Effect of Cu Addition on Mechanical Properties of Tempered Martensitic Steels: Retardation of Fatigue Crack Initiation by Cu Precipitation

This study investigates the effects of copper (Cu) addition on the bending fatigue strength of lath martensite, focusing on dislocation structures, Cu precipitates, and crack initiation sites. Steel specimens with varying Cu contents of 0, 1, 2, and 4 mass% were prepared and tempered at 523 K or 723 K. Fatigue tests revealed that Cu addition significantly enhances fatigue strength, particularly at 723 K, where the 4Cu steel (4 mass% Cu) exhibited superior performance compared to the 0Cu steel (<0.01 mass% Cu). Microstructural analysis by SEM and TEM showed that, after fatigue testing, the Cu precipitates at grain boundaries had undergone plastic deformation, indicating local stress relief at the grain boundaries. This stress relaxation effectively suppressed crack initiation at the grain boundaries and shifted the sites of crack initiation to the grain interior. Intragranular Cu precipitates were found to pin dislocations, delaying crack initiation within grains. It is also speculated that retardation of dislocation alignment during tempering, which is caused by Cu in solid solution and as fine precipitates, may further delay crack initiation, although direct confirmation of this effect is still required. At 523 K, Cu addition did not significantly improve fatigue performance, presumably due to the absence of Cu precipitates at grain boundaries. These findings suggest a dual mechanism of grain boundary strengthening by plastically deformable Cu precipitates and matrix pinning by intragranular Cu precipitates, which would explain the observed improvement in the crack initiation life and fatigue limit of Cu-added lath martensite.