**3. Eurofer steel — Complexity microstructure and property**

Eurofer steel, also called Eurofer 97 due to the year 1997 when its chemical composition was defined, is the end product of a synthesis of four steels of reduced activity that were studied in the European Union (MANET, OPTIMAX, BATMAN and OPTIFER I and II) (Lucon, *et al*, 2006). Although, a huge progress in the improvement of this steel has been developed in recent years (Lindal, 2005), studies with Euroferno with regard to optimization of the reduced activity, increased fluency and decrease in the ductile-brittle transition temperature after radioactive damage remain under constant growth, evolving thus to EUROFER 2 and 3. The aim is that EUROFER 3 becomes a residue of low level of radiation after 80 or 100 years when applied to the DEMO demonstration reactor (Daum and Fischer, 2000; Huang *et al*, 2007).

The mechanical and corrosion resistance properties of martensitic stainless steels can be seriously impaired particularly in function to the precipitation of complex phases, generally rich in chromium, operating temperature, or even during processing; thus, the thermal cycles to which these steels are submitted should be conducted under absolute control.

To provide the steel mechanical strength, hardness and toughness needed, the most common heat treatments consist of tempering followed by single or double thermal treatment. The main parameters involved in this case are: heating and cooling rate, austenitization temperature and time and tempering thermal cycles (Mariano, 2007).

In the specific case of Eurofer steel, the improvement of its chemical composition aiming at reduced activity was achieved by replacing niobium by tantalum and molybdenum by tungsten. Nb, Mo, Ni, Cu, Al and Co were restricted to ppm values. Calculations aided by computer simulation programs indicate that Eurofer is a very promising reduced-activity alloy.

The mechanical properties of ferritic-martensitic steels restricted their use to temperatures above 550°C. Subsequently, the addition of fine and homogeneously dispersed particles allowed their use at higher temperatures (650°C) to give origin to ferritic-martensitic alloys hardened by oxide dispersion (ODS) (Lindau, 2005). To date, the most promising of this series is the ODS-EUROFER alloy. This reduced-activity alloy was developed by the Karlsruhe Research Center (Forschungszentrum Karlsruhe, FZK), in cooperation with France and Russia aiming nuclear applications.

EUROFER steel (9Cr-1W) can be used in turbines for power generation, pressure vessels, nuclear reactors or applications where the material is submitted to temperatures between 250 and 550°C. One way to improve the properties of steel, so that it works at higher temperature or become more stable is to add second-phase particles into the matrix. These particles can be in the form of oxides, carbides, nitrides, or even in solid solution when certain chemicals are added to the material.
