Study of microstructural evolution and strength–toughness mechanism of heavy-wall induction bend pipe

Abstract

The induction bending and subsequent tempering process has recently been proven to be an optimal technological route for the manufacture of bend pipe with a better combination of strength and toughness. In this work, high temperature quenching followed by tempering treatment from 500°C to 700°C has been applied to heavy-wall bend pipe steel produced by the ultra fast cooling technology. The evolutions of microstructure and dislocation were characterized by means of an optical microscope, the positron annihilation technique (PAT), SEM, TEM, XRD and EBSD. Microstructure observations showed that fine and homogenous M/A islands as well as dislocation packages in a quasi-polygonal ferrite (QPF) matrix after tempering at 600–650°C generated an optimal combination of strength and toughness. At higher tempering temperature, the yield strength decreased dramatically; however, the impact toughness still remained at a high value of more than 300J. Dislocation analysis by means of TEM, EBSD and PAT suggested that the decrease and pile-up of dislocation could provide better toughness and tempering stability. EBSD analysis indicated that the average misorientation angle enlarged and the effective grain size diminished with the tempering temperature increasing, and these caused more energy cost during the microcrack propagation process with subsequent improvement in impact toughness. Microcracks mainly originated from the interfaces between ferrite matrix and M/A islands. The ring-type join-up of microcracks and the large number of branches formed during the propagation process effectively improved the toughness. All these results benefit the cost-effective commercialization of heavy-wall induction bend pipe with high performance.