Size effect studies of the creep behaviour of a pressure vessel steel at temperatures from 700 to 900 °C

The study of size and scale effects in plastic flow and failure is of great importance for the evaluation of the pressure vessel behaviour in severe accidents. The forged reactor pressure vessel material 20MnMoNi55, material number DIN 1.6310 (heat number 69906) was subjected to creep investigations under constant tension load using geometrically similar smooth specimens covering a scaling factor of 4. The tests were performed at 700, 800 and 900 °C in an inert atmosphere. The temperatures cover the phase transformation regime. The mechanical stress varied from 10 to 30 MPa, depending on the temperatures. The creep curves and characteristic data are size dependent to a varying degree, depending on the stress and temperature level. The size effect is largest at 700 °C at the lowest stress level and it implies that the above mentioned times are considerably longer for the small specimens. With increasing stress the size of the effect decreases and at the highest stress a reversal of the effect may occur. The type of fracture is strongly temperature dependent.