Abstract: Black image on the base material of HP Low C static casting pipe elements in ethylene cracking furnace of a petrochemical enterprise appeared in the process of radiographic testing (RT). Through verification of optical and electronic metallographic observation combining with radiographic testing and penetrant testing (PT) and other non-destructive testing methods, it is determined that the black image is non-defective image display due to intergranular diffraction caused by local coarse grain. This paper points out that non-defective image display from RT testing can be reduced or eliminated by changing the incident angle and energy of X-ray and adding the film hanger wave filtering.