Influence of long-term ageing on the microstructure and mechanical properties of T24 steel

The paper presents the results of research on the microstructure and mechanical properties of 7CrMoVTiB10 - 10 (T24) steel. The examined steel was subject to ageing at the temperature of 550 and 600 degrees C for 70,000 h. Performed tests of mechanical properties (YS, TS, YS550, HV10) of T24 steel have shown that, after ageing at the temperature of 550 degrees C, the observed decrease was not larger than 20% in comparison with the as-received state the properties were higher than the required minimum, Ageing of the T24 steel at the higher temperature resulted in a greater reduction of its mechanical properties - they were lower than the required minimum. Performed microstructural tests have shown that the degradation of the microstructure of the examined steel was mostly connected with the processes of recovery and polygonization of the matrix, disappearance of lath bainitic microstructure, the growth of the size of M23C6 carbides, and precipitation of the secondary M2C and M6C precipitates. The magnitude of these changes depended on the temperature of ageing. The changes observed in the microstructure of the steel aged at the higher temperature, i.e. 600 degrees C were more advanced than those observed at the temperature of 550 degrees C.