Effect of baking temperature and air exposure on the outgassing rate of type 316L stainless steel

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Abstract
The outgassing rates of a vacuum remelted type 316L stainless steel were measured as a function of baking temperatures, baking durations, and air exposure/baking cycles. Long-time baking, as was expected, reduced the outgassing rate of the stainless steel. A subsequent air exposure and short-time baking in vacuum was found to reduce the outgassing rate much more than the preceding long-time baking. The outgassing rate after baking qAB was reduced with the repeated air exposure/baking cycles at a constant baking temperature. After a few air exposure/baking cycles, qAB was not reduced and remained constant with a further air exposure/baking cycle. This indicates that the surface of the stainless steel reached the steady state with respect to the outgassing as a result of the repeated air exposure/baking cycles. For this thoroughly degassed state, the effect of baking temperature was evaluated in the range of 150 to 450 °C. Six-hour baking at 150 °C was sufficient to attain the outgassing rate of 1.3×10−10 Pa m3/s m2. The relation between the outgassing rate after baking q*AB and the maximum baking temperature TBmax was approximately expressed by the equation log q*AB =−1.87 log TBmax−4.51. The Auger electron spectroscopy (AES) analysis showed that the Cr enrichment at the surface became more evident as the maximum baking temperature increased. Further for a given baking temperature, little significant difference in AES spectra was observed between the thoroughly degassed state and the intermediately degassed state.

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