Valve hot forging die adopts medium frequency induction heating power supply for heat treatment

The forming of the valve is realized by the mold. By heating the valve head, it is forged and formed in the range of 1100-1250 ° C. The streamline of the valve head meets the requirements, without cutting and damage, and maintains the valve required. The performance of the valve is similar to that of precision forging. The hot forging mold is used to forge the garlic-shaped valve blank after hot forging into the required valve shape. The heating temperature of the valve material is as high as 1100-1200 ℃. The surface temperature is also 600-700℃. The surface of the cavity is in repeated contact with the hot metal. During the forming process, it not only has to bear the impact force and friction force, but also bears periodic impact effects such as bending, stretching, compression, and extrusion. Therefore, its working conditions are harsh. The performance requirements are very strict. For this reason, many manufacturers use medium-frequency induction heating power supply to heat-treat the valve hot forging die, and the effect is very good.

The mold is composed of four parts. The lower mold is usually made of 3Cr2W8V steel, which contains alloy carbide forming elements such as chromium, tungsten and vanadium, so it has high hardness, impact toughness and thermal fatigue resistance, and good thermal conductivity. Therefore, it is suitable for making concave molds for valves.

(1) Heat treatment of valve hot forging die. The forging die after forging should be subjected to good spheroidizing annealing treatment, the purpose is to improve the structure and reduce the hardness of the matrix, eliminate internal stress, and obtain pearlite ten spherical cementite structure, in order to facilitate Machining to prepare the structure for the final heat treatment, the annealing process is 830-850°C, and the hardness is 207-255HBW.

1. Technical requirements for valve hot forging die

According to the working conditions of the valve forging die, combined with its structure and shape, a reasonable heat treatment process is specified to meet the technical requirements. The basic principle is that for valve lower molds (concave molds) that require high fracture strength, thermal fatigue and thermal wear resistance, as well as the upper limit and higher heating temperature that need to be processed after quenching; for small deformation and fine grain size , The upper mold with high impact toughness adopts low-limit heating. The usual technical requirements are: the hardness of the substrate is 46-52HRC; the surface has no decarburization and no bump damage.

2. Common heat treatment process of valve hot forging die

With the development of new materials and the improvement of technological levels, high-performance alloy steels with good heat resistance, high strength and corrosion resistance have been mass-produced in China. The representative steel type is 4Cr3Mo3W4VNb (referred to as GR steel), which is used for The production of hot forging dies, hot extrusion dies, etc., which have a long contact time with the workpiece, high temperature and are prone to deformation, collapse or thermal wear failure, compared with the valve hot forging die made of ordinary hot work die steel (such as 3Cr2W8V), The service life has been significantly higher.

The upper mold (or convex mold) usually has a spherical mold surface to reduce the weight of the valve. The quenching process is: use a medium frequency induction heating power supply, the temperature is 1080 ° C, and then oil cooling, the hardness is 46-52HRC.

Lower mold quenching and tempering treatment: heating temperature is 1140℃, air cooling and then oil cooling, and then tempering heat treatment twice, the hardness is 46-52HRC. Considering that the forging die itself is subject to strong impact, the thermal stability of the lower die is required to be high, so it requires high hardness and good wear resistance. Increasing the heating temperature to about 1140°C can fully dissolve the carbides. In practice It has been proved that the service life of the treated forging die is 3-10 times longer than that of the low-temperature treatment. This process is currently widely used in the production of valve manufacturing plants.

(2) Analysis and implementation key points of heat treatment process of valve hot forging die

① Primary preheating is carried out using a medium frequency induction heating power supply. Due to the large number of alloy elements inside, a large number of alloy carbides and metal compounds are formed, so its thermal conductivity is poor. In order to reduce the effect of thermal stress, the preheating temperature is usually set at 500-550 ℃ (elastic deformation area), the secondary preheating heating temperature is 840-860 ℃.

② Quenching heating can also be carried out by medium frequency induction heating power supply. The heating temperature of the upper mold of the valve is 1070-1080 ℃, and the heating temperature of the lower mold is 1130-1140 ℃. This is considering that the upper mold should have good impact toughness, while the lower mold should maintain high hardness, red hardness and sufficient strength, so it is required to dissolve as many carbides as possible into the austenite, so the quenching of the upper and lower molds There is a big difference in heating temperature.