Quenching and intercritical tempering of microalloyed pipeline steel

Full text: Samples from a thick plate of microalloyed steel, intended for API X65 pipeline, were studied with regard to its quenching and tempering behavior. The steel was solution treated at 1200°C, temperature high enough to solubilise the microalloying elements, quenched in water and tempered at different temperatures. The resulting tempering curve presented evidence of secondary hardening, even at temperatures as low as 300°C, and softening up to 650°C, with microstructures presenting tempered martensite and spheroidized carbides. Surprisingly, the results of tempering at 700°C did not show the softening expected; there is a surge of hardness with times up to 15 min and after that a slow softening. Thermodynamical calculations using ThermoCalc indicated that 700° C is already inside the intercritical region, so at equilibrium there will be a certain amount of austenite, no cementite and a small amount of microalloying elements rich carbides. In fact, the microstructure found presented no spheroidized cementite; the martensite lath were intercalated with layers of a microconstituent that could not be resolved, but it is probably a mixture of fresh martensite and retained austenite. In order to study this structure, induce retained austenite (RA) and better understand the intercritical tempered martensite plate morphologies, a series of heat treatments were conducted using a quenching dilatometer. The RA content was determined using X-Ray diffraction and compared to a model for C and Mn partitioning during intercritical tempering. It was found that maximum RA was obtained at 700°C for 60 min. The mechanical properties of samples intercritically tempered at 700°C for 60 min were determined: yield strengths of 720 MPa with 12% elongation were found. An extraction replica of this last condition was observed on a SEM-FEG showing the presence of nanometer-sized carbides of titanium, niobium and vanadium. (author)