Issue 21

R. Valentini et alii, Frattura ed Integrità Strutturale, 21 (2012) 30-36 ; DOI: 10.3221/IGF-ESIS.21.04 30 Hydrogen Re-Embrittlement of Aerospace grade High Strength Steels R. Valentini, C. Colombo, M. De Sanctis, G. Lovicu Department of Chemical Engineering, University of Pisa, Pisa (Italy) r.valentini@diccism.unipi.it A BSTRACT . Hydrogen Re-Embrittlement on anodically coated high strength steels is a relevant risk for aerospace structures due to the possibility of hydrogen uptake during the operative life of the components. AISI 4340 and Maraging 250 unnotched tensile specimens were subjected to SSRT in order to evaluate the influence of test environment on time to failure. Fracture surfaces were examined by SEM analysis to evaluate the degree of embrittlement and to correlate it with hydrogen diffusivity of the tested steels. S OMMARIO . Il Re-Infragilimento da idrogeno di acciai altoresistenziali con rivestimenti sacrificali anodici è un rischio concreto per le strutture aerospaziali a causa della possibilità che si verifichi l’assorbimento di idrogeno durante la vita operativa dei componenti. Per valutare l’influenza dell’ambiente sul tempo a rottura, sono state svolte prove SSRT con provini di trazione lisci realizzati in AISI 4340 e Maraging 250. Le superfici di frattura sono state esaminate al SEM per valutare il grado di infragilimento e correlarlo con la diffusività dell’idrogeno degli acciai testati. K EYWORDS . Hydrogen Re-Embrittlement; AISI 4340; Maraging 250; High Strength Steels; Bolt failure. I NTRODUCTION his work falls within the context of brittle fractures associated to hydrogen embrittlement for anodically high strength steels (HSS) widely used in the aerospace field. Concerning environment induced failures, HRE is the most dangerous attack for anodically coated HSS components because, differently from Hydrogen Embrittlement (HE), HRE is an irreversible phenomenon that can't be avoided by means of preventive degassing. As a matter of fact, if the protective surface coating is damaged, localized corrosion may lead to embrittlement during its service. Regarding aerospace structures, the risk for HRE is relevant in bolt thread regions, where the coating is likely to be mechanically scratched during assembling. Therefore, the exposed steel becomes the cathodic area of the corrosion reaction and hydrogen reduction takes place on it. Once atomic hydrogen enters the steel, it diffuses toward regions of high stress states, and depending on the aqueous environment corrosivity and on the material hydrogen embrittlement susceptibility, the steel component could suffer for brittle cracking. This article presents the results of the second experimental activity inspired by the failure of a Cd-plated AISI 4340 steel bolt coupled with a IN718 nut due to HRE. Thanks to the first work[1], it was demonstrated that, in presence of a crevice, metals nobler than cadmium strongly enhance local hydrogen reduction on exposed steel areas. The susceptibility of anodic coated HSS to HRE, if loaded in the above mentioned environments, has been largely proved with different kind of anodic coatings[2-4]. Conversely, Maraging steels resistance to HRE hasn't been characterized yet. The aim of this experimental campaign is comparing the resistance of Maraging 250 to AISI 4340 as far as HRE susceptibility is concerned. T