THE EFFECTS OF REACTIVE ELEMENTS ON THE OXIDATION OF ALUMINA-FORMING IRON-CHROMIUM-ALUMINUM ALLOYS AT HIGH-TEMPERATURE

A study is being undertaken into the influence of sulphur and reactive elements on the growth and adhesion of alumina scales on iron-chromiu m-base alloys during thermal cycling in air from 1300 degrees C. A tow -sulphur-(< 10 ppm) containing alloy has shown considerable improvemen ts in scale adhesion compared with an alloy containing a higher concen tration of sulphur. Additions of reactive elements to the low sulphur- containing alloy had little further effect on scale adhesion, although second-phase precipitates facilitated preferential inward growth of t he scale and the development of a very irregular scale/alloy interface . All the alloys showed evidence for stress development during scale g rowth, but this was often relieved by flow of the weak alloy. Although not extensive, all the low sulphur-containing alloys exhibited some s cale failure, involving both a decohesion, buckling and tensile cracki ng route and a shear cracking and wedging route.