A block six or eight inches cube may require three to five hours to bring it up to the color and have it heated through, and sufficient time should be given; but as soon as it is hot through it should be removed from the fire.
A six-inch block may be brought up to a medium orange color in twenty minutes or less in a hot furnace, and then if it be kept in such a furnace until it is hot all through, the surface and edges will almost certainly be brought to a bright lemon color, with bad results. To do good annealing a piece should never be hotter in one part than in another, and no part should be hotter than necessary, usually the medium orange color. Annealing, then, is a slow process comparatively, and sufficient time should be allowed.
There are many ways of annealing steel, and generally the plan used is well adapted to the result desired; it is necessary, however, to consider the end aimed at and to adopt means to accomplish it, because a plan that is excellent in one case may be entirely inefficient in another.
Probably the greatest amount of annealing is done in the manufacture of wire, where many tons must be annealed daily.
For annealing wire sunken cylindrical pits built of fire-bricks are used usually; the coils of wire are piled up in the cylinders, which are then covered tightly, and heat is applied through flues surrounding the cylinders, so that no flame comes in contact with the steel. For all ordinary uses this method of annealing wire is quick, economical, and satisfactory. The wire comes out with a heavy scale of oxide on the surface; this is pickled off in hot acid, and the steel should then be washed in limewater, then in clean water, and finally dried.
If it be desired to make drill-wire for drills, punches, graving-tools, etc., this plan will not answer, because under the removable scale there is left a thin film of decarbonized iron which cannot be pickled off without ruining the steel, and which will not harden. It is plain that this soft surface must be ruinous to steel intended for cutting-tools, for it prevents the extreme edge from hardening—the very place that must be hard if cutting is to be done.
Tools for drills, lathe-tools, reamers, punches, etc., are usually annealed in iron boxes, filled in the spaces between the tools with charcoal; the box is then looted and heated in a furnace adapted to the work. This is a satisfactory method generally, because the tools are either ground or turned after annealing, removing any decarbonized film that may be found; the charcoal usually takes up all of the oxygen and prevents the formation of heavy scale and decarbonized surfaces, but it does not do so entirely, and so for annealing drill-wire this plan is not satisfactory. It is a common practice in annealing in this way to continue the heating for many hours, sometimes as many as thirty-six hours, in the mistaken notion that long-continued heating produces greater softness, and some people adhere to this plan in spite of remonstrances, because they find that pieces so annealed will turn as easily as soft cast iron. This last statement is true; the pieces may be turned in a lathe or cut in any way as easily as soft cast iron, for the reason that that is exactly what they are practically. When steel is made properly, the carbon is nearly all in a condition of complete solution; it is in the very best condition to harden well and to be enduring.
When steel is heated above the recalescence-point into the plastic condition, the carbon at once begins to separate out of solution and into what is known as the graphitic condition. If it be kept hot long enough, the carbon will practically all take the graphitic form, and then the steel will not harden properly, and it will not hold its temper. To illustrate: Let a piece of 90-carbon steel be hardened and drawn to a light brown temper; it will be found to be almost file hard, very strong, and capable of holding a fine, keen edge for a long time.
Next let a part of the same bar be buried in charcoal in a box and be closed up air-tight, then let it be heated to a medium orange, no hotter, and be kept at that heat for twelve hours, a common practice, and then cooled slowly. This piece will be easily cut, and it will harden very hard, but when drawn to the same light brown as the other tool a file will cut it easily; it will not hold its edge, and it will not do good work.
Clearly in this case time and money have been spent merely in spoiling good material. There is nothing to be gained, and there is everything to be lost, in long-continued heating of any piece of steel for any purpose. When it is hot enough, and hot through, get it away from the fire as quickly as possible.