| The effect of austenitizing temperature and tempering condition on the plane strain fracture toughness K_(Ic) and tensile properties was studied for AISI 4340 and 4130 steels in the austenitized temperature range between 870℃∼1200℃. In the as-quenched condition, plane strain fracture toughness measurements exhibited approximately 70% increase in K_(Ic) values without significant loss in strength for specimens austenitized at 1200℃ compared to those austenitized at 870℃. The specimens tempered at various temperatures after austenitizing at 1200℃ showed, in general, higher K_(Ic) values than those tempered at the same temperatures when austenitized at 870℃. However, the former showed a severe decrease in K_(Ic) values because of temper embrittlement compared to toe latter when they were tempered at 300℃. Under constant austenitizing temperature, specimen with the smaller grain size showed a higher value of toughness, on the other hand, when austenitizing temperature was increased, specimens with the same austenite grain size, the toughness value was significantly increased. These results were also discussed in terms of impurity segregation to prior austenite grain boundary in connection with fracture surface topography. It was concluded that enhancement in K_(Ic) values by high temperature austenitization was attributed to a reduction in the impurity segregation at the grain boundary as compared to conventional austenitizing treatments. |
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