Experimental and Analytical Investigation on the Effect of Heat Treatment Parameters on the Mechanical Properties of an API 5L X65 Steel

Abstract High Strength Low Alloy steels (HSLA) for oil and gas pipelines should display high mechanical strength, toughness, ductility and weldability. In this work we studied the influence of quenching and tempering temperature on the yield strength, ultimate tensile strength, percent elongation and hardness of API 5L steel pipes in order to optimize heat treatments to be performed after hot induction bending of the material. The thermal cycles involved soaking temperatures of 880, 920 and 960 °C, cooling water at 15, 23 and 31 °C and tempering at 530, 600 and 670 °C. From this, experimental design techniques were used to reduce the number of experiments. The results from contour maps suggest that soaking temperatures of 910 and 950 °C and tempering between 540 and 610 °C were the most suitable for treatment, regarding mechanical strength. The variation of the water temperature was not significant for the assumed cooling conditions. The prediction regression models of the mechanical properties from the variables involved in the heat treatments showed a good fit between the experimental and predicted results, with correlation coefficient between 0.89 and 0.94.