Mould is the core tool of modern manufacturing industry and an indispensable molding tool in industrial manufacturing. In the past 20 years, the mold industry in China has developed very rapidly, especially in recent years, the demand for molds has been increasing at an annual rate of about 15%. The rapid development of the national economy has put forward higher and higher requirements for the mold industry, and also provided a strong impetus for its development. Die steel, as the main material of die, is the basis of die manufacturing. With the rapid development of die industry, higher and newer requirements are put forward for the quantity, quality, variety, specification and performance of die steel. Cr 12MoV steel is the most widely used cold working die steel. Although the strength and hardness are high and the wear resistance is good, its toughness is poor, which requires high thermal processing technology and heat treatment technology. Improper treatment technology will easily lead to premature failure of the mold [2-3].

Cr 12MoV is a national standard, and the German standard is called X 165CrMoV 12.

Chemical composition:

C: 1.45 ~ 1.70

Silicon Si: ≤ 0.40

Manganese Mn: ≤ 0.40

Sulfur content: ≤ 0.030

Phosphorus P: ≤ 0.030

Chromium Cr:11.00 ~12.50

Nickel Ni: allowable residual content ≤0.25.

Copper and copper: allowable residue ≤0.30.

Vanadium: 0. 15 ~ 0.30

Mo: 0.40-0.60

Electric furnace vacuum refining production, forging cogging, uniform carbide, high hardenability, high wear resistance, high toughness, small volume deformation during quenching; So its market consumption is very large.

(1) Reducing the content of Cr, Mo and V directly reduces the cost and seriously affects the performance, such as using Cr8 and Cr 12 as Cr12mov; ;

② Changing its production mode, using intermediate frequency furnace instead of electric furnace for refining, resulting in excessive impurities, using continuous casting method instead of spheroidizing annealing, reducing calendering ratio and other methods to reduce costs, zui end customers can not achieve the expected effect when using materials, and the life of the mold is shortened, which directly leads to serious cracking and scrapping.

It is found that the martensite+lower bainite multiphase structure obtained during quenching has better strength and toughness than the single martensite or lower bainite structure. In addition, the toughness of the material can be improved to some extent by containing a proper amount of retained austenite in the quenched structure. For alloy steel, the type and content of alloying elements also have a significant influence on the amount of retained austenite after quenching [5; Reasonable quenching temperature will make the steel retain the required high-temperature microstructure and fine grains, thus ensuring good comprehensive properties after tempering.

In recent years, scholars at home and abroad have conducted extensive research on the new heat treatment process of Cr 12MoV steel [68]. The results show that the morphology and distribution of carbides in Cr 12MoV steel have great influence on its toughness (precipitation strengthening of dispersed carbides). Therefore, by controlling the shape, quantity, size and distribution of carbides in the material structure through appropriate tempering process, the strength and toughness can be improved and the comprehensive mechanical properties can be obtained. In addition, different tempering temperatures have great influence on the tensile and impact properties of alloy steel. Generally speaking, increasing tempering temperature will improve impact toughness and decrease tensile strength. Due to the secondary hardening phenomenon, increasing tempering temperature between 500 ~600℃ can also improve the hardness of alloy steel to some extent. To sum up, the development of heat treatment process of Cr 12MoV has achieved certain results, but there are also some shortcomings such as complex process and high energy consumption during heat treatment. In this paper, the microstructure and mechanical properties of Cr 12MoV steel under different tempering process parameters were studied, and then a more energy-saving heat treatment process was found.

Cr 12MoV steel used in the test is a typical high carbon and high alloy steel, and its chemical composition is shown in table 1. Cr 12MoV steel for heat treatment was processed into cylindrical samples with the size of $ ＄b20 mm×50mm, and the quenching and tempering test was carried out. The specific process is quenching at 1025℃ and holding at 490℃ and 5 10℃ for 0.5 h and 3 h respectively. The mechanical properties and microstructure of the samples after heat treatment were analyzed. In order to test the machinability and wear resistance of the samples after heat treatment, the hardness was measured by MHT- 10 microhardness tester (loading weight 100 g, loading time 10 s). The residual stress is measured by riga ku PSPC/ micro-stress analyzer, and the specific location is shown in Figure 1. JEOLJXA-8 100 electron probe (EPMA) is used to determine the distribution of elements. The distribution of microstructure was observed by ZEISS Axiovert 200 MAT optical microscope. The phases of different diffraction peaks were calibrated by Rigaku Smartlab X-ray diffractometer, and the volume fraction of retained austenite was calculated by relative strength method.

Deformation and mechanical properties analysis

Fig. 2 shows the measurement results of deformation, residual stress and hardness distribution of Cr 12MoV steel samples under different tempering conditions. As can be seen from Figure 2, when the tempering time is increased from 0.5 h to 3 h, the residual stress and deformation are significantly reduced regardless of the bottom surface or side surface of the sample. Generally speaking, the higher the surface compressive stress, the higher the fatigue strength and the worse the cutting performance. Therefore, the machinability of steel can be improved by increasing tempering time and reducing surface compressive stress. By comparing the measurement results when tempering temperature is 490℃ and 5 10℃, it is found that the influence of tempering temperature on deformation and residual stress is less than that of tempering time.

Fig. 2( c) shows the measured hardness results. It can be seen that although the maximum hardness value decreases with the increase of tempering time, the longer the tempering time, the more uniform the hardness distribution at different positions of the sample. At present, the hardness of Crl2MoV steel before heat treatment is 654HVO. 1, and the hardness of each measuring point after heat treatment is greater than this value, and the hardness has not been reduced due to tempering treatment. In addition, as can be seen from Figure 2( c), the hardness results measured at different tempering temperatures have little change.