Molybdenum’s strengthening potential cannot be used fully, since creep ductility decreases strongly with increasing molybdenum content. Another limitation in the application of Mo steels is decomposition of iron carbides above 500°C, known as graphitization. A solution to both problems was to alloy with chromium in combination with molybdenum. In fact, CrMo steels were the first to allow steam temperatures in power stations to exceed 500°C. The properties of the classical CrMo steels 13CrMo4-5 (T/P11) and 11CrMo9-10 (T/P22) are illustrated in Figure 2(b). The creep-rupture strengths of these alloys exceed those of the simple Mo steels by a substantial margin [Figure 2(a)] because of their higher Mo content. CrMo steels form chromium carbides that are stable above 500°C, which prevents graphitization. Chromium also improves oxidation resistance at higher temperatures. The newly developed steels 7CrMoVTiB10-10 (T/P24) and T/P23 shown in Figure 2(b) have extremely high strength properties. These alloys are based on and have a microstructure similar to T/P22. Their strengths are raised considerably by additional alloying with titanium, vanadium and boron in the case of T/P24, and tungsten, vanadium, niobium and boron in T/P23.
The increase of chromium to above 7% in CrMo steels leads to a group of steels containing martensite. This microstructure introduces a new element of structural hardening. Martensite is characterized by a high dislocation density and a fine lath structure stabilized by M23C6 precipitates. Thus, structural hardening is responsible for the large increase in strength of X11CrMo9-1, as compared to 11CrMo9-10 [Figure 2(c)]. Further improvements, especially of the creep strength, have been achieved by alloying with vanadium, niobium, tungsten and boron, as also shown in Figure 2(c). The introduction of X20CrMoNiV11-1 at the beginning of the sixties allowed major increases in power plant efficiency. The transformation behavior and microstructure of this alloy are comparable to those of X11CrMo9-1. The higher creep-rupture strength of X20CrMoNiV11-1 results mainly from the larger volume of M23C6 carbides in the microstructure, a result of the alloy's higher carbon content. The modified 9 % Cr steel T/P91 (EN designation: X10CrMoVNb9-1) invented in the USA is now used in power plants all over the world, both in new plants and in refurbishment work of high-pressure/high-temperature piping systems. Although the carbon content of T/P91 is lower than that of X20CrMoNiV11-1, its creep rupture strength is distinctly higher. This improvement is achieved by alloying with vanadium and niobium. T/P91 takes advantage of finely dispersed type MX Nb/V- carbonitride precipitates for additional strengthening. It was essential to balance the alloy’s composition because the optimum MX-precipitate dispersion and particle size can be achieved only by optimizing the Nb/V ratio and nitrogen content. Subsequently, new steel grades like X11CrMoWVNb9-1-1 (T/P911), T/P92 and T/P122 have been developed based on T/P91. These grades represent the current state of development for creep-resistant ferritic steels.