Quenching and Partitioning-Based Heat Treatment for Rolled Grinding Steel Balls

A Quenching and Partitioning (Q&P)-based heat treatment has been applied to rolled grinding steel (0.75 wt pct C-Mn-Cr) balls of 104-mm diameter to prevent their cracking during heat treatment. The conventional route of ball manufacturing includes rolling, water quenching interrupted when the bulk average temperature is 220 degrees C to 250 degrees C, and self-tempering in a large pile. This technology leads to cracking when the balls are made of steel of high hardenability. To overcome this deficiency, the Q&P principle is herein adopted by (a) adjusting the quenching duration to cool the ball center below the martensite start (M-s) temperature and (b) instead of self-tempering applying furnace tempering. The target bulk temperature of the ball that ensures an optimal martensite/retained austenite ratio inside the ball at the moment of water quenching interruption is determined as 125 degrees C to 170 degrees C. Quenching should be followed by tempering at 200 degrees C to 250 degrees C for stress release at the surface and carbon partitioning from martensite to austenite in the core. The resulting microstructure varies from tempered martensite in the shell layers to a mixture of martensite-bainite and retained austenite (RA) in the inner zones, where the austenite content is 25 to 30 vol pct. The carbon enrichment in austenite toward the core of the ball leads to balanced stresses and prevention of ball cracking. Q&P-heat-treated balls in an industrial-scale trial attained a uniform hardness of approximately 55 HRC through the cross section and a high fracture resistance under repetitive impacts of 6.8 kJ energy each.