SURFACE HARDENING OF STEEL PARTS

Abstract

SURFACE HARDENING, a process that includes a wide variety of techniques (Table 1), is used to improve the wear resistance of parts without affecting the more soft, tough interior of the part. This combination of hard surface and resistance to breakage on impact is useful in parts such as a cam or ring gear that must have a very hard surface to resist wear, along with a tough interior to resist the impact that occurs during operation. Further, the surface hardening of steel has an advantage over through harden-ing, because less expensive low-and medium-carbon steels can be surface hardened without the problems of distortion and cracking associ-ated with the through hardening of thick sec-tions. There are three distinctly different ap-proaches to the various methods for surface hardening (Table 1): • Thermochemical diffusion methods, which modify the chemical composition of the sur-face with hardening species such as carbon, nitrogen, and boron. Diffusion methods allow effective hardening of the entire surface of a part and are generally used when a large num-ber of parts are to be surface hardened. • Applied energy or thermal methods, which do not modify the chemical composition of the surface but rather improve properties by altering the surface metallurgy; that is, they produce a hard quenched surface without additional alloying species. • Surface coating or surface-modification methods, which involve the intentional buildup of a new layer on the steel substrate or, in the case of ion implantation, alter the subsurface chemical composition Each of these approaches for surface harden-ing is briefly reviewed in this chapter, with emphasis placed on process comparisons to facilitate process selection. More detailed infor-mation on the various methods described can be found in subsequent chapters.