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Aircraft main landing gear (MLG) components are commonly manufactured from low-alloyed, martensitic, ultra-high strength steels (UHSS) that have to be coated for corrosion protection, representing an expensive and environmentally harmful production step. To avoid already partly banned corrosion protection plating, the new high-alloyed UHSS, Ferrium S53 (UNS S10500), has been designed to replace lowalloyed legacy materials and has been subjected to a limited field test over five years. As with the legacy alloys, UNS S10500 has a fully hardened martensitic microstructure known to be susceptible to hydrogen assisted cracking, per se. Containing about 10 wt% Cr, steels such as S10500 are at the lower limit for corrosion resistant alloys. Similar to super-martensitic stainless steels used in the oil and gas industry, a common failure sequence in marine environments represents pitting and subsequent hydrogen assisted stress corrosion cracking (HASCC). For addressing such phenomena quantitively, as required for respective lifetime assessments of MLG components and systems, the tolerance of such materials dependent on the absorbed hydrogen concentration must be evaluated quantitatively. However, there is a lack of such valuable materials data, as well as of the fractographic behavior dependent on the hydrogen concentration that might be absorbed during HASCC. To provide an improved understanding of the hydrogen dependent mechanical and fractographic behavior, samples of the legacy AISI 4340 and the new S10500 MLG steels have electrochemically been hydrogen-saturated and subjected to tensile testing. In contrast to a previous study, this contribution for the first time focuses on materials that have been salvaged from real service used landing gear components. In this study, it has been demonstrated that the service-applied S10500 steel has not only a higher strength, but also an improved ductility in comparison to the legacy AISI 4340 steel after similar service durations that provides a higher tolerance against hydrogen concentrations that might be absorbed during potential pitting and HASCC in marine environments. In addition, it has been found that the absorbed hydrogen concentration significantly affects the fracture behavior. Interestingly, hardening of the hydrogen charged low-alloyed AISI 4340 steel changes the fracture topography from trans- toward intergranular, while hardening of the S10500 steel turned the fracture topography from inter- to transgranular at respectively high hydrogen concentrations.
Carrier gas hot extraction (CGHE) is a commonly applied technique for determination of hydrogen in weld joints using a thermal conductivity detector (TCD) for hydrogen measurement. The CGHE is based on the accelerated hydrogen effusion due to thermal activation at elevated temperatures. The ISO 3690 standard suggests different specimen geometries as well as necessary minimum extraction time vs. temperature. They have the biggest influence on precise hydrogen determination. The present study summarizes the results and experience of numerous test runs with different specimen temperatures, geometries (ISO 3690 type B and small cylindrical samples), and factors that additionally influence hydrogen determination. They are namely specimen surface (polished/as-welded), limited TCD sensitivity vs. specimen volume, temperature measurement vs. effects of PI-furnace controller, as well as errors due to insufficient data assessment. Summarized, the temperature is the driving force of the CGHE. Two different methods are suggested to increase the heating rate up to the desired extraction temperature without changing the experimental equipment. Suggestions are made to improve the reliability of hydrogen determination depended on the hydrogen signal stability during extraction accompanied by Evaluation of the recorded data. Generally, independent temperature measurement with dummy specimens is useful for further data analysis, especially if this data is used for calculation of trapping kinetics by thermal desorption analysis (TDA).