Deformation and Fracture Behaviour of 20MnMoNi55 Pressure Vessel Steel

Abstract

The deformation and fracture behaviour of 20MnMoNi55 low alloy reactor pressure vessel (RPV) steel has been studied at room and elevated temperature. This steel showed positive strain rate sensitivity at room temperature, whereas negative strain rate sensitivity at elevated temperature (300oC), indicating dynamic strain ageing (DSA). Detailed investigation at elevated temperatures revealed signatures of DSA phenomenon in the temperature range of 200-300oC and in the strain rate range of 10-4 s-1 to 10-3 s-1, thus explaining the negative strain rate sensitivity at elevated temperatures. The magnitude of its influence on subsequent deformation properties, however, was noted to vary with the temperature-strain rate combination. Influence of strain rage on cyclic deformation behaviour of this steel has been investigated at both room temperature and at 300oC. The material showed a non-Masing behaviour throughout. The deviation from Masing has been determined by both the change of plastic strain energy and proportionality limit, using both theoretical and experimental methods. The analyses showed that during DSA a correlation of theoretical and experimental measurements does not exist. The presence of DSA during cyclic deformation was manifested in an increase of stress with temperature, negative strain rate sensitivity of stress parameters and exhibition of prominent serration in hysteresis loops. The DSA and its influence were noted to be more prominent at 235oC and at a strain rate of 10-4 s-1. The stress response increased and fatigue life decreased with increase in total strain amplitude at this temperature. The monotonic fracture resistance was low at 300oC compared to room temperature value, due to dynamic strain aging. Ductile fracture studies using three sizes of compact specimens with varying crack tip constraint showed that the fracture toughness values decreased with decrease in specimen size. The variation in fracture parameters were small in miniature quarter CT specimens. The stretch zone width showed insensitivity to crack tip constraint. The Q parameter showed negative values, implying a loss of constraint, with decrease of crack length in three specimen sizes. The Q value was generally on a lower side for small size specimens implying a loss of constraint due to higher stress gradient near crack tip owing to reduction in size. The fictive J-Q toughness locus reflected the specimen size effect. The T0 temperature was measured to be -120.74oC from single temperature master curve tests, where T0 was initially estimated by experimental determination of Charpy DBTT curve. Cyclic fracture behaviour of 20MnMoNi55 steel was evaluated and the data analysed employing different methods suggested in literature. A comparative assessment of different procedures revealed that a method similar to standard ASTM procedure for monotonic fracture analysis is best suited for cyclic fracture problems. This method has been found to be relatively straightforward to adopt and makes the comparison to monotonic fracture results direct. The applicability of factor for cyclic J-R curve calculation was experimentally verified. The adverse effect of cyclic loading was evident from decrease in fracture toughness due to increase in extent and frequency of compressive unloading. The fracture toughness saturated beyond an extent of compressive unloading due to crack face contact and direct load transfer both at room temperature and 300oC. The degradation in fracture resistance was more severe at elevated temperature due to thermally activated mechanism. The stretch zone formation was found sensitive to load excursion nature both in appearance and in magnitude. At higher loading rate the cyclic fracture toughness improved, making the quasistatic cyclic fracture toughness, a conservative estimate.