The precipitation behavior of carbides of modified 9Cr-1Mo steel (Grade 91) by low-temperature tempering and the influence of those carbides on the mechanical properties at room temperature were investigated. As-quenched sample (AQ) showed a small amount of MC carbide in the martensite microstructure. Three types of carbides were formed by low-temperature tempering at 300-500ºC that were chosen for the purpose of suppressing recovery and growth of the dislocation substructure. These carbides

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... e identified as Fe 4 C, hP8 type Fe 3 C, and oP16 type Fe 3 C using replica samples for transmission electron microscopic and extracted residue analyses. Double low-temperature tempering at 500ºC for 5 min followed by at 300ºC for 1 h (DLTT) formed a large amount of carbide compared to AQ while AQ and DLTT had similar lath widths. The hardness of the DLTT sample was higher than that of the AQ sample, whereas the tensile strength of the DLTT sample was slightly lower than that of the AQ sample regardless of strain rate. The reason why the precipitation strengthening did not work for the improvement of tensile strength is considered to be the early formation of microvoids due to the delamination of carbide/matrix interfaces under tensile test.