Weldability Simulation And Cold Cracking Susceptiblility Prediction Of X80 Pipeline Steel

World oil and gas pipeline construction and development trend is to build long-distance, high-pressure, large volume transportation pipeline, which determines the steel to high grade, large diameter, and large wall thickness direction. X80 pipeline steel with its excellent strength and toughness became the first choice of steel grades for oil and gas transportation in the 21St century. In this work using phase change thermal dynamics, metal physics and the basic theory of plastic mechanics and by means of the combination of the physical modeling and numerical simulation, the microstructure and properties of X80 pipeline steel coarse grain HAZ were investigated, stress field of X80 pipeline steel weld joint was calculated . X80 pipeline steel cold cracking sensitivity was studied by implant tests.Taking into account the influence of heat input and preheat temperature, using a regression orthogonal experiment and welding thermal simulation technology, Nb-Cr system X80 pipeline steel coarse grain HAZ microstructure and properties in different welding parameters were studied. The results show that, the main microstructure of Nb-Cr system X80 pipeline steel coarse grain zone is bainite, for which there are 3 forms with the increase of cooling rate, namely non-carbon bainite, granular bainite and lath bainite. CGHAZ hardness and toughness prediction models were established, and they predict good agreement with the measured values.Based on welding heat transfer theory, heat elastoplasticity theory and phase transformation theory, the stress fields of Nb-Mo system X80 pipeline steel were simulated using SYSWELD software taking into mutual influence and nonlinear relationship between temperature and mechanical properties. The results show that, with the increase of heat input and preheat temperature, the peak value of the residual stress decreases. So high heat input and preheat temperature should better be used in order to reduce residual stress.Nb-Cr, Nb-Mo system X80 pipeline steel weld cold cracking sensitivity was studied through implant tests. The results shows that both steels have good resistance to cold cracking capacity, and To further enhance the crack insistence of Nb-Mo system X80 pipeline steel, diffusible hydrogen content should be controlled at a low level, and a low heat input with high preheating temperature or high heat input with low preheat temperature should be adopted. Through regression analysis, Nb-Mo system X80 pipeline steel weld cold cracking criteria formula of the critical stress were established, with which the cold cracking tendency of Nb-Mo X80 pipeline steel weld joints were predicted.