Steel may be good or bad in quality due to the substances it contains, and steel which is good when tapped from the furnace may be made bad by subsequent casting, reheating and mechanical treatment. Rolling improves the strength of a metal and tends to cover up defects.

Specifications for metals, particularly iron and steel, usually require (1) surface inspection, (2) physical testing, which is the actual pulling and bending of specimens of the metal, to determine its strength and ductility, and (3) chemical analysis to determine its chemical constituents. It is intended to mention here only the requirements of surface inspection, as the defects so disclosed are closely associated with casting and subsequent rolling or forging of metals.

Specifications usually provide that the surfaces of metals shall be free from the following-named defects:

(1) Slag.

(2) Foreign substances.

(3) Brittleness.

(4) Laminations.

(5) Hard spots.

(6) Sand or scale marks.

(7) Scabs.

(8) Snakes.

(9) Pits."

These defects are usually caused by:

(1) Slag and oxides which entered the ingot or billet mould from the ladle.

(2) The chilling of ingot corners in cold moulds, and surface laminations and shot caused by pouring metal against the sides of the mould and thus chilling it.

(3) The sudden chilling of the surface of a hot ingot, billet, bloom, or slab when it is taken from the mould or reheating furnace, causing small surface cracks called snakes. These cracks, when not too numerous nor too deep, are chipped from the surface before the material is rolled or forged, as their sides do not weld together when the metal is rolled.

(4) Failure to discard enough of the ends of an ingot or a billet.

(5) Squeezing metal in rolling so that fins press out between the rolls, and subsequently rolling these fins down as laminations or hard streaks.

(6) Not cleaning the mill scale from the upper surface in rolling plates, or other wide shapes, from which scale will not readily fall away due to the movements and jolting of the rolls.

Further than the defects named, rolled products should not be crooked or warped, nor should plates show undue variation of thickness across their width nor along their length.

It is a common practice to keep a record of the identity of every ingot and billet cast, as in this way many defects in finished products may be traced back to their causes.