The use of H-13 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 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. It will also review technology that is not yet used by die manufacturers but which offers interesting alternatives to current techniques such as the use of titanium-nitride (TiN) coatings, vanadium-carbide coatings, complex and duplex coatings and 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 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 are comparisons of the various nitriding processes currently available.
Comparisons of Nitriding Processes: General 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 reversible, and ammonia will decompose into its component parts (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, which combines to form molecular nitrogen. It is in that fraction of a second 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, then the nitrogen will combine with iron to form iron nitrides.
We will continue to evaluate the various nitriding process methods in part 2.