The characteristics of phase constitution near the interface of Fe3Al/Q235 diffusion bonding are researched by means of scanning electron microscope, X-ray diffraction and transmission electron microscope, etc. The test results indicated that an obvious diffusion transition zone forms near the Fe3Al/Q235 interface as a result of vacuum diffusion bonding (heating temperature 1050-1080℃, holding time 60min and pressure 9.8MPa). The maximum value of the Al content 13 in the transition zone was 16.6wt.% (about 28.5 at.%). The micro-hardness in the diffusion transition zone was HM 14 200-400. The transition zone consists of Fe3Al and a-Fe(Al) solid solution. There was no brittle phase of high hardness 15 near the diffusion interface. This is favorable to the enhancing of the toughness of Fe3Al/Q235 diffusion joint.

The distribution of hydrogen in the welding zone of HQ130 high strength steel is calculated by using finite element method (FEM). The finite element program of hydrogen in the welding zone is worked out. In the program, the effects of welding heat input (q/v), temperature and surface-escaping coefficient of hydrogen, etc., are taken into account. Crack morphology in the fusion zone and effect of diffusion hydrogen on cracks are analyzed. Cracks originated in the partially melted zone at about 20-60μm from the melting interface line and propagated parallel to the fusion zone or turned into the weld metal. Hydrogen accumulates seriously near the fusion zone, particularly at the root fusion zone. This is one of the importance reasons for the forming of cracks in this zone. The test results and analysis indicate that welding heat input should be controlled about q/v=16.0kJ/cm to prevent cracks being produced and propagated for HQ130 steel.

The microstructure near a diffusion interface was studied by means of scanning electron microscopy and electron probe microscopy, and the results indicated that the interface transition zone of Fe3Al/Q235 dissimilar materials was composed of a diffusion interface, a mixed transition region, and A/B transition regions at the sides of the interface. Microstructures of the interface and base materials were interlaced to form the microstructure of layer characteristic. With increased heating temperature and holding time, the width of the Fe3Al/Q235 interface transition zone increased and the microstructure gradually became coarse. The microhardness in the diffusion transition zone was decreased and there was a peak value at the diffusion interface. The distribution of Al, Fe, and Cr in the interface transition zone was increased or decreased monotonically with some local concentration fluctuation. There was nearly no change in the concentration of C element near the interface.