Broaches should, as a rule, be made from a good grade of crucible tool steel. Several of the alloy steels work exceptionally well for broaches that are to be subjected to heavy pulls; this is especially true of vanadium tool steel, the vanadium renders the steel stronger and tougher, and its presence in the steel also increases the range of heat that can be employed when hardening, without augmenting the brittleness. The manufacturers of these steels recommend a hardening temperature of from 1350° F. to 1425° F., grading the heat according to the diameter of the broach. The temper should be drawn to a full straw color- 460° F.
There are several oil-hardening steels that work well for many kinds of broaches. Their nature varies so much that it would not be wise to give specific instructions for their use. In case they are employed, it is best to obtain instructions for their treatment from the individual makers.
High-speed steel is used for some classes of broaches, but it is not advised unless the designer is familiar with the limitations of this steel for this particular class of work. In some cases where conditions are favorable, high-speed steel broaches used on malleable cast iron give exceptionally good results.
Regular carbon-tool steel when used for draw broaches should ordinarily contain from 1.0 to 1.1 per cent carbon, although excellent results follow the use of steel containing 1.25 per cent carbon, if the pull is not too great; in the latter case, the lower carbon content is to be preferred.
For broaches that are not to be subjected to great pulling strain, a good grade of basic open-hearth steel containing thirty points carbon works well, especially where the broaching is done directly from a cored or forged hole and where the broach is to be subjected to considerable vibration. Broaches made from this material must be pack hardened.