The effect of hydrogen on the fracture toughness of Alloy X-750

Journal Article · Sat Mar 01 04:00:00 EST 1997 · Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science

DOI:https://doi.org/10.1007/s11661-997-0068-8· OSTI ID:484837

Symons, D M ^[1]; Thompson, A W ^[2]

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1. Westinghouse Electric Corp., West Mifflin, PA (United States). Bettis Atomic Power Lab.
2. Lawrence Berkeley National Lab., CA (United States)

Alloy X-750 is widely used in both pressurized water and boiling water nuclear reactors. The effect of hydrogen on the fracture toughness behavior of this nickel-base superalloy in the solutionized and aged condition was investigated. Notched bend specimens were tested to determine if the fracture process was stress or strain controlled. The fracture was observed to initiate at a distance between the location of maximum stress and maximum strain, suggesting that fracture required both a critical stress and strain. The effect of hydrogen was further investigated and modeled using fracture toughness testing and fractographic examination. The fracture toughness of the noncharged specimen was 147 MPa{radical}m. Charging with hydrogen decreased the fracture toughness, K{sub Ic}, to 52 MPa{radical}m at a rapid loading rate and further decreased the toughness to 42 MPa{radical}m for a slow loading rate. This is consistent with the rate-limiting step for the embrittlement process being hydrogen diffusion. The fracture morphology for the hydrogen-charged specimens was intergranular ductile dimple, while the fracture morphology of noncharged specimens was a mixture of large transgranular dimples and fine intergranular dimples. The intergranular failure mechanism in Alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. One role of hydrogen was to reduce the void initiation strain for the fine intergranular carbides. Hydrogen may have also increased the rate of void growth. The conditions ahead of a crack satisfy the critical stress criterion at a much lower applied stress intensity factor than for the critical fracture strain criterion. A model based on a critical fracture strain criterion is shown to predict the fracture behavior.

• Sponsoring Organization: USDOE
• OSTI ID: 484837
• Journal Information: Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science, Journal Name: Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science Journal Issue: 3A Vol. 28; ISSN 1073-5623; ISSN MMTAEB
• Country of Publication: United States
• Language: English

Related Subjects

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS
36 MATERIALS SCIENCE
AGING
BWR TYPE REACTORS
CHROMIUM ALLOYS
FRACTURE PROPERTIES
GRAIN BOUNDARIES
HYDROGEN EMBRITTLEMENT
IRON ALLOYS
NICKEL BASE ALLOYS
PWR TYPE REACTORS
REACTOR MATERIALS
STRESS INTENSITY FACTORS