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Research on Vacuum Heat Treatment Technology of Cr12 Steel

Cr12 steel is one of the widely used cold work die steels. The steel has the advantages of good hardenability, high hardness, wear resistance, and small heat treatment deformation. It is often used to make cold work dies that bear heavy loads, large production batches and complex shapes, such as cold stamping, embossing, cold heading, and cold extrusion dies. Wait. However, the significant disadvantage of this steel is that it is brittle, which often leads to early failure of the mold. The die failure analysis shows that the heat treatment factor has the largest influence. Vacuum heat treatment has the characteristics of preventing heating oxidation and no decarburization. At the same time, because vacuum heat treatment can effectively control heating and cooling rates, the deformation of parts or workpieces is smaller than that of ordinary heat treatment. Vacuum heat treatment can significantly improve the toughness of the material and greatly improve the life of the mold under the premise of making the material have the same strength and hardness.

1 Experimental materials and methods

1.1 Experimental materials

The material used in the experiment is Cr12 steel. Its chemical composition (mass fraction, %) is: 2.32C, 0.82Si, 0.73Mn, 0.052S, 0.097P, 12.62Cr, 0.17Ni, 0.62Mo.

1.2 Treatment equipment and heat treatment process

A vacuum sintering furnace is used. The same group of samples with a diameter of 16mm x 15mm were heated to 850, 900, 950, 1000, 1050 and 1100 °C respectively for vacuum oil quenching (oil temperature 40 °C. The square Pa of the cold room vacuum degree 10). Measure the hardness value of the sample at each temperature, and find the best quenching temperature according to the hardness value. According to the best quenching temperature, quench at different oil temperatures, and then measure the hardness to find the best oil temperature. The effect of cold room vacuum degree on the material was studied under the optimum quenching temperature and optimum oil temperature. The specific heat treatment process is shown in Figure 1.

2 Experimental results and analysis

2.1 Influence of vacuum quenching temperature on material hardness

The hardness values at different quenching temperatures are shown in Table 1. It can be seen that with the increase of quenching temperature, the hardness gradually increases, and when the quenching temperature reaches 1000 ℃, the hardness gradually decreases with the increase of quenching temperature. When the quenching temperature is lower than 1000 ℃, with the increase of temperature, due to the increase of alloying elements and carbon in the austenite, the carbon content in the martensite after quenching increases, so the hardness increases; when the quenching temperature is higher than 1000 ℃ At the same time, due to the further increase of alloying elements, the M point decreases and the content of retained austenite increases. At the same time, the growth of austenite grains reduces the hardness after quenching. Therefore, the optimum quenching temperature is 1000°C.

2.2 The effect of quenching oil temperature on the hardness of Cr12 steel

In the experiment, three oil temperatures of 40, 60 and 80 ℃ were selected to quench the samples in vacuum at 1 000 ℃, and the vacuum degree of the cold room was controlled at 10 square Pa. The obtained hardness changes are shown in Table 2.

2.3 The effect of cold room vacuum on the hardness of Cr 12
In the experiment, the cold room was filled with pure nitrogen gas to 30, 50 and 100 kPa, and the heating temperature (1000 °C) and oil temperature (40 °C) of the previous experiment were selected for vacuum heat treatment experiments. Three groups of samples were tested for hardness, and the results are shown in Table 3.

Practice has proved that for some low hardenability steels, if the gas pressure is increased to above atmospheric pressure, a higher cooling rate will be obtained. This is due to the further thinning of the vapor film during quenching, shortening the vapor film stage with slow heat transfer. Due to the better hardenability of Cr12, it can be seen from Table 3 that the degree of vacuum in the cold room has little effect on the hardness of Cr12 steel.

2.4 Influence of tempering temperature on material properties

Ten groups of samples were tempered at different tempering temperatures for 1 h at different tempering temperatures, and the cooling method was air cooling. The hardness at different tempering temperatures was measured, and the results are shown in Figure 2. It can be seen that the hardness decreases gradually with the increase of tempering temperature. This is mainly due to the transformation of the microstructure from martensite to tempered martensite, and the hardness is lower than that of the untempered sample after quenching.

2.5 Effect of tempering temperature on impact toughness

The size of the sample used is a square sample of 10 mm x 10 mm x 55 mm, with a U-shaped straight groove in the middle, and the impact test method adopts GB229-84. The effect of tempering temperature on impact toughness is shown in Figure 3. It can be seen that the steel has obvious temper brittleness. When tempered at around 350 ℃, the toughness is low, which is the first type of temper brittleness; when tempered above 450 ℃, the toughness decreases, and it is the second type of temper brittleness.

3 Conclusions

(1) The optimum quenching temperature of Cr12 steel is 1000℃; the degree of vacuum and quenching oil temperature have little effect on its hardness.

(2) Compared with the traditional heat treatment method, the Cr12 steel after vacuum heat treatment has a bright surface, no decarburization, carbon increase and other adverse phenomena, and basically no deformation.

Selection of vacuum heat treatment equipment: RVS vacuum sintering furnace provided by SIMUWU is an excellent product for processing such processes, with good temperature uniformity and high temperature control accuracy. SIMUWU provides a professional team of engineers who can solve various problems encountered in the production process and are committed to giving customers the most convenient and efficient experience.