The Use of Microstructural Analysis for the Evaluation of Quality of Double-Wall Tubes

Abstract

Double-wall copper brazed steel tube is an economic alternative of copper and copper nickel tubes, that are used widely in automotive and AC&R applications, offering satisfying rigidity against high pressure, flexibility for bending and flaring, and rather good heat conductibility. When comparing in detail commercially available tubes of such type, significant differences in quality were revealed. This contribution is based on case studies concerning most common causes of failure of double-wall tubes due to several factors. Series of double wall-tubes from different suppliers were studied using light and electron microscopy in order to reveal defects which led to the failure of each set of the tubes. Tubes made of combination of steel and copper were analysed in the initial state and after heat treatment. Heat treatment performed by the suppliers was not further specified and varied between suppliers. Metallographic analysis revealed unsuitable microstructure of steel layers and heterogeneity in Cu layer. More detailed analysis revealed precipitation of Cu-based intermetallic phase on the Cu-steel interface resulting from heat treatment process, but for one set of tubes, precipitates were observed even in the initial state, probably due to higher temperatures used during the fabrication process.