Study of cyclic heat treatment on carbon steel

Abstract

In this work, an annealed Ck85 carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (3.4 minutes) holding at 8000C (above Ac3 temperature) followed by forced air cooling. After 8 cycles (about a total 1 hour and 10 minutes duration of heating and cooling cycles), the microstructure mostly contained fine ferrite grains (grain size of 7 μm) and spheroidized cementite. This microstructure possesses an excellent combination of strength and ductility. The disintegration of lamellae through dissolution of cementite at preferred sites of lamellar faults during short-duration holding above Ac3 temperature, and the generation of defects (lamellar faults) during non-equilibrium forced air cooling were the main reasons of accelerated spheroidization. The strength property initially increased, mainly, due to the presence of finer micro constituents (ferrite and pearlite) and thereafter marginally decreased with the elimination of lamellar pearlite and appearance of cementite spheroids in the microstructure. Accordingly, the fractured surface initially exhibited the regions of wavy lamellar fracture (pearlite regions) along with dimples (ferrite regions). By increasing number of heat treatment cycles, the regions of dimples gradually consumed the entire fractured surface.