Effect of Heat Input and M-A Constituent on Microstructure Evolution and Mechanical Properties of Heat Affected Zone in Low Carbon Steel

Microstructure evolution and impact toughness of simulated heat affected zone (HAZ) in low carbon steel have been investigated in this study. Thermal simulator was used to simulate microstructure evolution in HAZ with heat input of 10 - 100 kJ / cm welding thermal cycle. Results indicated that microstructure of HAZ mainly consisted of acicular ferrite (AF) inside grain and high volume fraction of grain boundaries ferrite (GBF) at prior austenite boundaries; the size of GBF and effective grain size increased with increasing heat input. Excellent impact toughness (more than 150 J at - 40 degrees C) was obtained in HAZ with heat input less than 50 kJ / cm. When heat input was 100 kJ /cm, the impact toughness of HAZ decreased to 18 J because of the presence of large M-A constituent with lath-form in HAZ, assisting the micro-crack initiation and decreasing the crack initiation energy seriously. Effect of inclusions on acicular ferrite transformation in HAZ was also discussed.