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

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.

Investigations of medium-temperature heat treatments to achieve low outgassing rates in stainless steel ultrahigh vacuum chambers

“…Three types of baking schemes are commonly employed to degas hydrogen from stainless steel: Vacuum firing, in which the entire vacuum chamber is placed in a vacuum furnace operating at >950°C and pressures below 10 −3 Pa; 2,5–9 medium heat treatment vacuum bake, in which the vacuum chamber is evacuated and heated to 400–500°C, typically with the outside of the chamber in air at atmospheric pressure; 10–13 and a medium heat treatment air-bake, in which the vacuum chamber is baked entirely in air at atmospheric pressure at a temperature of 400°C or greater. 6,7,13,14 Hydrogen diffuses through stainless steel as atomic H, 15,16 and the diffusion coefficient for hydrogen in stainless steel depends exponentially on temperature; 17 increasing the temperature greatly decreases the time it takes for the hydrogen to migrate from the stainless steel bulk to the surface, where it recombines to form H 2 and desorbs from the surface. 18 The time it takes to remove most of the hydrogen from the stainless steel bulk depends on the temperature and material thickness.…”

A Review of Outgassing and Methods for its Reduction

“…Baking is either carried out under vacuum or in air and generally, the higher the temperature during bakeout, the lower the outgassing rate afterwards since higher temperatures lead to faster outgassing as long as any gas removed from the surfaces during bakeout are pumped from the chamber. Empirically, this relation was found for stainless steel by Odaka, Ishikawa and Furuse [31] to be given by:…”

Long-term stability and zero drift of digital barometric pressure gauges

“…Assuming the diameter of the volume to be about 60 mm, we can obtain a surface area of S ~ 6.3 × 10 −3 m 2 . Then, the gas emission q is calculated as follows: The typical gas emission for stainless steel (SUS316L, electro-polished) has been reported to about 2.0 × 10 -9 Pa m 3 s −1 m −2 or less after surface baking at 150 °C for 24 h [13].…”