Failure Evaluation of Galvanized High Carbon Steel Spring Wires

Abstract

High carbon steel wires are subjected to patented heat treatment after the surface cleaning process to obtain sufficient mechanical strength and toughness properties. As a result of this process, thin lamellar pearlitic microstructural features suitable for subsequent diameter reduction processes are obtained. Before entering the hot dip zinc bath, the surface is again subjected to pre-surface preparation. Afterwards, the wires reach the targeted coating thickness depending upon the dipping time in the molten zinc bath at 450 degrees C and a bright and smooth surface finish is obtained by cooling with air or water after stripping at the exit of the bath. Various discontinuities can be observed in the galvanized layer depending on the cooling rate and surface preparation process quality. The risk of failure to the galvanized wire due to these discontinuities during subsequent shaping or during diameter reduction is very high. In this study, a failure analysis was carried out on galvanized spring steel. The results showed that the failure is related to two main factors: the relatively poor surface quality and the unsuitable cooling rate of the wires after exiting from the galvanizing bath. In order to explain the origin of the failure, systematic metallographic investigations were performed by means of scanning electron microscope on both the wire surface and zinc layer cross-section. Mechanical behavior of wire was investigated on lifespan testing.