Study On Microstructure And Mechanical Properties Of Welded Joint Of L485 High Strain Pipeline Steel

L485 high-strain pipeline steel is mainly used in complex geological environments such as submarine and polar regions,which can effectively resist the plastic deformation of pipelines during geological disasters.However,the formation of heataffected zone(HAZ)during welding process will lead to the degradation of performance in local area.And strain aging will change its mechanical properties during the course of service.In this paper,the microstructure transformation and mechanical properties of HAZ under different welding thermal cycle were explored with the aid of welding thermal simulation technology.In addition,the effect of strain aging on the microstructure and properties of girth welds of L485 high-strain marine pipeline steel was explored.The results show that the microstructure of coarse grain heat-affected zone(CGHAZ)is mainly composed of lath bainite(LB)and granular bainite(GB),and the microstructure of inter-critical heat-affected zone(ICHAZ)is related to the austenite transformation degree.With the increase of austenite transformation degree,the tempered structure decreases continuously and the proportion of new phase increases,which alters the effective grain size.After the second thermal cycle,the prior austenite grain boundaries(PAGBs)in intercritically reheated CGHAZ(ICCGHAZ)are annihilated.The untransformed lath coarsens and the new phase leads to grain refinement.The experimental results of crack tip opening displacement(CTOD)show that both CGHAZ and ICCGHAZ are local embrittlement zones.The fracture toughness of ICHAZ is related to austenite transformation degree.In ICHAZ,the fracture toughness of 20%-austenitized ICHAZ is the best(0.777 mm),and that of 100%-austenitized ICHAZ is the worst(0.467 mm).It is found that the coarse LB and GB with M-A constituent is the main reason for the embrittlement of CGHAZ.ICCGHAZ still presents brittle fracture characteristics under the combined action of grain refinement and tempering coarsening of lath,but the fracture toughness of ICCGHAZ is increased by about 19% compared with CGHAZ.In ICHAZ,the linearly aligned M-A constituents in 0%-austenitized ICHAZ and the chain-distributed ones in 100%-austenitized ICHAZ can distinctly worsen the fracture toughness.The excellent fracture toughness of partially austenitized ICHAZ was related to the fine grains,high density of high-angle grain boundaries(HAGBs),homogeneous distribution of local strain as well as the high percentage of small-deformed grains(SDGs).After strain aging,the microstructure of welded joints did not change obviously.However,the softening phenomenon in HAZ disappeared and the fracture toughness showed a downward trend.When the nominal strain degree is 0.5% and 1.0%,compared with the original state,the fracture toughness of weld metal decreases by12.6% and 25.9%,respectively;the fracture toughness of HAZ decreased by 15.2% and56.4% respectively.It was found that the increase of dislocation density,strain concentration as well as the geometrically necessary dislocation density(GNDs)are the main reasons for the decrease of plastic deformability and the deterioration of fracture toughness of welded joints.