Research on the Outgassing Rate of Stainless Steel Pipes

Outgassing rate of the stainless steel pipe is a key parameter for the design of the ultra-high vacuum system, and reducing the outgassing rate is one of the efficient ways to raise the vacuum level. An ultra-high vacuum level (~10⁻¹⁰ Pa) inside the vacuum pipes of the synchrotron ring is required for the heavy-ion upgrade project of the Xi'an 200 MeV proton application facility (XiPAF). It is further demanded that the outgassing rates of stainless steel pipes must not exceed 1.5×10⁻¹³ Pa·m³/(s·cm²). To guarantee that the outgassing rate of the stainless steel pipe is not higher than this value, the method to lower the rate is researched and a treatment proposal is presented in this paper. Three major concerns are involved in the proposal to lower the outgassing rate: the selection of stainless steel material, the choice of high-temperature vacuum furnace baking scheme, and the choice of heating band baking scheme. An experimental process is designed. Four stainless steel pipes are customized with the same size, with two pipes made of 304 and another two 316L. The four pipes are divided into two groups, and each group is baked inside a vacuum furnace at 500℃ and 950℃, respectively. Four heating-band baking tests are conducted for each pipe with the temperatures progressively set at 150℃, 200℃, 250℃, and 300℃. Ultimately, the final outgassing rate of each pipe is measured. A dedicated platform is constructed to measure the outgassing rates of the stainless steel experimental pipes. The measurement results indicate that, for the two types of stainless steel material—304 and 316L—there is no particular pattern regarding the relationship between the type of material with the initial outgassing rate of the pipes provided by the processing manufacturer. The outgassing rate can be reduced by one or two orders of magnitude after baking in a vacuum furnace or with heating bands. After undergoing successive baking at 950℃ in the vacuum furnace and 300℃ with heating bands, the outgassing rate can reach below 5×10⁻¹⁴ Pa·m³/(s·cm²), satisfying the requirement of the XiPAF heavy-ion upgrade project.