| Duplex stainless steel realizes the excellent combination of corrosion resistance,mechanical properties and welding properties by virtue of the coexistence of austenite and ferrite.However,the fatigue failure caused by frequent start-stop of equipment and fluctuation of working pressure still seriously threatens the strength safety of duplex stainless steel welded joints.Moreover,complex thermal cycle and high alloy element content of material lead to a serious non-uniform microstructure morphology of the welded joint,which has a great influence on the fatigue performance.Therefore,it is of great significance for engineering practice to study the microstructure forming mechanism of duplex stainless steel welded joint and its influence on fatigue properties.In this paper,the gas tungsten arc welding(GTAW)thick plate welding and electric resistance welding(ERW)thin plate welding are studied respectively for SAF2205 duplex stainless steel.The microstructure characteristics,forming mechanism and fatigue properties of two kinds of welded joints before and after heat treatment are studied.Then,the relationship between fatigue fracture behavior and microstructure of the joints is established.The main conclusions are as follows:(1)The heterogeneous microstructure of duplex stainless steel GTAW and ERW welded joints were revealed.The content of ferrite in the heat affected zone of the GTAW welded joint is higher than 62%,and the austenite is mainly composed of elongated grain boundary austenite and tiny Widmanstatten austenite.Contrary to the heat-affected zone,the austenite content of the weld exceeds the standard,reaching 71%,due to the existence of a large number of Intracrystalline austenite.There is a serious phenomenon of excessive ferrite,and the local ferrite content can reach 72%.The grain is refined at the weld material.Meanwhile,a large number of high-angle grain boundaries are retained in austenite phase,and the large stacking fault in ferrite phase can lead a large number of high-angle grain boundaries to transform into into low-angle grain boundaries.(2)Revealing the fatigue properties of two kinds of duplex stainless steel welded joints:for GTAW welded joints,the fatigue life is much lower than base material under the same fatigue amplitude due to the connect pieces of austenite which reduces the fatigue life greatly.At the same time,it further explains the reason for the difference in life from the perspective of fracture morphology;For ERW welded joints,the fatigue life gradually decreases with the increase of stress level and has an obvious turning point at 0.7 k N.Firstly,the austenite connected into strips at the weld edge greatly promotes the initiation and propagation of fatigue cracks,which is the dominant factor of fatigue fracture.Secondly,the large plastic deformation and high angle grain boundaries in the welded joints also provides more initiation points for fatigue cracks,which weakens the fatigue performance to a certain extent.(3)Study on the effect of post-weld heat treatment on microstructure and fatigue properties of two kinds of duplex stainless steel welded joints.After heat treatment,the grain size of GTAW is refined and the amount of austenite at the slender grain boundary is reduced,but the austenite content is increased and the phase disproportion is more serious,reaching82%.Meanwhile,the fatigue life is significantly reduced after heat treatment.Therefore,post-weld heat treatment is not suitable for GTAW welded joints.After heat treatment of ERW welded joint,the phenomenon of excessive ferrite in each area is alleviated,the microstructure distribution is more uniform,and the phase proportion is basically balanced.At the same time,the deformation of the two phases is more coordinated in the fatigue process.The plastic deformation is improved,and the resistance of crack initiation is increased,so the fatigue life is greatly increased.Therefore,post-weld heat treatment is recommended for ERW welded joints. |
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