Evaluation of welded joints of austenitic stainless steels at high temperatures in the presence of SO2, SO3, and O2

• Published in: Engineering failure analysis 2024-04, Vol.158, p.107959, Article 107959
• Main Authors: Resende, Flávio Augusto Coelho, Almeida, Luiz Fernando Maia de, da Silva, Leonardo Rosa Ribeiro, Vilarinho, Louriel Oliveira, Franco, Sinésio Domingues
• Format: Article
• Language: English
• Subjects: Austenitic stainless steel; Corrosion; Mechanical engineering; Sigma phase; Stress corrosion cracking; Superheater; Welding
• ISSN: 1350-6307; 1873-1961
• DOI: 10.1016/j.engfailanal.2024.107959

•Sigma phase nucleated cracks formed at high temperatures reduced toughness in welds.•Presence of SO2, SO3, O2 accelerates sigma phase corrosion in reduced Cr regions.•Notched specimens revealed E308L weld susceptibility, with faster crack formation.•Synergistic stresses, microstructural changes, and corrosion caused weld failure.•Materials with higher resistance to sigma phase formation should be investigated. This study undertakes a comprehensive examination of crack formation in welded joints of austenitic stainless steel 304, particularly in high-temperature corrosive environments prevalent in sulfuric acid production units. Initially, an investigation was conducted on a steam superheater that had been operational for about 19 years, where a crack formation was discovered. This unit, subjected to a working temperature of approximately 620 °C and exposure to gases composed of SO2, SO3, and O2, allowed for the analysis of corrosion products formed during the crack propagation. Building on the initial findings, a more controlled study was undertaken, focusing on the cracks formed in welded joints using the coated electrode E308L for joining 304H stainless steel plates under similar corrosive conditions. The specimens, constructed using materials typically utilized in sulfuric acid production units, were designed based on the double-beam principle, enabling exposure to both tensile stresses and a corrosive environment at high temperatures. Over a span of 122 days, or roughly 3000 h, microstructural analyses were conducted, revealing that the formation and propagation of cracks preferentially occurred through fragile microconstituents, notably the sigma phase. These findings mirrored the sigma phase formation observed in the initial superheater study and point to a stress corrosion process acting synergistically with sigma phase development, and consequently seen as a probable cause of failure. This comprehensive evaluation, therefore, indicates a significant role of the sigma phase and corrosive environments in influencing crack formation and propagation in welded joints of austenitic stainless steels, which require further studies for preventative strategies in industrial setups.