The assessment of thermal history in pressure vessel steel

A component of the present work involves attempts to simulate the microstructures of 1Cr-0.5Mo steel sample removed from service after extended exposure to elevated temperatures (10⁵ h at 535°C). The aim is to establish a basis for assessing the thermal history of service components and service weldments. Previous work has established that it is not possible to adequately simulate service microstructures using accelerated isothermal heat treatments alone. A selective mechanical testing program at elevated temperature has thus been investigated to superimpose the effect of stress on heat treatment. Qualitative comparison within each of these sets of micrographs suggests that the intraferritic precipitation in the creep samples is in each case refined and of a higher density compared to the sample subjected to isothermal heat treatments. It has been suggested that a comparison of the composition of the pearlitic M₃C in creep test samples with the empirical relationship may provide a means of assessing the average thermal history of the ex-service sample.