The surface treatment of H13 hot-work tool steel for extrusion and forging dies is a technique that has been practiced for many years to extend the performance of the die surface. Many different methods of surface treatment have been tried with varying degrees of success.
This presentation will review the different surface-modification techniques that are available to the engineering industry. We will also review technology that is not yet fully utilized by die manufacturers, but it offers very interesting alternatives to the current techniques (e.g., titanium-nitride coatings, vanadium-carbide coatings, complex and duplex coatings and DLC (diamond-like coatings) treatments).
The toolmaking industry has tried many different methods of metallurgical surface-treatment techniques to both improve and extend the useful life of the die with minimal downtime. The nitriding process has proven to be the most commercially acceptable process that has displayed any degree of success.
This has been largely due to the fact that the process is conducted at low temperature and that no quench is necessary to accomplish high surface-hardness values. This means that the likelihood of distortion occurring during the surface-treatment process will be limited to the relief of internal and residual stresses. This makes the process very popular where complex shapes and sections are involved due to the reduction of distortion. Figure 1 compares the various nitriding processes that are currently available.
General (Gaseous) Nitriding
The general principles of the nitriding process are based on the decomposition of ammonia by heat in the presence of a steel catalyst. The reaction of decomposition is a reversible reaction, and ammonia is used as the process gas. The process gas will decompose into its component elements of nitrogen and hydrogen in the ratio of one part nitrogen to three parts hydrogen.
For a minute fraction of a second, the nitrogen will exist as atomic nitrogen followed by its combination with another nitrogen atom, thus forming a molecule of nitrogen. It is that fraction of a second when the atomic nitrogen exists that, in combination with the process temperature, the nitrogen will diffuse into the surface of the steel, forming stable nitrides with the appropriate alloying elements. If these alloying elements are not available, the nitrogen will form with the iron to form iron nitride.
The process chemistry of gas nitriding is consistent with the process medium being used – gas or salt-bath nitriding. When using the gas nitriding process, for example, the source of nitrogen is anhydrous ammonia, which decomposes into a fixed ratio of one part of nitrogen and three parts of hydrogen.
This means that with a fixed gas chemistry the results will be fixed surface metallurgy. However, the decomposition ratio can be varied by dilution of the process with a supplemental gas of nitrogen or hydrogen.
The immediate surface metallurgy is seen as a compound zone (alternatively known as the white layer). This layer will consist of two phases: gamma prime and epsilon nitride. These phases that will be seen in the immediate surface of the steel are a direct result of the process-gas ratio hydrogen to nitrogen (3-to-1).
As previously stated, it is possible to manipulate that ratio by dilution. When this is accomplished then the immediate surface metallurgy will be changed.
The gamma-prime phase within the compound zone is a ductile phase, which will accomplish hardness values of approximately 800 VPN. The epsilon phase is very much less ductile, yet it is extremely brittle. This phase exhibits a very hard surface hardness but with a very low impact value. The epsilon phase has extremely good abrasion resistance to aluminum oxide formed during the hot extrusion of aluminum or die casting.
The carbon content of the steel is a contributing factor to the formation of the epsilon phase of the steel that is being treated. Medium- to high-carbon steels tend to cause the promotion of the epsilon phase, whereas low-carbon steels will tend to promote the gamma-prime phase in the compound layer.