**1. Introduction**

There is a worldwide need for the sustainability of current energy sources in order to ensure the viability of future generations, as long as these sources are environmentally friendly. In this sense, power plant designs for the future should ensure a cost-efficient reduction of CO2 emissions and improvements in efficiency of fuel consumption.

The essential function of a power station is to convert energy from fuel (fossil or nuclear) into electrical energy. In the steam power plant, this conversion involves consuming the fuel to produce heat which is then used to produce steam to drive a turbine. The mechanical energy of the turbine is then converted to electrical energy by an alternator. The steam temperature on the entrance of the turbine is essential to increase the efficiency of the conventional steam cycle.

The maximum steam temperature and pressure are limited by the performance of certain components. The main components which are critical are steam headers, superheater and reheater tubing in boilers, turbine valve chest, rotors and casings, main steam and reheat pipework, generator rotors, and bolts used for high-temperature applications. The boiler components are limited by corrosion and creep. Pipework also suffers creep as well as weld cracking and thermal fatigue. Turbine components are subjected to creep and fatigue (both thermal and mechanical).

Therefore, the development of improved structural materials to increase in thermal efficiency has been the driving force to develop new generations of 9-12Cr ferritic/martensitic (FM) steels [1–3]. The most relevant in-use properties that heat-resistant steels employed to manufacture components in power plants should fulfill are good mechanical properties, fabricability, corrosion resistance, and creep strength. As indicated above, creep strength has been the most studied and has led to innumerable research activities, aiming at improving the creep strength in 9-12Cr FM steel developments [4–6]. The disadvantage of these steels is their loss of strength beyond 600°C, so they need to be optimized to guarantee their use in the future power plants. In this chapter one of the most promising ideas described is applying a thermomechanical treatment (TMT) instead of a conventional treatment. The main contribution of the TMT is the ausforming, which, as other authors have reported, allows increasing considerably the number density of the thermally stable precipitates, i.e., MX nanoprecipitates. Consequently, the creep strength has improved greatly.
