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. Keeping N and C in solid solution, the yield and flow stresses were measured at varying strain rate and temperature. The following conclusions were obtained. In the N-charged specimens, with a grain diameter of 60 μ , the yield stress increased sharply with the increase of N content from zero to 40 ppm, beyond which it increased slightly in proportion to N content. In the sample of grain diameter 700 μ , it increased nearly in proportion to N content from zero N. In the C-charged specimens, with a grain diameter of 30 μ , the yield stress showed a maximum at about 60 ppm, while with a grain diameter of 700 μ , it increased nearly in proportion to C content from zero C. The grain size dependency of yield stress increased sharply with the increase of N content from zero up to 30 ppm, beyond which it increased slightly. In the case of C, the dependency showed a maximum at about 50 ppm C content. As to the strain-rate dependency, both yield stress and flow stress increased with strain-rate, their absolute values decreasing with N or C content. As to the effect of temperature, these stresses increased greatly as the temperature decreased, the major portion of which was strain-rate dependent. Under 250°K, the effect of about 30 ppm N or C content was little. The data was analyzed by Conrad's theory and the activation energy and pre-exponential term were determined.
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