Solid-state recycling for machined chips of iron by hot extrusion and annealing
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Solid-state recycling for machined chips of pure iron by hot extrusion at 823 K and annealing at 1073–1273 K was carried out. The as-extruded solid recycled specimen without annealing was fractured prior to yielding at room temperature. However, high ductility was recovered by annealing at 1073–1273 K. This is because the oxides at the interface of the machined chips dispersed in grain by annealing. The annealed solid recycled specimens showed higher yield stress than the annealed virgin extruded specimens. Grain refinement for the solid recycled specimens was enhanced by the high dislocation density in the machined chips, resulting in higher strength in the recycled specimens. Thus, the solid-state recycling is a low energy upgrade recycle process.
I. INTRODUCTION
Recycling of metals, especially iron and steel, is one of the important technologies for materials circulation because of their large consumption.1 In current recycle processes, iron and steel scraps are recycled by remelting.2,3 For example, in Japan, most steel scraps, whose total amount is about 40 million tons per year, are recycled by re-melting process.2 However, it is metallurgically difficult to eliminate detrimental elements such as copper and tin from steel scraps by re-melting process using redox reaction,4 resulting in a reduction in service properties.5 Hence, the current recycling processes are downgrade recycling.6 The desirable recycling is non-downgrade, or upgrade, recycling, in which service properties of a recycled material are the same as or higher than those of a virgin ingot, achieved with low energy consumption. In particular, upgrade recycling may be attained for scraps in manufacturing facilities such as machined chips because contamination of detrimental elements is less for scraps in manufacturing facilities. Solid-state recycling7–14 is one of the possibilities for upgrade recycling. In solid-state recycling, metal scraps are directly recycled by plastic deformation process such as hot extrusion,7–11 forging,12 and BMA (bulk mechanical alloying).13,14 It should be noted that re-melting is not needed for solid-state recycling. In previous works, the solid recycled magnesium alloy7–9 and aluminum alloy11 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0204 1524
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 5, May 2004 Downloaded: 17 Mar 2015
showed high strength due to microstructural control by the hot extrusion. The aims of the current paper are to apply solid-state recycling as recycling of machined iron chips and to investigate the mechanism of improvement or recovery of mechanical properties in the recycled iron. The current paper reveals that high dislocations density generated by machining causes grain refinement, resulting in high strength. This points out the importance of effective use of the internal energy stored in scraps for upgrade recycling. Also, annealing causes recovery of high ductility of the recycled iron. The energy for annealing is much lo
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