Strengthening mechanism in fatigue of maraging steel at elevated temperature

In order to investigate fatigue properties of 18%Ni maraging steel at elevated temperature, rotating bending fatigue tests were conducted for plain and drilled specimens at room temperature (RT) and 673K in air. The specimens involved single-aged ones under an under-aging condition at 753K and double-aged ones in which the second aging was performed at 673K to the single-aged ones. Fatigue strength of the single-aged plain specimens was higher at 673K than at RT, though the static strength was inversely decreased at the elevated temperature. On the other hand, the single-aged drilled specimens exhibited nearly the same fatigue strength at both temperatures. It was found that hardness measured at RT increased with time in the single-aged plain specimens fatigued at 673K, which also appeared in the static aging at 673K. This hardness increase was considered to be attributed to precipitation hardening which might occur due to the existence of excess Mo solute atoms. On the other hand, crack observations revealed that crack initiation in the plain specimens was retarded markedly at 673K in comparison with that at RT, whereas crack propagation rate did not differ significantly between these temperatures and between the plain and drilled specimens. The marked delay of crack initiation in the plain specimens at 673K arose from the oxidation of their surfaces. It was also observed that the double-aging led to a large increase in fatigue strength at RT, but resulted in a slight increase at 673K. Based on these results, it was shown that the main reason of the increase in fatigue strength at 673K in the plain specimens was the suppression of crack initiation due to oxidation, while the increase in hardness observed at this temperature played a minor role.