Identification of Phases in Stainless Steels Using Selective Etchants

The lecture covers the use of selective etchants to identify phases in austenitic, ferritic, martensitic, duplex and precipitation-hardened stainless steels, in the as-cast or in the wrought condition. The phases include the matrix phases (austenite, ferrite and martensite) plus carbides, delta ferrite, sigma and chi phases. Examples of phase identification in weldments and in failure studies are also covered. The study of how the delta ferrite in the AISI 308 stainless steel cladding on the lower head of the Three-Mile Island Unit 2 nuclear reactor changed with thermal exposure during the accident on March 28,1979 will be discussed. The delta ferrite was evaluated by selective etching and quantitative metallographic measurements of specimens removed from the head under where the molten uranium dioxide solidified. Results were compared to specimens removed from a nearly identical lower head from the Midland, Michigan nuclear reactor, whose construction was stopped after the TMI-2 accident. Midland samples were heated to a range of temperatures from 800 – 1100 °C and held for times from 1 to 100 minutes before cooling at a rate typical of the samples from the TMI-2 lower head after water was poured into the vessel when the operators discovered that the water level in TMI-2 had decreased substantially due to valve problems.