ENHANCING THE LIFE OF SHREDDER HAMMER TUNGSTEN CARBIDE TIPS

THIS paper reports on a computational and experimental investigation into the dynamic impact of steel debris against sugar mill shredder hammer tips. During cane preparation, billets of cane are comminuted against tungsten carbide (WC) tips with impact velocities approaching one-third the speed of sound. Foreign debris (often hardened steel pins) in the input stream, result in a high probability of fracture to the WC tiles. During fabrication, the WC tiles are brazed to a mild steel or white iron backing-block using a tri-ply laminate. The laminate is several orders of magnitude less stiff than the WC. Besides acting as a fastener the laminate serves to minimise the build-up of residual stress in the WC due to the marked difference in coefficient of thermal expansion between adjacent materials. The laminate also influences the impedance as compression waves are reflected and refracted. The impact process when steel debris comes into contact with the WC is modelled using explicit finite element dynamics. A Rankine rotating crack model is used to characterise the WC and the projectile is modelled as elasto-plastic. The effect of the WC geometry on resistance to crack initiation is noted and explored numerically. The mechanisms, which dominate failure, are reported. Laboratory experiments confirm the computational modelling. Factory observations involving novel tip geometries at Tully Mill provide evidence of throughputs exceeding 750 000 tonnes of cane between replacements.