Identification of the effect and mechanism of austenitizing temperature, sprocket rotation speed, and quenching medium on microstructure and hardness of flame-hardened gear steel

The object of this study is a medium-carbon gear steel commonly used for sprockets and gears in mechanical drive systems. The problem to be solved in this study is the limited understanding of the integrated effect of austenitizing temperature, sprocket rotation speed, and quenching medium on surface hardness and microstructural evolution, that makes parameter selection of flame hardening in industry does not optimum. The experiments were conducted by heating the medium-carbon gear steel into austenitizing temperatures of 850°C and 900°C, rotation speeds of 1503 rpm and 1977 rpm, and using water and oil as quenching medium. Specimens were evaluated by using Rockwell hardness tester (B scale) and optical microscopy. The result of this study reveals that the highest surface hardness of 120.08 HRB is achieved at austenitizing temperature of 900°C, rotation speed of 1503 rpm, and quenched by using water. This is also supported by the result of microstructural observations which show very fine martensite. The surface hardening is most affected by quenching medium, while rotation speed has no significant effect on the hardness enhancement. At austenitizing temperature of 900°C, the steel has been in austenite phase, so the increasing rotation speed reduces hardness due to excessive heat input and austenite grain coarsening. The distinctive feature of these results lies in the identification of interaction mechanisms between thermal exposure time and cooling rate. The findings can be practically applied to medium-carbon gear steel components subjected to controlled flame hardening with continuous rotation at speeds of 1503–1977 rpm, followed by water or oil quenching, particularly in small- and medium-scale industrial heat treatment of gears and sprockets