ARGON DETERMINATION IN STEEL SAMPLES USING ULTRAHIGH-VACUUM SYSTEM

Argon solubility is determined by means of ultrahigh vacuum (UHV) syst em 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 m easured is less than 0.1 ppb, irrespective of composition. With these quantities, a significant partial pressure increase of argon in entrap ped bubbles is not likely. The formation of new bubbles at other defec ts present in the steel is not possible either. The measured value is in good agreement with the theoretical solubility limit of argon in st eel containing numerous small nonmetallic precipitates. Desorption mea surements revealed that differences in gas composition exist between b ubbles in a slab and in the final cold rolled product. Argon is the ma jor gas constituent in bubbles in slabs and blowholes in rolled produc ts (93 and >99 at.-%, respectively). The gas from the bubbles in slab samples also contained nitrogen (5 at.-%) and hydrogen (2 at.-%). On t he other hand, in blowholes only small amounts of methane are present. The dissociation kinetics of molecular gas inside the cavities has ye t to be clarified sufficiently to rule out completely pressure buildup in bubbles by hydrogen absorbed during downstream processing. IS/1375 . (C) 1998 The Institute of Materials.