Because of the analysis of H13 chromium hot-work tool steel, and in particular because of chromium, molybdenum and vanadium, H13 will nitride very successfully to produce a highly wear-resistant surface. The type of nitriding (gas, salt or plasma) will depend on availability of equipment and the type of die being processed. On extrusion dies, it is recommended that a nitrided case be between 0.008-0.012 inch (0.2-0.3 mm) maximum. The monitoring of the process-gas analysis needs to be of concern in order to determine the compound-layer formation.
Heat checking is of particular concern especially on items such as shot sleeves. The molten aluminum will be injected into the sleeve at approximately 1100°F (590°C), and then the molten aluminum is ejected from the shot sleeve into the die cavity. Because of the repeated injection of the molten aluminum, the shot-sleeve face temperature begins to increase to a surface temperature probably higher than the tempering temperature of the H13 material.
The aluminum at high temperature causes a diffusion of surface oxygen, which will cause intergranular oxidation and a network of fine surface cracks. This leads to surface cracking and a softening and cracking of the surface of the H13 shot sleeve.
In addition, there are temperature differentials between the surface and the core of the H13 aluminum shot sleeve. This will develop compressive surface stress conditions and thermal differential expansion, which contributes (once again) to the surface cracking.
Where heat checking is problematic, good heat treatment and metallurgical process control is mandatory to ensure:
- Good uniform microstructures
- Fine carbide distribution
- Uniform martensitic structure
- High hardness
- Good impact toughness
The surface of the shot sleeve can be enhanced by nitriding, followed with a post-oxidation surface treatment immediately after the completion of the nitriding cycle.