Relationship between charpy impact toughness and microstructure in heat affected zone of high heat input low carbon steel weldment

:Although it has been known that toughness of heat affected zone (HAZ) in low carbon steels lowers with increasing heat input, high heat input welding has been demanded from economical reason. Toughness of HAZ has been often investigated using synthetic HAZ but thermal cycle in high heat input welding has not been well known. The thermal cycle in tandem submerged are butt welding in a low carbon steel was measured using thermocouples specially designed. Cooling rate of HAZ markedly changed in around A(1) transformation temperature in high heat input welding, that is high cooling rate above the temperature and low cooling-rate below the temperature. Toughness of synthetic HAZ of high heat input welding with high cooling rate above 700 degrees C and low cooling rate below 700 degrees C deteriorated compared with that of synthetic HAZ cooled at a constant low cooling rate. This result was related with degenerated pearlite precipitation in synthetic HAZ of high heat input welding. Charpy impact absorbed energy of actual HAZ scatters in a wide temperature range of transition zone. This has disturbed to understand toughness of HAZ. The scattering of absorbed energy occurred in a narrow temperature range in synthetic HAZ and in a wider one by Izod impact test. It appears that the scattering of absorbed energy in actual HAZ is caused by strength heterogeneity of actual joints because of no strength heterogeneity in synthetic HAZ and larger strength heterogeneity by Izod impact test compared with Charpy impact test.