Surface Hardening of Steels

Surface hardening a process which includes a wide variety of techniques 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 and breakage upon 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 steels has an advantage over through hardening because less expensive low-carbon and medium-carbon steels can be surface hardened without the 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): methods that involve an intentional buildup or addition of a new layer and methods that involve surface and subsurface modification without any intentional buildup or increase in part dimensions.

Table 1. Engineering methods for surface hardening of steels.

Layer additions	Substrate treatment
Hardfacing Fusion harcifacing Thermal spray Coatings Electrochemical plating Chemical vapor deposition (electroless plating) Thin films (physical vapor deposition, puttering, ion plating) Ion mixing	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

The first group of surface hardening methods includes the use of thin films, coatings, or weld overlays (hard-facings). Films, coatings, and overlays generally become less cost effective as production quantities increase, especially when the entire surface of work pieces 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. For tool steels, for example, TiN and Al2O3 coatings are effective not only because of their hardness but also because their chemical inertness reduces wear and the welding of chips to the tool. Overlays can be effective when the selective hardening of large areas is required.

The second group of methods on surface hardening is further divided into diffusion methods and selective hardening methods. Diffusion methods modify the chemical composition of the surface with hardening species such as carbon, nitrogen, or 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. 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 nitriding, ion implantation and ion beam mixing) are based solely on compositional modification.

As previously mentioned, surface hardening by diffusion involves the chemical modification of a surface. The basic process used is thermo-chemical because some heat is needed to enhance the diffusion of hardening species into the surface and subsurface regions of part.

The depth of diffusion exhibits time-temperature dependence such that:

Case depth ≈ K √Time

where the diffusivity constant, K, depends on temperature, the chemical composition of the 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 such as gas, liquid, or ions. These process variations naturally produce differences in typical case depth and hardness (Table 2). Factors influencing the suitability of a particular diffusion method include the type of steel (Table 3).

It is also important to distinguish between total case depth and effective case depth. The effective case depth is typically about two-thirds to three-fourths the total case depth. The required effective depth must be specified so that the heat treatment can process the parts for the correct time at the proper temperature.

Table 2: Typical characteristics of diffusion treatments

Process	Nature of case	Process temperature (°C)	Typical case depth	Case hardness (HRC)	Typical base metals
Carburizing Pack	Diffused carbon	815-1090	125μm-1.5mm	50-63*	Low-carbon steels, low-carbon alloy steels
Gas	Diffused carbon	815-980	75 μm-1.5mm	50-63*	Low-carbon steels, low-carbon alloy steels
Liquid	Diffused carbon and possibly nitrogen	815-980	50 μm-1.5mm	50-65*	Low-carbon steels, low-carbon alloy steels
Vacuum	Diffused carbon	815-1090	75 μm-1.5mm	50-63*	Low-carbon steels, low-carbon alloy steels
Nitriding Gas	Diffused nitrogen, nitrogen compounds	480-590	12μm-0.75mm	50-70	Alloy steels, nitriding steels, stainless steels
Salt	Diffused nitrogen, nitrogen compounds	510-565	2.5μm-0.75mm	50-70	Most ferrous metals. Including cast irons
Ion	Diffused nitrogen. nitrogen compounds	340-565	75μm-0.75mm	50-70	Alloy steels, nitriding steels, stainless steels
Carbonitriding Gas	Diffused carbon and nitrogen	760-870	75μm-0.75mm	50-65*	Low-carbon steels, low-carbon alloy steels, stainless steels
Liquid (cyaniding)	Diffused carbon and nitrogen	760-870	2.5-125μm	50-65*	Low-carbon steels
Ferritic nitrocarburizing	Diffused carbon and nitrogen	565-675	2.5-25μm	40-60*	Low-carbon steels
Other Aluminizing (pack)	Diffused aluminum	870-980	25μm-1mm	< 20	Low-carbon steels
Siliconizing by chemical vapor deposition	Diffused silicon	925-1040	25μm-1mm	30-50	Low-carbon steels
Chromizing by chemical vapor deposition	Diffused chromium	980-1090	25-50μm	Low-carbon steel < 30; High-carbon 50-60	High- and low carbon steels
Titanium Carbide	Diffused carbon and titanium, TiC compound	900-1010	2,5-12.5μm	> 70*	Alloy steels, tool steels
Boriding	Diffused boron. boron compounds	400-1150	12,5-50μm	40- > 70	Alloy steels, tool steels,Cobalt and nickel alloys

*  Requires quench from austenitizing temperature.

Table 3. Types of steels used for various diffusion processes

Diffusion substrates
Low-carbon steels	Alloy steels	Tool steels	Stainless steels
Carburizing Cyaniding Ferritic nitrocarburizing Carbonitriding	Nitriding Ion nitriding	Titanium carbide Boriding Salt nitriding Ion nitriding Gas nitriding	Gas nitriding Titanium carbide Ion nitriding Ferritic nitrocarburizing