Inconsistency in Transition Temperature by Testing Methods of Low Carbon Steels

Abstract

The fracture process at ductile-brittle fracture transition region of low carbon steels has been compared for J-integral tests and instrumented V-notch Charpy tests. Three steels of ferrite-pearlite structure, A and B, and of mixed structure with bainite, C, were employed. Only differences of the compositions are respective Ni to Mn ratios, and characteristic differences in the ductile-brittle transition behaviors such as the transition temperature, R-curve and brittle fracture initiation stage have been observed. From the analysis of the instrumented test, a Charpy energy is decomposed into the energies dissipated in involved fracture processes. On contrary to steel A and B, decreases of the energy transition temperature and of temperature dependence of stretched zone width at Charpy test compared with at the fracture toughness test are observed with steel C. These differences are originated in Region II where blunting of the notch tip proceeds associated with the extension of the stretched zone. Referring to the stress analyses, the estimated local stress at the brittle fracture initiation site of Charpy tests is about a half of that of J-integral tests. The inconsistency of the transition temperature takes place for steel C which shows pop-in at the J-integral test. It is discussed that the inconsistency of the transition temperature is caused by the suppression of pop-in at Charpy test because of its low local stress.