关键词: SNCM616钢, 活塞, 断口, 热处理, 晶粒度, 碳化物, 性能

Abstract: Composition and microstructure of the fracture surface of a curshing hammer piston provided by the user were studied by means of direct reading spectrometer, metallographic microscope and scanning electron microscope, and the cause of the piston fracture was analyzed, and then the chemical composition and heat treatment process of the SNCM616 steel for pistons were optimized according to the user requirements. The results indicate that the fracture of the piston is caused by the combined effect of coarse grains and abnormal precipitation of carbides in the surface carburized layer and its nearby zones, as well as thermal ablation during use, resulting in crack initiation in the carburized layer, followed by fatigue propagation under alternating stress until fracture. Based on the JIS G 4053:2016 standard, SNCM616 steel with P and S content of only 0.005% and V content of 0.12% is prepared by the design concept of low P, low S and addition of trace V element. On the basis of the original heat treatment process, the heat treatment process is improved by adding normalizing process (860 ℃ for 2 h, air cooling) after carburizing and cryogenic process (-140 ℃ for 14 h) after low-temperature tempering. After machining and improved heat treatment, the grain size of the carburized layer and at core of the piston trial-produced by the newly developed SNCM616 steel is relatively fine, not less than grade 5, no abnormal carbide precipitates in the carburized layer, and the hardness meets user requirements.

Key words: SNCM616 steel, piston, fracture, heat treatment, grain size, carbides, properties