熱間金型鋼SKD62の疲労き裂進展特性に及ぼす焼入冷却速度の影響

抄録

The fatigue crack growth behavior was investigated on hot work die steel SKD62, austenitized at 1025°C and subsequently cooled at various rates, and then tempered at 560°C or 630°C. The fatigue crack growth tests were carried out with a ΔK control servohydraulic test machine, using compact-tension specimens.
When the specimens were continuously cooled from 1025°C to room temperature, the fatigue fracture toughness, K_fc, for both 560°C and 630°C tempered specimens gradually decreased with a decrease in cooling rate. The decrease in K_fc correlated well with microstructural change; i.e., the formation of bainite in quenching, and the precipitation of coarse carbides along bainite grain boundaries in tempering, and a fracture morphology change; i.e., the occurrence of a larger quasi-cleavage facet.
When the specimens were rapidly cooled from 400°C to room temperature after slow cooling from 1025°C to 400°C, K_fc for a 560°C tempered specimen was the same as that for a specimen rapidly cooled from 1025°C to room temperature. This may be attributed to the suppression of bainite formation due to the rapid cooling below 400°C. However, K_fc for a 630°C tempered specimen was lower than that for a specimen rapidly cooled from 1025°C to room temperature, though higher than that for a specimen slowly cooled from 1025°C to room temperature. The result indicates that K_fc for a 630°C tempered specimen is not only affected by the cooling rate below 400°C but also by the cooling rate above 400°C.
The fatigue crack growth rate, da/dN, in what is called region 2 was not affected by cooling rate, but slightly decreased with an increase in tempering temperature. This is closely related to the fact that yield strength is not affected by cooling rate, though affected by tempering temperature. As tempering temperature rises, yield strength decreases and crack closure level rises, consequently decreasing da/dN in region 2.