TABLE OF CONTENTS One of the most important factors to be considered when evaluating the possibility of heat treatment for any particular material is the effect it may exert on the size of its grain. Since all fine-grained structures display much better toughness and less inclination to warpage at higher attained hardnesses, such materials are definitely preferable for this procedure.
The greatest danger of the emergence of grain-related irregularities may be encountered at temperatures above the critical range, in parts previously cold-worked, when an effect known as grain growth may occur. This condition was discussed in Sec. 9-11-4, "Grain Growth."
Suitability of Materials for Heat Treatment. The suitability of materials for heat treatment is given by the ease of the hardening process or by the depth of hardness penetration achievable within that material. Such suitability, otherwise called hardenability of the material, is closely tied to the carbon content of the particular stock: With greater carbon content the hardenability is improved. Low-carbon steels sometimes must first be saturated with carbon elements, or carburized, in order to attain the necessary hardness range.
Hardenability is further affected by the cooling rate of steel, or the speed at which the material must be cooled in order to harden. The depth of hardness penetration with regard to the length of exposure to hardening influences is often assessed as an indicator of hardenability. The depth is always more pronounced in materials with higher carbon content, where the difference between the hardened case and softer...
