The effect of austenitizing temperature on the fracture toughness of machine structural steels, SCM440 and S45C was investigated. Steels were austenitized at 780°C or 760°C, 1050°C and 1200°C and then quenched and tempered. The elastic-plastic fracture toughness JIc tests by the stretched zone method were performed to obtain the fracture toughness. Also the cross section of the stretched zone was observed to evaluate the effect of the microstructure.
The results obtained are summarized as follows.
(1) For SCM440 steel the fracture toughness decreased with increasing the austenitizing temperature, while it increased slightly for S45C steel.
(2) The decrease of the fracture toughness of SCM440 steel at high austenitizing temperature was caused by embrittlement of the grain-, martensite- and acicular ferrite·pearlite-boundaries.
(3) The critical stretched zone width, SZWc of S45C steel hardly changed because the variation of the strength of pearlite grains was very small under the present heat treatment conditions.