B distribution in steel was investigated using neutron imaging techniques, on a millimetric scale with a spatial resolution below 100 mu m. Steel structural components joined using the transient liquid-phase bonding (TLPB) process, with Fe-based B-alloyed foils as filler material, were observed. By means of neutron radiography and with the proposed calibration method, a quantitative profile of B concentration was attained across the joint, in particular in the region where the completion of the TLPB process was achieved. It is noteworthy that B concentration did not exceed the typical specified range of B-alloyed steels. This result explains why prior austenite grain boundaries are boride-free. To test the validity of the measured B concentrations, they were compared with those of the numerically simulated TLPB process. As a result, the calculated and experimentally measured values are in good agreement, within the estimated error. On the other hand, in regions where the TLPB process was not finished, athermally solidified liquid (ASL) was found at the joint, in the form of borides. In addition, neutron tomography enabled us to attain a three-dimensional visualization of the size and distribution of ASL, highlighting its potential as a non-destructive testing technique of TLPB weldments.