Revealing Prior-Austenite Grain Boundaries in Heat Treated Steels

Revealing prior-austenite grain boundaries in heat treated steels is probably the most difficult, and most frustrating, task facing the metallographer or failure analyst. Historically, many different etchants have been proposed for this work. They may work reasonably well for one composition but not at all for many others. To test a steel to find out if it has fine-grained characteristics at either the normal austenitizing temperature or at the carburizing temperature is a simpler task as the McQuaid-Ehn carburizing test can be used, or the oxidation method can be used. But, when we cannot perform a heat treatment, but must reveal the PAGBs only by etching, that is a far more difficult task. This paper describes an etchant and etching procedure that has a wide range of applicability. The most successful PAGB etch in the past was aqueous saturated picric acid with a wetting agent, usually sodium dodecyl benzene sulfonate, at room temperature. This etch works reasonably well if the carbon content is >0.35%, if the phosphorous content is >0.010%, and if the steel was tempered <1060 °F/571 °C, and if the microstructure is martensite or lower bainite. With a modification of this etchant, PAGBs have been revealed in steels with <0.1% C, <0.010% P, and with tempering temperatures >1060 °F/571 °C.