1989 年 38 巻 434 号 p. 1296-1302
The fatigue crack growth behavior of hot work die steel SKD62, which was austenitized at 1025°C, 1100°C, and 1200°C and subsequently quenched and tempered at 560°C-650°C, was investigated. The fatigue test was carried out on a ΔK control servohydraulic test machine, by using compacttension specimens. The impact test was carried out on an instrumented Charpy impact test machine. The results are summarized as follows.
(1) For steels with high hardness (about Hv 600) after tempering at 560°C-570°C, the fatigue fracture toughness Kfc was found to increase by raising the austenitizing temperature from 1025°C to 1100°C. The increase of Kfc is presumably attributed to the decrease of undissolved carbides due to increased austenitizing temperature. However, there was no further increase in Kfc for the increase of austenitizing temperature from 1100°C to 1200°C.
(2) For steels with low hardness (about Hv 400) after tempering at 640°C-650°C, the Kfc was found to decrease with increasing austenitizing temperature. This may be attributed to the embrittlement of grain boundaries by raising both the austenitizing and the tempering temperatures.
(3) At the linear region in the da/dN versus ΔK curve, the crack growth rate was not affected by the austenitizing temperature but decreased slightly with the rise of tempering temperature. The crack growth rate in this region could be correlated with the effective stress intensity factor range ΔKeff, regardless of tempering temperature.
(4) The Charpy impact value decreased with the rise of austenitizing temperature, regardless of tempering temperature. This trend did not agree with the Kfc mentioned above, because the Charpy impact value in this case mainly reflected the resistance to crack initiation.