EFFECT OF CYCLIC HEAT TREATMENT ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MICROALLOYED STEEL

Abstract

Microalloyed steels have wide range of applications in shipbuilding, storage tanks, automobile sectors etc. In this present study, effect of cyclic heat treatment on microstructure and mechanical properties of microalloyed steels was analyzed. Steel samples were first subjected to homogenizing annealing at 1200ºC for 8 hours and were then subjected to cyclic heat treatment consisting of 3,5,7,9 and 11 cycles, which involved repeated heating and cooling 50ºC above and below the Ac3 temperature (i.e. 845°C). After the completion of cyclic heat treatment, the heattreated samples were air cooled at room temperature and water-quenched using tap water to verify the effect of cooling rate on the microstructure and mechanical properties. In case of the air-cooled samples, with increase in number of cycles there was a decrease in the grain size, whereas in case of water-quenched specimens, martensite (~16.45%) and proeutectoid ferrite (~83.54%)were found to form. For the air-cooled specimens, there was a continuous but very little increase in the yield strength, hardness and toughness with increase in number of cycles; this was due to the grain size reduction associated with the cyclic heat treatment. The quenched sample showed high hardness, yield strength and low toughness due to the formation of some fraction of martensite along with ferrite. The specimen, which was air-cooled after 11 cyclic heat treatment, showed best combination of mechanical properties, such as better Charpy impact toughness (19.80 J), yield strength & ductility (233 MPa, 37.78%) and hardness (170 VHN). This is due to the formation of mixed-microstructure of pro-eutectoid ferrite and fine pearlite. Fractography analysis showed that the normalized samples underwent ductile failure, while quenched sample showed the mix-mode failure. This is found to correlate well with the tensile strength, ductility, Charpy impact toughness and microstructure of the corresponding specimen. In this microalloyed steel, cyclic heat treatment can be used to replace the traditionally used thermomechanical processing for the grain refinement