Conventional ferritic and martensitic steels

Conventional ferritic and martensitic steels as out-of-core materials for Generation IV nuclear reactors... 

Conventional ferritic and martensitic steels are attractive candidate materials for out-of-core components of Generation IV nuclear plants. This is mainly because they achieve excellent creep properties with lower costs compared to traditional austenitic stainless steels. 

As described above, conventional ferritic and martensitic steels are well-balanced materials with excellent properties and field experience. The major structural design codes around the world incorporate them. They are basically ready to be applied to the Generation IV nuclear plants. However, there remain some technical challenges to fiiUy meet the requirements of Generation IV designs. This section briefly reviews flie past achievements and ongoing activities to overcome the challenges. 

For what concerns the ODS alloys it is not possible to draw final conclusions, since the development of this class of material in terms of selecting the most appropriate manufacturing process or procedure and compositions is still ongoing. However, some prehminary results of ODS with a ferritic/martensitic matrix tested in liquid Na have shown that their behavior is comparable to that of conventional ferritic/ martensitic steel. 

Figure 16.5 Comparison of (a) yield strength and (b) total elongation versus test temperature for conventional 9% Cr-2% W (Grade 92) ferritic-martensitic steel and two experimental heats of 9% Cr-2% WMnV next-generation steel designed using computational thermodynamics. Heat treatment conditions include both standard normalized and tempered (N T) and hot-roU thermomechanical treatment (TMT) conditions [87].

As described above, in order to apply the conventional ferritic-martensitic steels to Generation IV nuclear plants, information specific to their application is necessary. Thanks to the past R Ds conducted worldwide on fast reactor applications, particularly for the ASME Grade 91 steels and their equivalents, most of the necessary information is already available. However, there remain some challenges to be resolved which will be described in the subsequent sections. 

For components subjected to cyclic thermal transients, for example, due to start ups and shut downs, such as those of sodium-cooled fast reactors, the prevention of creep-fatigue failure is one of the most important points for a 60-year design. Conventional ferritic-martensitic steels show lower thermal expansion coefficients, which is advantageous in terms of thermal stresses. However, unlike conventional austenitic stainless steels, Grade 91 steels are a cyclic softening material, and this point is to be taken into account appropriately in the evaluation of creep-fatigue. 

Grade 91 steels and their equivalents, along with some other conventional ferritic-martensitic steels, are implemented in the major structural design codes in the world, such as the ASME Code, RCC-MRx, and the JSME Code. These codes are continuously improving their provisions to further meet the requirements of Generation IV projects. The current major issue on ferritic-martensitic steels application is the extension of time-dependent allowable stresses to 500,000 h. In conjunction with this, provisions on items that involve time-dependent material properties such as weldment... 

Selection of material. As dealt with in previous sections, conventional stainless steels, with martensitic, ferritic, austenitic or ferritic-austenitic (duplex) structure, are sensitive to crevice corrosion (Table 7.4). Newer high-alloy steels with high Mo content show by far better crevice corrosion properties in seawater and other Cl-containing environments (see Section 10.1). 

Section 6.7 is dedicated to the recent improvements concerning the knowledge of the long-term fatigue and creep behavior in conventional austenitic stainless steels, stiU in relationship with the specihc in-service conditions of interest for the out-of-core components. The properties of tempered martensite-ferritic steels and conventional austenitic stainless steels can then be compared. The creep resistance of advanced austenitic stainless steels and Incolloy 800 is also discussed in this section. Finally, Section 6.8 sums up the main results of interest and highlights further research works which are required for the design of components of Generation IV reactors. 

Purthermore, as mentioned in Section 9.1, FM steels have also been chosen as structural materials for high dose components of other nuclear systems such as fusion reactors. The fusion community has developed new FM steels, called reduced-activation ferritic-martensitic (RAFM) steels, derived from conventional FM steels, with the objective to achieve enhanced radioactive decay resulting in reduced activation following the end-of-life of the components. To this end, radiologicaUy undesirable elements, such as Mo and Nb, were replaced by W and Ta [24]. The physical metallurgy and mechanical... 

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