It require right etchent. please check the concentration, time of application, right magnification of microscope

Velaphi:

I responded to a similar question on the detection of the prior austenite grain size in steels a while ago. Here's a synopsis of my answer.

You can determine the prior austenite grain size in steels the same way as you measure any other grain size, using standard line intercept methods or equivalent, in accordance with the ASTM standard mentioned in the preceding replies. The main issue though is how you actually see the prior austenite grains in the first place. They are not that easy to see. Typical etchants for steels, such as nital and picral, will generally not reveal the prior austenite grain boundaries.

As Dr. Mirzadeh mentioned above, a good way to etch the prior autensite grain boundaries in steels is to use an etchant of a saturated solution of picric acid containing a few ml of "teepol" (sodium alkyl sulfate) at 75C, although it can work at room temperature as well. This acts to "groove" the grain boundaries, particularly if there has been some segregation there during the austenitizing treatment. Be careful though as the picric acid will stain you hands yellow if you get it on them (you'll then look like you have nicotine stains from smoking too much!), and picric acid can become explosive if it is left too long, so it's wise to dispose of it once you're finished. I hope that this helps.

ROR

Hello,

Maybe the reason is not directly related with etching or etchants. As our experience, only a few seconds etching with even 2% Nital after a proper sample preparation results with clear PAGB image.

Before quenching, a coarse grinding followed by a 400 grit SiC abbrasive application on the surface will help you to reveal grain boundaries. Also, it is important to prepare a flat surface after quenching. Since the boundaries will form in a very thin layer, the preparation should be very gently.

Then, you may use Nital or Picral solutions for etching. My recommendation is also same as above; using saturated picric acid solution with a small amount of teepol and applying the solution above 40 °C will give the best result.

Kind regards,

Emre