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Synopsis: The Charpy impact properties of an 80kg/mmm2 grade low carbon high tensile strength steel were examined using an instrumented Charpy impact testing machine in the temperature range from -196 to 80•Ž with the V-notch half size specimens austenitized at 900, 1 000, and 1 100•Ž for 30 min followed by water quenching and tempering at 650•Ž for 1 h. The austenite grain size (dA) and the packet size (dP) of martensite lath bundle were 19 and 8.1 (in case of austenitizing at 900•Ž), 67 and 21 (1 000•Ž), and 150 and 93 ƒÊm (1 100•Ž), respectively. The crack initiation energy (E1) , the propagation energy (Er), and the total energy (Et) were calculated from the load-deflection record and the transition behavior of these values was examined. The lower austenitizing temperature with the finer austenite grain size provided the higher value of upper shelf energy and the lower transition temperature. The Hall-Petch relation was recognized between the energy transition temperature and the inverse of root of dA and dP; △Tr/△dA-1/2=-18 and △Tr/△dP-1/2 =-9.8°C/mm-1/2,respectively. The micro-fractography by S.E.M. revealed that the propagation of brittle crack in the low temperature range proceeded on (100)a in the cleavage mode with localized shear deformation in a packet. The refined grain of austenite (2.4 ƒÊm) could be obtained by the up-quenching method. This material resulted in the excellent resistance to brittle fracture because of the long total path of crack through packets. The apparent dynamic fracture toughness was related to the value of crack initiation energy per unit area in ligament under notch. The lateral expansion quantity was linear with the absorbed energy in the temperature range lower than the transition temperature, and the upper limit value of lateral expansion quantity in the relation may be considered as a characteristic.

Effect of Microstructure on Charpy Impact Properties in 80kg/mmm<SUP>2</SUP> Grade Low Carbon High Tensile Strength Steel