Dear Dr. Spyridon Chaskis,

at the following web-site by Paces Technologies: https://www.metallographic.com/Metallographic-Etchants/Metallography-Etchants.htm

you can find the etchant for your purpose.

Alternatively, as said before in a previous discussion present in RG by Dr. Alain Guidet, the observation of prior austenite grain size of microalloyed low carbon steel is indeed difficult. He suggested to prepare a water based saturated of picric acid at high temperature. Heat the solution up to the boiling temperature (80-90°C), add at this temperature picric acid up to saturation (you must see some picric crystals at the bottom of your flask). Also add some wetting agents (like the one used to wash dishes), some authors also mention the addition of few drops of HCl.  Quench for few seconds (5-15) the sample in the solution at a temperature slightly below the boiling point of the solution, rinse with water, then alcool and dry. Remove the black residue gently with cotton while rinsing. Check under the microscope and if it does not work try again... Be careful while handling picric acid.... please check the Safety Data Sheet before use.

Another method was suggested by Dr. Manidipto Mukherjee: the aqueous CrO3+NaOH-picric acid solution, heated to 120–130°C, will reveal prior-austenite grain boundaries in carbon steels, alloy steels (including alloys with more than 1% Si), and high-carbon, high-chromium steels with different microstructures. For more details, please see at:  http://www.sciencedirect.com/science/article/pii/104458039390078A http://digital.csic.es/bitstream/10261/78440/4/19_Mater.Char._capdevila.pdf

Best regards, Pierluigi Traverso

Etchant: Ferric chloride (45 gr), Copper ammonium chloride (9 gr), Hydrochloric acid (150 mL), Distilled water (75 mL).

Conditions: Immerse for several seconds.