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.