Ingenieurwissenschaften und zugeordnete Tätigkeiten
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- 2019 (6) (entfernen)
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- Hydrogen (2)
- Martensite (2)
- AISI 304L (1)
- AISI 321 stainless steel (1)
- Carrier gas hot extraction (1)
- Deuterium (1)
- Editorial Board (1)
- Experimental design (1)
- Hydrogen Assisted Cracking (HAC) (1)
- Hydrogen assisted cracking (1)
Organisationseinheit der BAM
A seamless pipe made of AISI 321 stainless steel represented a part of a transportation Pipeline system for hydrogen-containing hot gas in a hydrocarbon cracking unit. After a service period of approximately 21 months, a segment of such pipe demonstrated the cracks, causing leakage and respective fire. For clarification of a failure root cause, various metallurgical investigations combined with numerical simulations have been applied. The results revealed that the rupture of seamless pipe was evidently influenced by hydrogen assisted cracking (HAC). An increased susceptibility of the alloy to HAC had to be attributed to its sensitive microstructure which was related to the occurrence of slip bands with a high quantity in austenite grains, particularly in the specific region underneath the outer wall surface. In addition, the intensity of restraint resulting from the T-joint weld configuration caused respectively higher triaxial stresses in the confined area on the outer wall surface where the crack started. The numerical simulations of hydrogen diffusion revealed that a uniform hydrogen concentration profile over the pipe wall thickness was reached when the service period was more than 20 months. This duration agreed well to the timeto-failure of the actual component. Considering additionally that the final stage of rupture by overload was preceded by severe HAC, as confirmed by the respective intergranular fracture topography.
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.
Supermartensitic stainless steels (SMSS) are a commonly used material nowadays for building offshore structures, i.e. pipelines in the oil and gas industry. The harsh and corrosive environments in oil and gas applications require the correct combination of alloys to attain the desired properties of steel, including high strength and good corrosion properties, even in severe sour service conditions. Welding is the most commonly used method in joining offshore components, depending on requirements requiring strength or fitting. It has been shown that the heat affected zone (HAZ) is more susceptible to certain types of corrosion, including pitting corrosion, especially during severe sour service where a high pH and lower H2S values in the flow medium can lead to pitting corrosion in the HAZ of welded structures. Subsequent hydrogen uptake in the pits can cause cracks to initiate and propagate, leading to rupture of pipelines or catastrophic failures of structures, even at low mechanical loads. Offshore standards allow a certain amount of corrosion, including pitting, to be present before action is required, however the extent of pitting corrosion is not identified by performing visual inspection alone as the subsurface pit diameter may be vastly greater than the pit diameter at the surface. The critical conditions which lead to crack initiation and propagation from a pit with hydrogen uptake are currently not known. Therefore, pitting corrosion and subsequent crack initiation are a danger to the safety of structures. The interest in this phenomenon has resulted in many experimental studies and numerical simulations.
Several numerical models of pitting corrosion and hydrogen uptake resulting in crack initiation are already in existence, but these two phenomena are regularly modelled individually. Thus, a model enabling simulation of both phenomena simultaneously would be of great benefit. Hence, the goal of this study is to develop a model enabling simulation of pit growth and crack initiation, considering hydrogen uptake in the pit from a corrosive environment.
As a first step, this paper presents an investigation into various parameters, which influence crack initiation at pits. These crack critical parameters include: pit geometry, pit location, mechanical load and hydrogen transport into the microstructure. The results will help to identify critical conditions for crack initiation starting at the pit and developing measures to avoid hydrogen assisted cracking (HAC).
The damaging impact of hydrogen on the austenitic stainless steel AISI 304 L was analysed.
To this aim, samples were charged electrochemically with the hydrogen isotope deuterium (2H, D) and examined with time-of-flight secondary ion mass spectrometry (ToF-SIMS) and electron backscatter diffraction (EBSD). The fusion of the obtained chemical and structural information revealed local enrichment of deuterium in austenite, transformation into martensite, crack formation and severe roughening of the specimen surface. The results indicated that martensite was not only formed during charging but also during Desorption and ToF-SIMS examinations. Furthermore, cross-sections of deuterium-charged samples revealed that in preferred deformation bands a g/ε/a 0 evolution is taking place.
By means of microscopic analyses and carrier gas hot extraction (CGHE), it was found that the use of NaAsO2 as recombination poison decreased the uptake of hydrogen significantly and resulted in severe precipitation on the specimen surfaces. This is in contrast to the popular presumption that NaAsO2 enhances the uptake of hydrogen (and
deuterium) during electrochemical charging by hampering its recombination from Atoms to molecules.
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).