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Effect of Vacuum Heat Treatment on Microstructure and Properties of Superhard High Speed Steel M2Al

M2Al is a kind of cobalt-free super-hard high-speed steel. Vacuum heating promotes abnormal grain growth. Filling with inert gas for vacuum heating can increase the heating speed, hinder the formation of abnormally large grains, and improve the strength and toughness of M2Al steel. The most important factor affecting the strength and toughness of M2Al steel is not the size of the mixed crystal and austenite grain size, but the shape and distribution of carbides in the steel. In order to take advantage of the superhardness of M2Al steel, the vacuum quenching temperature can be increased to make the hardness above 66HRC.

W6Mo5Cr4V2Al (M2Al) steel is a cobalt-free super-hard high-speed steel developed by my country during the Seventh Five-Year Plan period. It has good grinding performance, but it has a long-term mixed crystal problem. When mixed crystals appear, the toughness and bending strength of the material decrease significantly. For many years, people’s research on M2Al steel has been carried out under salt bath graded quenching conditions. There are few reports on the effect of vacuum heat treatment on the structure and properties of M2Al steel. This article will discuss the influence of heat treatment process on the structure and properties of M2Al steel under vacuum oil quenching, and compare it with salt bath graded quenching to provide a technical basis for actual production.

Tab 1 Chemical composition (mass fraction) w (%) of M2A1 steel

1. Test Method

The quenching adopts vacuum oil quenching and salt bath graded quenching. X1 is 850°C preheating 1050°C preheating T vacuum oil quenching; X2 is 850°C preheating 1050°C preheating T vacuum oil quenching. After the workpiece is put into the furnace, it is evacuated to 2.3Pa and then filled with 6. 1X 104Pa pressure inert gas; X3 is 850°C preheating T salt bath 600°C graded quenching (T value is 1200°C, 1210C, 1220C, 1230°C, 1240°C, 1245C). According to the role of Al in high-speed steel, all samples are tempered at 560°CX4 times after quenching [2]. The heat treatment process of the sample is shown in Table 2. The size of the impact sample is 10mmX 10mmX 55mm, and the standard sample without notch is used, and the MK-30 impact sample machine is used. The size of the bending specimen is 5.0mmX 5.0mm X 55mm, the span is 24mm, and the test uses a 30t hydraulic bending material testing machine. Each group uses 5 samples to get the average value.

Tab 2 Heat treatment process of M2A1 steel sample

3. Conclusion

(1) Vacuum oil quenching is compared with salt bath hierarchical quenching, vacuum quenching promotes abnormal growth of crystal grains. However, inert gas is filled during vacuum heating to improve heating efficiency, which can hinder the appearance of mixed crystals and improve the strength and toughness of M2Al steel.

(2) The most important factor affecting the impact toughness and flexural strength of M2A1 steel is not the size of the mixed crystal and austenite grain size, but the shape and distribution of carbides in the steel.

(3) M2Al steel 1245C vacuum oil quenching, the austenite grain size reaches 6 to 7, but there is no overheated structure, so the superhard performance of M2A1 steel can be exerted by increasing the vacuum quenching temperature.