Mr. David Pye (View Profile)
Pye Metallurgical International Consulting.
The use of H13 steel for both extrusion and die casting dies is a technique that has been practiced for many years to extend the performance of the surface of the die during its operations. Many different methods of surface treatments have been tried with varying degrees of success. This presentation will review the different surface modification techniques that are available to industry. The presentation will also review technology that is not yet used by the die manufacturers but which offers interesting alternatives to current techniques such as the use of Titanium Nitride coatings, Vanadium carbide coatings, complex and duplex coatings and the DLC treatments (diamond like coatings).
The tool making industry has sought and 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 process will be limited to the relief of internal and residual stresses. This makes the process popular where complex shapes and sections are involved due to the reduction of distortion. The following comparisons are shown of the various nitriding processes that are currently available.
Comparisons of the various nitriding processes.
General Principles of Nitriding
The general principles of the 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 will decompose into its component parts of nitrogen and hydrogen in the ratio of 1 part nitrogen to 3 parts hydrogen. For a minute fraction of a second, nitrogen will exist as atomic nitrogen, followed by its combination with nitrogen atom nitrogen, thus forming molecular nitrogen. It is that fraction of a second when the atomic nitrogen exists, that, in combination with the process temperature that 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, then the nitrogen will form with iron to form iron nitrides.
Evaluation of the Nitriding Process Methods
The process chemistry of nitriding is consistent with the process medium being used, be it gas or salt bath nitriding. When using the gas nitriding process for example, the source of nitrogen is anhydrous ammonia, which decomposes to a fixed ratio of one part of nitrogen and three parts of hydrogen. This means that with fixed gas chemistry, the results will be a fixed surface metallurgy. However this decomposition ratio can be varied by dilution of the process gas with a supplemental gas of nitrogen or hydrogen.
The immediate surface metallurgy is seen as a compound zone. (Also known as the white layer) This layer will consist of two phases known as gamma prime and epsilon nitride. In this next phase is as a direct result of the ratio of the process gas, hydrogen to nitrogen (3 to 1 ratio) As previously stated it is possible to manipulate that ratio by dilution. When this is accomplished then the 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 is a very hard phase, yet with a very low impact value. The epsilon phase has extremely good abrasion resistance to aluminum oxide formed during the hot extrusion of aluminum.
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 too 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.
The subject of ion nitriding is not a new subject, as the procedure has been known for many years, in fact since 1932. The phenomenon of plasma was first commercialized by Drs. Weynheldt and Berghaus in Germany. The technology was based on the generation of the gaseous plasma using continuous DC electrical power. The process of ion nitriding is governed by the same laws of physics for gaseous diffusion into steel at elevated temperatures. These are the same laws of diffusion that govern the laws of diffusion of nitrogen derived from salt bath or gaseous nitride systems.
The speed of the reaction of decomposition and diffusion appear to be faster with ion nitriding drama with conventional techniques. This is an incorrect assumption. However, the speed all of gaseous preparation is considerably faster due to the immediate gaseous ionization without the need for time for catalytic reaction as is observed with salt bath and gaseous nitriding techniques. In addition to this, the process gases involved in plasma nitriding can be a varied in terms of gas ratios, thus giving the ability to the process to create both the necessary and appropriate surface metallurgy that will best suit the working environment of the die.
The surface of the steel is prepared for the nitriding procedure in a very different manner than is seen for the conventional surface treatment preparation. The surface preparation method for ion nitriding is to make use of a phenomenon known as a “sputter cleaning.” This phenomenon can be likened to atomic shot blasting. Instead of using steel shot carried in a blast of air to the work piece surface, the process now uses ionized gas electrons that are carried electrically to the work piece surface at a very high speed. The steel surface is therefore prepared for nitriding in a more efficient manner than when using conventional degreasing methods. The overall cycle times for the process using the ion nitriding technique, are generally faster than those of the more conventional nitriding methods.
The main process control parameters for the ion nitriding process are listed as follows
- Process temperature
- Process time
- Process gas ratios
- Process voltage
- Pulse duration (power on time)
- Pulse off time
- Pulse duty cycle
- Process pressure
Emerging technologies are focusing on deposition methods of coating the steel surface with a hard metal (or combinations of metals) which is harder than that of the steel. These coatings are known as Ultra Hard Coatings, or Surface Deposition Technique.
There are many different methods of the application of coatings that can be put down onto a metal substrate to improve its wear and corrosion characteristics. Generally the coatings can be subdivided into many different categories, for example, the simple groupings:
- Decorative coatings
- Hard coatings
- Ultra hard coatings
These coatings can be further categorized into the following temperature ranges, which will be determined by the selected coating type:
- Low temperature coatings, up to a process temperature of 500 ° C.
- Medium temperature coatings, up to a process temperature of 1000 ° C.
- High temperature coatings, up to a process temperature of 1700 degrees C.
This will be followed by the following categories in the relation to the method of application of the coat to the substrate material:
- Chemical vapor deposition
- Plasma assisted vapor deposition
- Physical vapor deposition
A further subdivision can be made according to the nature of the method of application of the deposited costing, which is:
- Pack Cementation
- Reactive gases
With the advent, developments and growth of plasma technology, one is now able to process under vacuum conditions and accomplishes both controllable and repeatable results.
Chemical Vapor Deposition (CVD)
This technology is both a developed and versatile application method which is utilized to deposit layers of almost any metal on to a substrate material. These metals candy and most of the metals found in the periodic table of elements and that within the metal group. Other compounds such as metallic oxides, nitrides, carbides and intermetallics can be deposited relatively easy.
Because there are numerous metallic elements and derivatives of these elements that are used in CVD, there are equally as many numerous reactive chemistries. These can include:
- Thermal decomposition
A CVD reaction is controlled by the following factors:
- Thermo dynamic, mass transport, and kinetic considerations
- Chemistry of the reaction
- Process parameters such as temperature, pressure, and chemical activity
When considering the application of a surface deposition method, it is necessary to exercise good control procedures of the selected method to produce repeated and consistent depositions. It should be remembered that they are depositions and not diffusion processes. This means that pre-cleaning of the part prior to the coating application is of paramount importance. The degree of cleanliness accomplished, will determine the success of the entire deposited coat.
With a diffusion process, a volumetric change in size is often seen which could lead to the necessity of further machining after the diffusion treatment. The need for post machining can be reduced by careful selection of the process temperature.
The deposition methods guarantee that there will be a surface size change. The size change will be of course, dependent on both the time and temperature. Some predictability can allow the engineer some latitude in allowing the part to grow into size.
The choice between a deposited coat or a diffused case is one which must be made after careful consideration of the operating conditions and environment that the component will be working under.