Subtleties Behind Hydrogen Embrittlement of Cadmium-Plated 4340 Steel Revealed by Thermal Desorption Spectroscopy and Su
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RODUCTION
HYDROGEN Embrittlement (HE) was first observed more than one century ago on steel protected against corrosion by an electroplated metal coating. Such electroplated steel parts were more brittle than non-plated parts, but the reasons behind this behavior were not understood. Johnson[1] found evidences that the problem was related to the presence of hydrogen introduced in the steel parts during cathodic charging or simply after soaking the parts in an acid solution. In the aerospace industry, high-strength steels used for critical parts such as landing gear and helicopter transmission shafts are plated with cadmium, chromium, nickel and other metals. It is still a challenge to prevent HE in all circumstances, especially in high-strength steels, where a fraction of ppm of hydrogen can lead to HE.[2] It is possible to detect potential HE in plated high-strength steel parts using standard sustained-load tests (SLT) such as the ASTM F-519.[2–4] However, this standard test takes 200 hours (more than 8 days), which is logistically restrictive, especially if the part fails the test and must be re-manufactured. A better understanding of the hydrogen distribution in high-strength steels plated with cadmium would help to prevent HE more
J. BELLEMARE, S. LALIBERTE´-RIVERIN, D. ME´NARD, M. BROCHU, F. SIROIS are with the Polytechnique Montre´al, Montreal, QC, H3T 1J4, Canada. Contact e-mail: [email protected] Manuscript submitted April 29, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
effectively, and may also lead to the development of faster non-destructive methods to detect HE. At the microscopic scale, three different mechanisms, HEDE, HELP and AIDE are generally proposed in the literature to explain HE. The HEDE mechanism (Hydrogen-Enhanced DEcohesion) occurs when hydrogen remains trapped in grain boundaries or at interfaces with precipitates and reduces the grain or interface cohesion.[5–8] It frequently results in intergranular fracture[9] or interface decohesion.[10] The HELP mechanism (Hydrogen-Enhanced Localized Plasticity) occurs when hydrogen is trapped in dislocations and facilitates their movement[11–13] under low mechanical stresses. In this case, a large number of dislocations can be found under the fractured surfaces as compared to within the bulk material.[14] Finally, the AIDE mechanism (Adsorption-Induced Dislocation Emission) occurs when hydrogen is adsorbed at crack tips and facilitates the nucleation of dislocations in its vicinity.[15] Microvoids can form close to the crack tip, facilitating the crack propagation. It usually results in a ductile fracture surface with small dimples, as the microvoids cannot grow due to the fast crack propagation rate.[9] In high-strength steels, HEDE is more commonly observed, so intergranular features are expected on fractured surfaces.[16] Hydrogen introduced in a steel part during its service life (in a gaseous hydrogen environment or during cathodic reactions) is called environmental hydrogen, whereas hydrogen introduced during manufacturing or p
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