Hydrogen assisted cracking of UHSS for military aerospace applications
Creators: name: Steffens, Benjamin R.
Publisher: name: Universitätsbibliothek der HSU / UniBwH
Year: 2018
Subjects: subject: 620 Ingenieurwissenschaften

description: Ultra-High Strength Steels (UHSS) are the predominantly used material on aircraft landing gear systems, to combat the high mechanical and partly impacting loads and to match the criteria of light weight design in airplane engineering. Legacy systems have been commonly manufactured from the low alloyed steel AISI 4340, also known as 36CrNiMo4 with the DIN classification number 1.6511. Another material, 300M, has also been widely used in this application – it is a close variant of AISI 4340 steel that has been alloyed with silicon and vanadium. Both alloys represent fully hardenable low alloyed steels which can reach ultimate strengths of about 2100 MPa and that match the design criteria regarding strength and toughness for landing gear applications, but had poor corrosion resistance. Therefore, these materials were commonly cadmium plated and/or painted with chromate paints to protect them against corrosive environments. These surface treatments worked well to protect the components against corrosion, but have been known for potential introduction of hydrogen into the components at the coating-substrate interface due to industrial processes. A second detrimental effect is the environmentally and personally harmful aspects of the chemical compositions of the surface treatments that have virtually prohibited their use. Ferrium S53, a computationally designed alloy that was tailored specifically for ultra-high strength applications in aerospace systems, has become a viable alternative to the legacy materials. Ferrium S53 met or exceeded all previous criteria for use in landing gear structures while adding an inherent corrosion resistance, which made the previously mentioned surface treatments unnecessary. Ferrium S53 has a highly martensitic microstructure which is susceptible to hydrogen assisted cracking per-se, and this at relative low hydrogen concentration levels. Quantitative investigations of hydrogen dependent properties of aircraft landing gear materials have only scarcely been carried out in the past, although they are essential for respective component life time assessments. To develop an improved scientific understanding of such steels in landing gear components which can be subjected to corrosive media, samples have been taken from materials in the as-delivered condition and, especially, in the used service applied condition. To maintain completely objective test program, the “new barstock” steel was purchased directly from the manufacturer without information about any future testing on the material. The “used component” steel was taken directly from the main landing gear struts of actual U.S. Air Force aircraft with at least 5 years of use. In the case of the “used component” material, selection of where the specimens were extracted would represent the most highly-stressed areas of the component. Based on a substantial analysis of the present state of knowledge regarding hydrogen assisted cracking in martensitic steels, the test program has carefully been selected by following a more multi-disciplinary approach. By such, the materials have been subjected to test series they have not seen up to the present and by which the effects of hydrogen on the mechanical and fractographic behavior in service have been investigated. Among these, a focus has been laid on tensile tests of hydrogen-saturated specimens, also in comparison to the AISI 4340 legacy steel, and on slow strain rate tests (SSRT). The SSRT have been carried out predominantly in artificial but also in natural seawater, partly at elevated temperatures and also at cathodic protection, to evaluate the behavior of the used and service-applied steel Ferrium S53 at extreme conditions in marine environments and to determine potential simple protection measures. Subsequent carrier gas hot extractions, as well as fractographic and metallographic analyses, conveyed in depth results about the cracking behavior of this novel material within presence of hydrogen in the martensitic microstructure.The investigations of the material Ferrium S53 after several years of service can be regarded as globally unique. They also correspond to the approach used by the CAStLE (Center for Aircraft Structural Life Extension) program, located at the U.S. Air Force Academy, to determine and to counteract potential weaknesses and damages of in-service or retired components or systems by suitable analyses. The results of the present contribution confirm that the new computer-designed material Ferrium S53 has superior properties as compared to materials of the type AISI 4340, in all analyses that have been carried out. Among other results, it has been shown by the tensile tests with hydrogen-saturated specimens that the Ferrium S53 steel, in contrast to the AISI 4340 steel, has an improved ductility after similar service durations, even if hydrogen is present in the martensitic microstructure. Also in the as-delivered condition, the Ferrium S53 steel has an improved ductility as compared to the AISI 4340 steel. From SSRT, it turned out that the phenomenology of potential damage of the steel under extreme service in marine climates, as for instance on aircraft carriers in tropical seas, is principally pitting corrosion and subsequent hydrogen assisted stress corrosion cracking. Interestingly, the as-delivered Ferrium S53 steel demonstrated a more brittle intergranular cracking behavior due to the extremely high amount of retained austenite, as compared to the used material with several years of service. Furthermore, a cathodic protection at about [– 600 mV (Ag/AgCl)] proved to be partly effective controlling the pitting corrosion and subsequent hydrogen assisted stress corrosion cracking. However, as compared to previous investigations with other materials which did not consider it, it turned out that such a cathodic protection is accompanied by a minor hydrogen absorption which has to be dealt with during long term usage. However, the SSRT showed a much better behavior of the used and serviceapplied Ferrium S53 steel than at open circuit potentials or respectively increased or decreased potentials. To sum up, the present investigations exhibit a significantly improved behavior of the novel computer-designed Ferrium S53 steel after a service period of more than five years with respect to potential hydrogen assisted stress corrosion cracking, as compared to the new as-delivered condition and also as compared to the new and used legacy AISI 4340 steel.