Study On Microstructure And Properties Of Welded Joints Of443Ferritic Stainless Steel Along With Trailing Intense Cooling

As a new ferrite stainless steel,443ferritic stainless steel was developed in recent years which have ultra-low carbon content and high chrome content, due to its excellent performance, it was regarded as the substitute of the304stainless steel in future. But compared with the austenitic stainless steel, the grains of the heat-affected zone were grown up,organization structure and properties become poor,which become the most serious problem in the welding structure,especially the impact property was poor, and these disadvantages become more obvious with the increase of the thickness, these disadvantages seriously affected the applications of443ferrite stainless steel in engineering structure field,which was an important technical difficulty at the present;In this paper,443ferritic stainless steel were welded along with air cold(ANC)、water cooling(TWC) and liquid nitrogen cooling (LNC) respectively,the thickness of the443ferritic stainless steel is4mm;the microstructure、phase composition、mechanical properties and the corrosion resistance of ANC、TWC and LNC welding joints were analyzed,and the impact fracture morphology of the different cooling conditions were observed;The results of microstructure show that the microstructure of the base metal was polygon ferrite,the grain size was about levels5;the seam organization of ANC、TWC and LNC welded joints were consisted of the duplex organization,containted austenitic and ferrite,and the acicular ferrite was formed in welding seams,the grains in the heat-affected zone were refined along with the trailing intense cooling,the level of the grain size was3～4;Microhardness results show that the average hardness of the base metal was167.9HV0.1,the hardness of ANC、TWC and LNC welding joints have declined from welding seam to base metal,the highest hardness appears at the weld center, the hardness is above200HV0.1,and the hardness of heat affected zone is rosen compared with base metal,its hardness is about185-200HV0.1;Tensile tests show the tensile strength of the welding joint using ANC method、TWC method and LNC method up to above95%of the base metal,the elongation reached77.6%、84.5%and87.2%of the base metal respectively; bending test indicated that the bending angle of the welded joints using ANC method、TWC method and LNC method and the base metal all can reach180°, whereas there is no crack on weld surface of the specimens;The notched impact tests show that the impact energy of the base metal and the weld using ANC method、TWC method、LNC method all can reach more than30J, impact energy of the heat affected zone using ANC method only reach4.867J, but impact energy of the heat affected zone increased compared with the ANC welding joint using TWC method and LNC method, which reach16.5J and17.3J respectively; SEM morphology of impact fracture surface shows that the heat-affected zone using ANC method had characteristics of the eleavage fracture,SEM morphology of the impact fracture of the heat affected zone using TWC method and LNC method show characteristics of the eleavage fracture and the ductile fracture simultaneously,and the base metal shows the characteristic of the ductile fracture obviously;Intergranular corrosion test shows the surface of the welding joints using ANC method、TWC method、LNC method and the base metal didn’t change greatly after corrosion test,through the bending test evaluation showed that the bending angle all can reach180°,whereas there was no crack on weld surface of the specimens which was observed under the magnifying glass;Electrochemical corrosion test shows the the corrosion potential Ecorrof the welding joints of the ANC method、TWC method、LNC method and the base metal is-0.999V、0.909V、0.803V and0.770V respectively,corrosion current density icon is7.08×10-6A·cm-2、2.51×10-6A·cm-2、0.63×10-6A·cm-2and0.5×10-6A·cm-2respectively;NSS test shows the corrosion extent of the welding specimen using LNC method and the base metal is level D,TWC welding specimen is grade E,ANC specimen is level F,the corrosion rate of the welding joints of the ANC method、 TWC method、LNC method and the base metal is1.41×10-8g·mm-2·h-1、0.635×10-8g·mm-2·h-1、0.52×10-8g·mm-2·h-1and0.27×10-8g·mm-2·h-1respectively after corrosion test480h; Based on the obove,the microstructure and the properties of the LNC welding joint was markedly improved, especially the impact energy of the heat affected zone was up to twice nearly than the ANC method, this is due to the liquid nitrogen as the cooling medium,the peak temperature was reduced and the holding time of the high temperature was shortened, and that inhibit the grain growth at certain extent, so the properties of the welding joint was improved.