Sensitization and Desensitization (Healing) in Austenitic Stainless Steel: A Critical Review

摘要

Austenitic stainless steel (ASS) or Rust free steel has become a predominant part in the finishing stages of superheater/reheater tubing of supercritical boilers, where intergranular corrosion (IGC) resistance turns out to be significant. Application of ASS is limited up to 600 degrees C due to IGC failure led by sensitization. The word sensitization states the failure in corrosion resistance due to chromium (Cr) depletion which further resulted from the chromium-carbide (Cr23C6) precipitation at grain boundaries. Sensitization takes place when an un-stabilized ASS is cooled gradually from the solution annealing zone (1100 degrees C) or re-heated (welding and post-weld heat treatment) between the temperature ranges from 400 to 800 degrees C. The reduction in the distance of the Cr depleted zone across the grain boundary is referred to as recovery from sensitization or healing or desensitization which may occur owing to several reasons. This review paper introduces a research work on sensitization recovery/healing in ASS and its (corrosion) measurement techniques. The analysis of sensitization and its healing is studied in terms of the degree of sensitization affected by several variables such as the effect of prior cold work, martensite, grain size, grain orientation, the effect of alloying elements, and temperature along with time. These parameters expressively change the activities of carbon (C) and Cr in the ASS and alter the carbide nucleation and growth. Further, various destructive and non-destructive sensitization measurement methods have been studied and appropriate practice is suggested which can quantify both Cr segregation and Cr depletion width precisely to identify desensitization. According to the present review, improvement of IGC resistance assisted by desensitization studied and needs for forthcoming investigations has been recognized to reinforce the capacity and abilities of the ASS in Super Critical Boiler technology.