Degradation of microstructure and mechanical properties in martensitic cast steel after ageing

The paper presents the results of research carried out to investigate the influence of ageing on the degree of microstructure degradation and mechanical properties of martensitic GX12CrMoVNbN9-1 (GP91) cast steel. The tests were performed on GP91 cast steel after heat treatment (as - received condition) and ageing at times to 20,000 hours at the temperature of 600 degrees C. In the as-received condition, the microstructure of the examined cast steel was a microstructure of lath tempered martensite with numerous precipitates. In the microstructure not only the retained lath substructure of martensite was observed, but also the areas of polygonized ferrite. After the process of ageing, in the microstructure of the examined cast steel, the following was observed: advanced processes of recovery and polygonization of the matrix, revealed by the disappearance of lath microstructure of martensite in favor of the polygonized microstructure of ferrite, large precipitations of Laves phase, and single CrNbN particles. The changes running in the microstructure of GP91 cast steel contributed to an insignificant decrease in the strength properties on the level of maximum 5% and around 70% decrease in the impact energy KV, compared to the as-received condition. Considerable fall of impact energy KV, in comparison with other properties, should be mostly associated with the precipitation of Laves phase on the boundaries of grains, but it is also influenced by the precipitation of M23C6 carbides and the disappearance of lath microstructure of martensite.