Study On Weldability Of12CrNi Ferritic Stainless Steel

Low temperature toughness of12CrNi stainless steel developed recent years is poor after welding. In order to improve the weldability of12CrNi stainless steel, in this paper, metallography behavior during welding of12CrNi stainless steel was researched, and effect of chemical elements on the microstructure, properties and weldability of12CrNi stainless steel was also investigated. According to the results, T4003sainless steel with excellent weld toughness was developed, and fabrication and service weldability of T4003stainless steel was investigated. The chief original work of the present paper is as follows:(1) Microstructure transformation, precipitating phases and grain growth of12CrNi stainless steel during welding were investigated. The a ferrite of12CrNi stainless steel in coarse grain heat-affected zone (CGHAZ) transfers to y austenite and then transfers to8ferrite during the heating process of welding thermal cycle. The microstructure in CGHAZ at very high temperature is fully8ferrite. During the cooling process of welding thermal cycle,8ferrite transfers to y austenite and then transfers to martensite. Grain edges or boundaries crystal defects are the preferential location for the nucleations of αâ†'y and δâ†'y phase transformations. Grain growth of8ferrite grain is effectively inhibited by the existing autensite in high temperature. When12CrNi stainless steel was heated up to fully8ferrite zone, grains dramatically coarsened as a result of disappearance of austenite grains and finely particle dissolution.(2) Ferrite factor (FF) is the chief factor on the width and microstructure in CGHAZ of12CrNi stainless steel. When FF is above9.0, the microstructure at CGHAZ is fully ferrite or a small amount of martensite net likely distributing along delta ferrite grain boundaries. On the other hand, if FF is lower than9.0, the percentage of martensite increases with the decreasing of FF. Width in CGHAZ is significantly influenced by FF. Width in CGHAZ is determined by δâ†'γ tranformatino point A5. The relationship between A5and FF follows the equation As=1536.7-30.397FF. With the increase in FF, A5point decreases, so the fully ferrite phase zone enlarges; therefore the width of CGHAZ increases.(3) Ultra low carbon martensite in the steel containing0.004wt.%C is characterized by dislocation cells substructure. The formation of dislocation cells is attributed to high martensite finishing point (above400℃) and low interstitial atoms content. Carbon content apparently influences on weldability of12CrNi stainless steel. First, carbon is a strong austenite forming element inducing the expansion of austenite loop, with the increase in carbon content, FF decreases and then the grain size and width in CGHAZ decrease. On the other hand, increase in carbon content is harmful to the impact property of martensite in heat-affected zone. Therefore, with the decrease in carbon content when FF is in a low level, the impact toughness of martensite in HAZ increases, and the impact toughness of12CrNi stainless steel in welded joint increases.(4) Delta ferrite deteriorates the impact toughness of12CrNi martensite stainless steel. In order to improve the toughness of12CrNi martensite stainless steel, soaking temperature before hot rolling should be at fully austenite zone.(5) Addition of Nb in12CrNi stainless steel inhibits the ferrite grain recrystallation. So the strength of12CrNi stainless steels after annealing is respectively high, but their plasticity and toughness decrease. The precipated phases in annealed steel containing0.28%Nb are and Nb(C,N), but the precipated phases in annealed steel containing0.49%Nb are and Nb(C,N).12CrNi stainless steel containing high Nb after annealing occurs laves embrittlement. The inhabitation of recrystallation decreases the effective grain size; therefore, the ductile brittle transition temperature (DBTT) decreases. Various martensite morphologies of martensite have been observed in CGHAZ of12CrNi stainless steel with Nb stabilization:(1) grain boundary allotriomorphs,(2) Widmanstatten structures developed from allotriomorphs,(3) secondary sawteeth nucleated and grew from allotriomorphs. The martensite net likely distributes along delta ferrite grain boundaries.(6) Grain size in CGHAZ was refined as a result of Ti addition. TiN particles can promote intragranular austenite formation inside delta ferrite. Therefore, the primary austenite grain will be refined by TiN particles. TiN particles can also promote martensite nucleation. In conclusion, the addition of Ti in12CrNi stainless steel refines the grain size in CGHAZ and minishs the width of CGHAZ, so the impact toughness in welded joint will be improved.(7) Intergranular corrosion (IGC) was only observed in finely grain heat-affected zone (FGHAZ) adjacent to base metals at the back of the welded joints in unstabilized steel. On the Other hand, IGC was not observed in the welded joints of stabilized steels. IGC is attributed to both the precipitated phase along the grain boundaries inducing the presence of chromium-depleted zone and the welding residual tensile stress promoting the corrosion.(8) The impact energies of welded joint of T4003stainless steel welded using ER316L are higher than the welded joint which welded using ER308L. Its reason is the impact energies in fusion-line (FL) and HAZ could be inproved by welded metal, and the impact energy of ER316L welded metal (WM) is higher than the value of ER308L welded metal. The impact energies of welded joint at a heat input of1.0kJ/mm in fusion-line and HAZ at-20℃and-40℃are below10J. Heat input would be controlled not above0.8kJ/mm so that the welded joint of T4003stainless steel has good low temperature toughness. The impact energy of welded joint of T4003stianless steel is nonsensitive to groove angle.(9) The PWHT of low and high tempering could improve the impact energy of welded joint. The welded joint of T4003stainless steel after tempering at450℃for2hours was embrittled. This embrittlement belongs to high temper embrittlement. The Cr, Ni and Mn contents in T4003stainless steels is repectivly high, so their welded joint is sensitive to this embrittlement. Accordingly, The heat treatment of T4003stainless steel should avoid middle temper temperature range, and the impact toughness of welded joint of T4003stainless steel can be improved through low tempering or high tempering above650℃.Acccording to above results, Baosteel has developed low carbon12CrNi stainless steels designated T4003for railway wagons with excellent weldability and acquired the authentication of Railway Ministry. These steels have been applied in the fabrication of C80B open wagons.