Gr.91 castings are used in high temperature valves for main steam lines and elsewhere, while Gr.91 forgings are essential products for power boilers and in turbine construction. However, scrap steel is very often used as a raw material in the manufacturing of Gr.91 castings and forgings, suggesting the inclusion of high levels of residual tramp elements in the products when the quality of the raw material and process is out of control. The influence of residual elements such as P, S, Cu, Sn, and Sb on creep properties is a very important issue from the standpoint of using Gr.91 castings and forgings in high temperature components. This study focuses on the effect of the amount of tramp elements on the creep properties of Gr.91, investigating the creep mechanisms of tempered martensite lath structures with the above-noted elements. As a result, it was found that creep strength increased by either increasing the content of P or S, or by decreasing the amount of Sn or Sb. The Cu content did not affect creep strength at 600°C. At 650°C, the creep rate was unaffected by the tramp elements. Whereas the carbide distribution was hardly influenced by the proportion of the tramp elements, the martensitic lath spacing was observed to be dependent on the content, and was also related to the minimum creep rate obtained in creep testing at 600°C.

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