Measurement of outgassing rate for GTAW welded SS304 materials

Abstract

Outgassing plays an important role to achieve Ultra High Vacuum (UHV) and to maintaining the required vacuum level of the vessel. For a large size machines like Tokamak, accelerators, space simulation chambers, outgassing from the structural materials and their welding sections need to be checked during the design. Hence studies were carried out for the measurement of outgassing rate for the Gas Tungsten Arc Welding (GTAW) welded SS304 materials at OutGassing Measurement System (OGMS) at IPR Extension Lab. The system consists of two chambers, pumping chamber and sample chamber, made up of 304 grades Stainless Steel pre-air baked at 400 °C. The pumping chamber connected to a turbo molecular pump having pumping speed 240 l/s, backed by a rotary vacuum pump having pumping speed 5m3/hour. Pumping chamber and sample chamber connected through a 100 CF flange having a circular aperture of 5.2 mm diameter. The conductance of aperture is 2.47 l/s. Bare SS304 sample & GTAW welded SS304 samples are prepared with 100mmX50mmX5mm and 95mmX55mmX5mm dimensions respectively. The base outgassing rate of the blank system is 2.34×10_-11 mbar l/s-cm2. The calculated outgassing rate is 3.66×10_-10 & 4.37×10_-10 mbar l/s-cm2 for bare sample & welded sample. From the partial pressure analysis it has been found that hydrogen and nitrogen are in the partial level of 3.35×10_-9 & 2.36×10_-9 for bare sample and 3.14×10_-9 mbar & 2.33×10_-9 mbar for welded sample. It has been observed that, the GTAW doesn't have major effect on outgassing rate and one can use welded joint for designing a large vessel welding sections.