Macro Defects in Steel - Forging Defects

Forging Defects

Discontinuities in forgings can originate from the defects in the slab, bloom, or billet being forged. These defects are modified during the forging of the material. The discontinuities can also result from the forging process itself. Some of the defects that can occur in forgings are similar to those in castings. Some specific defects of forgings are given below.

Laminations – These are large porosity, pipe and non-metallic inclusions in slabs, blooms, or billets which are flattened and spread out during forging process.

Forging laps – These are the discontinuities caused by the folding of metal in a thin plate on the surface of the forging. They are irregular in contour.

Centre bursts – These are ruptures which occur in the central region of a forging. They can arise because of an incorrect forging technique (e.g. very low temperature or drastic reduction) or because of the presence of segregation or brittle phase in the steel being forged.

Clinks (thermal cracks) – They are cracks due to stresses arising from excessive high temperature gradients within the material. Cracks formed during too rapid cooling originate at the surface and extend into the body of the forging while those formed during too rapid heating occur internally and can be opened up to become diamond-shaped cavities during subsequent forging.

Hairline cracks (flakes) – These are very fine internal cracks of circular shape. They develop and extend with time and are associated with the presence of hydrogen in steel. There is greater susceptibility in larger forgings than in smaller forgings and in certain grades of alloy steel than in carbon steel. They can be avoided by proper treatment.

Hot tears – These are surface defects due to the steel being ruptured and pulled apart during forging. They are normally associated with the presence of local segregation, seams, or brittle phases.

Overheating – It is normally identified by the facets seen on the fractured surfaces of a test-piece, but in extreme cases, can manifest itself as a severely broken-up surface.

Pipe – In case of forging of ingot, if there has been insufficient discard from the original ingot, remnant primary pipe normally show up axially. Secondary pipe which has never been exposed to the atmosphere gets welded-up if there is sufficient forging.

Reference: Website http://ispatguru.com/metallurgical-processes-and-defects-in-steel-products, 2017.