Degassing pretreatment approach for the sealing metal of a metal Berthelot tube

Abstract

The authors made an automatic temperature cycle repeater (ATCR), which could repeat the successive heating and cooling procedure of a metal Berthelot tube over the appropriate temperature range (temperature cycle) required for the generation of negative pressure. By operating the ATCR, they observed trends in negative pressure for water contained in a stainless steel tube, which enabled them to study the effect of the degassing pretreatment of the sealing material. Using a plug made from Ni degassed at 500 degrees C for 30 h under a vacuum of 10-6 Torr, the negative pressure was raised to -125 bar at 47 degrees C after a total of 3800 temperature cycles, which is the highest level ever reported for water in a metal tube; it is notable because it was raised for about a gram of water in contact with rough metal surfaces. The result supports the working assumption that the generation of a high negative pressure in a metal tube is limited by the supply of gas from sources in the bulk to crevices on the surface. The generally accepted gas-trapping crevice model should be supplemented with this working assumption.

Stagnations of increasing trends in negative pressure with repeated cavitation in water/metal Berthelot tubes as a result of mechanical sealing

“…The experiment employed here is similar to that reported before [8,9]. So we describe points specific to this paper in detail and the others briefly.…”

“…The pressure transducer was calibrated by applying nitrogen gas of known positive pressure from outside of the diaphragm [9]. Its linearity against pressure was assured to ca +15 MPa at every 10˚C in 20-70˚C.…”

“…The reason of the immersion was as follows. Too high an immersion temperature tended to cause a troublesome thermal shock [9], and too low the temperature did cavitation just after the immersion. After many trials, we found negative pressures were almost independent of the temperature so far as the absolute values of the pressures by then were smaller than 10 MPa in the all-stainless-steel tube.…”

“…By observing trends in negative pressure for water/ stainless steel tube sealed with poly-crystalline metal plug Berthelot systems, we disclosed (1) negative pressures increased with the number of temperature cycles (cavitation history effect), though scattered wide, (2) the effect was interpreted by a gas-trapping crevice model supplemented with a gas-supply assumption (gas-being-replenished crevice model) and (3) de-gassing pre-treatments for plugs were effective to raise negative pressures higher than their surface pre-treatments alone, though maximum negative pressures were achieved only after several thousands of temperature cycles [9][10][11][12].…”

“…A direct effect of mechanical sealing was once noted [9]. Gas inclusions having survived the cavitation history effect were dug out upon sealing and caused low negative pressures at the onset of a given run.…”