Rapid Cyclic Heating of Mild Steel and its Effects on Microstructure and Mechanical properties

Abstract

The effect of subjecting mild steel to several cycles of rapid heat treatment on its mechanical properties and microstructure was investigated. Mild steel of 0.213 wt % carbon was subjected to transformation heat treatment from austenite to pearlite and quenched in running water. Rapid heating was achieved by preheating an electric muffle furnace to 840 °C before charging the samples into it. Each cyclic heat treatment was for a period of 200 seconds held at 840 °C and cooled to 700 °C which was repeated four times. The effects of cycle numbers were evaluated by testing for impact, hardness, ultimate tensile strength and microstructural properties. The results showed that after 4th cycle of rapid heating the samples had impact energy 64.6 J, Brinell hardness number 563 and ultimate tensile strength 1257.78 N/mm2. The samples after one cycle had ultimate tensile strength 1027.45 N/mm2, % elongation 10.396% and impact energy 286.174 N.m before failure. Through cyclic heating, grain refinement was achieved by the fast simultaneous nucleation at the grain boundaries and the fast martensite to austenite transformation due to the fast heating rate which prevented austenite grain growth. Mechanical properties of the studied steel sample were improved with the rapid heat treatment cycles given.