In this work an annealed 0.6 wt.% carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (6 min) holding at 810°C (above Ac 3 temperature) followed by cooling in a flowing air medium (flow rate: 6 m 3 /h). After 8 cycles (about 1 h and 20 min), the microstructure mostly contains spheroidized cementite and ferrite along with trace amount (3%) of pearlite. In addition to the diffusion within lamella, the disintegration of lamellae through dissolution of cementite at preferred sites of lamellar faults during short-duration holding above Ac 3 temperature, and the generation of defects (lamellar faults) during non-equilibrium cooling in a flowing air medium are the main reasons of accelerated spheroidization.
In this research work annealed AISI 1080 steel bars are subjected to a typical combined cyclic heat treatment involving two cycles. The first cycle consist of inserting the specimen in an electric resistance furnace at a temperature of 775 8C (in fully austenitic region) and holding for a short duration (6 min), followed by forced air cooling to the room temperature. In second cycle holding temperature and time being same, the specimen is subjected to oil quenching. Such a typical combined thermal cycling results in the evolution of a novel microstructure that comprises of submicroscopic cementite spheroids and fine microscopic cementite clusters in a matrix of martensite. Accordingly, very high hardness (894 HV) is achieved which envisaged a new heat treatment route of the development of plain carbon tool steel.
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