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Sigma, a hard intermetallic phase in the iron-chromium system, has a complex tetragonal structure and was formerly thought to be the cause of 885 F embrittlement. Although previous investigators have found that sigma can form in ferritic stainless steels at chromium contents as low as 13 per cent and at temperatures as high as about 1500 F, the alpha — alpha plus sigma boundary of this system was not known with certainty at low chromium concentrations where equilibrium is established very slowly. The present study concerns the effects of exposures at 900, 1050, and 1200 F for durations up to 76,000 hr on annealed specimens and on cold-reduced specimens from a series of laboratory heats of simulated commercial purity steels containing nominally 12, 13, 14, 15, and 16 per cent chromium.
In the cold-reduced condition, all the steels developed sigma during exposure at 900 F, all but the 12 per cent chromium steel developed sigma at 1050 F, and none of the steels developed sigma at 1200 F. In the annealed specimens, sigma had formed within the time studied only in the 14, 15, and 16 per cent chromium steel specimens exposed at 1050 F.
As would be expected, the cold reduced specimens softened during exposure. However, precipitation hardening (885 F embrittlement) occurred in the annealed specimens containing 15 and 16 per cent chromium during exposure at 900 F. The 16 per cent chromium steel, and to a lesser extent the 15 per cent chromium steel, overaged after prolonged exposure at this temperature.
These results show that the alpha — alpha plus sigma boundary for commercial purity chromium steels is located at a lower chromium concentration than was heretofore reported. In addition, the observation—for what is believed to be the first time—of the reversal of 885 F embrittlement by overaging substantiates the prediction made by other investigators.
Ludwigson, D. C.
Technologist, Applied Research Laboratory, U.S. Steel Corp., Monroeville, Pa.
Link, H. S.
Chief research engineer, high strength steels, Applied Research Laboratory, U.S. Steel Corp., Monroeville, Pa.