The Effect of Cold Work on the Creep Properties of Copper
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The Effect of Cold Work on the Creep Properties of Copper Asa Martinsson and Henrik CM Andersson-Ostling Department of Manufacturing and Strength, Swerea KIMAB, Box 55970, 102 16 Stockholm, Sweden ABSTRACT Spent nuclear fuel is in Sweden planned to be disposed of by encapsulating in waste packages consisting of a cast iron insert surrounded by a copper canister. The waste package is heavy. Throughout the manufacturing process from the extrusion/pierce-and-draw manufacturing to the final placement in the repository, the copper is subjected to handling which could introduce cold work in the material. It is well known that the creep properties of engineering materials at higher temperatures are affected by cold working. The study includes creep testing of four series of cold worked, oxygen-free, phosphorus doped copper (Cu-OFP) at 75 °C. The results are compared to reference series for as-received material carried out in a recent study. Two series of copper cold worked in tension (12 and 24 %) and two series cold worked in compression (12 % parallel to creep load axis and 15 % perpendicular to creep load axis) were tested. The results show that pre-straining in tension of copper leads to prolonged creep life at 75 °C. The creep rate and ductility are reduced. The influence on the creep properties increases with the amount of cold work. Cold work in compression applied along the creep load axis has no effect on the creep life or the creep rate. Nonetheless the ductility is still impaired. However, cold work in compression applied perpendicular to the creep load direction has a positive effect on the creep life. Cold work in both tension and compression results in a pronounced reduction of the initial strain on loading. Yet the high value of the area reduction, 90 %, is unaffected by the degree of cold work. INTRODUCTION Spent nuclear fuel is in Sweden planned to be disposed of by encapsulating in cast iron inserts inside sealed copper (Cu-OFP) canisters. The cast iron is load bearing and the copper canister is protection against corrosion from the outside. The copper canister consists of an extruded or pierce-and-draw formed tube that is sealed by welding of a forged lid. Throughout the manufacturing process to the final deposition in the repository the copper is subjected to handling which could introduce cold work in the material. Some cold work has been observed after the hot forging of the lids. Also machining of the copper components is a potential source of cold work. A concern is to what extent cold work affects the mechanical properties of copper and especially the long term creep properties. The effect of surface damaged induced cold work on copper canister has been investigated in a recent study [1]. It was concluded that the affected area was dependent on both the depth of the imprint and the shape of the indenter. This was also supported by modelling work [2]. It is well known that the creep properties of engineering materials can be affected by cold work. Different creep responses due to cold work, at varyi
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