Rapid Cyclic Oxidation Tests, Using Joule Heating of Wire and Foil Materials

As part of a programme to establish a thermal cyclic oxidation test standard for metallic materials at elevated temperatures (COTEST) a programme of work has been undertaken on the rapid cyclic oxidation of wire and foil materials, using Joule heating.By way of introduction, alternative technologies for rapid thermal cyclic tests are reviewed. Following this the benefits of adopting a modified existing ASTM standard is discussed. The aims of the project were, first, to define a suitable test matrix to evaluate the effect of critical material parameters, to undertake the test to a prescribe standard and then evaluate the performance of materials under rapid thermal cycling, using Joule heating. The performance of the testing methodology is assessed using two materials, Kanthal Al (reference temperature 1250 degrees C) and Alloy 800 (reference temperature 1000 degrees C). In this paper tests on Kanthal A I at 1200, 1250 and 1300 degrees C are reported. These tests have been undertaken in laboratory air. Other parameters include the upper dwell time (2, 5 and 10 min), the lower dwell time (constant at 2 min) and specimen geometry. Kanthal Al material was available as 0.4 mm diameter wire, 0.7 mm diameter wire and 70 mu m x 1.25 mm ribbon.The results of these tests were analysed statistically using a 3 x 3 x 3 test matrix with triplicate repeat specimens. The lifetime of the wire or foil samples could be measured using either of two parameters: 1) the number of cycles to failure or the accumulated hot time to failure (accumulated upper dwell time). The cyclic lifetime was critically dependent on temperature, hot dwell time and sample geometry.For these rapid cycle tests on Kanthal A I wire and foil cycle life decreased with increase in temperature and increase in hot dwell time. The wire/foil endurance (accumulated hot time to failure) decreased slightly with increase in temperature, but increased with hot dwell time (fewer cycles). The endurance of foil samples was shorter than wire samples. Thus for wire/foil endurance only hot dwell time was a statistically significant parameter, over the temperature range studied.