Evolution of Fretting-Induced Cracks and Delamination Wear on Alloy 690 Steam Generator Tubes During Mixed Fretting Regime in Air at Room Temperature

Abstract

The occurrence of fretting-induced cracks and delamination wear processes in a mixed fretting regime on Alloy 690 steam generator tubes against 304L stainless steel pads was analyzed. Fretting tests in 90° cross-cylinder configuration were performed in air at room temperature. Changes in the contact conditions were evaluated using the energy ratio and the sliding ratio criteria. Damage was characterized using light microscopy, scanning electron microscopy, and optical profilometry. Vickers microindentation was applied to characterize the initial plastic deformation process. It was found that during the initial 1 × 103 cycles, plastic deformation constitutes the main damage mechanism. Thereafter, between 1 × 103 and 1 × 104 cycles, cracks nucleation and propagation accompanied by negligible wear were detected. Beyond 1 × 104 cycles, the dominant main damage mechanism consists of delamination wear assisted by cracks coalescence, and a change from a U-shaped to a W-shaped scar profile was observed in correspondence with the appearance of the delamination wear process.