硫化水素溶液中の負荷応力の増加に伴う炭素鋼のHICからSSCC型割れへの遷移

抄録

Transition mode of hydrogen sulfide cracking from hydrogen induced cracking (HIC) to sulfide stress corrosion cracking (SSCC) types has been investigated as a function of the ratio (σ/σ_0.2) of applied stress to proof stress. The tensile specimens of SS 400 carbon steel have been used for sulfide stress corrosion (SSC) testing in accordance with the NACE TM 0177 standard in 5 mass% NaCl solution containing saturated hydrogen sulfide for 720h with/without various applied stress ratio. After the tensile test of the SSC specimens, the observation of the number of cracks, crack angles, crack arrangement and hydrogen content has been carried out against the various values of σ/σ_0.2. The SSCC susceptibility increases with increasing in applied stress ratio from 0.1 and agrees with the stress ratio dependence of crack numbers and hydrogen contents. Mode transition from the HIC to SSCC types takes place beyond 0.35 in σ/σ_0.2. As the applied stress ratio goes up to over this critical value, the crack angles over 45 degree appear in addition to lower angles, and crack arrangement ratio of interval between two cracks to the crack length exceeds 0.12 which value brings about the change from coalescence to no-coalescence of adjacent cracks. It has become apparent that the evaluation of the mode transition from the HIC to SSCC types in hydrogen sulfide cracking of carbon steel is possible from the observation of applied stress ratio, crack angles and crack arrangement ratio.