Assessment of the master curve approach on three reactor pressure vessel steels

This study aims at assessing the applicability of the Master Curve procedure to the measurement of the reference temperature for three well-characterised reactor pressure vessel steels (22NiMoCr37, JSPS, JRQ). The following aspects of the methodology were investigated, using statistical tools such as the Generalised Maximum Likelihood (GML) and Monte Carlo methods: independence of T_o from test temperature and specimen type (configuration/dimensions); formula given in ASTM E1921 for evaluating the standard deviation of the reference temperature, and possibile expressions for estimating the standard deviation of the other Weibull parameters (m and K_min); proposed relationships for estimating the median toughness and standard deviation for T_o(_To) in the case of multi-temperature Master Curve analysis. In reference to the three Reactor Pressure Vessel Steels (RPVS) investigated, the independence of T_o was assessed from the test temperature (within the range prescribed by the following revision of the ASTM standard, T_o±50 °C) and, as far as C(T) specimens are concerned, from the sample dimensions; the well-known 10÷15 °C difference was however found between PCCv and C(T) geometries. Furthermore, using the Monte Carlo method, we assessed the relationship proposed by the ASTM standard for estimating the standard deviation of the reference temperature, which results fairly conservative; an alternative analytical function has been proposed. Clear trends have also been identified for the standard deviation of m (with respect to the number of valid data r) and K_min (with respect to the median toughness). Finally, the proposed relationships for estimating K_Jc,med and _To for the multi-temperature analysis have been validated by comparison with the results of the Monte Carlo method: an excellent agreement was found in terms of _To (better than 0.2 °C).