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Carburizing and Quenching Process of 18CrNi8 Long Needle Valve Body

On the basis of a comprehensive understanding of the working principle and performance of the needle valve body coupler, this paper is based on the technical requirements of the needle valve body parts, the particularity of its geometric shape and the country’s requirements for exhaust emissions, and draws on domestic and foreign counterparts. Using the new heat treatment technology, combined with our company’s existing production equipment and production technology, the carburizing, quenching, ice-cooling and tempering treatment of 18CrNi8 needle valve body has been deeply studied and discussed.

The shape of the needle valve body is complex, the slenderness ratio between the diameter of the middle hole and the depth of the hole is very small, the shape of the head is small spherical, the diameter of the nozzle hole is very small, and the entire inner hole is similar to a blind hole, all of which bring great benefits to carburizing. Difficulties. Therefore, we compared gas carburizing, carbonitriding, internal solid external gas carburizing and low-pressure vacuum carburizing in the carburizing test. According to the technical requirements of heat treatment, the internal solid and external gas can meet the performance requirements of the product, but the process control is complicated, the operating environment of workers is harsh and the cleanliness is poor, and low-pressure vacuum carburizing can meet the above technical requirements, so the 18CrNi8 needle valve The best process for bulk carburizing is acetylene vacuum (low pressure) carburizing. Through the research on the low-pressure carburizing process of acetylene and the optimization of the vacuum carburizing process parameters of the 18CrNi8 needle valve body, the specific process parameters are obtained: the carburizing temperature is 930°C, and the heating method adopts convection heating; The acetylene flow in each carburizing period is 1500~4500 1/h; the strong infiltration and diffusion pulse periods are each 5 periods, the strong infiltration time is 14min, and the diffusion time is 69min.

In order to meet the requirements of needle valve body hardness, metallographic structure and dimensional deformation, three different quenching processes were also selected for comparison. These three processes are: multi-purpose furnace oil quenching, vacuum furnace oil quenching and vacuum furnace gas quenching According to various test results, vacuum high-pressure gas quenching can not only meet various technical requirements, but also minimize heat treatment deformation. Therefore, the quenching process of the needle valve body is optimal with the vacuum high pressure gas quenching scheme. The specific process parameters are: quenching temperature of 840°C, holding time of 70min, and quenching medium of 10bar high-purity nitrogen.

After determining the two main processes of carburizing and quenching, the scheme and specific parameters of ice-cold treatment and tempering were tested, and the entire heat treatment process and process parameters of the 18CrNi8 needle valve body were finally determined. This process solves the three major problems that plague the needle valve body heat treatment: the uniformity of the inner and outer infiltration layers, the control of the carbide of the ball head and the heat treatment deformation problem.