If using optical microscopy, then etching it using Vilella's reagent (1 gram Picric acid, 5ml Hydrochloric acid, 100ml Ethanol) should work.

If you have EBSD available then you could use that instead.

Dear Ravi

You can try with Fry reagent: 

5g CuCl2

40 ml HCl

30 ml H2O

25 ml ethanol (95%)

or Aqua regia: 

20-30 ml HCl

10 ml HNO3

30-20 ml glicerine

good luck

@harry coules: Thank you sir for your suggestion. sir i used  Vilella's reagent.  Vilella's reagent perfectly worked in heat affected zones. But, in weld fusion zone grain boundary was not clear. sir, can you please suggest any other option for grain size measurement. 

Hi Ravi,

I'm afraid I don't have any further suggestions for etchants. I would move on to electron microscopy at that stage.