Effect of Hardening and Tempering on the Microstructure and Mechanical Properties of the Tapered-Forged Leaf Spring Steel

Abstract

The sharp side of a steel axe is often broken. This part is usually forged into a tapered shape until the front edge is thin and sharp. The purpose of this study was to determine the effect of tempering on the structure and hardness of a steel axe made of AISI 5160 leaf spring steel which has undergone a forging and hardening process. The as-received samples were obtained from a commercial market after undergoing a forging process to reduce their thickness from 10 mm to 5.5 mm in the base part and 1 mm in the front edge part. The hardening process was carried out by heating the samples at a temperature of 850 °C followed by rapid cooling in an isorapid oil. The samples were then tempered with tempering temperatures of 300 °C, 350 °C, and 400 °C. The hardness of the tempered samples was evaluated using a Vickers microhardness tester in three different locations; the base, the middle, and the front edge of the tapered samples. To analyze the property change, the microstructure of the tempered samples was observed using SEM and EDS. The result showed that the hardness of the samples decreased with the increase in tempering temperature. The hardness of the front edge was higher than that of the middle and base part of the tempered samples. The faster cooling rate inside the thinner part was responsible for the higher hardness. The microstructure showed that the martensitic structure that emerged after the hardening process turned into fine tempered martensitic and secondary carbides. The size and amount of carbides increased with increasing tempering temperature