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Structural materials used for a nuclear reactor have various carbide precipitates in their matrix which contribute to a strengthening and toughening of these materials under service. Many investigations that focused on the role of a carbide for a fracture strength and toughness have been progressed extensively and they have established that large carbides on the grain/lath boundary are considered as major detrimental microstructural features. Nanometer sized carbides have attracted little attention in spite of their effect for an increase in the yield strength. However, an application of nanometer sized carbides in nuclear structural materials has emerged recently due to its significant capacity for a resistance to a material degradation as a consequence of an irradiation. In SA 508 Gr3 steel used as a nuclear reactor pressure vessel (RPV) steel, it is well known that large carbides are formed on the grain/lath boundary whereas nanometer sized carbides with a needle shape are distributed densely in the ferrite matrix. Needle shaped carbides with a several nanometers width have been identified with b-Mo2C carbides. But according to studies on a carbide precipitation in a Fe–Mo–C ternary alloy, the formation of other carbides such as Ksi-carbide (Fe11Mo6C5 or Fe2MoC) and cementite (Fe3C) is also feasible at a low Mo content. 7,8) Since an intensive characterization of nanometer sized carbides in SA508 Gr3 steel has been insufficient, its evolution in SA508 Gr3 steel still remains far from being fully answered. The objective of the present work is to reveal the evolution of a needle shaped carbide in SA508 Gr3 steel used for a nuclear pressure vessel material in the hope of developing the design process for advanced structural steels. The chemical composition of the SA508 steel for the present work was: Fe–0.19C–1.35Mn–0.82Ni–0.51Mo– 0.17Cr–0.009Al–0.008Si (wt%). This SA508 steel ingot was fabricated through a vacuum induction melting and then it was forged. A subsequent heat treatment of this steel includes a homogenization at 910°C for 9 h, a tempering at 650°C for 9 h, an ausenitizing at 880°C for 8 h, a quenching and a tempering at 650°C during 1 h. The last tempering treatment is performed for a carbide formation and a stress relief. To analyze the characteristics of the needle shaped carbide precipitates, we used a transmission electron microscope (TEM) with an Energy dispersive spectrum equipment (EDS). The foils used for the TEM examination were prepared by an electro chemical thinning and an ion milling. Thermodynamic calculation was carried out for an estimation of the phase fractions and the driving forces of the equilibrium carbides at a tempering temperature by the CALPHAD method. Thermo-Calc program was used for the calculation. The calculations allowed for the potential existence of cementite (Fe3C), b-Mo2C (HCP), Ksi-carbide, M6C and M23C6 in addition to ferrite. Figure 1(a) shows the small needle shaped carbides observed in the experimental steel. A high resolution (HR) image taken by TEM was converted with a FFT image

Evolution of a Needle Shaped Carbide in SA508 Gr3 Steel