EFFECT OF CARBON CONTENT ON GRAIN REFINEMENT THROUGH THERMAL CYCLING IN PLAIN CARBON STEEL

Abstract

The work investigated the grain refinement achievable In carbon steels by re austenitization and alternate thermal cycling treatments and the properties developed in these carbon steels after cycling at different austenitizing temperatures. Re-austenitization thermal cycling consists of repeated heating to the austenite range; soaking for a short time at the peak temperature and then quenching in water, while the process of alternate thermal cycling consists of alternate heating between the austenite range and the two phase (a+y) range with intermediate quenching in water. The mechanical and microstructural properties of carbon steel with six different carbon levels (0.1%C, 0.22%C, 0.39%C, 0.54%C, 0.81%C, and 1.04% C) were studied at different austenitizing temperatures of 880°C, 900 "c and 920°C. The soaking times were 5, 10 and 15 minutes for 2, 4 and 6 number of thermal cycles. The effect of carbon content on grain refinement of the carbon steels was investigated. It was observed that an optimum temperature exists for grain refinement by thermal cycling treatments and this temperature decreases with increasing number of thermal cycles. For all the holding times investigated, grain size increased with increasing soaking time at the peak temperature, while higher number of cycles requires less holding time. However, with favorable condition of soaking time and peak temperature, grain size decreased with increasing number of thermal cycle up to a certain number after which further cycling does not provide any observable grain refinement but grain coarsening. Repeated austenitization, though accompanied by poor ductility gave higher strength than alternate thermal cycling but the latter produced slightly better grain refinement and better combination of strength and ductility.