Abstract:The austenite grain size of subeutectoid steel C72D2 was determined by direct quenching and hardening method. Using picric acid solution and ferric chloride solution of different ratio as corrosives,the corrosion effects of different corrosives on C72D2 were observed and compared. The results showed that the austenite grain boundary of the sample corroded by ferric chloride solution (ferric chloride 5 g+concentrated hydrochloric acid 5 drops+water 100 mL) was relatively complete and dark in color,which was convenient for observation and determination. It could substitute picric acid solution for austenite grain boundary corrosion of subeutectoid steel C72D2.