Reducing the Surface Degradation of Aluminum Extrusion Dies During Preheating

Aluminum extrusion dies are usually made from H13 steel that is ferritically nitrocarburized to minimize wear and pick-up. Before being placed in the extrusion press, the dies are preheated to minimize thermal shock at the start of the extrusion cycle. During the preheating time, the nitrocarburized layer oxidizes. Some of this layer can break away during extrusion leaving marks on the product. Although inerting the preheat furnaces with nitrogen has been found to reduce the oxidation, it does not solve the problem completely. Experiments have shown that a small addition of ammonia to the preheating protective atmosphere could eliminate oxidation and prevent nitrogen loss from the surface nitride layer.