1. A method of detecting defects in a metal part arising from processing of the metal part, the method comprising: obtaining a reference image for the metal part before processing the metal part; acquiring a thermal image of the metal part after processing the metal part; determining a difference image by comparing the thermal image to the 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 by displaying a filtered gradient image indicating regions of the metal part having a temperature difference gradient greater than a threshold value.