On Microstructures And Properties Of Recycled Composites

This paper investigated the microstructures and properties of recycled composites ring samples prepared by the centrifugal casting of WC_p/Fe-C composite roller scraps after remelting treatment, so as to understand the effect of the remelting process on them.Experimental results show that tungsten carbide particles (WC_p) in the viscous Fe-C melt segregated towards the outer region under the action of centrifugal force during the centrifugal casting, leading to the formation of a functionally graded composites thick-walled ring selectively reinforced with the WC_p towards the outer region, where the WC_p were distributed in the matrix homogeneously. The hardness value is up to 63.3 HRC in the outer region and the impact toughness is up to 6.9 J/cm2 in the inner region. The volume fraction of WC_p in the outer region with a thickness of 10-15mm is strongly influenced by the rotation velocity of centrifuge: it reaches 54 vol.% at 780 r/min and 70 vol.% at 920 r/min, respectively. Theoretical analysis reveals that the convection velocity of WC_p in the viscous Fe-C melt towards the outer region is proportional to the square of angle velocity of the rorating centrifuge, that is to say, with an increase in the angle velocity resulted from the increased rotation velocity, the convection velocity increases sharpely, contributing to the segregation of WC_p in accordance with the actual results. SEM observation presents that small amounts of short rod-like crystallites have separated from matrix in the outer region of the casting, a good number of short rod-like and bone-like crystallites and nodular graphite phases have separated from the matrix displaying needle-like bainitic structure in the inner region, and a lot of coarser in-situ crystallites with snowflake-like or branched shapes have separated from the matrix in the intermediate region between the outer and inner region of the casting. EDS analysis shows that these in-situ crystallites contain Fe, W and Ni elements and the matrix contains W elements, which indicates that the dissolution of partial WC_p surface during the remelting treatment caused the supersaturated Fe-C-W alloy melt and then some W-contained carbides crystallites separated from the supersaturated Fe-C-W alloy melt during the casting process. The W elements retained in the Fe-C-W alloy melt during the casting process have made the matrix material high alloyed, which is benefial to improving the comprehensive mechanical properties of the castings.Dry sliding friction and wear tests with high sliding speed show that the dry sliding friction and wear behavior of the recycled composites ring surface is strongly influenced by the sliding speed compared to that of the applied load; with the same sliding speed and applied load, the friction coefficient of the recycled composites are much lower than those of the high-speed steel. SEM observation of the recycled composites worn surface indicates that when the sliding speed is relatively low, the main wear mechanism is fatigue wear, furrow and micro-cutting; when the sliding speed is relatively high, the main wear mechanism is spalling, three-body abrasion, adhesive wear and oxidation wear.