Fig. 1 Schematic illustrations of a a welded plate of S355J2W steel, b a cylinder dog-bone-shaped fatigue sample cut from a, c the SMRT process. The red dot line on b marks the position for surface hardness measurements in Fig. 6. WM and HAZ denote weld metal and heat-affected zone, respectively (unit: mm)

Fig. 2 Typical SEM morphologies of different regions (as indicated in Fig. 1) in the as-welded S355J2W steel plate: a BM, b HAZ, c WM

Fig. 3 Cross-sectional SEM morphologies of different regions in the SMRT sample: a BM, b HAZ, c WM

Fig. 4 Typical TEM images of the topmost surface layers of different regions in the SMRT sample: a, b BM, c, d HAZ, e, f WM. a, c, e are bright-field TEM images with corresponding SAED patterns inserted. b, d, f are dark-field TEM images taken from the (110) diffraction of ferrite (the first ring of the SAED pattern) in a, c, e, respectively

Fig. 5 Variations of microhardness with the distance from the treated surface for different regions in the SMRT sample: a BM, b HAZ, c WM. Measurements were taken on the surface layer from a cross-sectional view. Each point denotes an independent measurement

Fig. 6 Distributions of surface hardness along the distance from the welded-joint center on the SMRT and as-welded samples. Measurements were taken on the sample surface from a planar view, along the red dot line marked in Fig. 1b. Each point was averaged from at least 3 measurements

Fig. 7 S-N curves of the SMRT and as-welded samples. Open symbols (with arrows) indicate tests terminated without fracture after 2 × 106 cycles

Fig. 8 SEM morphologies of fracture surface of a the SMRT sample fatigued at 320 MPa, b the as-welded sample fatigued at 280 MPa. The regions of crack initiation, crack propagation, and instant rupture are marked by A, B, and C, respectively