| Recently,with the rapid development of energy conservation and environmental protection,marine engineering,petrochemical industry,chemical industry and other fields,the demand for high-end equipment is increasing.Generally,the service environment of these high-end equipment is very harsh,such as high temperature,high pressure and strong corrosive medium.This puts forward high requirements for the corrosion resistance and mechanical properties of the equipment materials.Super austenitic stainless steel(SASS)has excellent mechanical properties and corrosion resistance,and it becomes the key material for these high-end equipment.However,there are many problems in traditional fusion welding of SASS,such as the precipitation of ? and other harmful phases,N escape and loss,welding defects,serious deterioration of the mechanical properties and corrosion resistance of the weldments,and the short-life of the weldments.These welding problems seriously restrict the industrial application of SASS.As a solid phase joining technology,friction stir welding(FSW)does not undergo the transition from solid to liquid phase.Thus,it can effectively restrain precipitate formationand reduce N loss and welding defects,significantly improving the weldment quality(especially mechanical properties and corrosion resistance)and prolonging its service-life.At present,FSW is widely used in aluminum,magnesium alloy and low alloy stainless steels,but less in high alloy stainless steels.In particular,the research on FSW of high alloy SASS thick plate is basically blank.Therefore,this paper focuses on the research of double-sided FSW of SASS 654SMO thick plate.The influence of welding process on the microstructure and properties of weldments is studied systematically,which provides a new way to solve the welding problem of SASS and further promotes the industrialization of 654SMO.In this study,654SMO was friction stir welded by three kinds of double-sided welding processes(360 rpm-50 mm·min-1?360 rpm-50 mm·min-1-water-cooling and 360 rpm-100 mm·min-1).The weldments with excellent surface quality,uniform weld structure,no defect and N loss were obtained.After welded by the three welding processes,the structure of the front and back sides of the weld was distributed symmetrically,,and their characteristics were basically the same.Under the combined action of thermal cycle and strong mechanical agitation,the dynamic recrystallization and grain refinement took place in the stir zone.The order of grain size is as follows:BM>360-50-SZ>360-50-water cooling-SZ>360-100-SZ.Due to the wear of the welding tool,W-Re particles were incorporated into the stir zone to form banded structure.The degree of wear is 360-50>360-50-water cooling>360-100.In addition,a small amount of? phases was formed in the stir zone due to its high heat input.The number and size order of precipitates are 360-50>360-50-water cooling>360-100.The combination of grain refinement,high-density dislocation and substructure improved the hardness of weldments.The main strengthening mechanism of weldment was the precipitation strengthening caused by W-Re particles and intergranular precipitates,and there are also some fine grain strengthening and dislocation strengthening.Under the combined action of several strengthening mechanisms,the yield strength of weldment increased,while the tensile strength and plasticity decreased apparently.There are more concentrated W-Re wear particles in the 360-50 and 360-50-water-cooling stir zones,so the increase in yield strength and the decrease in tensile strength and plasticity were more apparent.W-Re particles were relatively small in the 360-100 stir zone.As a result,its strength was equivalent to that of the base metal,and its elongation was slightly reduced.Grain refinement and W-Re wear led to a significant decrease in the impact toughness,whose order is 360-100>360-50>360-50-water cooling.Pitting mainly occured at the interface between W-Re particles and matrix and extended to the inside of the W-Re particles.There was a positive correlation between the corrosion resistance and the total amount of wear particles in the stir zone.The order of pitting resistance is 360-100>360-50-water cooling>360-50.The formation of precipitates and wear particles in stir zone caused intergranular corrosion,and the order of intergranular corrosion resistance is 360-100>360-50-water cooling>360-50.Comprehensively consider the uniformity of the weld structure,the superiority of the mechanical properties and corrosion resistance,the optimal friction stir welding process for SASS 654SMO thick plate is:360 rpm-100 mm·min-1. |
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