Rosebury, F.

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XX. SHOP NOTESHYDROGEN PROCESSING OF STAINLESS STEELExperience in the Tube Laboratory over a long period has shown that contaminationof the hydrogen atmosphere in the regular ceramic muffle furnaces is almost invariablypresent, and that stainless and other steels are particularly sensitive to it. The exactnature of all of the contaminants has not been determined, but it is reasonably certainthat air (i.e., oxygen) is one of them. It is also believed that water vapor is present,either from the air or from condensation and re-evaporation in the water-jacketedsections of the furnaces.It is generally believed that some commonly used metals such as copper, nickel,kovar, tungsten, and molybdenum are apparently not affected by small amounts of watervapor and/or oxygen; indeed, the manufacturers of kovar recommend wet hydrogen forfiring this alloy preparatory to glass sealing. However, stainless steel is ordinarilylightly coated with green chromium oxide after firing in the regular furnaces, and thiscoating prevents wetting by brazing materials (a property that is turned to advantagewhen stainless steel jigs and fixtures are used to hold copper, nickel, and other metalsfor brazing, and that also prevents the sticking of tungsten and molybdenum to previouslyoxidized stainless or steel mandrels on which they are to be annealed).We have found that very little or no oxidation is produced on stainless steel whenordinary tank hydrogen is used in a glass, silica, or nonporous metal retort. As anote of caution, however, the dew point of the regular-grade tank hydrogen (Air Reduc-tion Co.) should be measured, since some variation in moisture content is to be expected.For satisfactory results, the dew point should not be higher than -50'C. The use of adrying train and a "De-Oxo" (Baker and Co.) catalytic cartridge will assure good results.The drying train should be cleaned and replenished frequently; we have found that a spentdrying tube can actually introduce moisture to the hydrogen.The tube shown in Fig. XX-1 can also be used in a vertical position, with the hydro-gen inlet at the top. In this case the work-piece can be supported in the center from awire which passes upward through the inlet tube and is hooked over the edge. ConnectionsSEAL WITH VINYLELECTRICIANS'TAPE RF COILTO EXHAUST OOOO0 H2 INSTO DRY H,WORK SUPPLY\000 TYGON TUBINGRUBBER VYCOR ORSTOPPER SUPPORTS GLASS TUBEFig. XX-1. Method of firing stainless steel in dry hydrogen.170(XX. SHOP NOTES)from the hydrogen supply are made with Tygon (U. S. Stoneware Co.) tubing becauserubber tubing has been found unsatisfactory.Although the moisture content of ordinary tank hydrogen varies, it has been possible,by this technique, to fire austenitic stainless steel without oxidation, and even to removeentirely a moderate degree of oxidation that was previously present. It has also beenpossible to braze unplated and untreated stainless steel directly with silver-coppereutectic solder and other solders with higher melting points by the same procedure. Inall cases the temperature of the stainless steel must be raised to at least 1000 0 C; atthis temperature the dry hydrogen exercises a strong reducing effect. Measurementsof the dew point of the hydrogen at the time of performing these operations gave valuesbetween -60 0 C and -70 0C.F. Rosebury171

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