Development of a new generation of 12% Cr steels: Z-phase strengthened steels

Licentiatavhandling, 2015

Fossil-fuel fired steam power plants provide more than 60% of the electricity generated worldwide, and account for about one third of the global CO2 emissions. The thermal efficiency of the steam power plants is limited by the maximum allowed steam temperature and pressure, which in turn are determined by the long-term corrosion and creep resistance of economically viable materials. Today’s best martensitic steels contain 9% Cr and can be used at ~ 600°C/300 bar. All attempts so far to reach 650°C with 11-12% Cr steels have failed, and the reason is the formation of a complex nitride, Z-phase, after a few years of service. In this thesis it is shown that Z-phase can be used as strengthening rather than weakening phase to develop a new generation of martensitic steels. These contain 12% Cr for better corrosion resistance and densely distributed fine Z-phase precipitates for creep resistance. A high Cr content together with Ta and N additions were used to stimulate the formation of a fine distribution of Z-phase in two trial steels. Atom probe tomography, transmission electron microscopy, and scanning electron microscopy were employed for a detailed characterization of the microstructure. In the first trial steel, 12Cr7CoTa-uLC, the C content was limited to 0.005 wt.%, which resulted in a fast transformation from TaN to CrTaN Z-phase, and subsequently a fine distribution of Z-phase was achieved. In the second trial steel a higher C content of 0.06 wt.% was used, which resulted in a slower phase transformation from Ta(C,N) to CrTaN Z-phase. In the 12Cr7CoTa-uLC trial steel Laves phase formed continuously at the prior austenite grain boundaries, which gave poor impact toughness. In the second trial steel, 12Cr3CoTa-HC, the addition of Cu and higher C content enhanced the distribution of Laves phase, and equiaxed Laves phase particles of a few hundred nanometer in size formed resulting in improved toughness.