Experimental investigation into the wear resistance of tungsten carbide-cobalt liners in a full scale pneumatic conveying rig

Abstract

The purpose of the investigation was to compare the relative wear resistance of various grades of sintered tungsten carbide liners against a mild steel standard in a full scale oneumatic conveying testing rig. Specimens ranging in cobalt content from 6i to 30% and in grain size from 0.56 to 2.98 micrometers, including a mild steel standard, were placed on a specially designed holder which fitted into a tee type 100 mm Jjureter bend. The specimens were tested under various operating conditions i.e. air velocity ranging from 28 m/s to 52 m/s, impact angles of 30° to 70°, mass flow rates of 35 kg/min to 83 kg/min and phase densities of 1.2 to 2.9, using a 4 mm nominal size crushed granite rock. The experimental results show that the ultrafine grained, low cobalt (6%) tungsten carbide displays little sensitivity to varying velocities, impact angles, mass flow rates or phase densities, and consistently gave the best wear resistance under all testing conditions. The coarse grained high cobalt (30%) tungsten carbide's wear resistance was found to be the most s e n s i t v re to ant * increase in conveying air velocity * decrease in phase density * decrease in solids mass flow rate * decrease in impact angle. This material consistently showed the least wear resistance under all testing conditions and performed only slightly better than mild steel. The effect of the carbide grain size was found to be small. However, the medium grained alloy displayed a higher erosion resistance than the fine grained alloy. This is due to the effect of plastic deformation, which determines the WC grain size that yields optimum erosion resistance, (if one excludes the ultrafine grained alloy which is expensive to produce). The effect of cobalt content was such that the lower cobalt specimens (6% range) consistently performed better than the higher cobalt contents (10%, 15%, 30%) under all testing conditions; the wear resistance decreasing with increasing cobalt content. Microstructurally it has been shown that there is a definite relationship between erosion resistance and the inverse of the magnetic coercivity of the tungsten carbide alloys. Maximum erosion occurring below 90° has been explained in terms of a combination of three energy mechanisms i.e. removal of cobalt, plastic deformation of the target specimens and fracture of the erodant particles.