Microstructure and Mechanical Properties of API 5CT L80 Casing Grade Steel Quenched From Different Temperatures

The continuous development of line pipe and casing grade steels should be complemented by development of more effective welding methods. A special high temperature high speed forge welding technique called Shielded Active Gas Forge Welding (SAG-FW) has been developed to weld steel pipes for a range of applications. This article focuses on the microstructure development at different welding conditions in L80 steel with 0.25%C. Specimens with dimensions 100 mm × 11 mm × 6 mm were extracted from the wall of a large diameter L80 pipe. A SMITWELD thermal simulator was used to simulate heat treatment conditions using electrical resistance heating. The specimens were heated to peak temperatures ranging from 600°C to 1350°C within 10 s and subsequently quenched to 50°C at a constant rate of 60°C/s to simulate the heat-affected zone conditions for the real SAG-FW process. Martensite with small fractions of bainite was observed for higher peak temperatures. Mixed microstructures were observed in the specimens heated in the intercritical temperature range. Microstructures and phase fractions developed after heating to different peak temperatures have been analyzed by optical microscopy and scanning electron microscopy. Charpy V-notch tests and Vickers microhardness measurements have been carried out for the weld simulated specimens. The observed toughness values, hardness values, microstructures and phase fractions have been correlated to the respective weld simulation parameters.