Wear Beahaviour of Nitrided Microstructures of Aisi H13 Dies for Hot Extrusion of Aluminium

Due to their lightness and good mechanical properties various Al-alloy products obtained by casting, rolling and hot extrusion are still gaining in importance. Hot extrusion of Al-alloys (Figure 1.a) enables the production of a very varied range of profi le shapes, which, due to the above mentioned excellent properties are used mainly in the automotive and electro-industries, in aeronautics, in the building trades, in household goods, etc. The opening on the die (bearing surface, Figure 1.b), through which the extruded piece leaves, gives the extruded profi le the desired shape and dimensions.The most frequently used die steel is AISI H13. Beside the quality of the bearing surface its tribological conditions determine the surface quality of the product. Namely, the die life is determined by allowable tolerances of the cross sectional dimensions and by the appearance of the surface on the extruded piece [1 4]. Nitriding of bearing surfaces on dies (tools, AISI H13) for hot extrusion of aluminium is technologically a very sensitive process with regard to achieving a constant quality of the nitrided layers. This study was based on the analysis of microstructure on dies with intentionally prepared deep and narrow gaps which were nitrided by various manufacturers of equipment for gas and ionic nitriding. The manufacturers chose their own nitriding parameters in order to achieve an optimal wear resistant microstructure. The microstructures obtained showed differences with regard to the presence or absence of a compound layer (white layer), its thickness and its ε/γ′ phase ratio (XRD), nitriding depth and microhardness profi le. The measured nitriding depths and the maximum microhardness values on nitrided surface layers were quite similar on dies of the same manufacturer, while for different manufacturers these values differed. Differences with regards to compound layer characteristics were also found on the same die. The die samples with these various nitrided microstructures were then laboratory tested for wear resistance using equipment that provides simulation of the tribological conditions during hot extrusion of aluminium. The wear testing results show differences in behaviour of the nitrided samples. The differences in the actual structures, microstructures, hardness, etc. explain the high level of scattering in die life in actual industrial applications.