Microstructure of two different 18Ni Co-free maraging specimens and their electron beam weld joints were investigated comparatively by optical microscopy and SEM. It is showing that both of the steels are typical lath martensite，however，one grain size is about three times as another one，and XRD reveals that the amount of the retained austenitic phase in the former is less then the latter. The austenite distributes in plate form along granular and lath boundaries while some in fine particle within the matrix. The microstructural difference between two specimens led to diverse behaviors in electron beam welding. The first specimen is weldable well but thesecond shows obvious welding defects of pits and burn-through holes in weld face. The welding microstructureexhibits a typical dendritic morphology，and the grains in the heat-affected zone recrystallized and grew upobviously for high temperature heated by welding electron beam. The weldablity is relative to the thermalconduction performance of the base materials，which is contributed greatly for Grain size and austenite content.
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