Effect of addition of 4 Al on the high temperature oxidation and corrosion of a 20Cr 25Ni austenitic stainless steel

摘要

In high temperature applications, one class of alloys that recently has gained more focus is the alumina forming austenites due to good mechanical properties and the attractive oxidation properties of Al in e. g. FeCrAI:s. Two experimental alloys with austenitic structure and the nominal composition 20 Cr-25 NI-1Mn-0.5 Si-Fe (wt %) were prepared. To one of the alloys 3.77 wt % Al was added, thus reducing the amount of Fe. The alloys were studied in three different oxidising atmospheres: syn thetic air, air + 45 vol % water vapour and a sulphidising/chlorinating environment comprising Ar-0.1 % O_2-26 % H_2O-0.35 % HCI-1 % SO_2. The exposure temperatures were 700 °C and 1000 °C, apart from the sulphidisin g/chlorinating environment which was studied at 700 °C only. The time of exposure wa s set to 100 h, with one excep tion in air/water atmosphere where exposure time was extended to 1000 h. At 700 °C in air and air/water environment, the allo y with Al additions dis played lower mass gain than the reference material. This was also true for the sul phidising-chlorinating environment, but in this environment the material displayed a layered oxide that had spalled to a large extent even though the alumina layer ap peared to be intact. In experiments performed in air or air/water at 1000 °C, internal aluminium nitride and alumina formation occurred, appreciably reducing the sound metal thickness. Even though behaviour of the alumina forming alloy was not successful in all environments, its oxidation properties at 700 °C were excellent in both air and air/water environment. This may be regarded as a step towards understanding the optimum level interval of Al in an austenite subjected to moderate temperatures.