Behaviour of Fatigue Crack Growth of High Yield Strength Steels in Various Kinds of Water Environments

Abstract

A parameter γ_0.167Hz which means the degree of the effect of water environment on fatigue crack growth rate has been evaluated using six kinds of 80120 kgf/mm² grade steels at the stress ratio of 0.10. γ_0.167Hz≡(dadN)_cor/(da/dN)_air, where (da/dN)_cor is fatigue crack growth rate in each water environments at the frequency of 0.167 Hz and 30°C, and (da/dN)_air is that in air. Air-bubbling sea water, N₂ gas-bubbling pure water, air-bubbling pure water, air-bubbling 3% NaCl solution, and air bubbling 3%, , NaCl solution with zinc sacrificial anode have been used as environments. The following results are mainly obtained.
1) When the maximum value of γ_0.167Hz is less than 34 fracture surface almost shows transgramtlar. When the value is more than 34 fracture surface shows intergranular.
2) The maximum value of γ_0.167Hz less than about 2 is not obtained for six kinds of high yield strength steels used and under the above five water environments.
3) Under the zinc sacrificial anode condition, the effect of corrosion control against fatigue crack growth is observed when ΔK<3040 kgf/mm^3/2.
However, the value of γ_0.167Hz is higher under the condition than under another water environments when ΔK> 3040 kgf/mm^3/2. This fact can be explained by hydrogen arised due to cathodic reaction.
4) Under the zinc sacrificial anode condition, the maximum value of γ_0.167Hz are 513 depending on prior austenitic grain sizes, which is explained by relating to cyclic plastic deformation size at the tip of crack.