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MSRR 6665 is a Rolls Royce specification for Jethete M152

Jethete M152 is a creep and corrosion resistant, hardened and tempered steel with an usual upper temperature limit of 560°C. It combines very good toughness and creep rupture properties making it an excellent alloy for use in highly stressed turbine blades, discs, screws, bolts and rings.