12.2 MA STEELS

Many MA materials are now commercially available, but steels produced using MA show particular promise in a variety of applications. For example, MA 956 (Fe-20% Cr-4.5% Al-0.5% Ti-0.5% Y ₂O ₃) is a chromium-rich, ferritic stainless steel containing aluminium for oxidation resistance, (aluminium forms an Al ₂O ₃ protective layer at high temperatures) together with a dispersion of Y ₂O ₃ particles for creep resistance. An alternative ferritic stainless steel variant MA 957 (Fe-13.5%Cr-0.3%Mo-1%Ti-0.3%Y ₂O ₃) contains titanium rather than aluminium, and is designed for application in the nuclear industry. MA 956 can be used at operating temperatures of over 1300 C in corrosive atmospheres. The alloy is used as sheet material for aircraft and industrial gas turbine combustors, in burners, swirlers and heat exchangers of power generation equipment (Fig. 12.6), and in heat treatment equipment (Fig. 12.7) [5,1]. INCONEL MA 957 alloy has been developed for an application as nuclear fuel cladding material in a fast breeder reactor. Conventional austenitic alloys can not be used in this application due to swelling caused by the high neutron fluxes, while conventional ferritic steels generally have inadequate creep strength at the service temperature of 700 C. These steels are produced basically to meet these requirements. The steels after MA and consolidation have an ultrafine microstructure containing submicron grains of ferrite. The hardness in this condition is unacceptably high (in contrast to the MA nickel-base superalloys), so the steels are usually recrystallized into a...