Microstructural and Microchemical Characterization of Dual Step Aged Alloy X-750 and its Relationship to Environmentally Assisted Cracking

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When exposed to deaerated high purity water, Alloy X-750 is susceptible to both high temperature (> 249 C) intergranular stress corrosion cracking (IGSCC) and intergranular low temperature (< 149 C) fracture (LTF). However, the microstructural and microchemical factors that govern environmentally assisted cracking (EAC) susceptibility are poorly understood. The present study seeks to characterize the grain boundary microstructure and microchemistry in order to gain a better mechanistic understanding of stress corrosion crack initiation, crack growth rate, and low temperature fracture. Light microscopy, scanning electron microscopy, transmission electron microscopy, orientation imaging microscopy, scanning Auger microscopy, and thermal desorption spectroscopy were performed ... continued below

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5996 Kilobytes pages

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Young, G.A.; Lewis, N.; Hanson, M.; Matuszyk, W.; Wiersma, B. & Gonzalez, S. May 8, 2001.

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  • Lockheed Martin
    Publisher Info: Lockheed Martin Corporation, Schenectady, NY 12301 (United States)
    Place of Publication: Schenectady, New York

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Description

When exposed to deaerated high purity water, Alloy X-750 is susceptible to both high temperature (> 249 C) intergranular stress corrosion cracking (IGSCC) and intergranular low temperature (< 149 C) fracture (LTF). However, the microstructural and microchemical factors that govern environmentally assisted cracking (EAC) susceptibility are poorly understood. The present study seeks to characterize the grain boundary microstructure and microchemistry in order to gain a better mechanistic understanding of stress corrosion crack initiation, crack growth rate, and low temperature fracture. Light microscopy, scanning electron microscopy, transmission electron microscopy, orientation imaging microscopy, scanning Auger microscopy, and thermal desorption spectroscopy were performed on selected heats of Alloy X-750 AH. These data were correlated to EAC tests performed in 338 C deaerated water. Results show that grain boundary MC-type [(Ti,Nb)C] carbides and increased levels of grain boundary phosphorus correlate with an increase in LTF susceptibility but have little effect on the number of initiation sites or the SCC crack growth rate. Thermal desorption data show that multiple hydrogen trapping states exist in Alloy X-750 condition AH. Moreover, it appears that exposure to high temperature (> 249 C), hydrogen deaerated water increases the hydrogen concentration in strong hydrogen trap states and degrades the resistance of the material to low temperature fracture. These findings are consistent with a hydrogen embrittlement based mechanism of LTF where intergranular fracture occurs ahead of a crack tip and is exacerbated by phosphorus segregation to grain boundaries and grain boundary hydrogen trap states.

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5996 Kilobytes pages

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OSTI as DE00821679

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  • Other Information: PBD: 8 May 2001

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  • Report No.: LM-01-K034
  • Grant Number: AC12-00SN39357
  • DOI: 10.2172/821679 | External Link
  • Office of Scientific & Technical Information Report Number: 821679
  • Archival Resource Key: ark:/67531/metadc787190

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  • May 8, 2001

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  • Dec. 3, 2015, 9:30 a.m.

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  • April 28, 2016, 8:52 p.m.

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Young, G.A.; Lewis, N.; Hanson, M.; Matuszyk, W.; Wiersma, B. & Gonzalez, S. Microstructural and Microchemical Characterization of Dual Step Aged Alloy X-750 and its Relationship to Environmentally Assisted Cracking, report, May 8, 2001; Schenectady, New York. (digital.library.unt.edu/ark:/67531/metadc787190/: accessed December 11, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.