Effect of non-standard heat treatments on creep performance of creep-strength enhanced ferritic (CSEF) steel weldments

Yukinori Yamamoto, Michael L. Santella, Xinghua Yu, Sudarsanam Suresh Babu

Research output: Contribution to conferencePaperpeer-review

1 Scopus citations

Abstract

This paper summarizes recent efforts to improve creep performance in Grade 91 (Mod. 9Cr-1Mo, ASTM A387) steel weldments via non-standard heat treatments prior to welding. Such heat treatments offer a potential solution for minimizing Type IV failures in creep strength enhanced ferritic (CSEF) steels. A lower temperature tempering (LTT, 650°C) of the 9Cr steels prior to gas tungsten arc welding (GTAW) resulted in improved creep-rupture life at 650°C compared to the samples tempered at a standard condition (HTT, 760°C) before welding. From detailed characterization of precipitation kinetics in the heat affected zone, it was hypothesized that M23C6 carbides in the fine-grain heat-affected zone (FGHAZ) in the LTT sample were fully dissolved, resulting in re-precipitation of strengthening carbides during post weld heat treatment (PWHT). This was not the case in the HTT sample since M 23C6 in the FGHAZ was only partially dissolved prior to welding, which caused coarsening of existing M23C6 after PWHT and premature creep failure in the FGHAZ. However, it was also found that the LTT raised the ductile-brittle transition temperature above room temperature (RT). Two different thermo-mechanical treatments (TMTs); two-step tempering and aus-forging/aus-aging, of the modified 9Cr-1Mo steels were attempted, in order to control the balance between creep properties and RT ductility, through control of precipitation kinetics of the M23C6 carbides and/or MX carbo-nitrides. The hardness map of the TMT samples after GTAW and PWHT were evaluated.

Original languageEnglish
Pages1016-1024
Number of pages9
StatePublished - 2014
Event7th International Conference on Advances in Materials Technology for Fossil Power Plants - Waikoloa, HI, United States
Duration: Oct 22 2013Oct 25 2013

Conference

Conference7th International Conference on Advances in Materials Technology for Fossil Power Plants
Country/TerritoryUnited States
CityWaikoloa, HI
Period10/22/1310/25/13

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