Abstract
The microstructure and grain growth behavior were investigated in the hyper interfacial bonded joint of ultra fine-grained steels. The influence of M-A constituents on hardening and/or HAZ softening in hyper-interfacial bonded joint was also examined. Microstructure of the hyper interfacial bonded joint of ultra fine-grained steels was changed melted zone, coarse-grained zone, mixed-grained zone to base metal region in order from bond interface. Martensite and bainite were formed in coarse-grained zone, and ferrite existed at bond interface. M-A constituents and fine carbides were formed in the lower temperature region of HAZ. Grain growth in joint adjacent to bond interface was restrained below 12.0μm. The maximum hardness in the hyper interfacial joint was about Hv380, and HAZ softening was suppressed by M-A constituents formed in AC1-AC3 region. Hyper interfacial bonding process was superior to fusion welding processes such as TIG, LBW and EBW from the viewpoint of the restraint of grain coarsening.
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