Effect of bainite in microstructure on hydrogen diffusion and trapping behavior of ferritic steel used for sour service
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Min-suk Oha) Automotive Components & Materials R&D Group, Korea Institute of Industrial Technology, Gwangju 61012, Republic of Korea
Sung Jin Kimb) Department of Advanced Materials Engineering, Sunchon National University, Suncheon 540-742, Republic of Korea (Received 16 October 2016; accepted 28 November 2016)
To clarify the effect of bainite in microstructure on hydrogen diffusion and trapping behavior and susceptibility to hydrogen assisted cracking of API grade linepipe steel, three specimens with different fraction of bainite in the microstructure are used. Firstly, hydrogen diffusion and trapping behaviors of the steels are studied by utilizing the electrochemical permeation technique. For fundamental analysis on the experimental data, a variety of diffusion parameters were determined by curve-fitting with a theoretical diffusion equation based on numerical finite difference method (FDM). It indicates that the steel with higher fraction of bainite exhibits much higher sub-surface hydrogen concentration and much lower apparent hydrogen diffusivity. This behavior can be understood by the fact that the steel containing higher fraction of bainite in the microstructure has higher concentration of reversible traps and consequent larger diffusible hydrogen, leading to much slower diffusion kinetics of hydrogen atoms. Consequently, the susceptibility to hydrogen induced cracking (HIC) and sulfide stress cracking (SSC) of the steel with higher fraction of bainite increases significantly.
I. INTRODUCTION
Hydrogen assisted cracking attack has been one of the intriguing problems of high-strength ferritic steel equipments used in the oil and gas industry.1–3 This attack is primarily associated with hydrogen uptake and transport in the steels, and a substantial body of literature has been devoted to clarify the relationship between the hydrogen diffusion behavior and cracking failure of the steels.1,4,5 It is generally known that the hydrogen uptake in the steels is initiated by the hydrogen evolution and progressed by the ad/ab-sorption reactions. In sour environment, particularly, the atomic hydrogens reduced cathodically from H1 cation dissociated from H2S gas are readily adsorbed and introduced in the steels because of the poisoning effect.1,5–9 The hydrogen introduction mechanism proposed by Kawashima et al.9 is as follows:
Contributing Editor: Jürgen Eckert Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2016.480
Step 1: H2 Sads þ e ! H2 Sads
ð1Þ
Step 2: Hþ ! Hads þ
ð2Þ
Step 3: H2 Sads þ Hads þ ! H2 S ; Hads ðunstableÞ ð3Þ Step 4: H2 S ; Hads ! H2 Sads þ Hads
ð4Þ
The transport of hydrogen atom in the steel can be classified into 3 kinds of category; interstitial lattice diffusion, reversible trapping, and irreversible trapping. 10–12 As a result of complex interaction among hydrogen atom, trapping site and microstructure of matrix, the steel can be susceptible to HAC failure. For this reason, the steel industry has tried to propo
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