Good results are obtained by packing the steel in iron boxes or tubes, much as for case-hardening or carbonizing, using the same materials. Pieces do not require to be entirely surrounded by carbon for annealing, however. Do not remove from boxes until cold.
Steel to be annealed may be classified into four different groups, each of which must be treated according to the elements contained in its particular analysis. Different methods are therefore necessary to bring about the desired result. The classifications are as follows: High-speed steel, alloy steel, tool or crucible steel, and high-carbon machinery steel.
ANNEALING OF HIGH-SPEED STEEL
For annealing high-speed steel, some makers recommend using ground mica, charcoal, lime, fine dry ashes or lake sand as a packing in the annealing boxes. Mixtures of one part charcoal, one part lime and three parts of sand are also suggested, or two parts of ashes may be substituted for the one part of lime.
To bring about the softest structure or machine ability of high-speed steel, it should be packed in charcoal in boxes or pipes, carefully sealed at all points, so that no gases will escape or air be admitted. It should be heated slowly to not less than 1,450°F. and the steel must not be removed from its packing until it is cool. Slow heating means that the high heat must have penetrated to the very core of the steel.
When the steel is heated clear through it has been in the furnace long enough. If the steel can remain in the furnace and cool down with it, there will be no danger of air blasts or sudden or uneven cooling. If not, remove the box and cover quickly with dry ashes, sand or lime until it becomes cold.
Too high a heat or maintaining the heat for too long a period, produces a harsh, coarse grain and greatly increases the liability to crack in hardening. It also reduces the strength and toughness of the steel.
Steel which is to be used for making tools with teeth, such as taps, reamers and milling cutters, should not be annealed too much. When the steel is too soft it is more apt to tear in cutting and makes it more difficult to cut a smooth thread or other surface. Moderate annealing is found best for tools of this kind.
TOOL OR CRUCIBLE STEEL
Crucible steel can be annealed either in muffled furnace or by being packed. Packing is by far the most satisfactory method as it prevents scaling, local hard spots, uneven annealing, or violent changes in shape. It should be brought up slowly to just above its calescent or hardening temperature. The operator must know before setting his heats the temperature at which the different carbon content steels are hardened. The higher the carbon contents the lower is the hardening heat, but this should in no case be less than 1,450°F.