Introduction
The process of manufacturing a flexible exhaust coupling involves pressing studs into a flange to create an interference fit.
After pressing, it was found that the studs were failing at the interference fit location. This was characterised by significant fretting, with the result that the studs were loose in the flange.
Investigation
Samples of both the flange and the stud from a failed assembly, as well as samples from an intact assembly, were sectioned for metallographic examination, hardness testing and compositional analysis.
Results
The compositional analysis of the flange material shows significant differences in the silicon and manganese levels of the flanges in both the intact and failed assemblies. Manganese levels increase the hardenability of steels, thus the failed flange has considerably more hardenability than the intact flange.
The hardness of the flange in the failed assembly is considerably harder than that of the working assembly. This is also indicated by the microstructure. The intact sample has a fully annealed microstructure, while the failed sample shows evidence of a different tempering treatment having been conducted, which results in a harder material. The composition and microstructure of the failed and intact studs is similar and within specification.
In a working assembly, when the stud is pressed into the flange, the flange deforms to accommodate the stud in an interference fit. In the failed assembly, the flange does not deform to the extent that is required. Instead, the stud deforms by way of fretting.
The working flange shows grooves that extend most of the way down the hole. On the failed flange, initiation grooves are seen on the surface, but there is no extension of deep grooves into the hole.
Conclusions
Evidence suggests that the failed flanges were subjected to a tempering treatment which caused hardening of the material above the required level. On assembly, the difference in hardness between the stud and the flange was not sufficient to cause the deformation of the flange required for the interference fit. Instead, the knurls on the stud deformed in the form of fretting, resulting in a failed assembly.
