Microstructure And Mechanical Properties Of Friction Stir Welding Pipeline Steel

There are cold cracks,hot cracks,pores,inclusions and severe grain coarsening in welded joints in traditional fusion welding methods.A new type of solid state welding-friction stir welding was choosed to connect the pipeline steels in this paper.The tool rotation speed was controlled in 300-800 rpm,and welding speed in 100-200 mm/min to adjusting the welding heat input.Besides,three kinds of different cooling media(water,mixture of dry ice and water,dry ice)were applied to the 6mm thick X52 and X80 grade pipeline steels during the friction stir welding,and the microstructure and mechanical properties of the welded joints were studied.The main conclusions are as follows:(1)The welding heat input can be reduced by reducing the tool speed and increasing the welding speed,resulting in grain refinement of the prior austenite.The microstructure of X52 grade pipeline steel transformed from the coarse lath bainite and pearlite to the fine lath bainite with heat inputs decreasing.The welding peak temperature and high temperature staying time could be reduced by appling the cooling medium.In addition,the cooling rate after welding also changed significantly with the application of the cooling medium,which had a great influence on the final phase change structure.The martensite appeared in the tissue when it was colded by water,and undissolved ferrite appeared in the tissue when it was cooled by the dry ice.(2)The microstructure of X80 pipeline steel is prone to be lath bainite under the high heat input and to be granular bainite under the low heat input.With the decrease of cooling medium temperature,the structure of X80 pipeline steel transformed from lath martensite+lath bainite to lath martensite+undissolved ferrite.The shear textures of D1,D2 and F with different strength were observed in the stirring zone,and there were strong F texture and weak D1 and D2 texture in the advancing side,weak F texture and strong D1 and D2 texture in the retreating side.(3)The hardness in stirring zone of X52 pipeline steel welded joint was the highest under different welding parameters,followed by the hardness in inner heat affected zone and meddle heat affected zone,and the hardness inouter heat affected zone and base metal was the lowest.The hardness in stirring zone of welded joint increased with the decreasing of welding heat input,or with the decreasing of welding heat cycle staying time at high temperature.The impact energy of welded joints became worse,due to the presence of a large number of pearlites under 600-100 condition,however the impact energy reached 80%of the base metal under other air cooling conditions.The impact energy was further reduced under forced cooling.The strength coefficient of tensile specimens of welded joints reached 90%of the base metal,and the elongation decreased significantlycompared with the base metal under all welding parameters.The fracture location were all in the outer heataffected zone under air cooling conditions,and the fracture location were allin base metal under forced cooling.(4)The hardness in stirring zone of X80 pipeline steel welded joint was the highest under different welding parameters.The hardness of inner heat affected zone and meddle heat affected zone was lower than the stirring zone but higher than the base metal,and the hardness of external heat affected zone was the lowest.The hardness of stirring zone increased with the decreasing of welding heat input or with the decreasing of welding heat cycle staying time at high temperature.The impact energy in welded joints became worse due to the coarse granular bainite under 400-100 condition,however the impact energy reached 85%of the base metal under other welding conditions.Forced cooling was beneficial to improve the impact toughness of welded joints,and the impact energy is the highest under dry ice condition.Under all parameters,the strength coefficient of the welded joints exceeded 90%of the base metal,while the elongation decreased significantly compared with the base metal.The fracture location were all in the stiring zone under air cooling conditions,and the fracture location were all in base metal under forced cooling.