Temperingof Steel

Tempering of Steel is the process by which tools, etc., are brought to the requisite degree of hardness or temper. The article is first hardened by heating to redness and sudden cooling in water. It is then "let down" by reheating to a lower temperature. The ultimate hardness depends upon the temperature of this second heating, and this is estimated by observing the colour of the film of oxide which forms upon a part brightened for the purpose. The colours and corresponding approximate temperatures for various purposes are as follows:- Pale straw, 430° F., surgical instruments; straw, 460°, tools for brass and wood and penknives, etc.; brown, 500°, scissors, hatchets, saws; purple, 530°, table knives and tools for soft wood; blue, 580°, swords, axes, etc.; deep blue, 590°, watch springs. needles, saws.