Argon determination in steel samples using ultrahigh vacuum system

Argon solubility is determined by means of ultrahigh vacuum (UHV) system desorption measurements. The UHV system is capable of measuring the low quantities of gas present in steel and can be equipped with tools for mechanically opening bubbles under vacuum. The argon solubility measured is less than 0.1 ppb, irrespective of composition. With these quantities, a significant partial pressure increase of argon in entrapped bubbles is not likely. The formation of new bubbles at other defects present in the steel is not possible either. The measured value is in good agreement with the theoretical solubility limit of argon in steel containing numerous small non-metallic precipitates. Desorption measurements revealed that differences in gas composition exist between bubbles in a slab and in the final cold rolled product. Argon is the major gas constituent in bubbles in slabs and blowholes in rolled products (93 and > 99 at.-%, respectively). The gas from the bubbles in slab samples also contained nitrogen (5 at.-%) and hydrogen (2 at.-%). On the other hand, in blowholes only small amounts of methane are present. The dissociation kinetics of molecular gas inside the cavities has yet to be clarified sufficiently to rule out completely pressure buildup in bubbles by hydrogen absorbed during downstream processing.