First Principles Study On The Cracking Behavior Of Duplex Stainless Steel Induced By H₂S

According to the lack of research to describe the H2S hydrogen induced cracking behavior and crack growth mechanism of duplex stainless steel from electronic structure level, the first principles method based on density functional theory has been employed in this paper, theαa-Fe, γ-Fe and the surface properties of its low index surfaces have been analyzed firstly. Secondly, the adsorption behavior of H2S and its decomposition products on the α-Fe(110) and γ-Fe(111) surfaces, the dissociation behavior of H2S and HS free radical on the α-Fe(110) and yγFe(111) surfaces, the diffusion behavior of H atom on the α-Fe(110) and γ-Fe(111) surfaces, the diffusion behavior of H atom in α-Fe, γ-Fe bulk, and on the α-Fe(110)/γ-Fe(111) interface during the early period of H2S hydrogen induced cracking for duplex stainless steel have been systematically investigated from the microscopic field. The optimal adsorption configurations and energy characteristics of H2S and its decomposition products on the α-Fe(110) and γ-Fe(111) surfaces have been calculated, the transition states of the dissociation of H2S and HS free radical on the α-Fe(110) and γ-Fe(111) surfaces, the diffusion of H atom on the α-Fe(110) and γ-Fe(111) surfaces, and the diffusion of H atom in the α-Fe, γ-Fe and on the α-Fe(110)/γ-Fe(111) interface have been searched, the adsorption capacity of H2S and its decomposition products on the α-Fe(110) and γ-Fe(111) surfaces, the difficulty of the dissociation of H2S and HS free radical on the α-Fe(110) and γ-Fe(111) surfaces, the difficulty of H atom diffusion on the α-Fe(110) and γ-Fe(111) surfaces, and the difficulty of H atom diffusion in the α-Fe, γ-Fe bulk and on the α-Fe(110)/γ-Fe(111) interface have been compared, and the effects of the alloy elements such as Cr, Mo, Ni on the hydrogen diffusion behaviors of α-Fe, γ-Fe and the α-Fe(110)/γ-Fe(111) interface have been discussed. The microscopic intrinsic reason for the hydrogen induced crack generation of duplex stainless steel has been disclosed from the electronic structure level, and the H2S hydrogen induced cracking behavior and the crack propagation mechanism of duplex stainless steel have been explored, in order to provide a theoretical basis for the microscopic interpretation of hydrogen induced cracking mechanism of the duplex stainless steel in H2S and the development of new materials in duplex stainless steel with excellent resistance to hydrogen induced cracking of H2S. Through this study, the conclusions are as follows:(1) The researches on α-Fe, γ-Fe and the surface characteristics of its low index surfaces show that, the low index surfaces stability of α-Fe is:α-Fe(110)>α-Fe(100)>α-Fe(111), and the low index surfaces stability of γ-Fe is:γ-Fe(111)>γ-Fe(100)>γ-Fe(110);(2) The adsorption studies of H2S and its decomposition products on the α-Fe(110) and γ-Fe(111) show that, the interaction between H2S, HS, S and the two surfaces is:S> HS> H2S;(3) The dissociation studies of H2S and HS free radical on the α-Fe(110) and γ-Fe(111) show that, the dissociation barriers of H2S and HS free radical on the two surfaces are both lower, easy to decompose, and the second dissociation of HS radical is easier than the first dissociation of H2S;(4) The diffusion studies of H atom on the α-Fe(110) and γ-Fe(111) show that, the diffusion barriers of the H atom on the two surfaces are not high, indicates that H atom are easily to diffuse on the α-Fe(110) and γ-Fe(111);(5) The comparison for decomposition adsorption of H2S on the α-Fe(110) and γ-Fe(111) surfaces shows that, the α-Fe(110) surface exists more active H atoms. Due to the diffusion barrier of active H atom on the α-Fe(110) is lower, these active H atoms are more easily migrating into α-Fe lattice. As illustrate above, resulting in the generation of hydrogen induced crack;(6) The researches on the diffusion of H atom in the α-Fe and γ-Fe show that, the activation energy of hydrogen diffusion in α-Fe is more lower, thus accounting for the formation and expansion of hydrogen induced crack in α-Fe priority, and the studies of the effects of the alloy elements such as Cr, Mo, Ni on the hydrogen diffusion of α-Fe and γ-Fe show that, Mo element will significantly increase the hydrogen diffusion activation energies of α-Fe and γ-Fe, and Cr element only able to increase the hydrogen diffusion activation energy of γ-Fe. Hence, reasonable increase in the content of Mo element in α-Fe and Cr, Mo elements in γ-Fe will help to improve the hydrogen induced cracking resistance of duplex stainless steel in H2S;(7) The diffusion study of H atom on the α-Fe(110)/γ-Fe(111) interface shows that, in the process of H atom diffuses from α-Fe to γ-Fe on the interface, the activation energy of hydrogen diffusion firstly decreases, then increases, and the closer to the γ-Fe phase, the greater the diffusion activation energy is. This illustrates that H atom may bypass the γ-Fe phase and straightly diffuse along the two-phase interface when it diffuses at the interface;(8) The investigation on the effects of alloying elements Me on hydrogen diffusion of α-Fe(110)/γ-Fe(111) interface shows that, the Ni element has less impact on the hydrogen diffusion behavior of the interface, while Cr, Mo elements can significantly improve the hydrogen diffusion activation energy of the interface region. This also reflects that a reasonable increase in the content of Cr, Mo elements will help the duplex stainless steel to enhance the ability of hydrogen induced cracking resistance in H2S;(9) The research on the hydrogen induced crack propagation mechanism of duplex stainless steel shows that, the crack preferentially forms and expands in the ferrite phase, and exhibits transgranular cracking. When the crack extends to the two-phase interface of the ferrite and austenite, the austenite phase will hinder the crack propagation as a result of the diffusion activation energy is higher along the direction of the austenite phase. As illustrates above, the crack at the interface shows intergranular cracking. And the hydrogen induced cracking study of 2205 duplex stainless steel further confirms the reliability of the above calculation results.