The progress of degradation on the bearing surfaces of nitrided dies for aluminium hot extrusion with two different relative lengths of bearing surface

In this study progress of the wear on the bearing surfaces of nitrided dies for aluminium (AA6063) hot extrusion is demonstrated and explained. Two groups of nitrided AISI H11 tool steel dies were analysed, with relatively short and long bearing surfaces. It was found that bearing surface length in relation to the microstructure of the nitrided exit edge should be considered especially in the case of relatively short bearing surface. Cracking and adhesive removal of die material were identified as the main wear mechanisms leading to the formation of craters, both deep and shallow furrows and removal of the exit edge of the bearing surface. On the exit edges with relatively short bearing surface, initiation of wear on the two characteristic spots (distances) from exit edge was observed while on the relatively long bearing surface, wear observed was mainly in the central area of the bearing surface. Formation of deep furrows originates from previously formed craters which then expand in the longitudinal direction. On the other hand formation of shallow furrows is based on the high-density of adhesively removed islets on the nitrided layer that leads to its removal in longitudinal direction as a consequence of its decreased shear strength. Oxygen and aluminium ingress was detected on the grain boundaries far beyond the crack tip, which accelerated grain boundary decohesion.