Direct Observation of Austenite and Pearlite Formation in Thermally Simulated Coarse Grain Heat-Affected Zone of Pearlit
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JMEPEG https://doi.org/10.1007/s11665-020-05327-2
Direct Observation of Austenite and Pearlite Formation in Thermally Simulated Coarse Grain Heat-Affected Zone of Pearlite Railway Steel Adnan Raza khan, Yu Shengfu, and Muhammad Zubair Submitted: 7 July 2020 / Revised: 15 October 2020 / Accepted: 31 October 2020 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. Keywords
3–wires electroslag welding, coarse grain heat affected zone, austenitization, pearlite formation
1. Introduction 3–wires electroslag welding (ESW) is a new development welding method of joining pearlite railway steel (Ref 1), where coarse grain heat affected zone (CGHAZ) is considered as the most affected area (Ref 2). Unlike arc welding processes, 3– wires ESW is associated with the high-heat generation method that produces long peak temperature holding time and slow cooling rate following complete pearlite transformation in the CGHAZ, same as that of the base metal (Ref 2, 3). Khan et al. reported that by changing the process variables, variety of cooling rates can be achieved in the ESW (Ref 2) that significantly affects the microstructure transformation. In CGHAZ of 3–wires ESW joint, during heating the ferrite– pearlite microstructure of the railway steel transforms into austenite, whereas during cooling, austenite transforms into pearlite (Ref 4, 5). Formation of austenite and pearlite is of two stages, i.e., the nucleation and the growth (Ref 6). So far, both the phenomenon has attained little attention (Ref 7, 8), especially when the accelerated heating or cooling conditions
Adnan Raza khan and Yu Shengfu, State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology,
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