Hydrogen Re-Embrittlement of Aerospace grade High Strength Steels

R. Valentini et alii, Frattura ed Integrità Strutturale, 21 (2012) 30-36; DOI: 10.3221/IGF-ESIS.21.04 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) ABSTRACT. 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. SOMMARIO. 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. KEYWORDS. Hydrogen Re-Embrittlement; AISI 4340; Maraging 250; High Strength Steels; Bolt failure. INTRODUCTION T 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. 30 R. Valentini et alii, Frattura ed Integrità Strutturale, 21 (2012) 30-36; DOI: 10.3221/IGF-ESIS.21.04 EXPERIMENTAL PROCEDURES T wo series of Maraging specimen were heat treated to achieve the desired resistance, the first was peak-aged at 480°C for 8 hours and the second at 535°C for 1 hour. AISI 4340 was quenched and tempered to achieve a hardness of 50-52 HRC. The mechanical properties of the materials in exams are presented in Table 1. Material AISI 4340 Maraging 250 (peak-aged at 480°C) Maraging 250 (peak-aged at 535°C) Y [MPa] 1548 1720 1400 U [MPa] 1858 1740 1410 V20 549 509 441 Table 1: Average (3 specimens) yield (σY ) and ultimate strength (σU ) for SSRT tested steels. Cylindrical unnotched specimens (3 mm of diameter and 25 mm of gage) were used for HRE tests. Specimens were anodically coated with a layer of Zinc (15 µm) as visible in Fig. 1 (a) to obtain a high current associated to hydrogen reduction. To expose the underlying steel, Zinc was removed from an annular area (1 mm wide) shown in Fig. 1 (b). Mechanical tests (SSRT at a strain rate of 3.3·10-5 s-1 ) were performed in the following conditions: air, 3.5% NaCl aqueous solution and paint-stripper. Further tests in salt solution were conducted with the addition of a galvanic coupling to recreate the enhancing effect of a nobler material on HRE. For this particular test, the coupling was made of AISI 316 (austenitic stainless steel), for its high galvanic activity and its good availability and workability. The coupling was made in two symmetrical halves and designed with a cylindrical cavity with the same nominal diameter of the specimen. Specimen and coupling were assembled like shown in Fig. 1 (c) to provide a suitable crevice for HRE. (a) (b) (c) Figure 1: (a) Optical microscope picture of Zinc layer (500X), (b) Zinc coated round unnotched tensile specimen with the annular exposed steel area and (c) galvanic coupling made of AISI 316. 31 R. Valentini et alii, Frattura ed Integrità Strutturale, 21 (2012) 30-36; DOI: 10.3221/IGF-ESIS.21.04 RESULTS R esults of SSRT campaign are presented, as a function of time to failure and environment, in Table 2. Figures 2 to 3 show the measured stress as a function of time for the AISI 4340 and Maraging 250 respectively. From the curves of Fig. 3, it emerges as the shortest time to failure for AISI 4340 (551 s compared to 2490 s of the specimens loaded in air) was measured for the specimens subjected to painting and then stripped. Specimens, tested in salt solution, with and without galvanic coupling failed in comparable times. In particular, the specimens coupled to AISI 316 failed inside the galvanic coupling, even if externally to the exposed area. Fig. 4 (a) shows both the absence of necking and the ring of zinc corrosion products for one of the specimens involved in the latter test. Regarding Maraging 250, any of the curves related to the specimens loaded in corrosive environment differed significantly from the trend associated to the test performed in air. In detail, time to failure for Maraging peak-aged at 480°C were approximately equivalent in all the conditions, whereas the curves related to the Maraging peak-aged at 535°C differed slightly both for time to failure and sustained load. Furthermore, it was observed that all Maraging 250 specimens presented ductile necking like the one showed in Fig. 4 (b). Results from scanning electron in microscopy (SEM) analysis are shown in Figures 5 to 10 for the most significant tests. According to fracture surfaces of Fig. 5, test in air for AISI 4340 resulted in a ductile rupture. Conversely, Fig. 6 displays a brittle f (...truncated)