Creep and structural stability of nickel-chromium-iron-alloys at 1,600 degrees F.

A study was made of the creep characteristics at 1,600° F. (870° C.) of 15 alloys covering a range from 1 to 75 percent nickel and from 3 to 55 percent chromium. The results were compared with those of a previous investigation at 1,000° F. (538° C.) on similar alloys. In the investigation at 1,000° F. (538° C.) of the nickel-chromium-iron sys- tem, it was found that the alloys containing little or no iron, 50 to 80 percent nickel and 20 to 50 percent chromium exhibited the greatest resistance to creep. At 1,600° F. (870° C.), the strongest alloys are those containing approximately equal parts of nickel and chromium, and not more than 30 to 40 percent iron. As part of a metallographic study the attempt was made to distinguish between the effect on structure of elevated temperature alone and of elevated tem- perature and stress combined. A comparison was made between the structure in specimens used in the Greep test and that in the unstressed specimens of the same alloys annealed at 1,600° F. (870° C.) for periods ranging from 100 to 1,000 hours or quenched in iced brine from that temperature. In nearly all cases the quenched specimens were similar in structure to the annealed materials which indicates that these alloys were not readily heat treated. Carbide precipitation and agglomeration of the carbide at the grain boundaries were most pronounced in both the binary iron-chromium alloys and the ternary iron-chromium-nickel alloys of higher chromium content. Prolonged heating, of both stressed and unstressed specimens, did not produce any pronounced changes in the structure of the binary iron-chromium alloys or the ternary alloys except those containing 50 percent or more of nickel.