Study On The Microstructures And Properties Of HAZ In 00Cr12 Ferrite Stainless Steel

00Cr12 ferrite stainless steel is widely used in industries due to its high strength and ductility,good corrosion and wear resistance,and low cost.It is one of commercial stainless steels employed on railway vehicles.Although the material weldability of 00Cr12 is good,that is, normolly no welding cracks occur in welding,the grain size in HAZ could rapidly grow up if the chemical composition is not properly controlled, even if the changes are within the standard,which will induce the great decrease of toughness in HAZ.Therefore,the improvement of toughness of HAZ in 00Cr12 ferrite stainless steel was studied in this thesis.The effects of chemical composition of base material and welding process on the microstuctures and toughness of heat affected zone(HAZ) of weldment were studied in detail.Optical microscope(OM),scanning electron microscope(SEM),impact test,physical simulation by Gleeble Simulator and fatigue test were performed in order to analyze the relationship between microstructures and mechanical properties, especially the toughness.Several critical factors related to the HAZ toughness of welds were investigated.Methods of controlling the grain size in HAZ were suggested.The optimization of both base materials and advanced welding process was studied theoretically and experimentally.The results are shown as following:1.The chemical composition and Cr/Ni equivalent of 00Cr12 ferrite stainless steel were critical to the mechanical properties(impact toughness) of HAZ.The optimization of chemical composition of base material should induce a sound microstructure in HAZ,which were austenite+ferrite at high temperature,and few ferrite islands+low carbon martensite matrix in room temperature.The addition of Nb and Ti led to a smaller grain size,which can improve both strength and toughness of HAZ.2.Compared to the normal MAG welding process,Laser-MAG hybrid welding can suppress the coarsen tendency of grain size in HAZ, decrease the width of HAZ and hence improve impact toughness of HAZ at low temperature.3.Carbide particles tended to aggregate and grow larger in the range of 500℃-800℃and dissolve above 900℃.Equiaxed grains can be found when the temperature was above 800℃.The grain size increased with the increase of temperature,especially in the range of 900℃-1100℃.The transformation from ferrite to austenite happened if temperature was 900℃-1000℃.The hardness increased significantly when the temperature was above 1100℃due to the formation of austenite which will transferr into martensite during cooling time.4.Compared to normal MAG welding,Laser-MAG hybrid welding can short the high temperature range in thermal cycle,which was the main reason of finer grain in HAZ(Laser-MAG:t₉₀₀=1.6s,MAG:t₉₀₀=5.6s).5.The fatigue tests indicated that the fatigue limit of Laser-MAG hybrid welding was 153 MPa and 131 MPa of MAG welding.The improvement of fatigue was 17%,which shown Laser-MAG hybrid welding is a good welding process for the reliability and life of railway vehicles.