Third, by reference to the specific-gravity [table No. I, Chap. V], it will be seen that the change in volume due to differences of temperature is much less in mild steel than in high steel. This fact does not rest upon the evidence of this table alone; it is a fact of common knowledge to all steel-makers that mild steel is much more inert than high steel; therefore differences of heat and working that produce serious results in high steel are hardly appreciable in mild steel. As a rule all structural steels are comparatively mild, therefore they are generally in a fit condition for use when they leave the rolls or forge. In cases of special forging, where one part is heated and another is left cold, as in the forging of the heads of eye-bars, it would seem to be wiser to anneal such pieces to remove the area of strain that must exist between the unheated parts and those that were heated and forged.
The operation of removing strains and hardness by careful, uniform heating and slow cooling is known as annealing.
Annealing should not be confused with tempering. Tempering is the partial softening of hardened steel, to remove some of the exceeding brittleness of hardened steel, and so to make it strong and highly elastic while it is still very hard.
Annealing is the complete softening of a piece of steel; that is to say, as a rule, the obtaining of the utmost softness that is possible; or in any case to have the steel softer than any tempering would leave it.
Annealing, and tempering are frequently used synonymously. Such misuse of terms in speaking of technical matters leads to confusion of ideas and misunderstandings.
As a rule, the best heat to use for annealing is that which gives a medium orange color; it is a good heat to quench from; it is a little above the heat of recalescence, about 655° Cent. This heat is that which gives the finest grain to steel when it is hardened, and is known as the refining heat.
As steel is thoroughly plastic and soft at this heat, and as it yields the best and strongest grain when cooled from this heat, it is clear that there is nothing to be gained by heating any higher for annealing.
In annealing, the steel should be brought up to the right color, medium orange, and left at that heat until it is hot through, care being taken that the heat does not run any higher in any part of the piece. If the corners or edges or any part be allowed to run up to bright orange, or to medium or bright lemon, as is often done, then there is bad work; the result will be uneven grain and internal strains.
When steel is to be hardened afterwards, there may be no harm in heating up to an even lemon color; but where is the use in applying this excess of heat merely to make a coarse grain, when the lower, medium orange color will give just as good softness and a much better grain?
The time necessary for good annealing depends upon the size of the piece; a wire may be brought up to the right heat in five minutes or less, and heated through in another minute; then it should be removed from the fire, as every additional moment of heating will only injure the steel.