1. A method of detecting defects in a metal part arising from processing of the metal part, the method comprising: acquiring a thermal image of the metal part after processing the metal part; determining a difference image by comparing the thermal image to a reference image, the difference image being related to temperature differences between temperature data represented by the thermal image and reference temperature data represented by the reference image; determining a gradient image using the difference image, the gradient image representing temperature difference gradients within the difference image; and detecting defects in the metal part using the gradient difference image.