Internal surface roughness of cold pilgered zircaloy tubes

Cold pilgering imposes a mixed lubrication regime which gives a “plateaux and valleys” surface, with generally a high rate of plateaux, and related transfer, galling and scratching hazards. An investigation of the inner surface topography of zircaloy pilgered tubes is carried out. First, the evolution of topography along the pilgering process for given (reference) tooling and lubricant is illustrated. Then the final surface is studied as a function of lubricant nature and tool design. A mechanical model of pilgering (published elsewhere) and experimental roll force measurements are used to investigate the correlation between mechanics and surface topography. For a given tool, the higher the roll force (or contact stress), the higher the proportion of plateaux. Moreover, severe risks of defects (scratches, etc.) appear when the plateaux cover more than about 85% of the surface.