Creep-rupture tests at 1100, 1300, and 1500 F on seven bar-stock and seven sheet-stock, 18 Cr-8 Ni-Mo steels show that small variations in C, N, and P measurably influence both 100 and 1000-hr creep-rupture strength. Variation in amounts of C, N, and P accounts for at least one third of the variability in creep-rupture strength. The additional variability may be related to differences in thermal and mechanical history and in the inherent variability in creep-rupture testing. The bar-stock steels are slightly stronger at 1100, 1300, and 1500 F for rupture in 100 hr and at 1300 and 1500 F for rupture in 1000 hr. At 1100 F, essentially no difference is found between bar and sheet steels for the 1000-hr rupture strength.

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