Effect of Nb and Cu Additions on High Temperature Oxidation Behavior and Mechanical Properties of Al2O3-Forming Austenitic Alloys

Abstract

The addition of Cu to austenitic steels was found to be effective for decreasing the critical Al content and promoting protective alumina scale formation from our recent studies. It was also reported that Nb addition to the alloy promotes the formation of alumina. These two elements are also known to improve the mechanical properties of steels by forming fine precipitates of Cu and intermetallics such as Laves and γ” phases, thus combined addition of these elements is expected to improve both the oxidation and mechanical properties of steels, however, there is no systematic study on the effects of the combined addition of these elements on the oxidation resistance and mechanical properties of austenitic heat-resistant alloys. The aim of this study was to investigate the effects of Nb and Cu on the oxidation behavior and mechanical properties of austenitic heat-resistant alloys containing Nb and Cu.Fe-25Ni-17Cr-(5~8)Al-(0, 0.5, 1)Nb-(0, 3, 5)Cu (in at.%) alloys were prepared by Ar-arc melting pure constituent metals. The ingots were cold-rolled by 90% and heat-treated at 1200ºC for 1 h, followed by aging at 800 ºC or 600 ºC for up to 500 h. Oxidation samples were cut from the cold-rolled plates and polished down to a 3 µm diamond paste. Oxidation tests were performed at 800°C in a box furnace in air for up to 100 h. The alloys were also subjected to a Vickers hardness test at room temperature and tensile tests at room temperature and 600ºC. The tensile test was performed in Ar at a strain rate of 0.005mm/s. Samples were analyzed by SEM, STEM, TEM, EPMA, GD-OES and XRD.Fig. 1 shows the oxidation kinetics and room temperature hardness of the alloys with/without different Nb and Cu contents. The oxidation kinetics of the alloys with/without different Nb and Cu contents shown in Fig. 1(a) and (b) reveals that the oxidation mass gain of the alloys without Cu decreased significantly by the addition of Nb and it further decreased with increasing Nb content. The oxidation mass gain of the 1Nb doped alloys tended to increase slightly with increasing Cu content. From the STEM-EDS analysis, it was confirmed that the protective alumina scale was formed on the Cu and/or Nb added alloys. An α-alumina was confirmed to develop on the alloys with Nb addition at about 600 ºC during heating, which is much lower temperature than that reported for formation of an α-alumina, suggesting that Nb addition promotes α-alumina formation. Vickers hardness of all alloys at room temperature increased with aging time, however, the maximum hardness was higher for the alloys aged at 600 ºC than 800 ºC. The hardness of the 5Cu alloy was higher than 0Cu alloy, which might be attributed to a formation of very fine Cu precipitates confirmed by TEM observation. Figure 1