Effects of Tampering Heat Treatment on the Performance of Hardened S7 Tool Steel

Technological advancements increase the pressure on industrial activities where post processing activities like machining places high demand on tools with high performance and longer tool life. The unique mechanical and physical properties of tools are improved through heat treatment processes like tempering, hence, the effect of different tempering conditions on the performance of S7 tool steel was studied. Four S7 tool steel samples were subjected to different tempering conditions. One was used as-received while another was tempered without any heat treatment exposure. Other two samples were austinitized to 1050oC prior to quench-hardening in automobile lubricating oil. Among the two quenched samples, one was used in as-quenched condition while the other was tempered to 500oC before being cooled in still air. The four samples were subjected to tensile, hardness and metallographic analysis. Results showed that quench-hardening increased hardness by 91.17%. However, tempering the as-quenched sample reduced the excessive core hardness and surface microhardness by 14.04% and 16.11% respectively. Also, through tempering the hardened samples, the tensile strength, yield strength and Young Modulus were moderated by 89.26, 95.83 and 2.49% respectively. At these high mechanical properties, post-hardened tempering increased the ductility of the hardened sample by 33.33% leading to an improved toughness. Results also show a correlation between microstructural phase changes and mechanical properties. Metallographic studies revealed that transformation of retained Austinite to martensite within eutectic carbide matrix was responsible for the high mechanical properties obtained which must be tempered before use. Hence, tempering is highly recommended for hardened S7 tool steels.