Damage of repeatedly nitrocarburised steel dies for aluminium extrusion

The dies employed for the hot extrusion of aluminium alloys are subjected to repeated cycles of nitriding, or nitrocarburising, after service periods of given length. Three dies, fabricated with ISO X40CrMoV5-1 (AISI H13) steel and liquid nitrocarburised, were examined either in the asfabricated condition, or after one service period, or after 12 nitrocarburising and service cycles (end of life). Samples cut from the die subjected to one service period were re-nitrocarburised for increasing durations and similarly examined. The examination methods included X-ray diffraction, metallography, microhardness profile measurements and SEM fractography. The service temperature induced microstructural modifications in the core and in the case. Microcracks and wear craters, due to adhesive wear, occurred at the bearing surfaces. The wear mechanisms at a bearing surface, the microstructural evolution close to the same surface, the fatigue crack nucleation and early growth, and their interrelations and consequences during the whole die life are discussed.