Influence of various contact-materials on the fretting fatigue life of P91 steel

Abstract

Fretting-induced damage is an important failure mechanism in steam generator (SG) tubes in which there will be continuous contact and relative motion between tube and tube-support structures. In this work, the influence of various contact materials on the fretting fatigue (FF) behavior of modified 9Cr-1Mo (P91) steel, a widely used SG material has been studied. The test setup has been configured as a flat-on-flat contact between the fatigue specimen made of P91steel and the contact pad. The three different bridge type contact-pads namely, P91-steel, Inconel718 (IN-718), and aluminized IN-718 were employed. The contact pad applies constant pressure on the specimen while it is undergoing a fatigue cycle. All the tests were carried out at a frequency of 10 Hz, with maximum cyclic stresses ranging from 250 to 525 MPa and a stress ratio of R = 0.1 at a constant contact pressure of 100 MPa. The FF life of the alloy with the contact material made of aluminized IN-718 alloy was observed to be lower in comparison with the IN-718 and P91 contact materials. The life reduction in the former case was investigated through a comparative analysis of the formation and distribution of debris on fretted surfaces, surface roughness, and the fracture surface of the steel. The formation of the oxide phase such as Fe2O3 (hematite) and Fe3O4 (magnetite) was found on the fretted surface of the specimens.