Cleavage fracture toughness of tempered martensitic Ni–Cr–Mo low alloy steel with different martensite fraction

This study investigates the relationship between fracture toughness behavior and phase fraction using the master curve approach for SA508 Gr.4N Ni–Cr–Mo low alloy steel. The different phase fractions of the model alloy were obtained by controlling the cooling rate from the austenitization temperature to transformation temperature. The fracture toughness tests were carried out using ASTM E1921-09c, standard master curve method. Yield strength and the impact toughness at room temperature are improved when increasing tempered martensite fraction, but the transition region narrowed in a fully martensitic microstructure. Fracture toughness test results indicated that transition properties are improved when increasing the tempered martensite fraction by reducing the probability of discovering carbide larger than critical size, which can act as cleavage initiation site. The exponential fitting for datasets of the model alloys showed that the temperature dependency of the measured fracture toughness is steeper with higher fraction of the tempered martensite. The T0 values determined from the adjusted curves for the data distribution well reflect the variation of transition property with the phase fraction when compared to the T0 values from the standard master curves.