Surface Hardening Of Steels: Understanding The Basics

Surface Hardening of Steels J.R. Davis, editor, p1-16 DOI: 10.1361/shos2002p001 Copyright © 2002 ASM International ® All rights reserved. www.asminternational.org CHAPTER 1 Process Selection Guide 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 hardening, because less expensive low- and mediumcarbon steels can be surface hardened without the problems of distortion and cracking associated with the through hardening of thick sections. There are three distinctly different approaches to the various methods for surface hardening (Table 1): Table 1 Engineering methods for surface hardening of steels • Thermochemical diffusion methods, which modify the chemical composition of the surface 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 number 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 hardening is briefly reviewed in this chapter, with emphasis placed on process comparisons to facilitate process selection. More detailed information on the various methods described can be found in subsequent chapters. Diffusion methods Carburizing Nitriding Carbonitriding Nitrocarburizing Boriding Thermal diffusion process Applied energy methods Flame hardening Induction hardening Laser beam hardening Electron beam hardening Coating and surface modification Hard chromium plating Electroless nickel plating Thermal spraying Weld hardfacing Chemical vapor deposition Physical vapor deposition Ion implantation Laser surface processing Diffusion Methods of Surface Hardening As previously mentioned, surface hardening by diffusion involves the chemical modification of a surface. The basic process used is thermochemical, because some heat is needed to enhance the diffusion of hardening species into the surface and subsurface regions of a part. The depth of diffusion exhibits a time-temperature dependence such that: Case depth  K T im e (Eq 1) where the diffusivity constant, K, depends on temperature, the chemical composition of the 2 / Surface Hardening of Steels steel, and the concentration gradient of a given hardening species. In terms of temperature, the diffusivity constant increases exponentially as a function of absolute temperature. Concentration gradients depend on the surface kinetics and reactions of a particular process. Methods of hardening by diffusion include several variations of hardening species (such as carbon, nitrogen, or boron) and of the process method used to handle and transport the hardening species to the surface of the part. Process methods for exposure involve the handling of hardening species in forms su