Shozo Sekino1 and Toshiyuki Fujishima2
1Technical Research Institute, Yawata Works, Nippon Steel Corp., Kitakyushu. Present address: Fundamental Research Laboratories, Nippon Steel Corp., Kawasaki
Two kinds of pure iron of commercial grade were heat-treated to have different grain size, hydrogen-treated and N or C charged, the content of which was measured by the Snoek peak height. Strain aging index (SAI) was measured by 7.5% straining and aging. In this case the aging conditions, strain rates, and N or C content were changed. The following conclusions were obtained. SAI increased sharply from zero when N content increased from zero to about 10 ppm, and became almost constant when N content was over 20 ∼ 40 ppm. On the other hand, SAI continued to increase with the increase of C content even when it was over 20 ppm. As regards the effect of aging temperature and time, SAI showed a maximum at 20 min at 100°C, no maximum being attained even at 300 hr at 50°C. SAI increased with strain rate, but the degree of increase was larger, the larger the grain size and the higher N or C content. When N or C content was less than 10 ppm, the grain size showed almost no effect on SAI, but in the range of higher N or C content, SAI was larger, the finer the grain size. As to the effect of annealing and furnace cooling on SAI in the case of only C, SAI was dependent largely on the interstitial C remaining in that state after the annealing and furnace cooling.
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