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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 vTime

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

* 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