Direct Observation of Austenite and Pearlite Formation in Thermally Simulated Coarse Grain Heat-Affected Zone of Pearlite Railway Steel

Abstract

In the present study, coarse grain heat affected zone (CGHAZ) of the pearlite railway steel was thermally simulated at different cooling rates ranging from 1 to 4 °C/s to in–situ observe the austenite and pearlite formation by high temperature laser scanning confocal microscopy (HT–LSCM). During heating, austenite nucleation was started at 811 °C at fast heating rate of 9.1 °C/s. Pearlite was completely decomposed into austenite in 21 s at 1005 °C. At peak temperature of 1300 °C, austenite was composed of two microstructural compositions, i.e., high-carbon austenite and low-carbon austenite. Homogenization of austenite and formation of grain boundaries were continued even during cooling when the temperature was above the pearlite transformation. During cooling, Low-carbon austenite has increased the pearlite transformation by providing additional nucleation sites. By increasing the cooling rate from 1 to 4 °C/s, the pearlite transformation temperature was reducing, pearlite growth rate was increasing, and pearlite interlamellar spacing was becoming narrow, respectively. Mathematical interpretation of pearlite growth rate was developed that gives true changing behavior of pearlite growth rate with the varying cooling rate. Present study provides direct observation and unique quantitative information of austenite and pearlite formation in CGHAZ in pearlite railway steel.