Surface Hardening of Steels

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 softer, tough interior of the part. This combination of hard surface and resistance to breakage upon impact is useful in parts such as a cam or ring gear, bearings or shafts, turbine applications, and automotive components that must have a very hard surface to resist wear, along with a tough interior to resist the impact that occurs during operation.

Most surface treatments result in compressive residual stresses at the surface that reduce the probability of crack initiation and help arrest crack propagation at the case-core interface. Further, the surface hardening of steel can have an advantage over through hardening because less expensive low-carbon and medium-carbon steels can be surface hardened with minimal problems of distortion and cracking associated with the through hardening of thick sections.

There are two distinctly different approaches to the various methods for surface hardening (Table 1):

Table 1 Engineering methods for surface hardening of steels

Layer additions

Hardfacing:

^¡       Fusion hardfacing (welded overlay)

^¡       Thermal spray (nonfusion-bonded overlay)

Coatings:

^¡       Electrochemical plating

^¡       Chemical vapor deposition (electroless plating)

^¡       Thin films (physical vapor deposition, sputtering, ion plating)

^¡       Ion mixing

Substrate treatment diffusion methods: 

^¡       Carburizing

^¡       Nitriding

^¡       Carbonitriding

^¡       Nitrocarburizing

^¡       Boriding

^¡       Titanium-carbon diffusion

^¡       Toyota diffusion process

Selective-hardening methods:

^¡       Flame Hardening

^¡       Induction Hardening

^¡       Laser Hardening

^¡       Electron beam hardening

^¡       Ion implantation

^¡       Selective carburizing and nitriding

^¡       Use of arc lamps

Methods that involve an intentional buildup or addition of a new layer

Methods that involve surface and subsurface modification without any intentional buildup or increase in part dimensions.The first group of surface-hardening methods includes the use of thin films, coatings, or weld overlays (hardfacings). Films, coatings, and overlays generally become less cost-effective as production quantities increase, especially when the entire surface of workpieces must be hardened. The fatigue performance of films, coatings, and overlays may also be a limiting factor, depending on the bond strength between the substrate and the added layer.

Fusion-welded overlays have strong bonds, but the primary surface-hardened steels used in wear applications with fatigue loads include heavy case-hardened steels and flame- or induction-hardened steels. Nonetheless, coatings and overlays can be effective in some applications. With tool steels, for example, TiN and Al2O3 coatings are effective not only because of their hardness but also because their chemical inert-ness reduces crater wear and the welding of chips to the tool. Some overlays can impart corrosion-resistant properties. Overlays can be effective when the selective hardening of large areas is required.

This introductory article on surface hardening focuses exclusively on the second group of methods, which is further divided into diffusion methods and selective-hardening methods (Table 1). Diffusion methods modify the chemical composition of the surface with hardening species such as carbon, nitrogen, or boron. Diffusion methods may 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.

In contrast, selective surface-hardening methods allow localized hardening. Selective hardening generally involves transformation hardening (from heating and quenching), but some selective-hardening methods (selective nitrating, ion implantation, and ion beam mixing) are based solely on compositional modification. Factors affecting the choice of these surface-hardening methods are discussed in the section “Process Selection” in this article.

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Revised by Michael J. Schneider, The Timken Company, and Madhu S. Chatterjee, Bodycote.