Effect of Heat Input on Microstructure and Impact Toughness in the Simulated Coarse-Grained Heat-Affected Zones of X90 Pipeline Steel.

This study aims to describe the relationship among heat input, microstructural evolution, and impact toughness in the simulated coarse-grained heat-affected zones of X90 pipeline steel under different heat inputs varying from 10 to 50 kJ/cm. Optical microscopy, transmission electron microscopy, and electron backscatter diffraction were performed for microstructure observation. As the heat input increases, the lath bainite expenses at the increase of granular bainite, which makes the samples fracture from ductile to brittle. The impact toughness was increased to a peak value of 273 ± 17 J when the heat input was increased to 25 kJ/cm, and was markedly decreased with increasing heat input. The toughness was deteriorated by the coarse-stringer and massive martensite/austenite constituent with a large grain size when the heat input was larger than 30 kJ/cm. The average efficient grain size (EGS) was 3.6, 3.7, and 4.3 μm with heat inputs of 10, 25, and 40 kJ/cm, respectively. The EGS is significantly smaller than the prior austenite grain size of 22.8, 26.8, and 31.2 μm. In addition, the precipitate is Nb-rich (Ti, Nb) C with a circular morphology modified to Ti-rich (Ti, Nb) C with a rectangular morphology and the size obviously increases, which also causes the toughness to deteriorate. [ABSTRACT FROM AUTHOR]