Hydrogen-enhanced cracking of superalloys
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I.
INTRODUCTION
H Y D R O G E N embrittlement is the loss of ductility, delayed failure, or faceted fracture attributable to the presence of hydrogen. If the hydrogen enters the material before an external stress is applied, the phenomenon is called internal hydrogen embrittlement; otherwise, it is known as external or environmental hydrogen embrittlement. Internal hydrogen comes from a variety of sources: solidification of an ingot or weld, pickling baths and other corrosive media, or electroplating, r~l In the latter two instances, hydrogen must diffuse from the surface into the metal, and it may not be uniformly distributed on a macroscopic scale. Finally, hydrogen gas used as a fuel ( e . g . , in the space shuttle) or chemical reactant may be present at high temperatures and pressures, thus leading to enhanced diffusion rates of dissociated hydrogen into the metal lattice. Embrittlement may occur subsequently when the environment is removed. Whether or not internal and external hydrogen embrittlement are basically the same phenomenon is open to question because it is not known whether observed differences are due to experimental differences, microstructural differences, or different crack advance mechanisms. For example, Jewett e t a l . t2] noted that the most striking difference between alloys showing the two types of embrittlement is the behavior of relatively pure nickel compared with nickel-based alloys. INCONEL* 718 and RENI~** 41 (nickel-based alloys) were not embrittled by *INCONEL is a trademark of Inco Alloys International, Inc., Huntington, WV. **RENE is a trademark of General Electric Company, Fairfield, CT.
P.D. HICKS, formerly Research Assistant, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, is Senior Engineer, Nalco Chemical Company, Naperville, IL 60563t 198. C.J. ALTSTETTER, Professor of Physical Metallurgy and Associate Dean of Engineering, is with the Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted July 27, 1990. METALLURGICAL TRANSACTIONS A
severe electrolytic charging, while these alloys were among the most embrittled in hydrogen gas at 70 MPa. High-strength steels are more susceptible to both types of hydrogen embrittlement than low-strength steels. Low alloy steels are more susceptible to both types of hydrogen embrittlement than stainless steels, except those that transform to martensite during deformation. The present study attempts to clarify the role of microstructure and contribute to the understanding of the way hydrogen can lead to crack propagation. Three alloys, INCONEL 718 (nickel base), A286 (iron base), and INCONEL 625 (nickel base), were chosen, because in their commonly used conditions, they provide a contrast between precipitation-hardened alloys in different alloy systems (IN718 v s A286) and between agehardened and solid solution-hardened alloys in the same alloy system (IN718 v s IN625). The majority of hydrogen embrittlement research on
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