Analysis of the master curve approach on the fracture toughness properties of SA508 Gr.4N Ni–Mo–Cr low alloy steels for reactor pressure vessels

This study aims at assessing the fracture toughness behavior of tempered martensitic Ni–Mo–Cr low alloy steels for reactor pressure vessels in a transition temperature region using a master curve approach. The fracture toughness tests for model alloys with various chemical compositions were carried out following ASTM E1921-08. The microstructures, tensile properties, and Charpy impact toughness were also evaluated. Alloying elements such as Ni, Cr, and Mo affected the mechanical properties of alloys from changes in the phase fraction and precipitation behavior. In the fracture toughness test results, the data sets showed a deviation from the median curve and a smaller scatter than that of the prediction of the ASTM standard, especially in the lower transition region. The exponential parameter of the master curve equation was adjusted by an exponential fitting to data sets for expressing well the temperature dependency of the fracture toughness. The adjusted parameter provided good agreement for data distribution and the independence of T0 from test temperatures through an overall temperature range in contrast with the results from the standard master curve.