Application of flame hardened steel 7CrSiMnMoV on mould

Because it works under high temperature, high speed and high load conditions, the mold is usually made of difficult-to-machine materials such as alloy steel, heat-resistant steel and hard alloy which are resistant to high temperature, wear and high alloy composition. In order to improve the life, a specific heat treatment process or surface strengthening process is required, so the processing is very difficult. Special machining methods such as cutting, electric and chemical machining of superhard tools can solve some problems, but the cost is obviously increased. At present, cold-working die steels widely used in China include CrWMn and Cr12 steels. The disadvantage of the former is that it is easy to form reticulated carbides, and the latter eutectic carbides are severely segregated, resulting in a decrease in the toughness of the steel, which leads to easy chipping, fracture and early failure of the mold. In order to improve the toughness, a variety of toughening processes, such as a low temperature quenching process, are employed in the production. On the other hand, people are striving to find cold-working die steels with superior performance through alloying methods. Flame hardening die steel 7CrSiMnMoV (code CH-1) is a new type of die steel developed and used by industrialized countries in recent years. Its main features are as follows:

(1) Good toughness Good flame quenching has hardness and various properties similar to the overall quenching. There is a high-toughness matrix under the hardened layer as a liner, and the blade is not prone to cracking or chipping during operation. After the surface strengthening process, the hardened layer retains a certain compressive stress (304 MPa), which can improve the fatigue strength and make the mold obtain a higher service life.

(2) Small quenching deformation When the mold is completely processed and formed, it is heated to the quenching temperature by the oxygen-acetylene flame at the cutting edge, and then the air cooling reaches the quenching purpose, and no other processing is required, so the deformation is small.

(3) It is convenient to repair and weldability. When the manufacturing is biased, the corresponding welding rod can be used for repair welding. After grinding and trimming, the ideal effect can be achieved.

(4) Saving costs and reducing costs Because only the oxygen-acetylene gas source is needed, it is not restricted by the site and equipment, and the operation is convenient. The cumbersome process of heating and tempering the entire quenching is eliminated, and the productivity is improved. According to preliminary statistics, the mold made by the flame hardening process of 7CrSiMnMoV steel saves about 80% compared with the Cr12 steel mold, the labor productivity is increased by about 20%, the total heat treatment cost is reduced by about 70%, and the mold life is increased by more than 1.5 times.

Large die cutting edges for automobiles and motorcycles are usually made of Cr12 or Cr12MoV steel. Due to the large size of the mold and the large three-dimensional profiled surface, it is very difficult to process regardless of the overall structure or the mosaic structure. The overall structure wastes materials, and the feasibility of the process is limited by the size of the heat treatment furnace. After the heat treatment of the inlaid structure, the deformation is large, the deformation is difficult, and the precision of the mold is easily reduced. The emergence of flame hardened steel in the 1980s was first applied and promoted in the automotive industry, and then more and more applications in mold manufacturing.

In recent years, our company and the cooperative manufacturers have successfully adopted 7CrSiMnMoV steel and flame quenching process in the joint development of imported motorcycle fuel tank molds and other punching molds. The main chemical composition of 7CrSiMnMoV steel is shown in Table 1.

Table 1 Chemical composition mass fraction w (%) of 7CrSiMnMoV steel

Firstly, the 7CrSiMnMoV round steel is forged as required, or forged into various three-dimensional shapes (real shape) according to the requirements of the parts. After annealing, roughing and smoothing, the shape of the cutting edge is machined in the machining center, and the fitter is repaired and assembled, and the flame is quenched after installation and debugging. Figure 1 shows the cutting edge of the inner casing of the fuel tank. The quenching preheating temperature is generally controlled at 180 to 200 ° C and preheated for 1 to 1.5 hours. A single large, integral, closed cavity mold can be directly preheated with a spray gun. The quenching heating flame is a neutral flame with a flame length of 10 to 15 mm, an oxygen pressure of 49 to 69 MPa, and an acetylene pressure of 4.9 to 6.9 MPa. When heating, the flame is 1 to 3 mm away from the heating surface, 4 to 6 mm from the edge of the cutting edge, the heating belt is controlled at 2 to 3 mm, the edge of the cutting edge is 4 to 6 mm, and the heating speed is controlled at 160 to 200 mm/min. The final hardness of the mold can be controlled at 52-62HRC, and the depth of quenching is 1.5mm. Since the material size change after flame quenching is much smaller than the overall quenching, the insert structure mold with the length within 300mm, the gap between the joints is <0.12. Mm, which is perfect for the life and accuracy of the blanking die.

The above is some empirical technical data in the manufacture of motorcycle fuel tank moulds and motorcycle helmet moulds using 7CrSiMnMoV steel. It has been proved that it can be referenced in the production of other punching moulds, the quality and life of the mould are obviously improved, and it is greatly convenient and Meet the needs of production. Therefore, it is a worthwhile method to use 7CrSiMnMoV steel flame quenching process instead of CrWMn and Cr12 steel to make large three-dimensional curve mold.

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