1. FUNDAMENTAL PRINCIPLES OF THE PROCESS
This thermos-diffusion surface treatment is defined by the German industrial standard DIN 17014 as “an enrichment by thermo-chemical treatment of the surface of a workpiece”. By means of thermal energy, the atoms of the diffuser element are combined in the metal lattice of the workpiece surface where the corresponding compounds are formed with the atoms of the base material.
2. SUITABLE MATERIALS FOR TREATMENT
Vachrom TFB treatment has gained interest because in the case of parts where only the surface layer has to be hardened, it is not necessary to use high-alloy steels which are very expensive and also difficult to machine. In many cases a good result can be achieved by using low-alloy steel or even high-speed machining steels (AVP).
Nitriding alloy steels such as 34 CrAlNi 7 (no. 1.8550), steels with a silicon content of more than 0.5 % by weight and light alloys should be avoided.
Other metals such as cobalt, nickel and tungsten carbide can also be treated with Vachrom TFB. However, based on the results obtained to date, it is advisable to treat hard metals only if the binder metal content is > 6 vol.%.
Copper cannot be treated with Vachrom TFB. For this reason, in the form of self-adhesive strips or sheet metal, it is used effectively to cover areas or zones that do not require treatment.
3. TREATMENT CONDITIONS
Depending on the base material employed, temperatures between 800-1050° C may be used. An exception is cast iron and tungsten carbide. For cast iron, the treatment temperature must not exceed 850-880° C.
As far as tungsten carbide and other carbides are concerned, they should be treated Vachrom TFB at a temperature of 900° C max.
At higher temperatures, not only would the matrix undergo Vachrom TFB treatment, but tungsten and, if present, titanium and/or tantalum carbide would also be transformed into other harmful compounds.
The treatment temperatures for parts protected with copper must not exceed 930°C due to problems of interaction between the copper itself and the components used for diffusion.
4. SUBSEQUENT HEAT TREATMENT
Since the coefficient of thermal expansion of the Vachrom TFB treated layer is approximately 14 X 10 -6° C-1 for iron alloys or common technical steels, it is possible to temper parts conventionally in hot baths, in air or in oil and then subject them to tempering to increase the hardness of the core.
Therefore, tempering of treated layers to a thickness of 120 μ normally does not lead to cracking. Austenitizing and tempering must however be carried out in an inert atmosphere, preferably in a protective gas, in a vacuum or in a neutral salt bath.
5. OPTIMAL THICKNESS OF THE TREATED LAYERS
It has long been known that the optimal treated layer is not necessarily the one with the greatest thickness. The thickness should always be adapted to the desired application and base material:
- Thicker layers for erosion wear (e.g. ceramic pressing equipment)
- Thin layers for adhesion wear (e.g. metal stamping equipment)
In theory, the layers treated with a thickness of about 5 μ would be suitable for preventing adhesion wear but, since crystals by their nature have a strong tendency to combine with each other, it is not even possible to produce layers of this thickness on non-alloy or low-alloy steels.
For applications such as chip-free metal forming tools, it was found that the best results were obtained with high-alloy steels with layers ranging in thickness from 15 to 20 μ.
Considering all these elements, it can be concluded that the choice of the base material is determined by the application to be made, bearing in mind that the diffusion of Vachrom TFB in the steel becomes increasingly difficult as the content of alloying elements increases.
The Vickers hardness of the treated layers on ferrous-based materials varies from 1800 to 2100 HV, i.e. in line with the hardness values of aluminium oxide. On high-alloy steels, hardness values of up to 2400 HK 0.025 are sometimes found.
Micro-hardness tests are carried out either on sections polished in normal direction to the surface or, in exceptional cases, on the polished surface of the workpiece, provided that there are high diffusion thicknesses.
In the case of Knoop and Vickers tests, higher loads generally lead to inaccurate results as the treated layer cracks or splits. Brinnell or Rockwell test methods are totally inadequate for testing treated layers as they cause deformation of the base material and at the same time destroy the surface layer.
6.2 Resistance to acids
Vachrom TFB treatment can significantly increase the acid resistance of low-alloy ferrous materials. Our tests with various mineral acids not only confirm this, but also show that all austenitic stainless steels treated with Vachrom TFB are extremely resistant to hydrochloric acid. This property is already being used very successfully in technical applications.