Influence of Sulfur Content on Inclusion and Fatigue Properties of 18CrNiMo7-6 Gear Steels

Abstract

 In this paper， two kinds of 18CrNiMo7-6 gear steels with sulfur content of 0.002% （low sulfur content） and 0.022% （high sulfur content） were studied. Pseudo-carburizing samples were obtained by reference to gear heat treatment process， and the effects of sulfur content on mechanical properties， microstructure， fatigue properties and inclusion distribution of the steels were investigated. The results show that the strength of the two steels is basically similar， while the steel with high sulfur content has better plasticity and low temperature impact toughness. Meanwhile， its fatigue limit and fatigue life are better than those of the low sulfur content steel， the fatigue ratio increases from 0.445 to 0.479. Sulfur content significantly affects the distribution of inclusions in steel， the inclusions in low sulfur content steel are mainly composed of MnS and oxides with less quantity but larger size， while the inclusions in the high sulfur content steel are mainly complex inclusions composed of Cas， MnS and oxides with more than doubled quantity but smaller size. By employing Ca treatment technique， the inclusion size of high sulfur content steel is obviously refined， which favors better mechanical properties and fatigue performance of steel.

Keywords

• 18crnimo7-6 gear steel； mechanical properties； fatigue properties； non-metallic inclusion