Effect of Al Addition on the Microstructure and Phase Stability of P91 Ferritic-Martensitic Steel

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LEAD or lead-bismuth eutectic (LBE) has long been considered as alternate coolants for generation IV liquid metal cooled fast breeder reactors (LMFBR). The choice of LBE over sodium has certain inherent advantages such as (a) absence of exothermic reaction with water or air, (b) high thermal inertia, (c) low neutron moderation and (d) simple coolant circuit having effective natural circulation of the coolant.[1] However, two major issues to be addressed with respect to Pb or LBE are Liquid Metal Corrosion (LMC) of structural materials and accumulation of volatile Po210—a strong alpha emitter during reactor operation. Though the technology to trap Po210 has already been developed (in Russia),[1] the issue regarding LMC still remains unresolved.

S. HARIBABU, C. SUDHA, S. RAJU, R.N. HAJRA, R. MYTHILI, S. MURUGESAN, and S. SAROJA are with the Physical Metallurgy Division, Materials Characterization Group, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamilnadu 603102, India. Contact e-mail: [email protected] J. JAYARAJ is with the Corrosion Science and Technology Division, Materials Characterization Group, Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research. Manuscript submitted August 15, 2018.

METALLURGICAL AND MATERIALS TRANSACTIONS A

Interaction of structural materials with liquid metal coolant modifies the microstructure, local chemistry, and surface morphology. Consequent changes in physical and mechanical properties of the materials lead to shortened service life of the components.[2] LMC can be tackled either by developing advanced structural materials with modified composition or by adding corrosion inhibitors to the liquid metal.[1] Effectiveness of the latter method depends on multiple factors like composition of the structural material, continuous availability of the inhibitor in the liquid metal and strict oxygen control in the melt.[3,4] Corrosion tests carried out in Pb or LBE under both static and dynamic conditions in the temperature and oxygen concentration ranges of 573 K to 873 K (300 C to 600 C) and 1010 wt pct-saturation levels gave valuable insights on the compatibility of various structural steels with liquid metals. A brief overview of relevant literature and methodology adopted to develop advanced structural materials that are resistant to LMC is given below. Kurata et al.[5] tested various steels in oxygen saturated static LBE at 723 K (450 C) and arrived at the following conclusions: (1) depth of the corroded layer for both ferritic/martensitic (F/M) and austenitic steels decreased with increasing Cr concentration and (2) for temperatures exceeding 823 K (550 C), LMC was severe for austenitic steels due to Ni dissolution and ‘ferritization’, rendering them unsuitable for service in

Pb or LBE.[6] To combine the superior mechanical properties of austenitic steels with better corrosion resistance in liquid metals, Alumina Forming Austenitic (AFA) steels were developed[7] and the effect of Ni was specifically studied for fixed Al

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