Fatigue-crack propagation in modified 300-grade maraging steel

Abstract

Fatigue-crack-propagation studies were performed on an 18Ni (300-grade) maraging steel at room temperature in dry and humid argon environments (atmospheric pressure) to examine the effect of moisture on the rate of fatigue-crack propagation and on the fracture path through the microstructure. The results showed that the rate of fatigue-crack propagation was increased by about 30 per cent by moisture in the argon. Fractographic analyses showed that the crack path in the dry-argon atmosphere was transgranular. In the humid-argon atmosphere, the crack path was predominantly transgranular with respect to prior austenitegrain boundaries, but appeared to be partly intergranular with respect to subboundaries. The fracture surfaces of the specimens were covered with fatigue striations regardless of the testing environment, an indication that the mechanism of crack growth was similar in both environments, even though the fracture surfaces of the specimens fatigued in dry argon were rougher. Therefore, although moisture in the argon environment accelerated crack growth by about 30 per cent, it apparently did not change the mechanism of crack growth. Evidently the effect of moisture in the environment is to promote striation-type crack growth along paths such as subboundaries. A comparison of fractographic results with those of previous work on hydrogen-accelerated fatigue-crack growth in 250-grade maraging steel indicates that in 300-grade maraging steel the increased rate of fatigue-crack growth in the humid environment is not hydrogen-induced.