Development of superalloys for intermediate heat exchanger tubes in national research and development program of nuclear steelmaking

The first stage research and development on the direct iron-making technology utilizing high temperature reducing gas have been completed in 1980 after eight years and having spent 13.7 billion yen. For the purpose of establishing the fundamental technologies required for the realization of a pilot plant for direct iron-making, high temperature heat exchangers, heat resistant superalloys, high temperature insulating materials, reducing gas generating plant, reduced iron production plant and total system were selected as the main subjects. In this paper, the course and the results of the research and development concerning heat resistant superalloys are reported. As the target of developing the superalloys for heating tubes, the creep rupture strength in 1000 deg C helium after 5 x 104 h should exceed 1 kgf/mm2. These alloys must be worked into the tubes of 25 mm outside diameter, 5mm thickness and length exceeding 7 m. The selection of the candidate superalloys and the tests to confirm their properties were carried out. The alloy design and the structures of the proposed alloys, the creep rupture characteristics, high temperature corrosion characteristics and hydrogen permeability are reported. SSS 113 MA, KSN and R 4286 were selected as the most excellent alloys. (Kako, I.)