2% Nital should reveal ferrite phase boundaries.

Saturated aqueous picric acid solution containing,  0.5% sodium alkylsulfonate or sodium tridecylbenzene sulfonateor and  few drops of HCl as wetting agent. Etching should be done at  ~80-90°C for low carbon steels. Best results are obtained if the steel is slightly tempered.

Madhumanti, I have read elsewhere apart from your answer to this question that temepering has to be done for revealing the grain boundaries. Infact, tempering is also suggested for revealing the grain boundaries of a fully martensitic microstructure which are otherwise not possible to see after etching with nital.  Should this tempering be done before etching with the suggested reagents or after etching? What role does tempering play in getting the best results?

Abhinav, during tempering or I would say aging treatment (below  austenite transformation temperature during heating), several alloying elements like P, Sn, Mo, Ni tends to segregate to the prior austenitic grain boundaries. Temper embrittlement also comes with phosphorus segregation. The overall idea is to delineate the austenite grain boundaries in the  temperature range where embrittlement occurs due to grain boundary segregations. Thus, prior austenite grain boundaries can be revealed easily when they are etched after tempering.

According to ASTM E 407 [Standard Practice for Microetching], Etchant 81 [2 g picric acid, 1 g sodium tridecylbenzene sulfonate, 100 mL water] will be a good solution. The procedure is given as "immerse few seconds to a minute". Application in temperature ranges between 30 - 60 °C would be better for revealing grain boundaries.