Microstructure evolution of nanostructured bainite in tempered zone during welding and regeneration has been analysed. The effect of welding heat input on microstructure and mechanical properties is investigated. Results show that cementite precipitation and nanostructured bainite regeneration occur in tempered zone. When heat input is reduced, the size and amount of cementite decrease, while the volume per cent of retained austenite increases. At the same time, the tensile strength in tempered zone increases with welding heat input decreasing. A welded joint of nanostructured bainitic steel with tensile performance almost the same as base metal is obtained by laser beam welding with regeneration.
Because of the brittle martensite crystalline structure, nanostructured bainitic steel is very difficult to be welded and easily form cracks in the welded joint, which limits the scope of their application. Regeneration treatment can lead to nanostructured bainite formation in the welded joints, preventing further degradation of the welded joint. Detailed changes of microstructures and mechanical properties of the weld are characterized here. Coarse inter-dendrite structures appear in the weld due to welding segregation, and are confirmed to be retained austenite by TEM. Moreover, an extraordinary combination of strength and ductility of the weld is achieved. The ultimate tensile strengths are 1913MPa and 2115MPa when regeneration temperatures are 250 °C and 230 °C. The corresponding elongations are 5.14% and 2.3%. In addition, the tension fracture behaviour and crack propagation mode of the weld are investigated.
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