Abstract：The fatigue fracture surfaces of welded joints were observed and studied, and accordingly the fatigue mechanisms, governing fatigue failure of welded joints, were discussed. The microstructures on the base metal, the weld seam and the heat-affected zone of welded joints were investigated. It is concluded that the microstructures on the base metal are fine tempered martensite and dispersed secondary phase particles, whereas the ones on the weld seam are coarse lath martensite and few of secondary phase particles. The differences between their microstructures confirm the better mechanical properties of the base metal.
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