OUTGASSING CHARACTERISTICS AND MICROSTRUCTURE OF A VACUUM FIRED (1050-DEGREES-C) STAINLESS-STEEL SURFACE

Vacuum fired (1050-degrees-C) type 304 stainless steel (SS304) surfaces were investigated with scanning electron microscopy and Auger electron spectroscopy. The grain boundaries of the surface were vague and shallow, which occurred due to elemental diffusion at high temperature in vacuum. The newly formed oxide layer of the vacuum fired surface was much thinner, and could be said to be finer in microstructure than the native layer of an "as-received" surface. A vacuum fired (1050-degrees-C) SS304 chamber was evacuated by a sputter ion pump whose vessel was pretreated by vacuum firing (1050-degrees-C). An extremely high vacuum of 1.5 x 10(-9) Pa (N2 equivalent pressure) was indicated by an extractor ionization guage after a mild bake (170-degrees-C) following an air exposure. The outgassing rate of the chamber wall at the elapsed time of one day after a mild bake (170-degrees-C) was roughly estimated as low as 2 x 10(-11) Pa l/s cm-2 (N2 equivalent value) by an orifice method. Vacuum firing has the effects of degassing the gas molecules soluted in the interior of stainless steel wall, and of forming a new, thin, and fine (in microstructure) oxide layer on the surface.