Abstract
Nitride-strengthened reduced activation ferritic/martensitic (RAFM) steel is anticipated to have higher creep strength because of the remarkable thermal stability of nitrides. Such steels with different manganese contents are designed based on the chemical composition of Eurofer 97 steel but the carbon content is reduced to an extremely low level. The larger amount of vanadium-rich nitrides and more dissolved chromium in the matrix could be responsible for the strength being similar to Eurofer 97 steel. The steels have the microstructure of full martensite with fine nitrides dispersed homogeneously in the matrix and display extremely high strength but poor toughness. Compared with the steel with low carbon content (0.005 % in wt%), the steel with high carbon content (0.012 % in wt%) has not only the higher strength but also the higher impact toughness and grain-coarsening temperature. The complicated Al2O3 inclusions are responsible for the initiated cleavage fracture by acting as the critical cracks. The final part of the chapter is concerned with conventional nitride-strengthened martensitic heat resistant steel. It was surprising to reveal that the half-size impact energy greatly increases from several Joules to nearly 100 J, when the tempering temperature is increased from 650 to 750 °C.
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