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Effect of Applied Tensile Stress and Metallurgical Factors on Hydrogen Diffusion and Hydrogen Assisted Cracking Behavior in High Strength Steel

Effect of Applied Tensile Stress and Metallurgical Factors on Hydrogen Diffusion and Hydrogen Assisted Cracking Behavior in High Strength Steel
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The high–strength ferritic steels used in the petrochemical industry suffer frequently from hydrogen assisted cracking (HAC) problem when they are used in a sour environment containing H2S. The atomic hydrogens which are reduced from H+ ions dissociated from H2S tend to become the hydrogen molecule by the recombination reaction (H + H  H2). Since H2S dissolved in aqueous environment suppresses this recombination reaction, the hydrogen atoms are easily adsorbed on the steel surface and diffused into the steel matrix. The hydrogen atoms absorbed in the steel are reversibly or irreversibly trapped at various metallurgical defects in the steel, often resulting in the HAC failure. With the depletion of high quality oil and gas, the HAC failure may become serious engineering problem for the high–strength steel pipes transporting and/or processing the low quality oil and gas containing a large amount of H2S. The HAC problem can be classified into two categories depending on the source of hydrogen and stress level
one is hydrogen induced cracking (HIC) occurring under no applied stress and the other is sulfide stress cracking (SSC) occurring under applied tensile stress or residual stress. In order to understand clearly HAC failure of the steels, the combined effect of hydrogen and applied stress should be evaluated together. For this reason, many studies have been tried to evaluate the hydrogen permeation behavior under tensile stress using the electrochemical permeation technique equipped with a constant load testing device. However, most studies have been restricted to low strength metallic materials because of rupture problem of palladium coating layer deposited on the steel surface under applied stress. Therefore, the present author has developed a new and simple permeation technique which can be used with no problem of pre–mature failure of Pd coating layer under both elastic and plastic stress of the base metal. In the first and second parts of this dissertation, hydrogen diffusion behavior in high strength pressure vessel and TRIP steel under loading condition is investigated utilizing the newly devised permeation technique. The results indicate that the iron sulfide film (FeSx) formed on the pressure vessel steel acts as sites for hydrogen reduction reaction and that the applied elastic stress weakens the stability of the sulfide film due to lots of cracks forming in the local sulfur deficient region. It results in the increase in both the anodic dissolution and hydrogen reduction reaction. In addition, the applied stress in plastic range on ferritic steel leads to the decrease in permeation current considerably due to the trapping of hydrogen atoms by newly generated dislocations. Hydrogen permeation in the TRIP steel under application of plastic deformation is governed by 3 factors, trapping by newly generated dislocations, transport by the dislocation movement and the phase transformation from γ to α’. In particular, the steel becomes more susceptible to hydrogen embrittlement and/or delayed fracture because of the increase in the mobile (diffusible) hydrogen atoms released from the former γ–phase and introduced from the entry side of the steel membrane and of the decrease in the fracture toughness under plastic deformation. Furthermore, for the fundamental understanding of hydrogen transport in the steel, numerous theoretical models describing the hydrogen diffusion and trapping phenomena in the steel membrane have been proposed. These hydrogen diffusion models have contributed much to understand the mechanism of HIC failure. However, there has been no accurate model and equation which can be applied for the real engineering situation where the steel is subjected simultaneously to the hydrogen charging and loading. As a result, the hydrogen diffusion parameters under applied loading condition have not been determined accurately. For this reason, a newly modified diffusion equation which can be applied under the tensile stress even in plastic range is proposed in the third part of this dissertation. From the numerical analysis based on the diffusion equation, it is found that high level of tensile stress which causes the local plasticity or plastic deformation in the steel is concentrated at incoherent 2nd phase particles, and the interface between the particles and matrix is expanded. It provides the additional hydrogen trapping site, resulting in slower diffusion rate and a little decrease in the steady–state permeation current. This phenomenon appears predominantly in the steel containing larger size of inclusion and precipitate having sharp extremities. The diffusion parameters determined by curve-fitting to the diffusion equation indicate that, with the transition of mechanical domain from local to generalized–plastic, a big increase in hydrogen capture rate per irreversible trap site is observed. It suggests that transition probability for hydrogen transport from interstitial lattice site to irreversible trap site increases with the applied stress level. The effect of two kinds of metallurgical factors, microstructure and alloy composition, on the hydrogen diffusion and hydrogen related cracking behavior in the pressure vessel steel is discussed in the fourth and fifth parts of this dissertation. The results in the fourth part indicate that as the microstructure changed from bainite to ferrite/pearlite and also after tempering and post–weld heat treatment (PWHT), apparent hydrogen diffusivity (Dapp) showed a continuous increase while apparent hydrogen solubility (Capp) showed a continuous decrease. The value of Dapp was maximum whereas the value of Capp was minimum after PWHT, suggesting that additional heat treatment leads to the increase in HAC resistance. However, PWHT conducted at 620 oC for 1 hour makes the steel having acicular ferrite more susceptible to HIC. It is closely related with the fact that Fe3C bodies are newly precipitated along the grain boundary by the PWHT. Since the interface between the matrix and Fe3C acts as reversible trapping site for hydrogen atom, the new formation of Fe3C in the acicular ferrite during the PWHT leads to an increase in diffusible hydrogen content in the steel. From the fifth part, it is found that the Cu and Ni–containing steel shows higher corrosion and HIC resistance. It is attributed to the formation of thin and protective sulfide film on the steel surface. Since the protective nature of the Cu and Ni containing sulfide film reduces the anodic dissolution and the hydrogen reduction reaction of the steel, the value of Icorr, crack length ratio (CLR) and diffusible hydrogen contents of the steel are lower than those of the steel without Cu and Ni.
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