Structural materials martensitic steels

For example, a common process encountered in industry is the heat treating of alloy steel parts to produce a locally hard surface (e.g., bearing or gear tooth wear surface). Though only a very small fraction of the material actually needs to be hardened, conventional technology has required that the entire part be heated to about 1650°F, then quenched at 350°F in oil to produce a hard martensite structure in the steel. 

Sherby, O.D., Wadsworth, J., Lesuer, D.R., Syn, C.K., 2008. Revisiting the structure of martensite in iron-carbon steels. Materials Transactions 49 (9), 2016—2027 (The Japan Institute of Metals). 

Another important two-phase structure in steel is observed in tempered martensite. The parent material, martensite, is a supersaturated single-phase... 

Rawls J. M. et al., Assessement of Martensitic Steels as Structural Materials in Magnetic Fusion Devices , GA-A 15749 UC-20d, January 1980... 

Core structural materials Ferritic-martensitic stainless steel cladding and stmctures... 

The use of lead-bismuth as a primary coolant at relatively low core outlet temperature and coolant heatup allows the use of ferritic-martensitic steel EP-823 (12%Cr-Si) as a structural material for the core and steam generator. This steel was checked in practice for resistance against radiation swelling and radiation creep. 

A. Abdollah-Zadeh, A. Salemi, and H. Assadi, "Mechanical behavior of CrMo steel with tempered martensite and ferrite-bainite-martensite microstructure". Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 483-84, 2008 pp. 325-328. 

In general, it has been observed that neutron irradiation does not impact the corrosion mechanism and the rate of reference structural materials exposed to liquid Na, Pb and Pb-Bi. On the contrary, neutron irradiation and liquid metal synergetic effects on the degradation of the mechanical properties of structural materials have been observed. As an example, ferritic/martensitic steels when neutron-irradiated and in contact with liquid Pb-Bi show degraded mechanical properties attributable to both the irradiation defects and the Uquid metal impact. As already reported before, from the reactor operation point of view, the design criteria of reactor components have to include these phenomena. 

In this oxidation mode, austenitic steels can be used as structural material up to temperatures around 400—450°C and martensitic steels up to 450—500°C. 

A comprehensive literature review on the degradation of mechanical properties of structural materials exposed to liquid Pb and Pb-Bi is given in Ref. [16]. As reported in this reference most of the experiments have been performed in liquid Pb-Bi and the extension of these experimental findings to pure Pb cordd lead to incorrect estimations, since liquid Pb-Bi seems to be more aggressive than liquid Pb. Moreover, the main structural materials tested were the 9Cr ferritic/martensitic steel 791 and the austenitic steel AISI316. However, a generalization of the residts obtained in order to predict the behavior of other 9Cr ferritic/martensitic and austenitic steels is not feasible, since as, for instance, minor alloying elements in the steel impact the materials behavior in these liquid metals. 

The crystal structure of aU the austenitic steels is face-centered-cubic (fee) and, in this class of materials, the physical and mechanical properties of the 300-series steels and 15/15Ti derivatives are quite similar. From other Fe basis as body-centered-cubic (bcc) ferritic steels certain properties, such as thermal properties and mechanical strength at high temperature, are very different. So, compared to the ferritic-martensitic steels, the thermal expansion of austenitics is about 50% higher but thermal conductivity is clearly lower and the mechanical strength at high temperature, typically >550°C, is always higher. 

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... 

Fast reactor core structures are exposed to various environments during their in-reactor life liquid metal for the exterior surfaces and MOX (U, Pu)02 fuel for the internal layer of the cladding. Moreover, it is also necessary to check the impacts of these materials on the cleaning of fuel assemblies and on fuel reprocessing, especially during fuel dissolution in nitric acid. Only a few results are available on the behavior of ODS alloys in these environments, they can be complemented by results available on ferritic-martensitic steels. The influence of the fine dispersion of nanoparticles on the behavior of ferritic-mattensitic steels in the environment has to be considered. 

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. 

When ferritic-martensitic steels are used for structural components, usually dissimilar welded joints with austenitic stainless steels are used, too. When these are subject to elevated temperatures, creep property evaluation and creep-fatigue evaluation are needed. In the case of dissimilar welds, close attention should be paid to the location of failure. Under certain conditions, failure could occur at the interface between the two materials [27]. 

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... 

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