Probabilistic fracture toughness of a duplex stainless steel in the transition range

The fracture toughness of a 2205 duplex stainless steel (DSS) heavy section forged bar in the transition range (between 0 °C and -60 °C) was investigated. Experimental results were used to estimate the reference temperature T0 and to define a Master Curve according to the ASTM E 1921-11 Standard. Following this Standard, that assumes the Weibull exponent b fixed and equal to 4, the reference temperature obtained using data at T = - 60 °C is T0 = -110 °C; using data at T = - 40 °C is T0 = -103 °C and using data at T = - 20 °C is T0 = -110 °C. As the description of the experimental data distribution does not follow the theoretical Weibull slope of b = 4, the slope obtained by the best fit of the experimental data was also measured and its influence on the assessment of the toughness scatter in the transition range by the weakest link approach ascertained. The recalculated reference temperatures T0 for the data at -60 °C, -40 °C and -20 °C are -114 °C (with b = 6.5), -107 °C (with b = 6), and -113 °C (with b = 4.8) respectively. Experimental data are well fitted by the Master Curve for all the chosen temperatures. Fracture mechanisms were also investigated by means of scanning electron microscope in order to discuss the application of the ASTM E 1921-11 Standard to duplex stainless steels whose microstructure is only partially ferritic and whose fracture transition doesn’t result in a completely cleavage fracture at low temperatures because of the ductile behaviour of the austenite phase.