Fig. 1. (a) Optical micrographs of an as-welded sample (top left) and a comb-shaped sample (bottom left). The black diamond-shaped spots (top left) indicate the position where the neutron diffraction data were acquired, and the red and black dash rectangles (top left) show the position of the residual strain maps using NBEI. (b) Optical microscopy image of distinct FZ and HAZ regions of the top view of the as-welded sample. (c) Table for introducing the sample status and experimental arrangements.

Fig. 2. (a) A schematic of the neutron diffraction experiment setup and data acquisition position. Three through-thickness line-scans at different distances to beam incident surface were performed. The data was acquired at both filled- and unfilled-spots position for the as-welded sample, while only at filled-spot positions for the PWHT sample. The diffraction data was recorded by two detector banks, labelled ‘axial’ and ‘radial’, yielding the strains along TD and ND. In order to measure residual stress along LD, the sample was rotated 90° around ND, and three line-scans were performed at the same data acquisition position. (b) Lattice spacings were extracted by peak fitting. (c) Neutron Bragg edge imaging (NBEI) experiment setup. The red and black dashed rectangles illustrate the two mapped areas that cover the welding affected region. The two images on the left are the transmission data recorded and saved by the detector. (d) An example of Bragg edge fitting for the {110} crystal plane. The Bragg edge position is obtained as one of five fitting parameters: edge position, edge height, edge width, edge pedestal and edge asymmetry (equipment parameter).

Fig. 3. Microstructure of as-welded sample. (a) Microstructure at FZ, HAZ and BM regions acquired by SEM. The lath-like bainite, martensite and precipitate are labelled by B, M and P, respectively. (b) EBSD maps at the FZ, HAZ and BM regions. (c) Corresponding PF at the FZ, HAZ and BM regions, where the X direction is the TD, the Y direction the LD and the map normals are the ND.

Fig. 4. Microstructure of the PWHT sample. (a) Microstructure at FZ and HAZ regions after PWHT acquired by the SEM. The tempered martensite and precipitate are labelled as TM and P, respectively. (b) EBSD maps of PWHT sample at FZ and HAZ regions. (c) PF derived from relative EBSD maps at FZ and HAZ regions where the coordinate of X is TD, Y is LD, and the centre is ND.

Fig. 5. Residual stress of σxx, σyy and σzz components of as-welded sample derived from neutron diffraction by a single-peak fitting approach. (a) The through-thickness residual stress distribution extracted at three different distances to beam incident surface. (b) The residual stress on the segment plane.

Fig. 6. Residual stress of σxx, σyy and σzz components of the PWHT sample derived from neutron diffraction by single-peak analyses. (a) The through-thickness residual stress distribution extracted at three different distances to beam incident surface. (b) The residual stress on the segment plane.

Fig. 7. Residual strain maps and profiles of the as-welded sample measured by NBEI. (a1), (a2), (a3)and (a4) Strain maps along ND calculated from lattice spacings of {211} and {110} planes. (b1) and (b2) Comparison of residual strain distribution of {211} and {110} crystal planes which are measured by NBEI and neutron diffraction, respectively. (c1) and (c2) Comparison of Bragg edges of {211} and {110} crystal planes in the FZ and BM regions, respectively.

Fig. 8. Residual strain maps and strain profiles of the PWHT sample measured by NBEI. (a1), (a2), (a3)and (a4) Strain maps along ND (b1) and (b2) Comparison of residual strain distribution of {211} and {110} crystal planes which are measured by NBEI and neutron diffraction, respectively. (c1) and (c2) Comparison of Bragg edges of {211} and {110} crystal planes in the FZ and BM regions, respectively. The residual strain are plotted in the same way as in Fig. 7.

Fig. 9. (a) Micro-hardness distribution in the as-welded and PWHT samples correlated with the crystal size in the FZ, HAZ and BM. The micro-hardness at the teeth of comb-shaped reference sample is also included, which is labelled as as-welded d0 and PWHT d0. (b) and (c) Load-displacement curves derived from the FZ, HAZ and BM of the as-welded and PWHT samples, respectively. (d) The FWHM distritbution across the welding region of both as-welded and PWHT samples extracted at the thickness of 1.72 mm from the top surface.