The effect of ageing temperatures on microstructure and mechanical properties of GX12CrMoVNbN9-1 (GP91) cast steel

Abstract

Increase in the steam parameters, the so-called supercritical parameters, resulted not only in the growth of efficiency of power units, or reduction in the emission of pollutants into the atmosphere, but also it has contributed to the development of heat-resisting steels and cast steels. One of the newly developed and implemented cast steels in the power industry is martensitic GX12CrMoVNbN9-1 (GP91) cast steel, created on the basis of chemical composition of P91 steel. The cast steel ofamicrostructure and properties similar to the base material replaces in the power industry the grades of steel used previously which do not meet the increasingly high requirements. A study has been made with regard to the effects of ageing at 550 and 600°C and time up to 6000 hrs on the changes in microstructure and mechanical properties of GX12CrMoVNbN9-1 (GP91) cast steel. A detailed microstructural research and the analysis of secondary phases was carried out using the electron microscopy technique, whilst the research on mechanical properties includedahardness test, static tensile test and impact strength measurement. The microstructural observations showed that the lath martensite microstructure was maintained after 6000 hrs. The coarsening of M 23C 6 carbides was revealed. Ageing of the investigated cast steel resulted inaslight decrease in the strength properties (Rp 0.2, TS and HV30), i.e. of around 6%, which was accompanied byasignificant decrease in impact strength KV. Moreover, in the microstructure of investigated cast steel there were the processes of recovery and polygonization of the matrix observed.