Alloy 718 comprises 51% of the total weight of the Space Shuttle Main Engine. It is used for more than 1,500 individual parts in hydrogen, oxygen, and hydrogen-rich steam at temperatures ranging from-253°C to 760°C. The severe operating conditions of the SSME lead to potential problems such as hydrogen-cnvironment embrittlement and oxygen ignition. This paper will discuss how these potential problems are controlled through design,, protective barriers, and heat treatment. Operational experiences, fabrication tcchniqucs, and welding microfissures will also be discussed.

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HE data presented by these authors are valuable in showing the formation of certain compounds, in identifiable amounts, during the reactions of the cement minerals with water in pastes. The writer42 has pointed out that the reactions of the cement minerals with water are almost certainly solid-liquid reactions in which the solids react with water, without first going into solution, to form solid reaction products; i.e., product solids. These product solids tend to dissolve in water to form solutions from which either stable or metastable products crystallize. For example, the product solids formed by the solid-liquid reaction of CaA and CIS with water tend to dissolve and yield a solution containing the equivalent of CaO, Algol, and SiO2. It is readily understandable that the compound C3AHs containing some silica could crystallize from such a solution. A similar solution would

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