Stainless steel ball screw is one of the important parts of nuclear power plant equipment. It is required to have high hardness (52-56HRC), high strength, high impact toughness, and excellent corrosion resistance of the workpiece. 9Cr18 stainless steel was used to make ball screws in production. 9Cr18 steel is prone to quenching cracking due to serious network and band carbide defects, and grinding marks will occur during grinding, causing the workpiece to fail. By strictly selecting materials and improving the forging process to improve the distribution of network and band carbides in the workpiece, the performance of the workpiece has been improved. The workpiece is quenched using medium frequency induction heating equipment. However, it was found during production that the impact toughness of the 9Cr18 steel ball screw is low. After treatment, it is only 2-5J/cm2, which is far lower than the technical requirements. The workpiece is notch sensitive, and workpieces may occur during use. Twist failure. Therefore, we must start from two aspects: material selection and heat treatment process improvement to meet the technical requirements of the ball screw. Since then, PH15-7Mo semi-martensitic precipitation hardening stainless steel and GP5 martensitic precipitation hardening stainless steel have been tested successively, but they were abandoned due to insufficient hardness and strength after treatment. Then lCr15Co14Mo5VN stainless steel was tested, and after optimization, selection and improvement of the heat treatment process, Good results were achieved and the ball screw performance met the technical requirements.
lCr15Co14Mo5VN steel is martensitic precipitation hardening stainless steel. The mechanical properties in the annealed state are: high strength = 1357.5MPa, high impact toughness = 55.7J/cm2, and hardness 38-39HRC. After many process tests and improvements, it is concluded that the heat treatment process of lCr15Co14Mo5VN steel using medium frequency induction heating equipment is as follows:
(1) Annealing. (860+10)℃x (2-3)h, furnace cooling, hardness is 38 -39HRC.
(2)Quenching. Austenitizing, (1050+10)℃x60min, oil cooling, hardness is 47-48HRC.
(3) Cold treatment. -78℃x30min, air cooling, hardness is 48-49HRC.
During the heat treatment process, affected by many factors, the workpiece may produce some quenching defects. At this time, it is necessary to master the preventive measures for these defects. This article briefly introduces the preventive measures for quenching defects in stainless steel ball screws. After many manufacturers adopt the above optimization process, the mechanical properties meet the technical requirements. The performance of the workpiece is excellent, meeting the technical requirements and production needs, and the production runs well.
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GS-ZP-1200
