Effect of Sea Water on Fatigue Crack Growth Rate in High Yield Strength Steels

Abstract

Fatigue crack growth rates were obtained in air at 16.7 Hz, (da/dN)_air, and those in sea water at 0.167 Hz, (da/dN)_cor, for HT80, HY140, HY180, HY240, HP9-4-20, PH13-8Mo, and 18Ni(200) maraging steels. Stress ratios, R, of 0.10 and 0.50 were used. The ratio of (da/dN)_cor/(da/dN)_air, r_0.167Hz, were used as the degree of the effect of sea water on fatigue crack growth. The following results are obtained:
(1) The value of r_0.167Hz was lower in HY140 tempered at 615°C and HP9-4-20 steels.
(2) The maximum value of r_0.167Hz was obtained at ΔK = 50 to 100 kg/mm^3/2 for R = 0.10 in all steels used. At ΔK<2040 kg/mm^3/2 and ΔK>250 kg/mm^3/2, no effect of sea water on fatigue crack growth rate was obtained.
(3) The value of r_0.167Hz depended on K_max and ΔK at lower and depended on ΔK at higher crack growth rate. The maximum value of r_0.167Hz depended on K_max.
(4) A positive interrelation was obtained between r_0.167Hz and area fraction of brittle fracture in 18Ni (200) maraging steel.