The intelligent reading of industrial radiographic film defect information for welds has always been an important issue, usually divided into three steps: image pre-processing, feature extraction, and defect recognition. In the image preprocessing process, it is necessary to extract the weld area, reduce the number of parameters, and avoid interference from the base material area and lead identification on defect recognition. However, on most digital films after grayscale transformation, there is usually a grayscale gradient from white to grey to black from the weld to the base material, which causes the weld boundary to be blurred and the weld area cannot be directly extracted. In this article, we divided the digital films into three regions, "welding seam region", "uncertain region", and "non-welding seam region", based on the grey value changes, and assigned labels "correct", "unknown", and "wrong" to them respectively. We trained these three regions using the U Net, and the regions predicted as "unknown" and "correct" were combined as welding seam region. The experimental results showed that the proposed method achieved a reasonable prediction rate (RP) of 95.45% on the test set, with a minimum MAE index of 0.0028.