Effect of carbon content on creep rupture strength and microstructure in heat affected zone of heat resistant ferritic steel: Alleviation of decrease in creep rupture strength in heat affected zone of heat resistant ferritic steel

The effect of the carbon content on the creep rupture strength and the microstructural change in weld heat affected zone (HAZ) during high temperature services was investigated in order to alleviate the decrease in the creep rupture strength in HAZ of heat resistant ferritic steels. The test specimens were prepared from 9Cr–3Co–3–W–V, Nb ferritic steel plates with carbon content ranging from 0˙005 to 0˙1%. After the simulated HAZ thermal cycle treatments at the peak temperature of 1273 K was applied on these specimens, creep rupture tests, aging tests and microstructural examinations were conducted. As a result, it was clarified that the creep rupture time of simulated HAZ became longer and the decrease in the creep rupture strength in HAZ was alleviated by decreasing the carbon content. Then the mechanism to explain the effect of the carbon content was discussed from a viewpoint of the growth of precipitates, such as M₂₃C₆ and MX, during long term heating.