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The microstructure of E911 deposited metal was observed and the effect of heat input and postweld heat treatment on microstructure and impact toughness was investigated.The microstructure consists of tempered martensite and residual δ-ferrite.The morphology of tempered martensite is columnar and the residual δ-ferrite is polygonal.With the increase in heat input,the width of columnar martensite grain and the size of residual δ-ferrite increased,whereas the volume fraction of residual δ-ferrite varied slightly.The impact toughness decreased as heat input increased.The result reveals that coarsening columnar martensite grain and δ-ferrite have greater effect on impact toughness than volume fraction of residual δ-ferrite.As the time of postweld heat treatmeat is exceeded 8 h,aggregation of M23C6 occurs in some grain boundaries or lath interfaces.The partial aggregation of M23C6 results in the decrease in impact toughness.