On the Relationship of Microstructure, Toughness, and Hardness Properties in a Submerged Arc Welded API-5L Grade X65 Pipeline Steel Section

Abstract

In order to further improve welded pipeline performance, a detailed knowledge of the key and interlinking relationships between the chemistry, microstructure and mechanical properties of the weld joint is needed. In this paper, the results of optical emission spectroscopy analysis on the as welded chemical properties of a submerged arc welded API-5L grade X65 linepipe are first presented. The microstructure of the various weld regions is then assessed against the results of the chemical analysis using a series of microscopy techniques. A fine grained ferrite-degenerate pearlite microstructure was observed within the base metal of the linepipe along with large (1.5 μm) cuboidal Ti (C, N) precipitates. Within the heat affected zone (HAZ) close to the molten weld joint, grain growth occurred with small volume fractions of induced upper bainite present within the microstructure. The fusion zone of the submerged arc weld joint consists of predominantly acicular ferrite with a small volume of grain boundary phases and a high number of large (0.8 μm) spherical Ti (C, N) precipitates. The results of Vickers hardness tests carried out at two length scales (macro + micro) show clear relations between the hardening effects of the cementite enriched degenerate pearlite and induced upper bainite phases within the base metal and HAZ respectively. Fractography analysis of Charpy impact test samples across the submerged arc welded joint found that the large Ti (C, N) precipitates within the fusion zone appear to be acting as microvoid initiation sites for the ductile fracture and as such contributing to the relatively low toughness properties within the fusion zone. Finally, the potential benefits of reducing the Ti content in both the welding wires and X65 base metal for further improvement of the mechanical properties of the linepipe weld joint are discussed in regards to reducing the size of the coarse Ti (C, N) precipitates within the base metal, HAZ and fusion zone.