mechanical properties. 2) The microstructure of KMN steel after different quenching methods and high temperature tempering heat treatment is consistent, mainly tempered sorbite.

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