Vacuum Heat Treatment of Cast Steel Valve Seats

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Steel casting is a combination of casting forming process and steel material, which has both advantages. It can not only have complex shapes that are difficult to obtain by other forming processes, but also maintain various properties unique to steel. The performance and reliability of steel castings can meet particularly stringent requirements, and they have played an important role in heavy machinery, transport vehicles and power station equipment. Although there is a coarse and brittle ledeburite structure in the as-cast structure of high-speed steel, and its mechanical properties are lower than that of pressure-processed high-speed steel, cast high-speed steel also has its advantages. It can be produced by using high-speed steel chips and scraps, and the manufacturing process is relatively low. Simple, only a small amount of surface finishing work is required, and the material consumption can be reduced by 30-50% compared with the method of using forged billets or rolled materials to be formed by cutting.

The valve seat is the intake valve and exhaust valve assembly of a large gas engine. This type of gas engine has 48 intake valves and 48 exhaust valves respectively. The valve seat is made of cast steel W6Mo5Cr4V2. The valve seat is connected to the engine case cover, the valve moves at high speed under high temperature, the valve seat and valve are subjected to the erosion and corrosion of high temperature and high pressure gas, the stress state is complex and changeable, and it is required to have sufficient high temperature strength and wear resistance, good Therefore, after casting and heat treatment, it should ensure high comprehensive performance and dimensional stability.

The shape of the valve seat is annular, the external dimension is φ (external circle) = 66.5mm, the height h = 12.5mm, the inner hole is tapered, and the processing process is casting → annealing → semi-finishing → vacuum quenching → multiple tempering → Grinding plane and outer circle → CNC machining inner hole taper surface. The author made experiments and observations on the hardness and microstructure of the valve seat sample after vacuum quenching and multiple tempering in order to accumulate more experience in processing cast steel parts and improve the service performance of the parts.

W6Mo5Cr4V2 steel is the representative of tungsten and molybdenum series general high-speed steel. Mo replaces part of W to refine the as-cast ledeburite. Due to the relationship between resources and price, many countries use W6Mo5Cr4V2 steel instead of W18Cr4V steel to become the main high-speed steel steel. No. The toughness, wear resistance and thermoplasticity of W6Mo5Cr4V2 steel are better than those of W18Cr4V steel, and its hardness, hot hardness and high temperature hardness are comparable to those of W18Cr4V steel. Therefore, W6Mo5Cr4V2 steel can be used to manufacture various types of ordinary tools, as well as large-scale and thermoplastic forming. Tools and wear parts under high loads. The valve seat is annealed by powder sand casting and has a hardness of 35-39HRC to reduce the hardness and facilitate cutting.

The requirements for heat treatment of casting tool and die steel castings vary with their composition and service conditions. For example, casting tool steel castings generally have small cross-sectional dimensions and complex shapes. The selection of the heat treatment process should be based on ensuring strength, red hardness, and preventing deformation and cracking. The valve seat and the sample with the furnace are quenched in a vacuum quenching furnace. The size of the effective heating zone of the furnace is 900mm × 600mm × 600mm. The vacuum heat treatment process is shown in the figure. Cast steel W6Mo5Cr4V2 steel has high alloying element content and low thermal conductivity. In order to reduce thermal stress and structural stress, segmental preheating is adopted during heating. During quenching and cooling, measures such as cooling speed and oil temperature are controlled to ensure the surface and core of the part. The temperature difference is as small as possible to reduce the risk of part deformation and cracking.

The anisotropy of mechanical properties of cast steel is not significant, which is an aspect superior to forged steel. In addition, regardless of its weight and batch size, cast steel can be easily made according to the designer’s concept with a reasonable shape and interior. For parts with high profile, high stiffness, complex shape and insignificant stress concentration, appropriate molding process is adopted to make the castings have the required dimensional accuracy and surface quality, which are difficult to achieve when using forgings.

After the valve seat sample is tempered three times, the hardness is 52.0-54.0HRC, the hardness is uniform, the distortion is qualified, it meets the technical requirements of customers, and it is used well.

Equipment selection: The VOGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.