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The creep properties and the microstructure of two groups of austenitic steels containing manganese as a replacement for nickel have been studied at 700°C and for a range of stresses. In one group, nitrogen was added to stabilize the austenite; in the other, the content of manganese was higher and the steels were expected to be stably austenitic without nitrogen. The objective of the programme was to demonstrate that austenitic steels containing manganese could be produced having creep properties comparable with those of the more conventional austenitic steels. The work has shown that in terms of life to rupture the present steels compare favourably with equivalent steels containing nickel. The addition of carbide- and nitride-forming elements such as tungsten, vanadium, and molybdenum gave a significant improvement in creep strength relative to the base compositions, and X-ray analysis of extracted residues has been used in order to identify the precipitates formed.

Creep tests have been carried out at 750°C on sheet specimens of Nimonic 90 having thicknesses between 0·15 and 1·7mm and grain sizes in the range 0·04−0·07 mm, but mostly ∼0·05 mm. The results show that there is a significant decrease in creep performance when the thickness is less than about 1mm and that failure occurs as a result of grain-boundary cavitation, irrespective of section size. For specimen thicknesses of 0·28 and 1·7mm the stress dependence of the minimum creep rate was similar to that expected for bulk material, suggesting that diffusion creep was not responsible for the decrease in creep resistance of the thinner specimens. A simple mechanism is proposed to explain the effect of section size on life to rupture and the implications of the interpretation are discussed from the point of view of the practical applications of thin sections in turbine components.

The effects of trace amounts of Pb, Sn, Se, Te, and Tl on the creep and stressrupture properties of Nimonic 105 at 815°C have been investigated. The work confirmed that with the exception of Sn, all the elements have a deleterious effect when present in a sufficient amount. The most harmful impurities were Te and Pb. A generally more sensitive indication of the presence of harmful amounts of impurity was obtained with notched specimens. Optical metallography established that the deleterious effects were associated with greatly increased intergranular cavitation which became evident at an early stage of creep life when impurities were present. Auger electron spectroscopy showed that about one monolayer of lead was present on a cavitated grain boundary in a creep-tested specimen containing 34 ppm of Pb.