Influence of heat input on microstructure and fracture toughness property in different zones of X80 pipeline steel weldments

Abstract

In this paper, microstructure observations and mechanical behaviour of fusion line and offsetting positions from fusion line by 1, 2 and 3 mm were analysed. For the welding of X80 pipeline steel plates, different magnitudes of heat inputs such as high heat input (HHI) 25 kJ/cm, medium heat input (MHI) 20 kJ/cm and low heat input (LHI) 15 kJ/cm were employed. Critical values of J-integral (J(0.2)) and crack tip opening displacement (CTOD0.2) for predetermined regions in the X80 weldment were determined as per ASTM-E1820a. M-A constituents of different sizes such as small (1-2 mu m), large >2 mu m and slender (>4 mu m) were observed in the microstructure of subzones of weldments for different heat inputs. Formation of granular bainite, M-A constituents and inclusions of Ti, Si, Mo in the microstructure impaired fracture toughness property. In the X80 weldment, the fusion line (FL) for HHI was found weakest in terms of fracture resistance, which subsequently increases the risk of fracture.