Stress corrosion cracking of high‐strength steels

The threshold stress intensity of stress corrosion cracking (SCC) in the NaCl solution, K_ISCC, has been measured for five low alloy steels. The effects of yield strength, alloy elements, microstructure and grain size on K_ISCC were studied. The results showed that K_ISCC decreased exponentially with increasing yield strength, σ_ys, i.e., K_ISCC = 1.38 · 10⁶exp(‐8.26 · 10^‐3σ_ys) for 40CrMoV steel and K_ISCC = 1.42 · 10⁶exp(‐4.66 · 10^‐3σ_ys) for 30CrMnSiNi steel. For low‐alloy high‐strength steels with σ_ys = 1400 MPa, the effect of alloy elements, microstructure and grain diameter larger than 7 μm on K_ISCC was little. The threshold stress intensity of hydrogen‐induced cracking during dynamical charging for 40CrMoTi steel decreased linearly with the logarithm of the concentration of diffusible hydrogen, C₀, i.e., K_IH = 31.3‐9.1lnC₀. This equation was also applicable to SCC of a high‐strength steel in aqueous solution, and in this case, C₀ is constant. The critical hydrogen enrichment concentration, C_th, necessary for SCC of high‐strength steel in water decreased exponentially with the increase in yield strength. It was possible to deduce the relationship between K_ISCC and σ_ys, i.e., K_ISCC = Ak₁exp(‐k₂σ_ys), where A = 3RT√πρ /2(1 + ν) , k₁ and k₂ are constants, which depend upon the compositions and microstructure of the steel as well as the test conditions.