Degradation of Mechanical Properties in Austenitic Stainless Steel Pipe and Fittings for Corrosive Refinery Applications

Recent tests and data analysis have shown a correlation between the degree of hot working and elevated temperature mechanical properties of austenitic stainless steel piping and fittings, independent of the final solution anneal temperature and grain size. While this phenomenon is shown to occur in stabilized stainless steels by data in this paper, it has also been observed in non-stabilized austenitic stainless steel grades. It is understood that work-hardening results in higher material strength, and that annealing results in lower strength, as well as promoting dissolution of carbides and intermetallic phases. The “as-fabricated” mechanical properties of hot formed product are affected by the competition between strain-hardening, and softening due to recovery and recrystallization during hot working and subsequent solution annealing. It has been shown that increasing the amount of hot forming lowers the yield strength of austenitic stainless steel at elevated temperatures. Data is presented that calls into question the common belief that the solution anneal substantially eliminates strain-hardening resultant of prior forming. This paper discusses strengthening mechanisms, provides case histories, suggests mitigation practices, and stresses the importance of proper alloy characterization, and using conservative Code allowable safety factors.