Weldability Of Ultra-high Strength Pipeline Steel X120

X120ultra high strength pipeline steel with high strength and good low temperaturetoughness was obtained by means of ultra low carbon design, microalloying and TMCPprocessing. The microstructure of this steel is dominantly upper bainitic ferrite The addition ofmicroalloying elements (Nb, V, Ti, Zr) pinned the prior austenite grain boundary and led to smallaustenite grains. TMCP processing resulted in pan-cake shape austenite grains and finallyfine-grained microstructure in the base metal steel plate.The microstructural evolution in coarse-grained region of heat-affected zone (CGHAZ) ofX120ultra high strength pipeline steel was investigated by means of thermal simulation on aGleeble3500. The continuous cooling transformation curves of simulated HAZ (SH-CCT) wereobtained simultaneously. As the cooling rate ranged from0.1°C/s to60°C/s, the predominantlybainitic microstructure was obtained in the simulated CGHAZ. Bainite was obtained in thesimulated CGHAZ when the heat input of welding was1225kJ/cm. The Vickers hardness ofCGHAZ was between291303HV10. The impact toughness at room temperature was high andstable (208~225J). The simulated results revealed that X120steel can be welded in a wide rangeof welding heat input.A fast cooling process was introduced to improve the toughness of the CGHAZ. Thewelding thermal cycle was varied in which both the holding time at high temperature above Ac3(tH) and cooling time from800to500C (t8/5) were shorten. Shorten tHled to thin HAZ widthand fine prior austenite grain in fusion line (FL) and CGHAZ; while the shorten t8/5resulted inthe thinner bainite lath. The martenist-austinite (M-A) constituents in fusion line were reducedand their morphology was changed into thin film-like. All these factors played a beneficial rolein improving toughness of HAZ of pipeline steel X120.According to the SH-CCT, the heat input of21kJ/cm was chosen to weld the X120baseplate utilizing three-wire submerged arc welding and the above-mentioned fast cooling technique.The microstructure in the heat-affected zone was investigated by means of optical microscopyand scanning electron microscopy. The hardness of heat-affected zone was measured by Vickershardness indenter. Results show that the X120linepipe welding joint had superior toughness atlow temperature (-30C). The investigated X120steel had perfect weldability, which wasattributed to the ultra-low carbon design and the Zr-Ti microalloying.