2019 年 85 巻 873 号 p. 18-00431
In this paper, a ferritic CrMo steel SCM440 which satisfies the prerequisites of ASTM E1921 master curve (MC), but which this MC failed to characterize its fracture toughness KJc temperature dependence is introduced. The material was tested at four temperatures -55, 20, 60 and 100°C in the ductile-to-brittle transition temperature region. The obtained reference temperatures T0 (i.e., a temperature that 25 mm thickness specimen is expected to show KJc of 100 MPam1/2) from these tests differed in the range of -9 to 89°C. T0 obtained from all the valid test data was 17°C. Thus, MC using any of these T0’s failed to characterize the material’s KJc temperature dependence. One finding was that this material showed fracture toughness of 100 MPam1/2 at higher temperature than room temperature, which is a rare case reported for the materials that MC is successfully applicable. Remembering Kirk’s opinion that “ASTM E1921 MC exists because MC of the yield temperature dependence (Zerilli equation) exists,” it was noted that the Zerilli equation showed poor fitting performance of yield stress over room temperature for this material. If this yield stress temperature dependence is corrected (i.e., shift in temperature), ASTM E1921 MC will better fit the material’s fracture toughness test data. However, some discrepancy still remained. Because the Zerilli equation shows small yield stress change and might differ from the experimental results in the temperature range over room temperature, a possibility that ASTM E1921 MC might fail to characterize the fracture toughness temperature dependence over room temperature was suggested.
