Failure Analysis of Stainless Steel Sheets of Heat Shield Assembly of a Vacuum Degassing Furnace

Abstract

Abstract A twenty-five-year-old vacuum furnace is being used for degassing of ultra-high vacuum components. The heat shield assembly of the furnace comprised of one molybdenum (Mo) front sheet, followed by four 304 stainless steel (SS) sheets. During prolonged service of the furnace, SS sheets developed thick black oxide scale. As a part of refurbishment drive, the SS sheets were subjected to chemical cleaning in 20 % HNO3 + 1.5 % HF, which caused severe thinning along the edges of some sheets. Detailed analysis of a damaged SS sheet showed that the damage was caused by severe sensitization of edge region which underwent extensive intergranular corrosion during chemical cleaning. Important remedial measures suggested to avoid similar failures included: (i) use of type 316L or stabilized grades of austenitic SS sheets, (ii) use of two Mo front sheets in place of one and (iii) use of mechanical cleaning in place of chemical cleaning.