Analysis of crack tip transformation zone in austenitic stainless steel laser-MIG hybrid welded joint

Abstract

In this study, fatigue crack growth rates in weld center and Heat Affected Zone (HAZ) of laser-MIG hybrid welded joint are compared using standard compact tension (CT) specimens. Electron backscatter diffraction (EBSD) is applied to analyze crack tip transformation zone by characterizing phases, plastic strains and crystallographic orientations. This study provides interesting insights into the growth of fatigue cracks in austenite steel welded joint at micro-scale. Analysis of the plastic strain and Schmid factor indicates that mechanical energy provided by external load transforms into internal crystal energy firstly, which is stored within grains in the form of strain energy, then partly relieves to provide mechanical driving force for martensite transformation. With plastic strain accumulating within individual grain, dislocations begin to glide on slip system resulting in formation of persistent slip bands (PSB), then metastable austenite grains transform into martensite. Thus, strain-induced martensite transformation increases resistance to crack growth by dissipating energy and crack closure effect. Different crack growth rate in weld center and HAZ is attributed to variant volume of strain-induced martensite. Shielding effect produced by neighboring harder ferrite decreases martensite in weld center, as confirmed by nanoidentation and Atomic Force Microscope (AFM). Additionally, martensite phase is found to have a Kurdjumov-Sachs (K-S) orientation relationship with austenite phase according to Inverse Pole Figure (IPF) and Pole Figures (PF) derived by EBSD.