Impact of crack propagation path and inclusion elements on fracture toughness and micro-surface characteristics of welded pipes in DWTT

Abstract

Despite fundamental differences between the characteristics of base and weld metal and higher accuracies obtainable using original-size specimens in drop-weight tear test (DWTT), there still exists a research gap in fracture surface examinations of DWTT welded specimens. This study investigates the microscopic characteristics of the fracture surface of spirally welded API X65 steel with chevron notch (CN) and pressed notch (PN) DWTT specimens. Microstructures of different sub-zones were investigated, including, weld metal (WM), heat affected zone (HAZ), and base metal (BM). Scanning electron microscopy (SEM) investigations revealed the numerous inclusions in the WM that cause stress concentration. Micro-cracks are formed at the beginning of the fracture process when the energy level is high; nevertheless, in the shear fracture area, where energy is reduced, micro-cracks were not observed. Inverse fracture was located in the HAZ and BM in the PN and CN specimens, respectively. Comparison of the weight percentages of inclusion elements obtained by energy-dispersive x-ray spectroscopy (EDS) with API 5L standard target values showed that in most inclusions, Mn, Ti, and S were higher than standard values. After metallurgical and mechanical investigations, a framework for a new prospect has been introduced to conduct statistical analyses, based on which, the confidence intervals of the weight percentages of inclusion elements were carefully determined. Changes in the weight percentages of constituent inclusion elements in the WM are smaller compared to the BM and HAZ that would be reasons for better mechanical properties of the WM compared to the BM. Considering the impact of inclusions on weld-ability and toughness, industrial guidelines are presented to carefully control the elements within the confidence intervals of this research, during welding to minimize the formation of inclusions, in turn, reducing the formation and growth of micro-cracks, and significantly improving mechanical properties.