Microstructure and creep strength of simulated intercritical heat-affected zone of grade 91 steel

Yiyu Wang; Wei Zhang; Yanli Wang; Zhili Feng

doi:10.1080/09603409.2022.2120741

Microstructure and creep strength of simulated intercritical heat-affected zone of grade 91 steel

Yiyu Wang, Wei Zhang, Yanli Wang, Zhili Feng

Research output: Contribution to journal › Article › peer-review

Abstract

The intercritical heat affected zone (ICHAZ) has been reported as one of the most Type IV cracking susceptible regions in 9Cr creep-resistant steel weldments. However, creep degradation mechanisms within the ICHAZ itself need further clarifications. In this work, two ICHAZ specimens of Grade 91 steel, low-temperature ICHAZ (LT-ICHAZ) and high-temperature ICHAZ (HT-ICHAZ), were simulated using the Gleeble thermomechanical system by exposing to two peak temperatures (860 °C and 900 °C) between A_C₁ and A_C₃. Dramatically different creep strengths of two simulated ICHAZs were observed and studied. Dilation curve analysis indicates a high fraction of newly transformed martensite formed in the HT-ICHAZ, which results in a much higher hardness (360 HV0.5) of the HT-ICHAZ than 266 HV0.5 of LT-ICHAZ. Precipitates, especially M ₂₃ C ₆ carbides, were not fully dissolved in both ICHAZ specimens. After a typical postweld heat treatment (760 °C-2 hours), the faster recovery of low-carbon martensite and reduced precipitation strengthening due to M ₂₃ C ₆ carbides coarsening in the HT-ICHAZ led to a significant hardness reduction. These microstructural degradations in the HT-ICHAZ made its creep lifetime about 36 times shorter than that of the LT-ICHAZ tested at 650 °C. The remaining tempered martensite from base metal in the LT-ICHAZ was the primary contributor to maintain its high creep resistance.

Original language	English
Pages (from-to)	313-327
Number of pages	15
Journal	Materials at High Temperatures
Volume	39
Issue number	4
DOIs	https://doi.org/10.1080/09603409.2022.2120741
State	Published - 2022

Funding

This work is funded by the Department of Energy R&D Award (FWP-FEAA118). Research and Development work was performed at Oak Ridge National Laboratory, which is managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725. The authors would like to thank Mr. Alan Frederick, and Mr. Doug Kyle for their assistance in Gleeble and creep testing experiments. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide licence to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ).

Funders	Funder number
U.S. Department of Energy	DE-AC05-00OR22725, FWP-FEAA118
Oak Ridge National Laboratory

Keywords

Grade 91 steel welds
creep strength
intercritical heat affected zone
microstructure
power plants
type IV cracking

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10.1080/09603409.2022.2120741

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title = "Microstructure and creep strength of simulated intercritical heat-affected zone of grade 91 steel",

abstract = "The intercritical heat affected zone (ICHAZ) has been reported as one of the most Type IV cracking susceptible regions in 9Cr creep-resistant steel weldments. However, creep degradation mechanisms within the ICHAZ itself need further clarifications. In this work, two ICHAZ specimens of Grade 91 steel, low-temperature ICHAZ (LT-ICHAZ) and high-temperature ICHAZ (HT-ICHAZ), were simulated using the Gleeble thermomechanical system by exposing to two peak temperatures (860 °C and 900 °C) between AC1 and AC3. Dramatically different creep strengths of two simulated ICHAZs were observed and studied. Dilation curve analysis indicates a high fraction of newly transformed martensite formed in the HT-ICHAZ, which results in a much higher hardness (360 HV0.5) of the HT-ICHAZ than 266 HV0.5 of LT-ICHAZ. Precipitates, especially M 23 C 6 carbides, were not fully dissolved in both ICHAZ specimens. After a typical postweld heat treatment (760 °C-2 hours), the faster recovery of low-carbon martensite and reduced precipitation strengthening due to M 23 C 6 carbides coarsening in the HT-ICHAZ led to a significant hardness reduction. These microstructural degradations in the HT-ICHAZ made its creep lifetime about 36 times shorter than that of the LT-ICHAZ tested at 650 °C. The remaining tempered martensite from base metal in the LT-ICHAZ was the primary contributor to maintain its high creep resistance.",

keywords = "Grade 91 steel welds, creep strength, intercritical heat affected zone, microstructure, power plants, type IV cracking",

author = "Yiyu Wang and Wei Zhang and Yanli Wang and Zhili Feng",

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N2 - The intercritical heat affected zone (ICHAZ) has been reported as one of the most Type IV cracking susceptible regions in 9Cr creep-resistant steel weldments. However, creep degradation mechanisms within the ICHAZ itself need further clarifications. In this work, two ICHAZ specimens of Grade 91 steel, low-temperature ICHAZ (LT-ICHAZ) and high-temperature ICHAZ (HT-ICHAZ), were simulated using the Gleeble thermomechanical system by exposing to two peak temperatures (860 °C and 900 °C) between AC1 and AC3. Dramatically different creep strengths of two simulated ICHAZs were observed and studied. Dilation curve analysis indicates a high fraction of newly transformed martensite formed in the HT-ICHAZ, which results in a much higher hardness (360 HV0.5) of the HT-ICHAZ than 266 HV0.5 of LT-ICHAZ. Precipitates, especially M 23 C 6 carbides, were not fully dissolved in both ICHAZ specimens. After a typical postweld heat treatment (760 °C-2 hours), the faster recovery of low-carbon martensite and reduced precipitation strengthening due to M 23 C 6 carbides coarsening in the HT-ICHAZ led to a significant hardness reduction. These microstructural degradations in the HT-ICHAZ made its creep lifetime about 36 times shorter than that of the LT-ICHAZ tested at 650 °C. The remaining tempered martensite from base metal in the LT-ICHAZ was the primary contributor to maintain its high creep resistance.

AB - The intercritical heat affected zone (ICHAZ) has been reported as one of the most Type IV cracking susceptible regions in 9Cr creep-resistant steel weldments. However, creep degradation mechanisms within the ICHAZ itself need further clarifications. In this work, two ICHAZ specimens of Grade 91 steel, low-temperature ICHAZ (LT-ICHAZ) and high-temperature ICHAZ (HT-ICHAZ), were simulated using the Gleeble thermomechanical system by exposing to two peak temperatures (860 °C and 900 °C) between AC1 and AC3. Dramatically different creep strengths of two simulated ICHAZs were observed and studied. Dilation curve analysis indicates a high fraction of newly transformed martensite formed in the HT-ICHAZ, which results in a much higher hardness (360 HV0.5) of the HT-ICHAZ than 266 HV0.5 of LT-ICHAZ. Precipitates, especially M 23 C 6 carbides, were not fully dissolved in both ICHAZ specimens. After a typical postweld heat treatment (760 °C-2 hours), the faster recovery of low-carbon martensite and reduced precipitation strengthening due to M 23 C 6 carbides coarsening in the HT-ICHAZ led to a significant hardness reduction. These microstructural degradations in the HT-ICHAZ made its creep lifetime about 36 times shorter than that of the LT-ICHAZ tested at 650 °C. The remaining tempered martensite from base metal in the LT-ICHAZ was the primary contributor to maintain its high creep resistance.

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Microstructure and creep strength of simulated intercritical heat-affected zone of grade 91 steel

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