Microstructure and precipitates in thick-walled X90 bend related to mechanical properties

Abstract

The microstructure and precipitate characterizes in the API X90 hot bend related to mechanical properties was investigated by OM, TEM, EBSD and mechanical tests. X90 pipeline steel is consist of quasi-polygonal ferrite (QPF), acicular ferrite (AF), lath bainite (LB) and a small amount of M/A constituents. The width of bainite lath is about 0.2~0.3 μm. After hot induction bending, hardly observe AF in the bend zone. In the outer arc side, the width of LB was coarsened to 0.53˜1.34 μm, and sharp M/A constituents formed along the prior austenite grain boundaries. Compared to the parent pipe, the strength of X90 bend decreased 30˜80 MPa, and the Charpy impact energy increased 20 J. The outer arc side with the weakest low temperature impact toughness, is 153 J. The main component of the precipitate is NbC with a small amount of TiC, possibly (Ti, Nb) C, and the size is about 15nm. The fraction of the high angle grain boundaries (HAGBs) and the kernel average misorientation (KAM) value of the outer arc side is 21% and 0.62°respectively, which is the higher than the other positions.