Effect of Cooling Rate During Quenching on the Toughness of High Speed Steels

Abstract High speed steels are usually employed in cutting tools and forming dies. Heat treatment is an important step during the tool manufacturing process, being responsible for most of the final properties, mainly hardness and toughness. In this context, an especial interest relies on the effect of hardening variables on mechanical properties and tool performance. The present paper aimed to study the effect of cooling rate during the quenching process of standard designation AISI M2 high speed steel, under typical industrial conditions. The experiments were carried out in an industrial vacuum furnace, with high pressure nitrogen quenching. Several cooling rates were obtained by variation of the nitrogen pressure and by the use of test specimens with different dimensions. Toughness results were mainly evaluated through static bend test. Low cooling rates were shown to decrease material toughness and large parts presented a decrease in mechanical properties from surface to core regions. Carbide precipitation on grain boundaries are pointed as the main explanation for all these effects.