The post emulsifiable and water-washable fluorescent penetrant testing were carried out with ZL-27 A and ZL67 respectively.Ultrasonic cleaning by detergent were used for 30 minutes before penetrant. The parts were immersed and drained for 60 minutes. The macroscopic and microscopic characteristics of cracks were researched using the split mirror and scanning electron microscope. The results show that the outgrowth of high temperature oxidation plugs up the forging cracks. Thus the penetrant testing is not e...