Objective The traditional hot-extruded tees are manufactured through a complex process and exhibit unstable organizational properties and quality levels. These tees are susceptible to having strength or toughness levels below standard requirements, thereby significantly impeding the safe and stable operation of oil and gas pipelines. Hence, it is imperative to explore methods to enhance properties and ensure quality control for tees.
Methods This study introduced equal tees made of X80 steel with a diameter of 1 219 mm for the structural design optimization and safety evaluation of oil and gas pipelines, aimed at addressing local stress concentration issues. These equal tees were fabricated using Jinglei welding wires and the arc additive manufacturing (AM) technique. Various tests, including assessments of microstructures, mechanical properties, defects, and residual stresses, were conducted on the products. Additionally, pressure tests and reciprocating pressure fluctuation tests were performed on these equal tees and their circumferential welds.
Results The tee structure was optimized and improved on the prerequisite of fulfilling the weight control requirement. The maximum stress in the tees under design pressures decreased by 38 MPa, while the stress at the tee connections to the main pipelines was reduced by approximately 50%. The mechanical properties of the X80 tees, produced through additive manufacturing, demonstrated stability, and their behaviors and microstructures were observed consistent across all parts and directions, including their cores and surfaces. Their strength and toughness exhibited minimal anisotropy and were unaffected by thickness variations. Both phased array ultrasonic testing and conventional ultrasonic testing proved effective for defect detection in AM tees. Additionally, bulk heat treatment was found to be highly beneficial in significantly reducing residual stresses during the production of AM tees.
Conclusion The X80 AM tees and their circumferential welds for connection significantly outperform hot-extruded tees with the same steel grade. The maximum pressure capacity of these tees surpasses 57.5 MPa. Cracking upon blasting of welded nipples propagates within the fitting shoulder range of AM tees, demonstrating good toughness and crack arrest capability. The additive manufacturing technology contributes to the safety and stability improvement of tees and offers new insights and methods for the design and manufacturing of tees suitable for special scenarios, such as low-temperature service environments.
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