Study On X80 Pipeline Steel HAZ Grain Growth And Computer Simulation

With the delvelopment of economy, the demand for high-strength pipeline steelis increased.As the new generation of high strenth pipeline steel, the development andapplication of X80 pipeline steel has been thought much. Baced on phisicalmetallurgy and welding metallurgy,CGHAZ(coarse grain zone of heated affectedzone) is the most weakness area of weld joint and the grain growth of CGHAZ has agreat infuence on the mechanical property.In the paper, from the opinions ofmicrostructure simulations, the microstructure evolution in some process isresearched.Atfirst,considering influence of heat input,plate thickness and pre-temperature,a scheme about welding cycle based on linear orthogonal experiment is designed,andthe welding cycles of different welding conditions are measured.Using the measuredwelding cycles, by means of welding thermal simulation ,X80 pipeline steel CGHAZis simulated. The influences of different welding process and thermal cycles on themicrostructure evolution ,grain growth and second phase particles variation isresearched.The influence of microstructure evolution and grain growth on mechanicalproperty is also studied.The resultu shows that the microstructure of CGHAZ is mainly lath bainite andgranular bainite.The austenite grain size of CGHAZ is very fine, and manly in therange between 30μm and 40μm. The size distributing is belonged to normaldistribution approximately. X80 pipeline steel is the fine-grain-steel and the trend ofgrain growth is relatively small. The second phase particles in base metal is average47.84μm,the composition and shape of particles is greatly depended on the size.Afterthermal cycle of the welding, the particle numbers is decreased and the effects ofhindering grain growth is reducedThe research on the mechanical property shows that the range of variation of rigityis not wide and the toughness is influenced by many factors ,but the grain size is themost important. When the grain size is greater than 34μm,toughness is decreasedobviously with increment of grain size. When the grain size is less than 34μm,toughness increasing with increment of grain size is because that toughness is affectedby combined influence of grain size ,granular bainite and second phase particle.The division of lath bainite by granular bainite facilitates the improvement oftoughness.According to measured welding thermal cycle of different welding conditions,thethermal cycle eqution is established. Using welding heat simulation datas,the dynamiceqution of CGHAZ grain growth of X80 high strenth pipeline steel material areestablished.The result of dymamic eqution is fit to the experiment very well.Except using the welding thermal simulation, we using the methods of computersimulations to reappear the process of the grain growth.In this paper ,by usingMATLAB language ,the auther make the program of MC simulation of microstructureevolution with the Voronoi initial lattice and the Radhakrishnan growth method.Theprogram can simulate grain evolution at different time. A model is provided to buildthe relationship of between the Mento Carlo simulation time and the real time used inprocess of welding by using welding thermal simulation experiment data. The kineticsof grain growth in HAZ is simulated very well using mento carlo method.