Roll coatings formed during the hot rolling of aluminium with rolls of different materials

A small passive mill was used to study the roll coatings developed on rolls of different materials during the hot rolling of aluminium strip. The structure and composition of the coatings were examined by scanning electron microscopy and X-ray microanalysis. Rolls of solid alumina failed during the rolling experiments, but rolls with a plasma-sprayed layer of alumina on steel remained undamaged although they showed evidence of the formation of a roll coating. Steel rolls formed no detectable coating after the 56 rolling impressions given in these experiments and a thermally produced iron oxide film on a pair of steel rolls was removed by the rolling. Small amounts of discontinuous roll coatings were found on rolls of tungsten carbide, silicon carbide and silicon nitride. The largest amount of roll coating was observed on rolls having a plasmasprayed layer of chromium oxide, and this was associated with the complex structure of the original surface of the layer. Analysis of the roll coatings showed that they contained aluminium metal. The results indicate that in the presence of a lubricant-coolant during rolling it was the mechanical interactions, rather than chemical or thermal, that influenced the formation of a roll coating.