Evaluation of Nitriding Process MethodsGas Nitriding
The metallurgy of a gas or salt-bath nitride process is consistent with the chemistry of the medium being used. Gas nitriding, for example, uses anhydrous ammonia, which has a fixed composition of one part nitrogen to three parts hydrogen. This suggests that a fixed gas chemistry will yield a fixed surface metallurgy. The only time the surface metallurgy can be altered is by dilution or addition to the process gas with a supplemental gas of nitrogen or hydrogen.
In order to understand the significance of this, see Figure 1. This shows the formed case of steel nitrided by a conventional gas. The compound zone (sometimes known as the white layer) consists of two metallic phases, gamma prime (g¢) and epsilon ( e). The mixed phase is a direct result of the process-gas ratio of hydrogen to nitrogen (3:1). If that ratio is changed, then the surface metallurgy will also change.
The gamma-prime phase in the compound zone (or white layer) is a ductile phase that achieves hardnessess of ±800 VPN. The epsilon phase in the compound zone is brittle because it is a very hard phase with a very low impact value. This phase has good abrasion resistance, however. The two phases in combination will extend the life of a forging die. There is, however, a bit of a trade-off in the metallic phases formed, whereby the gamma-prime phase will enhance wear resistance and the epsilon phase will enhance impact resistance.
Another contributing factor to the formation of the epsilon phase is the carbon content of the steel being treated. Medium- to high-carbon steels will promote the formation of the epsilon phase in the compound zone, whereas low-carbon steels will promote the gamma-prime phase. Thus, treated medium- to high-carbon steels will be more wear resistant and treated low-carbon steels will have better impact resistance.
The ion-nitriding process is governed by the same laws of diffusion as conventional gas nitriding. It is a vacuum process that uses electricity and nitrogen gas to cause diffusion into the forging die. However, the ion-nitriding surface-reaction time (surface catalytic reaction time) is considerably faster than that of conventional nitriding because in gas nitriding the whole chamber must be brought to a temperature high enough to decompose the ammonia. This takes a little time. Consequently, cycle times for the process of ion nitriding, which can commence at the flick of a switch, are faster than those of conventional nitriding. In addition, the process gases can be varied at will to suit the necessary surface metallurgy required for the optimum performance of the steel.
For ion nitriding, the surface of the steel is prepared in a completely different manner to that required of conventional nitriding. The surface is sputter cleaned, rather than conventionally degreased. A simple analogy of sputter cleaning is likened to that of “atomic shot blasting.” Instead of using steel shot to clean the part, the part is cleaned by ionized hydrogen atoms bombarding the steel’s surface, which is also more efficient than conventional degreasing.
Emerging TechnologiesThe emerging technologies are usually deposition methods of coating the steel surface with a material harder than that of the steel. These are known as surface-deposition techniques. There are many different coatings that can be put down onto a metal surface to improve its wear characteristics and corrosion resistance. A comparison of these coating materials is shown in Table 3.
There are two primary methods of applying deposition coatings – physical vapor deposition (PVD) and chemical vapor deposition (CVD). The PVD method used to be a “line-of-sight” operation, meaning that only surfaces of the workpiece exposed to the deposition material source could be coated. The technology has matured, however, to the point where the workpiece can be placed on a rotating table to expose its different sides to deposition. PVD has more potential as a die-coating process, primarily because it is a low-temperature process and requires no phase changes to the steel substrate (die) material as it is heated, as would be the case with CVD.
The CVD method of deposition is not yet widely accepted as a forging-die treatment. It was first applied by immersing the part into a metallic compound (such as aluminum oxide or titanium dioxide) in a steel box, sealing the box and heating in a conventional furnace to an elevated temperature in order to release the metal gas to be condensed onto the part surface. This is a cumbersome and usually inconsistent method of application and has severe limitations on the materials that can be treated.
With the advent and subsequent developments in plasma technology, one is now able to process under vacuum-type conditions with a high degree of control and repeatable results. Diamond-like coatings are produced under vacuum conditions and are usually assisted with enhanced plasma. Enhanced plasma is a more intense form – almost a plasma within a plasma.
Two notable exceptions to the PVD and CVD processes are the application of vanadium carbide, which is usually applied using a salt-bath technique, and DLCs (diamond-like coatings), which require a vacuum-furnace technique in combination with enhanced plasma.
Diffusion or Deposition?When considering a metallic surface-deposition method, one needs extremely good control of the method to produce repeatedly consistent depositions since deposition processes are more erratic than diffusion processes. With a diffusion process, there is usually a surface volumetric change, but this volume change is usually very small (within 2 microns) and within product tolerances. It is dependent on:
- Time at the process temperature
- Process temperature
- Gas composition
With any of the technologies discussed here, the success of the process will be contingent upon the degree of cleanliness prior to whichever process method has been selected. This will determine how good the diffusion will be (with both gas and plasma process techniques) or how good the deposition hard coating will be. Another consideration to address will be the material selection for the die. The typical material group for most of the hot-work die applications is the H series (hot work) group of steels, often H-13.
In addition to material selection, the pre-heat treatment prior to coating of the manufactured die is absolutely critical. It is the die material condition (developed by the pre-heat-treatment procedure) that will determine the success of either the diffused layer or the thin-film hard-coat deposition. It is the pre-heat-treatment procedure and resulting metallurgy that will support either the diffused case or the deposited thin film.
ConclusionThe surface conditions of a forging die can only be enhanced by surface treatments – either diffusion techniques or deposition techniques. The nitriding process has not been fully utilized by many industries that are using hot-work applications for metal manipulation. The determination of the success of the forging die is dependent on the following criteria: pre-heat treatment, surface nitride composition and surface metallurgy (Figure 3). The forging industry has made attempts to develop appropriate and effective nitriding procedures.
The forging techniques – be they hot forge, warm forge or cold forge – are brutal to the base material, no matter what it might be. A great deal of research has been done on the base materials, whose cost is a major factor. But no matter how good the base material is, it will eventually fail either by longevity, cracking or heat checking. Heat checking relates to the contact time of hot billet and forging die. Repeated contact sets up thermal gradients between the core and surface of the die. Resulting thermal stresses can cause microcracks in the die, which can then oxidize and eventually result in catastrophic failure.
Author David Pye is president of Pye Metallurgical Consulting, Meadville, Pa. He can be reached at 814-337-0194; fax: 814-337-5939; or e-mail firstname.lastname@example.org