Developments of Extra-High Purity Stainless Steels for Nuclear Corrosive Environments
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Developments of Extra-High Purity Stainless Steels for Nuclear Corrosive Environments Junpei Nakayama1 and Kiyoshi Kiuchi2 1
Kobe Steel Limited, Kobe-shi, 657-0845, Japan
2
Japan Atomic Energy Agency, Tokai-mura, 319-1106, Japan
ABSTRACT The austenitic stainless steels so-called, the EHP steels with the extra high purity, are developed for improving the reliability of nuclear equipments materials used in the heavily corrosive irradiation environments. By considering the impurities segregation mechanism, the major impurities included in the EHP steels is controlled less than 100ppm by the new melting technology. It is a two-step refining process composed of CCIM and EB for effectively removing both non-volatile and volatile harmful elements. The risk to cracking on melting and welding processes is also effectively minimized by enhancing both the eutectic point and the metallic bonding at grain-boundaries. In the EHP steels, it is possible to select the appropriate composition of Ni and Cr for stabilizing austenitic phase and enhancing corrosion resistance. The characteristics of the welding joints are as good as those of the base metal because the same filler metal is used without the formation of residual delta ferrite. The resistance to IGC and SCC of the EHP steels is markedly improved by minimizing the refining effects, except for type 316 steels with Mo. The welding technique and the chemical composition range are selected for standardizing the EHP.
INTRODUCTION The degradation mechanisms like inter-granular corrosion (IGC), inter-granular stress corrosion cracking (IGSCC) and trans-granular stress corrosion cracking (TGSCC) of the structural materials made of stainless steels are mainly controlled with the non-metallic impurities and austenite (gamma) phase stability. From the former, the low carbon steels made of the present vacuum melting processes like VAR, VIM and ESR are mainly applied in the current nuclear plants for inhibiting the formation of Cr depletion zone along grain boundaries due to carbide formation during the thermal histories so-called sensitization. However, the L-grade steel is still suffering from the corrosion failures at the high corrosion potential by the segregation of residual minor impurities. The latter is relating to the poor austenite phase stability of type 300 series steels with the commercially standardized composition. For inhibiting the cracking during the steel melting and welding processes derived from the residual impurities, austenite phase stability of the commercial steels is adjusted in delta ferrite contents within 10%, by controlling the ratio of Ni to Cr with based on Shaeffler diagram. It is suffering from TGSCC like chloride cracking due to the gloss slip caused from planer type dislocation array. Moreover, the susceptibility to local attack like IGC is enhanced with the brittle sigma phase formation.
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Spinodal decomposition of ferrite/martensite during plant aging are enhanced as shown in an example of type 316 (see Figure1).
Figure 1. The metallurgica
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