Evolution of microstructure and mechanical properties of Fe-24Ni-0.3C TRIP steel during friction stir processing

In this study, the effect of friction stir processing (FSP) by various transverse welding speeds of 50-150 mm/min,on mechanical properties and microstructure of Fe-24Ni-0.3C TRIP steel was investigated. The results showed that defectless FSPed specimen was obtained at the transverse speed of 100 mm/min. Microstructural analysis was carried out by scanning electron microscope equipped with electron backscattered diffraction (EBSD) detector. It was found that by increasing the transverse speed, more significant grain refinement was observed in the stir zone. Furthermore, deformation induced martensite was not observed in the area subjected to FSP. Significant grain refinement resulted from dynamic recrystallization was found to be responsible for hindering formation of deformation induced martensite in the weldment section after tensile deformation test. Tensile test results showed that yield strength (YS) was increased from 145 MPa to 210 MPa in the FSPed specimens. However, tensile elongation was restricted in the FSP processed specimens which may be due to limitation of TRIP phenomenon.