Before specimens can be etched to reveal prior-austenitic grain boundaries, they must be prepared to a high-quality level. The most critical step is sectioning, which must be conducted to induce minimal damage. Use abrasive cut-off machines with a blade/wheel designed for metallography and for steels of the hardness level being prepared. Generally, mounting is performed, but may not be necessary if the structure at the edges of the sample is not important. Preparation methods can be found on websites such as www.struers.com. You can see a more detailed version of this note in my article in April 2010's Industrial Heating.
The writer and others (see references in the Industrial Heating article) have been using saturated aqueous picric acid plus a wetting agent and a small HCl addition (when steels have more than about 1% Cr) for some time, but formerly at room temperature. The specimen is placed polished face vertical in a beaker with at least 100 ml of the etchant in an ultrasonic cleaner. The water level in the ultrasonic cleaner should not be higher than the etchant level in the beaker or it will flip over. The timer would be set for 7 minutes with etching at room temperature. Results at room temperature for 8620, 4140 and 5160 in the as-quenched and tempered (400, 800 and 1200°F) conditions revealed PyGBs only in the 4140 and 5160 specimens in the as-quenched condition and after tempering at 400 and 800°F. Light back-polishing was always done to try to improved the visibility of the grain boundaries. As sodium tridecylbenzene sulfonate became difficult to obtain, the writer switched to the dodecyl version with no apparent difference.
In more recent years, I have used Nacconol 90G as the wetting agent. (Nacconal is a registered trademark of the Stepan Company of Northfield, Ill.) This is described as sodium alkyl benzene sulfonate. On the MSDS sheet the composition is given as 90-93% sodium dodecylbenzene sulfonate, 5% sodium sulfate, 1% sodium chloride and 1.5% water. In the past, I have used wetting agents from Wito Corp., ICN Biochemicals, Rhone-Poulene, Pilot Chemical Co. and Sigma Chemical Co. (see Materials Characterization, Vol. 35, September 1995, pp. 135-137). HCl was added in the amount of 6 drops per 100 ml of the saturated aqueous picric-acid solution (1-500 ml). After this was mixed, the excess picric acid was removed by filtering. Etching was conducted at room temperature for 7 minutes using the ultrasonic cleaner for agitation. However, the results were similar to before. Hence, the writer heated the solution to 80-90°C (below the boiling point to retard evaporation). Specimens were swab etched for 1-2 minutes and then lightly back-polished to remove smut and excessive structure etching. Some were swabbed a second time, followed by light back-polishing.
Results were exceptionally good. All of the 8620, 4140 and 5160 specimens (in the Industrial Heating article) showed prior-austenitic grain boundaries adequate for manual measurements. Other low-to-medium alloy steels were successfully etched with carbon contents as low as 0.07%, phosphorus as low as 0.005% and tempering as high as 1350°F (but still below the AC1) – a vast improvement. For questions, contact the writer at firstname.lastname@example.org.