Analysis of fracture toughness in the transition-temperature region of an Mn-Mo-Ni low-alloy steel

This study is concerned with the analysis of fracture toughness in the transition region of an Mn-Mo-Ni low-alloy steel, in accordance with the ASTM E1921 standard test method. Elastic-plastic cleavage fracture toughness (K _Jc ) was determined by three-point bend tests, using precracked Charpy V-notch (PCVN) specimens, and relationships between K _Jc , the critical component of J (J _c ), critical distance (X _c ), stretch-zone width (SZW), local fracture stress, and plane-strain fracture toughness (K _Ic were discussed on the basis of the cleavage fracture behavior in the transition region. The master curve and the 95 pct confidence curves well explained the variation in the measured K _Jc , and the Weibull slope measured on the Weibull plots was consistent with the theoretical slope of 4. Fractographic observation indicated that X _c linearly increased with increasing J _c , and that the SZW had a good correlation with K _Jc , irrespective of the test temperature. In addition, the local fracture stress was independent of the test temperature, because the tempered bainitic steel used in this study showed a propagation-controlled cleavage fracture behavior.