Effect of Microstructure on Charpy Impact Properties in 80 kg/mm2 Grade Low Carbon High Tensile Strength Steel

The Charpy impact properties of an 80 kg/mm2 grade low carbon high tensile strength steel were examined using an instrumented Charpy impact testing machine in the temperature range from --196 to 80 deg C; the V-notch half size specimens were austenitized at 900, 1000, and 1100 deg C for 30 min followed by water quenching and tempering at 650 deg C for 1 h. The austenite grain size (dA) and the packet size (dP) of martensite lath bundles were 19 and 8.1 (in the case of austenitizing at 900 deg C), 67 and 21 (1000 deg C), and 150 and 93 mu m (1100 deg C), respectively. The crack initiation energy (Ei), the propagation energy (Ep), and the total energy (Et) were calculated from the load/deflection record and the transition behavior of these values was examined. A lower austenitizing temperaute with finer austenite grain size led to a higher value of upper shelf energy and lower transition temperature. The Hall--Petch relation was observed between the energy transition temperature and the inverse of the root of dA and dP; Delta Tr/ Delta dA--1/2 = --18 and Delta Tr/ Delta dP--1/2 = --9.8 deg C/mm--1/2, respectively. Micro-fractography using S.E.M. revealed that the propagation of brittle cracks in the low temperature range proceeded in a (100) alpha cleavage mode, with localized shear deformation in a packet. Refined austenite grains (2.4 mu m) could be obtained by the up-quenching method. This material showed excellent resistance to brittle fracture because of the long total path of cracks through packets. The apparent dynamic fracture toughness was related to the value of the crack initiation energy per unit area in the ligament under the notch. The lateral expansion value varied linearly with the absorbed energy in the temperature range below the transition temperature, and the upper limiting value of the lateral expansion in the relation may be considered as characteristic. 17 ref.--AA