Abstract
It has recently been reported that ferrite grains coarsened to several hundred micrometers were occasionally observed in long-term serviced 9Cr-1Mo-V-Nb steel welds. To clarify the factors that cause ferrite grain growth in martensite during high-temperature exposure, alternating aging heat treatment and observation of the same field of view was performed using a welded 9Cr-1Mo-V-Nb steel. Such ex-situ observations revealed that the rapid grain growth of ferrite by consuming martensite occurred in the weld metal during aging at 740 °C. In the test material used in this study, some δ-ferrite grains were observed in the weld metal near the fusion line. In the region where δ-ferrite grains were observed, the concentrations of austenite-forming elements such as Mn and Ni were locally decreased in the matrix due to dilution by the base metal, promoting δ-ferrite retention after welding. Ex-situ observation indicated that no significant grain growth of δ-ferrite occurred during aging. Therefore, it was suggested that a new ferrite grain formation followed by rapid grain growth consuming martensite occurred during aging. The elastic strain energy density of the dislocations PMdis and the interfacial energy density PMsurf in martensite can affect the driving force for ferrite grain growth by consuming martensite. Based on the evaluation results for PMdis and PMsurf after 500 h of aging, PMsurf was considered to be the main driving force for ferrite grain growth. Although the ferrite-formation process could not be directly observed, it is possible that ferrite was formed by the recrystallization of martensite through the bulging mechanism.
Published Version
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