Back polish : light polishing after etching. halos 晕圈 are area with low C% Martensite and were etched deeper, so they are rougher and were not polished during back polish.
Thanks for your kindly instruction. I have ever prepared more than thousands metallographic specimen of ductile iron for production examination, but never tried to light polish the etched sample even one time. What a pity!
You make me understood why interesting is so important to achievement.