Effect of secondary peak temperature on microstructure and toughness in ICCGHAZ of laser-arc hybrid welded X100 pipeline steel joints

Abstract

Intercritically reheat-coarse grained heat-affected zone (ICCGHAZ), as a brittle zone in double-pass welding, its toughness is affected by the peak temperature of the secondary thermal cycle. To elucidate the effect of secondary peak temperature on the microstructure and impact toughness of ICCGHAZ in laser-arc hybrid welded X100 pipeline steel joints, thermal simulation studies were carried out. The experimental results indicated that the microstructure of ICCGHAZ consisted of tempered martensite and products of secondary thermal cycle along the prior austenite grain boundaries. The products of secondary thermal cycle were necklace-type M-A constituent, necklace-type lath martensite and granular bainite + acicular ferrite at 740 °C, 790 °C and 840 °C, respectively. Based on experimental observations, transformation mechanism of ICCGHAZ at different secondary peak temperatures was proposed. The higher half zone(790℃∼Ac3) in ICCGHAZ of the laser-arc hybrid welded joint has a higher impact energy than the base metal. Consequently, microhardness difference between the products of secondary thermal cycle and matrix was reduced such that the stress concentration and crack nucleation rate was decreased during impact. The lattice orientation was more dispersed and fraction of high angle grain boundaries was increased at 840 °C, which suppressed crack propagation. The method of pipeline steel by laser-arc hybrid welding can effectively reduce the width of low toughness zone in ICCGHAZ and increase the impact energy of the welded joints.

Keywords

• Microstructure
• Toughness
• ICCGHAZ
• Necklace-Type lath martensite