Microstructural modifications and changes in mechanical properties during cyclic heat treatment of 0.16% carbon steel

In this work an annealed 0.16 wt% carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (6 min) holding at 910 ◦C (above Ac3 temperature) followed by forced air cooling. A typical microstructural development, not so common for low carbon hypoeutectoid steel, was observed. The main features involved were: (i) a substantial grain refinement (mainly at initial stage), (ii) generation of dislocations (at initial stage) and its annihilation (at later stage) and (iii) generation of grain boundary network of cementite and cementite cluster owing to divorced eutectoidal reaction. In low carbon steel, the presence of large proportion of grain boundary areas promoted grain boundary diffusion of carbon during short-duration holding. This phenomenon finally led to the generation of grain boundary cementite network and cluster through divorced eutectoidal reaction. Unlike high carbon steel, the contribution of divorced eutectoidal reaction to spheroidization was not so significant. For higher cycles (5–8 cycles) substantial presence of grain boundary cementite and cementite cluster deteriorated the strength property. However, quite a high strength (UTS = 455 MPa) was achieved for this low carbon steel with two cycles of heat treatment due to fine ferrite grain size, high dislocation density and adequate proportion of fine lamellar pearlite in the microstructure