Effect of Cooling Rate on Microstructure and Mechanical Properties of Eutectoid Steel Under Cyclic Heat Treatment
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JMEPEG DOI: 10.1007/s11665-017-2779-3
Effect of Cooling Rate on Microstructure and Mechanical Properties of Eutectoid Steel Under Cyclic Heat Treatment Soma Maji, Amir Raza Subhani, Bijay Kumar Show, and Joydeep Maity (Submitted November 13, 2016; in revised form May 22, 2017) A systematic study has been carried out to ascertain the effect of cooling rate on structure and mechanical properties of eutectoid steel subjected to a novel incomplete austenitization-based cyclic heat treatment process up to 4 cycles. Each cycle consists of a short-duration holding (6 min) at 775 °C (above A1) followed by cooling at different rates (furnace cooling, forced air cooling and ice-brine quenching). Microstructure and properties are found to be strongly dependent on cooling rate. In pearlitic transformation regime, lamellar disintegration completes in 61 h and 48 min for cyclic furnace cooling. This leads to a spheroidized structure possessing a lower hardness and strength than that obtained in as-received annealed condition. On contrary, lamellar disintegration does not occur for cyclic forced air cooling with high air flow rate (78 m3 h21). Rather, a novel microstructure consisting of submicroscopic cementite particles in a Ôinterweaved pearliteÕ matrix is developed after 4 cycles. This provides an enhancement in hardness (395 HV), yield strength (473 MPa) and UTS (830 MPa) along with retention of a reasonable ductility (%Elongation = 19) as compared to as-received annealed condition (hardness = 222 HV, YS = 358 MPa, UTS = 740 MPa, %Elongation = 21). Keywords
cooling rate, cyclic heat treatment, eutectoid steel, lamellar disintegration, mechanical properties, microstructure
1. Introduction Cyclic heat treatment that involves repeated thermal cycling around a critical temperature has been proved to cause many novel microstructural changes, thereby providing enhanced mechanical properties in steel. In early studies on steel, thermal cycling consisting of repeated a-c phase transformations resulted in extensive grain refinement effect (Ref 1, 2). Thermal cycling around A1 temperature resulted in an accelerated spheroidization (Ref 3-6). Besides, accelerated bainitic transformation is observed with cyclic austempering (Ref 7). In recent years, a novel concept of incomplete austenitization-based cyclic heat treatment has been adopted by the research group of Maity (Ref 8-13). In this heat treatment process, each cycle consisted of holding for a short duration (6 min) in fully austenitic region (resulting in incomplete austenitization) followed by cooling to the room temperature in different cooling medium. In early approaches, cooling was carried out in a forced air medium with an air flow rate of 6 m3 h 1 for 0.16 wt.% C, 0.6 wt.% C and 1.24 wt.% C steels. These research works envisaged many novel microstructural changes and improvement of material properties. A grain refinement effect was observed in 0.16 wt.% C steel (Ref 8). An accelerated spheroidization was identified in 0.6 wt.% C steel where the spheroidization process was com
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