Improvements in Mechanical Properties of Spring Steel Through Surface Tempering and Hardening Assisted by SMAT

Abstract

In this paper, the microstructural, mechanical properties of hardened and tempered spring steel altered by surface mechanical attrition treatment (SMAT) were investigated. The effect of SMAT on surface microstructure, phase constituents, hardness, elastic and plastic properties of the surface: Young’s Modulus, stiffness, YS, UTS were analyzed. The SMAT was carried out for different time periods such as 15, 30, 45, 60 and 75 min respectively. SMAT introduced severe plastic deformation (SPD) over the surface with case depth of about 200–300 µm. The SEM microstructure revealed the presence of tempered lath martensite in as received samples while SMAT had resulted in the formation of twinned martensite at SPD regions. Further, it was also found that fine carbides (Fe3C) started to precipitate and led to the formation of alternate layers of tempered martensite and Fe3C indicating stress relaxation/tempering by SMAT. The X-ray and EDX results also confirmed the presence of martensite in as received samples and precipitation of carbides by SMAT. However, in contrast to a thermal tempering, the surface hardness was found to be increasing due to the effects of SPD. The SMAT had altered the mechanical properties of the steel significantly. The stiffness of the leaf spring was 1452 and 2032 N mm−1 respectively for the samples before and after 15 min of SMAT. This could be attributed to the resistance to elastic deformation caused by surface composite layer. It was also interesting to note that the toughness of the hardened and tempered spring steel increased from 242.45 × 104 to 337.12 × 104 J m−3 by 75 min of SMAT.