Influence of micro-alloying elements and deep cryogenic treatment on microstructure and mechanical properties of S5 cold work shock resisting tool steel

Abstract

Shock resistance cold work tool steel (S5) has many applications such as blades for sheet cutting, manufacturing of chisel and hammers etc. This Steel has the ability to accommodate residual stresses without premature failure. This work investigates the effect of carbide forming elements; V and Nb on the microstructure and mechanical properties of S5 tool steel. Three heats were prepared with different content of carbide forming elements via vacuum melting and the effect of heat treatment [hardening, tempering and deep cryogenic treatment (DCT) at - 196 °C] on the mechanical properties was investigated. Retained austenite for quenched samples were calculated using X-ray diffraction method, also impact test was used to determine the toughness of all samples after different heat treatment conditions. Microstructure observation showed that the basic structure for the tested samples are tempered martensite for traditional S5 samples and fine tempered martensite for other micro alloyed samples. Scanning electron microscope (SEM) was also used to ensure that the amount of retained austenite have been eliminated or reduced due to deep cryogenic treatment. Hardness (HRC) after quenching process reached 51 HRC, further increase in hardness values has been obtained after DCT due to reduction of retained austenite. The toughness results showed lower values for deep cryogenic treatment than conventional heat treatment samples; this is mainly due to the reduction in retained austenite content.