Surface preparation and hydrogen compatibility of an iron base superalloy
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THE
adverse effects of hydrogen on the mechanical properties of metals and alloys has been the subject of numerous investigations; many of these studies have shown that austenitic stainless steels often have excellent hydrogen compatibility. 1-3 Austenite stability and dislocation substructure play major roles in determining the susceptibility of many austenitic alloys to hydrogen damage 4 while the nature and morphology of second phase precipitation have been shown to be important to the susceptibility of age hardenable, iron base alloys of the near A-286 composition. 5,6 A-286 is not particularly hydrogen compatible, and hydrogen induced sustained-load crack growth has been observed; 7 however, slight modifications of the A-286 chemistry improved the hydrogen compatibility 5 and resulted in a patent for a more hydrogen compatible alloy with excellent welding characteristics? This alloy, called alloy 8 in Ref. 5 and referred to as JBK75 in this paper can be hardened to yield strength levels in excess of 1000 MPa by controlled working and precipitation of the ordered phase, 3" (Ni3(A1,Ti)). The improved hydrogen compatibility of JBK-75 over A-286 is thought to derive from an increase in lattice mismatch between 3" and the austenite matrix. 5 The hexagonal Ni3Ti phase, 7, may also form in JBK-75 and its presence is thought to increase the susceptibility of precipitation strengthened austenitic steels to hydrogen damage. 5,6 The effects of machining and surface preparation on the susceptibility of austenitic stainless steel to hydrogen embrittlement have been known since the 1960s. Tensile tests of type 304L samples in 69 M P a hydrogen gas showed that the ductility losses and extensive surface cracking which accompany plastic deformation are associated with the cold worked layer produced by machining. 9 This observation and related metallographic studies have led to the conclusion that c~' -martensite produced during cold working is the
J. A. BROOKS, currently on Educational Leave at CarnegieMellon University, Pittsburgh, PA, is Member, Technical Staff, Sandia National Laboratories, Livermore, Livermore, CA 94550, and M. R. LOUTHAN, JR., formerly Professor at Summer University, Sandia National Laboratories, is Professor of Materials Engineering, Virginia Polytechnic Institute, Blacksburg, VA 24061. Manuscript submitted April 22, 1980.
cause of hydrogen induced ductility losses in the metastable type 304L stainless steel5 ~ Electropolishing and annealing to remove the cold worked layer, prior to testing in hydrogen, increased the hydrogen compatibility of 304L in short time tests. 9 However, subsequent studies H showed that the beneficial effects of electropolishing are eliminated if 304L tensile specimens are precharged with hydrogen prior to testing. Related solubility and permeation studies indicate that the primary effect of electropolishing is to minimize hydrogen absorption during the tensile test. lz This observation coupled with the observation that vacuum evaporated coatings are also effective in increasi
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