Effect of aging condition on fatigue strength of maraging steel in long life region

A novel aging treatment was proposed to improve the fatigue strength of maraging steel by taking the effects of aging condition and humidity into account. Rotating bending fatigue tests were carried out for a 350 grade of 18% Ni maraging steel in the long life region up to 10 8 cycles in the relative humidity of 25% and 85%. Aging conditions under investigation included a conventional aging or the so called single aging at 753K, and a two–step aging or double aging by ageing at lower temperature of 473K succeeded to the single aging. Through the double aging, the susceptibility of fatigue strength to humidity was significantly improved without any deleterious effects. The main reasons for the decrease in fatigue strength in high humidity are due to the promotion of crack initiation and the acceleration of small crack propagation. The improvement of fatigue property in high humidity by the double aging was explained from the roles of the additional precipitation of supersaturated Mo atoms during the second lower temperature aging.