2019, Saffari, H.
THESIS (MSc, 2017-2019): Effects of butter layer on the microstructure and mechanical properties of API 5L X65/AISI304 dissimilar joint
This research aimed to investigate the effects of ERNiCr-3 butter layer on the microstructure and mechanical properties of API 5L X65/AISI 304 dissimilar joint, welded by gas tungsten arc welding (GTAW). The butter layer is applied to minimize carbon diffusion from the base metal to the welding pool and to eliminate the risk of the formation of a hard martensitic layer at the fusion line. The microstructures of buttered and non-buttered specimens were studied using optical microscope, scanning electron microscope (SEM), electron backscattering diffraction (EBSD), X-Ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDS). Mechanical properties of the joints were also evaluated using standard and sub-sized tensile and impact tests. Hardness profilometry was also conducted across the weldments. Results showed that the implementation of the butter layer results in the elimination of a rather large brittle martensitic layer from the base metal at the API side. Also, it appears that the weld zone in buttered sample hardly has any δ-ferrite, which has to do with high nickel content of the butter layer. Buttering overall decreases the tensile strength and increases the elongation/ductility of the joints. More importantly, the butter layer essentially acts as a barrier layer between the ferritic side of the weldment and the weld pool, and this in turn eliminates two hardness peaks at both sides of the welding pool. The correlations between microstructure and mechanical properties are comprehensively investigated and discussed.
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