| X120 pipeline steel with broad market prospects will be widely used in pipeline construction in the future.However,it is very difficult to meet the requirements of microstructure and mechanical properties using general arc welding.Compared with the traditional arc welding,the hybrid of laser-arc welding has many advantages such as high heat energy density,low heat input,small heat affected zone,small welding residual deformation and residual stress,and excellent mechanical properties.Therefore,it is a very urgent task to develop a new welding technology and manufacturing process with high quality and efficiency.Based on the characteristics of hybrid of laser-arc welding heat source,the hybrid welding heat source model with double-ellipsoid heat source and conical body heat source was established to simulate temperature field and stress field.The temperature field simulation results showed that the heating speed was fast,the high temperature halt time was short,and the hardening tendency was serious in laser-arc hybrid welding.The stress field simulation results showed that the transverse stress was less than the yield strength,and the longitudinal residual stress exceeded the yield strength of the material.The transverse residual stress of U-shaped groove was slightly larger than that of the V-shaped groove with rectangular shape,but the longitudinal residual stress value of two grooves has little difference.Therefore,in order to reduce the hardening tendency of the heat affected zone and the residual stress of the welded joint,it is necessary to preheat before welding and cool slowly after welding.The CCT curve based on conventional heat treatment and SH-CCT curve based on laser-arc hybrid welding were established respectively by thermal simulation equipment.The results showed that for the transformation lines of Ac1 and Ac3 of SH-CCT curve increased due to rapid heating,and there are globular precipitates due to shorten halt time of high temperature,which provided the core for the subsequent bainite transformation.In addition,due to the base metal with low carbon content and the low homogenization of the austenite composition,the Ms point of martensite transformation was increased so that the range of the martensitic transformation temperature was enlarged.From Ms point to room temperature,slow cooling can not reduce the number of martensite,but it can improve the strength and toughness of the microstructure by increasing “self tempering” time of martensite so as to reduce hardened tendency.In order to investigate the effect of groove types on laser arc hybrid welding process,the U-shaped groove and the V-shaped groove with rectangular were designed.The effects of preheating temperature on the microstructure and mechanical properties of welded joints were analyzed.The results indicated that the V-shaped groove with rectangular shape had the characteristics with small amount of filling metal and high welding efficiency.However,the window of welding process parameters was narrow,and the incomplete fusion and pores often easily occured.The U-shaped groove had a good process adaptability of hybrid of laser-arc welding,and the window of welding process parameters was wide.Moreover,it was easy to eliminate the defects such as pores,incomplete fusion and so on,which resulted in welded joint with good internal quality.With the increase of the preheating temperature,the microstructure of the fusion zone and the heat affected zone changes from martensite to granular bainite,and the microhardness of the welded joint is decreased.When the preheating temperature is 160 ?,the weld metal had the highest impact toughness of 182 J.The tensile strength and yield strength of the welded joint are nearly equivalent to that of the base metal.However,the elongation of the welded joint is slightly smaller than that of base metal.The bending performance is up to standard.The welded joint obtained can meet the requirements of welding quality. |
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