定速ひずみ法による軟鋼の硫化物割れの研究

Abstract

Effects of strain rate, applied potential, pH, temperature and H2S concentration on the sulfide cracking of mild steel were studied at constant strain rates by means of an Instron-type tensile test machine. The results obtained are as follows:(1) The susceptibility of sulfide cracking is highest in the strain rate region of 10−6∼10−5 sec−1 and decreases in the higher strain rate region.(2) In both solutions with and without H2S, the lower the pH of solutions the higher is the susceptibility to cracking, which varies almost linearly with pH. The H2S addition to the solution is most effective for the embrittlement at pH 4.7 and above pH 5 the embrittlement index decreases rapidly. The embrittlement indices below pH 4 are almost identical but slightly decrease with the lowering of pH.(3) The susceptibility to the sulfide cracking varies linearly with applied potential, decreasing with anodic polarization and increasing with cathodic polarization.(4) The embrittlement index is highest at 50°C.(5) The change in H2S concentration has little effect on the susceptibility to cracking.(6) The cracking susceptibility is not directly related to the presence of an iron sulfide film on the steel surface.(7) The dissolved species which promotes hydrogen embrittlement in hydrogen sulfide solutions has turned out to be an undissociated, molecular H2S.(8) The sulfide cracking of mild steel is caused by hydrogen embrittlement.(9) The constant strain rate method is suited for quantitative comparison of changes in susceptibility to the hydrogen embrittlement of steel.