Tritium distribution at the crack tip of high-strength steels submitted to stress corrosion cracking
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I.
INTRODUCTION
THE effect of hydrogen entry on the stress corrosion cracking (SCC) of high-strength steels has been extensively discussed in the literature and recently reviewed by Ch6ne and Brass. llj The common feature of the different mechanisms proposed to describe the propagation of a SCC crack in high-strength steels is an increased hydrogen concentration at or near the crack tip and the requirement of a critical hydrogen concentration to propagate a crack in a given microstructure. However, precise values of the critical concentration and of the kinetics of hydrogen enrichment at the crack tip f2-~z~are missing9 Direct measurements of local concentrations are unavailable to support the critical hydrogen concentration concept; they require nondestructive techniques with a high sensitivity and a very good spatial resolution. Moreover, the computed data on the hydrogen enrichment at the crack tip depend on assumptions on the stress and strain state at the crack tip and on its influence on hydrogen transport and trapping9 They can differ by several orders of magnitude depending on these assumptions. [4,11] The aim of this study is to take advantage of the capabilities of tritium autoradiography t13,~4~combined with electrochemical permeation experiments t~5~ to get quantitative data on hydrogen diffusion and distribution at a crack tip in samples undergoing SCC in a tritiated aqueous medium. The experimental results presented in this article were obtained on a high-strength steel where hydrogen was able to diffuse during SCC testing9 The steel was heat-treated so that a large plastic zone was present at the crack tip.
II.
EXPERIMENTAL
A. Materials The selected materials are 4120 and 4130 high-strength steels. The chemical composition is given in Table I9 The resistance of each steel to SCC has been extensively characterized as a function of the heat treatment in a previous study. ~6~ The specimens were given the heat treatments reported in Table II together with the corresponding mechanical characteristics, the size of the plastic zone computed according to the Irwin plastic zone correction t17} for plane stress condition and the experimental Klscc values given in Table II. The SCC response of the steels was determined on 11.9-ram-thick bolt-loaded double cantilever beam (DCB) type specimens in 3.5 wt pet NaC1 aqueous solution at room temperature, l~6J Specimens were immersed in the test solution, and the crack length was monitored optically to the nearest 0.1 mm on either submerged or wet specimens. Using a conventional compliance calibration, the instantaneous mode I stress intensity KI ahead of the crack tip was calculated from the measured crack length and loading line displacement, t~81 The SCC tests were continued under these falling K~ conditions, until a threshold value of the stress intensity K,h was reached; this threshold was defined either as the stress intensity for which no visibly detectable crack growth was observed in 24 hours or, for times greater than 24 hours, as when the calculated cra
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