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The influence of columbium on the impact properties of 18-8 steels has been studied in steels containing 8 to 16 per cent nickel and 0 to 3 per cent columbium. The addition of more than 2 per cent columbium decreases the toughness of the steel in the annealed condition due to the formation of M6C carbide and the compound Fe3Cb2. Long heating at 650 and 870 C. (1200 and 1600 F.) has no significant influence on impact toughness of steels containing up to 2 per cent columbium, but it is more detrimental to higher columbium steels. Positive X-ray identification of sigma phase was obtained after long periods at 650 C. (1200 F.) over the range 8 to 16 per cent nickel and 0.75 to 3 per cent columbium. However, the steels do not form large amounts of sigma even though strained at elevated temperatures. Cold working, followed by low-temperature annealing, accelerates the rate of precipitation of sigma phase and favors particle growth, whereas, high-temperature annealing inhibits precipitation and particle growth. The precipitation of sigma phase in a critical particle size is believed to have a beneficial effect on creep strength.
Binder, W. O.
Research Metallurgist, Union Carbide and Carbon Research Laboratories, Inc., Niagara Falls, N. Y.