Weibull analysis of low temperature fracture stress data of 20MnMoNi55 and SA333 (Grade 6) steels

Fracture stress data obtained by tensile testing of circumferentially notched round tensile specimens made of 20MnMoNi55 and SA333 (Grade 6) steels at different subzero temperatures down to −150 °C have been analysed following Weibull statistics. It is observed that between −50 and −100 °C the mean Weibull modulus of 20MnMoNi55 steel obtained by employing linear regression technique over the complete range of data population for every test temperature increases in a perfectly linear manner with lowering of test temperature. A sharp decrease in the modulus value with further decrease of test temperature to −150 °C is observed. The data population for −50 and −70 °C, however, do not agree well with single straight-line relationship of Weibull statistics. In case of SA333 (Grade 6) steel nearly two fold increase in the value of mean Weibull modulus is observed on lowering of test temperature from −100 °C to −150 °C. Scanning electron fractography reveals different failure modes in both the steels depending upon test temperatures.