Hello everyone,
I have been trying to etch my microalloyed steel with a solution including 4 gr of picric acid and 100 mlit of ethanol (4% Picral) but I could not get any success in it and the sample does not etch even after 10 mins of etching. Do you have any suggestion to help me out solving the issue I encountered?
Many Thanks in advance!
Sir,
The observation of prior austenite grain size of microalloyed low carbon steel is indeed difficult. I suggest you 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) your 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. Check the Safety Data Sheet.
Alain Guillet
Thank you Alain for your help. I helped me a lot. But, I've heard that I cannot make a 4% picric acid aqueous solution due to the solubility limit of picric acid in water is less that 1 wt.%. Is that correct? I could not find any resources that explains the solubuility limit? But I tried a few drops of HCl and it worked in some way! But I want to more clearly see the boundaries which I could not have reached yet to it!
Hello,
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.
Check the links
http://www.sciencedirect.com/science/article/pii/104458039390078A
http://digital.csic.es/bitstream/10261/78440/4/19_Mater.Char._capdevila.pdf
Thank you Manidipto. This is awesome. I will definitely try this. Just as an update, I could see PAGBs while using picral with few drops of HCl. I etched my microalloyed steels for 60-90 seconds. However, as some papers suggested, I aged the solution by leaving some small piece of steel in the solution for a short while and it gave me better revealing PAGBs in this way. Many thanks for all your help. But I'd like to get more comments and other potential ways if you guys have or find in the future.
Dear Mohsen,
I think following paper will be helpfull for your question regarding to alternative etching solution.Best Regards,
https://vacaero.com/information-resources/metallography-with-george-vander-voort/1405-revealing-prior-austenite-grain-boundaries.html
Dear Mohsen
It seems the following paper will be helpful to you
Article Revealing austenite grain boundaries by thermal etching: Adv...
Dear Mohsen,
Prior-austenite grain boundaries are those of the steel when it was austenitized prior to quenching and tempering. If the steel's microstructure is fully martensitic after hardening, or contains some retained austenite or lower bainite, the prior-austenite grain boundaries may be revealed.
Prior austenite grain size(PAGB) is important for understanding the microstructure–property characterization relationships in steels. The PAGB plays an important role in defining the microstructural scale of low-temperature phases and the mechanical properties (e.g., strength, ductility, fracture toughness, etc.) of steels in the final product form. It is recommended that swab etching with a solution of 100 ml saturated aqueous picric acid and 0.5 g sodium dodecyl benzene sulfonate(% can you easily determine equivalent to 0.5%) can worked remarkably well for delineating the PAGBs in this steel.
Refer the site:
https://www.jstage.jst.go.jp/article/isijinternational/58/10/58_ISIJINT-2017-485/_pdf
Hope content is useful for you.
Ashish
I am agreeing with Mr. Ashish Thakur. Swab etching with picric acid works. But I think, electrolytic etching may also works on austenitic structures.
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