Effect of Addition of 4 % Al on the High Temperature Oxidation and Nitridation of a 20Cr-25Ni Austenitic Stainless Steel

In high temperature applications, the alumina forming austenites (AFA) have recently gained more focus. These utilise the advantageous effect of Al on oxidation resistance, and also have good mechanical properties. Two experimental alloys [20Cr-25Ni-1Mn-0.5Si-Fe (wt.%)] were prepared. To one of the alloys 3.77 wt.% Al was added. The alloys were studied in air and air/water at 700 A degrees C and 1,000 A degrees C, in a sulphidising/chlorinating environment at 700 A degrees C and in a nitriding atmosphere at 1,000 A degrees C. The time of exposure was 100 h, except for one 1,000 h exposure in air/water. At 700 A degrees C in air and air/water, the AFA displayed lower mass gain than the reference material. After exposure in the sulphidising-chlorinating environment, the material displayed a surface alumina layer with some spallation. In air or air/water at 1,000 A degrees C, internal aluminium nitride and alumina formation occurred, appreciably reducing the sound metal thickness. The nitridation was enhanced in the nitriding environment.