Microstructural features of martensitic chromium steels and their influence on creep strength

Martensitic 9-11% chromium steels are used as structural materials for boilers, turbines, and pipe work in advanced high efficient steam power plants. The strength and damage development under a creep loading in those steels are strongly influenced by the processes in their microstructures. The damage evolution in those steels is different from that observed in high-temperature resistant low-alloy ferritic steels. The number of creep cavities is similarly influenced by creep stress (degree of multiaxiality) and material behaviour (creep strain) but the total number of cavities proves to be pronouncedly smaller. The configuration of the cavities differs as well. A description of the damage condition in martensitic 9-11% chromium steels additionally requires the knowledge of the dislocation density, subgrain size and, in particular, the precipitate characteristics. Those values can be determined by a combination of conventional TEM, EFTEM, EDX and diffraction studies. A comparison of specimens of different creep-rupture strengths has shown that the creep strength is dependent on the stability of the M23C6 and MX precipitates. The occurrence of the Z phase can be interconnected with unsufficient creep properties of the steel.