Grain Boundary Engineering Of 304 Austenitic Stainless Steel By Laser Shock Assisted

Intergranular corrosion of 304 stainless steel can result in serious problems like the premature of components. Grain boundary engineering (GBE) based on thermomechanical processing (TMP) can improve the 304 intergranular corrosion resistance of austenitic stainless steel. The microstructure of the surface layer of 304 austenite stainless steel would be modified through laser shocking processing, which could introduce prestrain in the surface layer of 304 austenite stainless steel, followed by annealing treatment at a higher temperature. EBSD (Electron Back Scatter Diffraction), SEM (Scanning Electron Microscopy) and OM (Optical Microscopy) were used to investigate microstructure evolution of the surface layer after laser shocking processing and laser shocking combined with annealing treatment, electrochemical potentiokinetic reactivation (EPR) test and a H2SO4+Fe2(SO4)3 test were carried out to investigate effect of the processing on intergranular corrosion resistance, and the corrosion resistant mechanisms were analyzed as well. The following conclusions were obtained.(1) Strain could be introduced into the surface layer of 304 austenite stainless steel by laser shocking processing, which resulted in serious plastic deformation of the surface layer of the specimens, martensite phase transformation, and increase of hardness, and the thickness of the affected layer is about 100-150μm. However, laser shocking processing made the intergranular corrosion resistance of 304 austenite stainless steel decrease.(2) Laser shocking processing followed by annealing treatment could modify the microstructure, specially grain boundary character distribution (GBCD) in the surface layer of the specimens, which led to formation of a great amount of special CSL boundaries and increase of thefraction ofΣ3n related boundaries. The intergranualr corrosion resistance of the specimens was improved remarkably compared with the based materials.(3) Long time annealing was beneficial for formation of annealing twins. The corrosion resistance of the specimen annealed at 947℃for 15h was better than those of annealed for 10h and 2h at the same temperature, because the fraction ofΣ3n related special CSL boundaries accounted for about 82% of all CSL boundaries. It should be noted that the fraction of total CSL boundaries of the specimen annealed for 2h was the highest, however the proportion ofΣ3n related special CSL boundaries was lower than that of the 15h-annealing specimen, which resulted in the poor intergranular corrosion resistance. At the same annealing time and a given laser shocking power,947℃compared 927℃and 967℃ was the moderate temperature for improvement of intergranular corrosion resistance of austenite stainless steel. The experimental results indicated that a higher laser shocking power benefited for improvement of the intergranular corrosion resistance.