Microstructure And Properties Of Friction Stir Welded High Nitrogen Martensitic Stainless Steel

High nitrogen martensitic stainless steels,which have broad application prospects,exhibit a favorable combination of excellent mechanical and corrosion resistance properties.In the process of conventional fusion welding,it is inevitable that several welding defects happen and damage the joint's mechanical and corrosion resistance,which deteriorate the quality of the whole joint and restrict the development and engineering application of high nitrogen martensitic stainless steel.In order to avoid the welding defects,Friction Stir Welding(FSW),as an innovative solid-state welding technology,has potential advantages for the welding of high nitrogen martensitic stainless steel,as the heat generated in FSW is not enough to melt the material and just softens it into solid state,which decreases the possibility of nitrogen desorption,hot cracking in fusing zones and nitride precipitation in heat-affected zone However,significant microstructure and properties gradients are observed of as-welded friction stir welded joints,which results in the transition zone between the stir zone and base metal always be the weakest part.Therefore,for weakening these gradients effectively,it is necessary to employ post-weld heat treatment on the jointIn this article,a high nitrogen martensitic stainless steel of 30Cr15Mo1N is friction stir welded with two different welding speeds(50 mm/min and 100 mm/min)and fixed rotating speed(300 rpm).In order to investigate the microstructure evolution,optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)and electron backscatter(EBSD)was used.Besides,the mechanical properties of weldments were evaluated by hardness tests and the corrosion resistance of the welds were determined by electrochemical impedance spectroscopy(EIS)and anodic polarization curve,respectively.The effect of microstructure evolution on the mechanical properties and corrosion resistance is also studied to explore the influence of welding speed on the mechanical and corrosion resistance of friction stir processed high nitrogen martensitic stainless steel.Subsequently,the weldments were oil quenching at 1000? for 40 min,deep cryogenic treatment twice at-80? for 2 h and tempering twice at 450? for 2 h.Then the microstructure evolution of the weldments after heat treatment was well studied using optical microscopy(OM),scanning electron microscopy(SEM)and X-ray diffraction(XRD).The weldments after heat treatment were subsequently undergo hardness tests and electrochemical experiments to evaluate the mechanical properties and the corrosion resistance,respectively.The effect of post-weld heat treatment on the mechanical properties and corrosion resistance of weldment was also well studied.The main conclusions are as follows:The friction stir welds of high nitrogen martensitic stainless steels at different welding speeds have little amount of deformation and no obvious surface defects such as grooves,holes are observed.The nitrogen content tests show that the FSW processed high nitrogen martensitic stainless steels did not exhibit nitrogen loss.The cross-section of each weld under two welding speeds can be divided into four areas:base metal(BM),the heat-affected zone(HAZ),the thermal-mechanical affected zone(TMAZ)and stir zone(SZ).The microstructure of BM is dominated by lath martensitic with some granular precipitates;less precipitates are found in SZ whatever in the top or bottom and the grain size in SZ-top is larger than that in SZ-bottom;as a result of the specific metal flow and stress distribution,the microstructure on the both sides of stir zone turns to be asymmetry:(1)it is clearly seen that there is a distinct boundary on the TMAZ/SZ-AS,while no obvious border on TMAZ/SZ-RS,(2)the TMAZ-AS is wider than TMAZ-RS;some white bands can also be found in the vicinity of the boundary on TMAZ/SZ-AS and the EDS analysis demonstrates that these bands contain W and Re elements which formed by the tool wear owing to high friction load between weldments and tool.The boundary on the TMAZ/SZ-AS at 100 mm/min is more distinct than that at 50 mm/min.The whole TMAZ at 100 mm/min is wider than that at 50 mm/min.While the whole HAZ at 100 mm/min is narrower than that at 50 mm/min.TEM observation showed that BM has low dislocation density and many precipitates;low dislocation density dominates the SZs at different welding speeds and no obvious precipitates could be observed in these areas.Based on the results of XRD and EBSD,in the process of FSW,austenitic transformation was occurred and the smaller the welding speed is,the higher the peak temperature is,the larger the fraction of austenite is.According to the EBSD results,due to more heat generated at the welding speed of 50 mm/min than 100 mm/min,which makes the grain size of former is larger than that of latter;after FSW,compared to the BM,SZ exhibits lower fraction of ?3 twin boundaries and higher fraction of LABs,which ascribes to the intense deformation,associated with friction stir process breaks up the initial microstructure of BM,which is an evidence that the SZ experiencing intense deformation and dynamic recrystallization.The hardness in BM is much higher than that in SZ,and a sudden change takes place in TMAZ/SZ on AS.On both sides of HAZ,the hardness value climbs considerably to that of the BM,which results from the elevated volume fraction of martensitic caused by temperature history.In the horizontal direction,the higher welding speed is,the narrower HAZ is,the narrower hardness-drop zone is;in the vertical direction,the higher welding speed is,the higher microhardness is.Potentiodynamic polarization curves and electrochemical impedance spectroscopy were employed to study the corrosion resistance of BM,SZ-top and SZ-bottom.The corrosion resistance of SZ is superior to that of BM and SZ-bottom exhibits better corrosion resistance compared to that of SZ-top.The corrosion resistance of SZ-top and-bottom of 50 mm/min is better than that of 100 mm/min,respectively.The smaller the welding speed is,the better the corrosion resistance is.According to the macro morphology,the joint after heat treatment does not exist divided zones and the original white profiles produced by etched is replaced by brown.However,tracks of tool wear still could also be found in the vicinity of the boundary on TMAZ/SZ-AS.SZ is dominated by martensitic structure.Under the observation of high power,SZ-top possesses more precipitates than SZ-bottom has.The boundary on the TMAZ/SZ-AS is more distinct at welding speed of 50 mm/min than that at 100 mm/min.After heat treatment,no matter in the horizontal or vertical direction,the hardness value of FSWed steel joint increases to the level of BM hardness.However,the previous sudden in TMAZ/SZ on AS disappears and the hardness gradients between SZ-top and SZ-bottom are fundamentally eliminated.The hardness level of the whole joint turns to undifferentiated.Corrosion resistance of SZ-top and SZ-bottom after heat treatment is slightly inferior to that of SZ-top and SZ-bottom without heat treatment,respectively,while still superior than that of BM.Corrosion resistance of joints at different welding speeds after heat treatment is similar.