Evolution of microstructure in P91-type steel in high temperature creep

The modified 9%Cr P91 steel is one of the materials actually employed in power plant pipework components. The detailed microstructural analysis of a trial melt produced by Vitkovice Steel, Ostrava is reported in the present work. Microstructure evolution during creep at 873 K was investigated by means of transmission electron microscopy (TEM) and computer image analysis on specimens subjects to creep tests conducted up to several predetermined creep life fractions. Two main microstructural elements, namely subgrains and secondary phase particles, were studied quantitatively. The separate contribution of stress free ageing and stress under creep conditions on particle coarsening and subgrain growth is determined. The microstructure evolution consists in a growth of subgrain size and shape changes. This process is retarded by the influence of the dispersed phases that are also subject to further evolution in the course of the creep and annealing process. The smooth increase of subgrain size resulting from stress free ageing is strongly accelerated by the applied stress. Contrary to the subgrain size, the evolution of particle mean size is not monotonic: the initial increase is followed by a final; decrease. Due to this fact, care should be taken over the evaluation of true particle growth rate values.