Strength and Microstructure of Copper Tube in Cross-Roll Straightening

Abstract

The diameter and wall thickness of copper tubes used for heat transfer in heat exchangers have been decreased in order to improve performance and save resources. One of the problems related to the reduction in tube wall thickness and diameter is ensuring the workability of the products. Copper tubes used in heat exchangers are subjected to various types of processing, such as hairpin bending, expansion, and flaring. Since the necessary stiffness cannot be achieved in copper tubes with thin walls, defects such as buckling may occur. Copper tube manufacturers deal with such problems by fine control of the mechanical properties of the materials. This paper focuses on straightening used in tube manufacture as a potential machining method for providing precise control of mechanical properties. The strength and microstructure of a work-hardened copper tube before and after straightening was investigated.

A decrease in tensile strength after straightening was observed regardless of the drawing conditions. The decrease in tensile strength was larger when the tubes were drawn to higher reduction ratios of cross-sectional area; the largest decrease in tensile strength was observed when the reduction ratio of the cross-sectional area was 98%. In this case, the tensile strength before straightening was 450 MPa, and after straightening, decreased by 21 MPa to 429 MPa. Vickers hardness testing showed that the surface layer of the copper tube decreased in hardness. Crystal structure analysis showed that this corresponded with a change in balance of the orientation density of Cu and Goss orientations developed during drawing. Straightener rolls rotate crystal grains in the surface layer of the copper tube, changing the Cu orientation to the Goss orientation. The study confirmed that the mechanical properties of copper tubes could be altered by changing the texture of the material, even using processing operations such as straightening, without significant alteration of the cross-sectional area.