Effect Of Tempering And Nitrogen Content On The Heat-treated Impact Toughness Of Sintered Steels

Abstract

Induction hardening and induction tempering gain increasing importance for heat treating of sintered steels due to high reproducibility and the chance for subsequent soft machining of not heat-treated part regions. Due to the high heating rates and the short processing times during the inductive heating the diffusion processes have to be accelerated by higher temperature. This results in higher austenitizing and tempering temperatures compared to classical furnace heat treatments. In the presence of absorbed nitrogen higher tempering temperatures might result in blue brittleness (300 °C embrittlement), an aging effect caused by the precipitation of nitrides. Based on that working hypothesis four different sintered steels were hardened and tempered under varying conditions. The hardness and the impact toughness were analyzed to check whether a drop of the toughness can be observed at a certain tempering temperature level. Moreover, the absorbed nitrogen contents were measured.