Outgassing characteristics and microstructure of a ‘‘vacuum fired’’ (1050 °C) stainless steel surface

Abstract

‘‘Vacuum fired’’ (1050 °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 °C) SS304 chamber was evacuated by a sputter ion pump whose vessel was pretreated by vacuum firing (1050 °C). An extremely high vacuum of 1.5×10−9 Pa (N2 equivalent pressure) was indicated by an extractor ionization gauge after a mild bake (170 °C) following an air exposure. The outgassing rate of the chamber wall at the elapsed time of one day after a mild bake (170 °C) was roughly estimated as low as 2×10−11 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.

Positron beam and RBS studies of thermally grown oxide films on stainless steel grade 304

“…As it was mentioned, the surface of ASS as a result of annealing is covered with ␣-Fe 2 O 3 on the outermost layer and spinel rich in the chromium oxide in the inner layer [6][7][8]. It was observed [30][31][32][33] that the temperature of 800 • C is closed to the limit above which the chromium oxide is a predominant layer because of diffusion of chromium from the bulk. In this way, with the reference of quoted information for samples studied here, annealing at 800 • C leads to formation of the multi-oxide films on the surface, where at the top ␣-Fe 2 O 3 containing defects appeared.…”

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

“…Most users of UHV and XHV systems employ a heat treatment to lower the hydrogen outgassing to achieve a flux in the range of 10 −10 to 10 −12 Pa l s −1 cm −2 . 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.…”

Thermal outgassing rates of low-carbon steels

“…14,39 At the same time, the quantities of surface impurities, especially carbon, are reduced to an extremely low level. The oxide layer is believed to be cleaner and smoother than layers that had not been exposed to high temperature heat treatment as in the stainless steel, 15 although further study is necessary to evaluate this possibility. The former surface in turn has fewer adsorption sites for water than does the latter.…”

“…For the RoR measurement, an SRG was used to prevent errors caused by either pumping or the outgassing action of an ion gauge. Because temperature control is extremely important during operation of the SRG to ensure precise measurements of extremely low outgassing rates, an oven with active temperature control unit that consisted of a cooling coil at constant temperature of 15 C and a proportional-integral-derivative controlled heater was used to maintain the temperature at 24 6 0.1 C. Throughput measurements were conducted in a room with the temperature actively controlled within 61 C. To investigate the high temperature annealing effect, all measurements were repeated after the specimens had been heat-treated at 850 C for 12 h in a vacuum furnace.…”

“…9,10,14,15 These differences may be due to difference in the steelproduction processes, in particular, the secondary metallurgy during which impurities are removed. 16,17 The reference values (Table II) for mild steels were published in the early 1970s.…”