Y. A. Al Shehri, H. Fadhli, B. S. Yilbas, M. S. J. Hashmi, Saudi Oil Company, Dhahran, Saudi Arabia
Thermal spray coating is a widely used process to protect the metallic surfaces from the high temperature, wear and corrosive environments. The powder emerging from the spray gun reaches almost the melting temperature of the constituting substrate material. Since the stand-off-distance between the gun exit and the base material surface is short, the splats changes form the round shape to the oval shape upon impacting onto the surface due to their elevated temperatures. Once the coating is build on to the base material surface, the coating temperature remains high while the substrate bulk temperature is low. During the cooling period, the thermal stresses are developed in the coating. This situation becomes important for multi-layer coating applied over welded surfaces. In this case, temperature gradient across the coating and the welded substrate changes in each layers and the residual stress field developed in the coating alters in the magnitude across the layers. However, the residual stress developed in the coating and the welded substrate influences the mechanical performance of the coating; consequently investigation into the metallurgical and mechanical properties becomes, necessary. The mechanical properties were evaluated through three-point bending under two conditions: (a) Surface welded and coated with one layer of HVOF Inconel-625, (b) Surfaces welded and coated with Multilayer of HVOF Inconel-625/WC. The surface morphology and the elemental composition of the coating and the welded surfaces before and after the mechanical tests were examined using SEM and EDS techniques. The results indicated that the welded-coated specimen exhibit lower residual stresses compared to the welded multi-layer coated specimen. Microscopic observations of the fracture surfaces showed that the cracks resulted from three-point bending test were formed at both the interface between coating and welded substrate, and within the coating itself.
Summary: This study is aimed to evaluate the effect of residual stresses resulted from applying multi-layer coating over welded surfaces.
