Fretting wear response of a nitrided 316L SS/304L SS interface: Effect of lithium/bore liquid environment

Abstract

In nuclear power plants (NPP), rod cluster control assembly (RCCA) tubes undergo impact under low-pressure contact against guides, leading to specific wear on the contact surfaces. To focus on this industrial issue, an experimental layout was designed to perform reciprocating fretting in different environments.A representative tube interface (nitrided AISI 316L)/plane (AISI 304L) was investigated in air and in a solution composed of 1000ppm of bore and 130ppm of lithium to be closed to a NPP primary water chemistry at atmospheric pressure and room temperature. Wear regimes were identified in terms of variation in parameters such as sliding amplitude (from ±40μm to ±160μm), normal loads (from 2N/mm to 5N/mm) and test duration (from 100,000 to 1.5millioncycles).Surface damage evolution was followed by 3D profilometry and several analyses (SEM, EDX, optical observation) were conducted on surfaces and cross-sections. Based on these results, fretting-wear mechanisms in the 304L SS plate and nitrided 316L SS tube were investigated. The first results in dry environment showed that, for δg⁎ <±120μm, plane profiles had a “W-shape” whereas for δg⁎ >±120μm there was a U-shape. Thus, a W-shape morphology parameter Mw was defined, taking into account the third-body wear volume. Moreover, the worn surfaces of the plane were smoother in solution, and wear was no longer detected on the tube specimens. A modified energy wear approach was then implemented to quantify reciprocating fretting wear rate according to ambient conditions. Finally, two wear mechanisms are proposed.