Experimental study of mechanical behavior of welded API X70 pipe steel using tensile and three point bend specimens

The welded zone of API X70 steel pipe, due to the inherent defects of welding, is a potential area for initiation and propagation of cracks which can eventually lead to damage of the structure. In this research, mechanical behavior of spiral seam weld was evaluated in API X70 pipe steel in three zones (base metal, HAZ and weld metal) using uniaxial tensile and three point bend (3PB) experiments. Three tensile specimens were used in each zone for measurement of mechanical properties. For studying the mechanical behaviour of the pipe, one specimen with 3PB geometry was tested for each zone. Specific values including yield, peak and final load for 3PB specimen were determined from load-displacement plots. The associated energy for each load plus initiation and propagation energies were calculated and the results were analyzed in relation to microstructure in each zone. With an analytical equation based on slip-line field analysis, yield strength was determined with 3PB specimen in each zone and compared with the result of uniaxial tensile experiment. From uniaxial tensile experiment, yield strength levels of 560, 514 and 507 MPa were found for base metal, HAZ and weld metal respectively and from 3PB experiment 604, 582, 575 MPa respectively. Keywords: Pipe line steel; API X70; Three point bending test; Spiral seam weld; Static fracture energy.