Pure Ti is adopted as a material for ducts and bellows at the 3 GeV-RCS (Rapid Synchrotron) in J-PARC (Japan Accelerator Research Complex) project, because of its small residual radioactivity.
Recently some reports focus on the thermal outgassing characteristics of Ti, related to the surface treatments. In the case of particle accelerator, however, the H2 outgassing due to surface bombardment by particles is often dominant gas release. As the reduction of hydrogen content probably leads to the suppression of outgassing by the particle bombardment, we decided to decrease the hydrogen content in the Ti by in-situ vacuum baking, and to investigate the influence of hydrogen in the bulk on the outgassing characteristics.
First of all, thermal desorption spectra have been measured to survey the desorption process. For instance, a major peak attributed to diffusible interstitial hydrogen was recorded when the Ti sample 0.5 mm thick was ramped up to 1300 K at the rate of 200 K/hr. H2 desorption started at about 720 K and peaked at around 800 K. Then it was completed up to about 1020 K. The hydrogen content was reduced from 35 ppm (wt) to less than 0.5 ppm.
Then, we have investigated the change in hydrogen content as functions of treatment temperature and duration time, with a parameter of sample thickness. Here, the mechanical properties have been monitored after the heat treatment, because a grain growth occurs at a high temperature and leads to the reduction of mechanical strength. Even after the treatment at 1120 K for 12 hr, the changes in tensile strength and yield strength were only a decrease of 10 percent and 15 percent, respectively. The elongation increased by 10 percent. Thus we have no anxiety about the reduction of mechanical strength after heat treatment.
After all, to reduce the hydrogen content to less than 1 ppm (wt) the plates for bellows (0.3 mm thick) and ducts (4mm thick) are vacuum fired at 923 K for 8 hr and at 973 K for 9 hr, respectively.
The removal of the hydrogen, which is carried into the near surface by electrolytic or chemical polishing, will be also discussed.
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