This slightly rough-and-ready surface inspection method is good enough for most purposes of steel billets which will go for further processing, eg. forging.
Four grinding wheels, one for each face, oscillate as the billet passes through the machine, exposing a sine-wave track of bare silver metal in its grinding path, which exposes linear surface defects. Which are seen in the light-tight inspection booth adjacent to the grinding head. Where bright lights shine directly and through mirrors onto the billet surface, with inspectors pressing a marker to the passing surface wherever defects are seen - which will be very thin linear indications along the axis of the billet.
Out in the billet finishing shop, for a any batches with flaw indications, the "chippers" roll-out the billets on trestles and gouge along the cracks identified. Using an air hammer with semi-circular chisel bit, you follow the centre of the crack given it splits the curling-off chipping in-two - and silveriness at the lower extent of the vertical split in the long chip you are gouging indicates you are "bottoming-out" the defect.
(R. Smith, 13Dec2017, 10Feb2018)