The mechanical properties of heat-treated alloy steels are strongly influenced by the grain size of the parent austenite phase before quenching. But revealing the prior-austenite grain boundaries (PgGBs or PAGBs) can be quite difficult depending upon the alloy and its microstructure.
There are a number of well-established (see ASTM E 112, for example) procedures that are used to decorate the PgGBs during a heat-treatment cycle, e.g., the McQuaid-Ehn carburizing test (Fig. 1) and the oxidation test. In some medium-carbon steels, at a specific cooling rate, proeutectoid ferrite will precipitate at the PgGBs, while in high-carbon steels (generally hypereutectoid tool steels), proeutectoid cementite will precipitate on the PgGBs upon slow cooling from elevated temperatures. These conditions are often seen in as-cast or as-rolled steels, as shown in Figures 2 and 3. But these methods cannot be applied to determine the prior-austenite grain size of a steel part or component that has already been heat treated, as these methods will produce a different grain size. For this problem – and this is a common situation in failure analysis – one can only use an etching technique to reveal the PgGBs. By George F. Vander Voort