Impact toughness of base metal and welded joints of X90 high-strength pipeline steel

X90 pipeline steel pipe is a new type of high strength pipeline steel pipe being researched and developed. With the increase of pipe strength, the structure and properties of submerged arc welding seam become the key of research and control. The impact toughness in low temperature, fracture morphology, microstructure characteristics, grain orientation and percentage of large/small angle grain boundaries of weld seam, heat affected zone (HAZ) and base metal in X90 steel grade spiral submerged arc welded pipe manufacture were investigated by OM, SEM, TEM, EBSD and Charpy impact experiments. The results indicate that the microstructure near fracture zone of weld seam specimen was composed of acicular ferrite(AF) and quasi-polygonal ferrite (QPF). The form of M-A constituents shows diversity, sharp-angled clearly and the size increased as length×width =1.8 μm×0.5 μm. The values of average grain size was 3.12 μm and the proportion of large-angle grain boundaries was 67.15%. The microstructure near fracture zone of HAZ specimen was composed of granular bainite (GB) and multi-morphological M-A constituents, the M-A constituents in the grain boundary and crystal. The values of average grain size was 4.52 μm and the proportion of large-angle grain boundaries was 85.95%. The microstructure near fracture zone of base metal specimen was composed of fine AF, fine QPF, lath bainite (LB) and a small fine granular M-A (martenite/austenite) structure. The values of average grain size was 2.1 μm and the proportion of large-angle grain boundaries was 93.75%. Concentrated distribution of large size M-A constituents and the relatively small proportion of large-angle grain boundaries were the main reason for the poor impact toughness of weld.