Effect of microstructures on notch fatigue properties in ultra-high strength steel sheet welded joint

•Notched fatigue properties of different regions in welded joints were investigated.•Microstructure effect was clearly prominent when notch root radius r>0.2mm(stressconcentrationfactorKt≤2.24).•Fatigue strength of softened fine grain heat affected zone (FGHAZ) was highest.•Grain boundaries of martensite laths seems to enhance crack initiation. The fatigue properties of different regions, namely, the weld metal (WM), the fusion line (FL) and the fine grain heat affected zone (FGHAZ), in welded joints with different surface notch root radii r=0.1,0.2and0.3mm were investigated by using welded joints of 980 MPa grade ultra-high strength steel sheets with a thickness of 2.3 mm. The fatigue strength of the FGHAZ was higher than those of the WM and FL despite the lower hardness of the FGHAZ, while the effect of the microstructure became prominent at r≥0.2mm, where the stress concentration factor Kt≤2.24. The strain gauge measurement and FE analysis imply that the effect of the microstructure arose only within 50 μm from the surface of the notch. In addition, the critical distance approach was applied to evaluate the relationship between the effective stress range Δσeff and the number of cycles to faliure Nf. As a result, the critical distance L of WM, FL and FGHAZ were 37.2, 59.7 and 26.7 μm, respectively. The fracture surface and cross-secctional observation suggested that fatigue cracks of WM and FL initiated at twin or prior austenite grain boundaries, then propagated along martensite laths in the crystallographic propagation stage (Stage I). In contrast, the fatigue cracks of FGHAZ initiated inside ferrite grains, and then the fine grain boundaries and martensite-austenite of the FGHAZ blocked and branched the cracks. These results can explain the fatigue strength reversal phenomenon with respect to the hardness of the FGHAZ.