Surface Hardening

Surface hardening is used to extend the versatility of certain steels by producing combinations of properties not readily attainable in other ways. For many applications, wear and the most severe stresses act only on the surface of a part. Therefore, the part may be fabricated from a formable low- or medium-carbon steel, and is surface hardened by a final heat treatment after all other processing has been accomplished. Surface hardening also reduces distortion and eliminates cracking that might accompany through hardening, especially in large sections. While an extensive review is beyond the scope of this article, two major approaches to surface hardening will be described below. One approach does not change composition and consists of hardening the surface by flame, induction, laser, or electron beam heating. The other approach changes surface composition and includes the applications of such diffusion techniques as carburizing, nitriding, and carbonitriding. Table 38 compares the relative benefits of the five most common surface hardening processes. More detailed information on surface hardening of steels can be found in Heat Treating, Volume 4 of the ASM Handbook.

Table 38 Relative benefits of five common surface-hardening processes

Process

Benefits

Carburizing

Hard, highly wear-resistant surface (medium case depth); excellent capacity for contact load; good bending fatigue strength; good resistance to seizure; excellent freedom from quench cracking; low-to-medium-cost steels required; high capital investment required

Carbonitriding

Hard, highly wear-resistant surface (shallow case depths); fair capacity for contact load; good bending fatigue strength; good resistance to seizure; good dimensional control possible; excellent freedom from quench cracking; low-cost steels usually satisfactory; medium capital investment required

Nitriding

Hard, highly wear-resistant surface (shallow case depths); fair capacity for contact load; good bending fatigue strength; excellent resistance to seizure; excellent dimensional control possible; good freedom from quench cracking (during pretreatment); medium-to-high-cost steels required; medium capital investment required

Induction hardening

Hard, highly wear-resistant surface (deep case depths); good capacity for contact load; good bending fatigue strength; fair resistance to seizure; fair dimensional control possible; fair freedom from quench cracking; low-cost steels usually satisfactory; medium capital investment required

Flame hardening

Hard, highly wear-resistant surface (deep case depths); good capacity for contact load; good bending fatigue strength; fair resistance to seizure; fair dimensional control possible; fair freedom from quench cracking; low-cost steels usually satisfactory; low capital investment required

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