Filtern
Erscheinungsjahr
Dokumenttyp
- Zeitschriftenartikel (55)
- Beitrag zu einem Sammelband (36)
- Beitrag zu einem Tagungsband (36)
- Vortrag (27)
- Forschungsbericht (5)
- Buchkapitel (4)
- Posterpräsentation (4)
Schlagworte
- Hydrogen (15)
- Hydrogen assisted cracking (11)
- ToF-SIMS (8)
- Duplex stainless steel (6)
- Microstructure (6)
- Numerical simulation (6)
- Titanium alloys (6)
- Hydrogen desorption (5)
- Hydrogen embrittlement (5)
- Restraint (5)
Organisationseinheit der BAM
- 9 Komponentensicherheit (19)
- 9.0 Abteilungsleitung und andere (19)
- 5 Werkstofftechnik (4)
- 5.1 Mikrostruktur Design und Degradation (4)
- 6 Materialchemie (3)
- 6.1 Oberflächen- und Dünnschichtanalyse (3)
- 9.4 Integrität von Schweißverbindungen (2)
- 9.2 Versuchsanlagen und Prüftechnik (1)
- P Präsident (1)
- P.0 Präsident und andere (1)
Paper des Monats
- ja (2)
Eingeladener Vortrag
- nein (27)
The relation between microstructure and corrosion behavior of GTA welded AZ31B magnesium sheet
(2007)
Welding of AZ31B magnesium alloy was carried out using gas-tungsten arc (GTA) welding. The microstructure and the corrosion behavior of welded magnesium AZ31B alloy were investigated. ac and dc polarization tests were carried out on the welded Mg sheet. The microstructure was examined using optical and electron microscopy (TEM and SEM), X-ray analysis and EDS. Scanning Kelvin probe force microscopy (SKPFM) was used in order to measure the Volta potential of different phases relative to the matrix. The results showed that the GTA process effected both the microstructure and the corrosion behavior. These results can be explained by the effects of the process on microstructure of AZ31B Mg alloy sheet such as grain size and precipitates caused by the change in precipitation and recrystallization behavior.
Analysis of Residual Stress Distribution in Welded Joints Depending on the Restraint Intensity
(2003)
Hydrogen assisted steel corrosion and cold cracking is still a major topic regarding the reliability of welded steel components, such as offshore platforms and pipelines. Measurement of local and crack critical hydrogen concentrations in a specific weld is difficult and not very precise. Numerical simulations of hydrogen uptake and diffusion developed within the last twenty years have to be regarded as much more effective to determine local hydrogen concentrations in crack critical regions of welds. As an overview of existing theories of the state of the art in modelling of hydrogen cracking in steels, the present contribution shows that the development of a comprehensive numerical model for both types of hydrogen assisted cracking in welds, hydrogen assisted stress corrosion cracking and cold cracking, is still a major research task. As a first step towards that direction, a basic procedure for numerical simulation of crack initiation and propagation has been developed and experimentally verified. The approach can be extended to a comprehensive quantitative model for life time prediction and risk based inspection of welded steel components. (Steel specimen used was S355 along with other materials such as AISI 321, Incoloy 800, Inconel 690 and 600.)
This chapter introduces conceptually Part C of the Handbook Technical Diagnostics of Machines and Plants. Although various prescriptions for organization and performing failure analyses are existing in literature [1–6], up to the present no approach is consistent to the concept of technical systems in the Sects 2.2 and 2.3 (Fig. 2.6) and thus, a comprehensive concept is outlined in this section, with a special attribute to the VDI-Guideline 3822 Failure Analysis.
Numerical investigations on hydrogen-assisted cracking in duplex stainless steel microstructures
(2014)
Numerical modelling of cold cracking initiation and propagation in S 1100 QL steel root welds
(2009)
Numerical investigation on hydrogen-assisted cracking in duplex stainless steel microstructures
(2014)
More than 400 iron hydrogen storage containers (also called bottles or cylinders exploded on the air strip Berlin-Tempelhof on May 25, 1894, leaving immense destruction. The Royal Prussian Materials Testing Institute was requested to investigate the material properties and to furnish an expertise, how an increased safety of such cylinders might be achieved for the future under protection of the interests of the air ship service, as for instance by improvement of delivery specifications or respective material inspections.
The studies conducted personally by the director Prof. Adolf Martens and his deputy Prof. M. Rudeloff represent one of the first comprehensive failure case investigations in history and initiated BAMs long tradition in failure analysis. Martens and his colleague elaborated quite detailed specimen plans and investigated original failure parts with a special emphasis on conspicuous fracture appearance, but also made comparison experiments with hardened as well as annealed samples. Experienced investigators might identify some first routines how to conduct failure analyses and the importance of Adolf Martens as a pioneer in this field becomes evident.
Martens publications about the original expertise Martens, 1896 [1] and [2] include detailed descriptions about the experimental procedures and specimen preparation. Also, quite modern materials testing technologies and machines have been utilized, as for instance light microscopy as well as the tensile testing machines developed by Werder and Pohlmeyer. As special features developed by Martens, precision strain measurements have been applied during respective tensile tests and the so-called micro-photographic apparatus has been adopted to produce photos of the investigated microstructures. Additionally, the publications contain at that time very valuable advices regarding appropriate materials selection for gas storage cylinders.
The present contribution provides a nearly complete and as exact as possible translation of the original report Martens, 1896 [1] written in Old German language. Only little changes have been made in the text for a better understanding.
Short Term Metallurgy and Hot Cracking During Laser Beam Welding of Austenitic Stainless Steels
(2011)
Industrial application of high alloyed austenitic stainless steel laser welding has grown significantly in the recent time due to the continuous improvement of compact and high power density lasers systems. The application of such processes meanwhile ranges from pipeline or railway car body manufacturing to the production of household wares. The largest advantages of the laser application to welding production are much higher welding speeds, reduction or complete exclusion of welding consumables, easy design of the weld joints, decrease of thermal distortions and thus, costs saving. In contrast to arc welding, laser beam welding might particularly be associated with metallurgical defects, like the formation of hot cracks. Such phenomena are related to an order of magnitude higher temperature gradients and cooling rates in the solidification zone, providing rapid solidification kinetics which may cause significant segregation of alloying elements such as Ni and Cr and respective undercooling of the solute at the solidification front. In specific metastable austenitic stainless steels alloys in vicinity of the so called eutectic rim of the Fe-Cr-Ni constitutional diagram, such effects might entail a change of solidification mode from primary ferrite to austenite, providing an increased risk of solidification cracking. Previous studies has shown that the primary solidification mode change during laser beam welding of Cr-Ni austenitic stainless steels such alloys could be effectively influenced by nitrogen absorption as well as by the laser plasma type and also proved the occurrence of metastable primary ferritic solidification. In the present contribution, such results are compared to recent investigations of laser welding newer austenitic Fe-Cr-Mn-Ni steel grades by identification of respective hot cracking critical welding parameter intervals and strain rates in the Controlled Thermal Weldability (CTW) Test.
Hydrogen might cause severe degradation of supermartensitic stainless steels, if they are activated during exposure to sour environments. Consistent and comprehensive data for hydrogen transport in these materials are thus required to support, in particular, modelling of hydrogen assisted cracking as a part of life time assessment of welded steel components. In addition to previously published diffusion coefficients and subsurface concentrations of a supermartensitic stainless steel further data dependent on heat treatment are provided by this contribution. Furthermore, a higher alloyed material has been investigated in the state as received and also in the quenched condition, in order to approach the influences of chemical composition on hydrogen transport in supermartensitic stainless steels. With respect to welding it turned out that the diffusion coefficient and the subsurface concentration are markedly dependent on heat treatment of the materials.
As an approach towards more rough industrial laser welding applications, the Nd:YAG laser welding process has been coupled to the plasma powder welding process for the first time. Process stabilities have been achieved by such hybrid welding which are even better than those known from previous coupling of the laser process with gas metal arc welding (GMAW), for instance.
The present contribution focuses on the technology and potentials of the novel laser hybrid welding technique and addresses the realisation and the perspectives for practical applications. The experimental results demonstrate that the new process coupling enables welding of austenitic stainless steels in the thickness range between 3 mm and 8 mm at various weld geometries and joint configurations and at welding speeds exceeding 2 m/min. In addition it has been proven that the laser plasma powder arc welding (LPPAW) technique can also be applied to out-of-position conditions. Continuous advancement of the welding equipment ensures a high gap bridging ability and levelling of misalignment due to higher powder feeding rates. The recent development of a new plasma powder torch even provides a higher efficiency of powder feeding and guarantees smoth weld surfaces. Up to the present, the investigations demonstrated that the novel laser plasma powder hybrid welding technique offers all perspectives for application in pipe fabrication, ship building and offshore as well as transportation industry.
The explosion of hydrogen gas storage cylinders on May 25, 1894, represents one of the most spectacular failure cases during the late industrialization period in Germany. With respect to modern applications to hydrogen storage as energy carrier, it has several times been referred to as precedent failure case for the whole industrial sector. The detailed investigation reports by Martens have thus gained interest in the last years, but also, because the publications in 1896 about his expertise already provided in September 1894 document one of the first and most comprehensive investigations which can be regarded as a nucleus for modern failure analysis.
After summarizing the newspaper reports in the introduction, the present contribution provides a review of Martens' reports targeted at the development of failure analyses and materials testing procedures as well as potential failure origins.
Due to the increasing global demand for pure silver, native wire silver aggregates in very high purities are gaining more industrial attention. Up to the present, no substantial metallurgical Investigation of natural wire silver exists in the accessible literature. To convey urgently needed cross-disciplinary fundamental knowledge for geoscientists and metallurgical engineers, twenty natural wire silver specimens from eight different ore deposits have been investigated in detail for the first time by EBSD (Electron Back Scattering Diffraction), supported by light microscopy and micro-probe analyses. The improved understanding of the natural silver wire microstructure provides additional Information regarding the growth of natural silver aggregates in comparison to undesired artificial growth on electronic devices. Clear evidence is provided that natural silver curls and hairs exhibit a polycrystalline face-centered cubic microstructure associated with significant twinning. Although the investigated natural wire silver samples have relatively high purity (Ag > 99.7 wt.-%), they contain a variety of trace elements such as, S, Cu, Mn, Ni, Zn, Co and Bi, As and Sb. Additionally, Vickers micro-hardness measurements are provided for the first time which revealed that natural silver wires and curls are softer than it might be expected from conversion of the general Mohs hardness of 2.7.
Aus wirtschaftlich-technischer Perspektive ergeben sich in den letzten zehn Jahren hinsichtlich der Werkstoffinnovation und -auswahl für geschweißte Rohrleitungen im Wesentlichen zwei Entwicklungsrichtungen:
Im Bereich der niedriglegierten Stähle werden immer höher festere Feinkombaustähle mit Streckgrenzen von inzwischen über 900 MPa verwendet. Wirtschaftliches Ziel ist hierbei eine möglichst hohe Kosteneinsparung durch Reduktion der Wanddicke.
Für korrosionsbeanspruchte Komponenten, unter anderem im Petrochemie- und Energieanlagenbau besteht das wirtschaftliche Ziel primär darin, eine höhere Kosteneinsparung durch neue Legierungskonzepte zu erzielen und zwar möglichst so, dass bei mindestens gleicher Festigkeit auf die Verwendung von teuren Elementen, wie beispielsweise Nickel, verzichtet werden kann. Beispiele hierfür sind insbesondere hochlegierte martensitische Cr-V-Stähle, supermartensitische Stähle, Lean-Duplexstähle, superaustenitische Stähle und austenitische Cr-Mn-Stähle.
Für einige der neu eingeführten Grundwerkstoffe sind die Zusatzwerkstoffe noch in der Entwicklung. In jedem Fall stellen diese Werkstoffinnovationen besondere Ansprüche hinsichtlich einer Rissvermeidung während der schweißtechnischen Verarbeitung, um von vornherein riss- bzw. fehlerfreie Rohrleitungen herzustellen, die eine hohe Sicherheit gegen ein Versagen während der Inbetriebnahme und im Verlauf der vorgesehenen Lebensdauer aufweisen. Fast alle dieser neu eingeführten Rohrleitungswerkstoffe weisen eine reduzierte Heiß- oder Kaltrissresistenz vor allem während des Schweißens mit modernen Hochleistungsprozessen, bspw. Laserhybridschweißen, auf. Dabei besteht nach wie vor Forschungsbedarf hinsichtlich der jeweiligen Entstehungsmechanismen. Da es in den letzten zehn Jahren immer häufiger zu Produktionsausfällen und Schadensfällen mit teilweise katastrophalen Konsequenzen gekommen ist, werden die Ursachen und die Vermeidung fertigungsbedingter Heiß- und Kaltrisse im Folgenden zusammengefasst.
Schadensfälle in den letzten zehn Jahren mit teilweise katastrophalen Folgen belegen, dass der Vermeidung von Rissen beim Schweißen von Rohrleitungen besondere Aufmerksamkeit zu schenken ist. In der Analyse solcher Schäden ist zu unterscheiden, ob die Risse bereits während oder kurz nach der schweißtechnischen Fertigung der Rohrkomponenten oder während des anschließenden Betriebes entstanden sind. Im vorliegenden Beitrag wird sich vor allem auf die Vermeidung einer Heiß- und Kaltrissbildung beim Schweißen von Rohrleitungen konzentriert und es werden weitere Forschungsperspektiven abgeleitet.
The present contribution highlights recent trends in investigating hydrogen diffusion and cracking in steel welds. For such studies, supermartensitic stainless steels (SMSS) have exemplarily been selected. These materials have been used for offshore and marine constructions for about two decades now. They present a versatility of improved properties such as high strength to thickness ratio, good weldability and good corrosion resistance. However, as shown by respective failure cases, SMSS welds might become prone to hydrogen assisted cracking and the degradation phenomena are more easily to investigate due to the nearly homogeneous martensitic microstructure than in other materials. Generally, it has to be distinguished between cracking that occurs during or shortly after fabrication welding, or during subsequent operation of SMSS components. In order to achieve crack avoidance during fabrication and service, conclusive test sequences have to be applied, ranging from field tests at real components and full scale tests investigating the materials behavior under real service conditions to basic and small scale tests, such as tensile and corrosion tests, oriented more towards a materials ranking. Considerable testing of SMSS welds has been carried out and the present paper particularly summarizes spotlights on 1:1 scale component testing of welded tubulars, slow strain rate testing and basic tests oriented to elucidate the hydrogen behavior and degradation in SMSS weld microstructures. Also, permeation tests, hydrogen dependent degradation of mechanical properties and thermal desorption spectroscopy are adressed. As a specific item, first results of lately conducted investigations for tracking hydrogen movement in such weld microstructures by using high energy synchrotron radiation are elucidated.
Hydrogen assisted stress corrosion and cold cracking represent still a major topic regarding the safety of welded steel components against failure in many industrial branches. Hydrogen might be introduced during fabrication welding or might be taken up from an environment during sour service or at cathodic protection. Additionally, understanding and avoidance of hydrogen entry into weld microstructures from gaseous pressurized environments becomes increasingly important for renewable energy components. There are two types of metallurgical mechanisms associated with hydrogen assisted cracking, i.e. the cracking as well as hydrogen transport and trapping mechanisms. For numerical modelling, it has to be considered that both types are not independent of each other, that the mechanisms are not yet completely clarified and that validation of such models strongly depends on implementation of the correct hydrogen related materials properties. However, quite significant achievements have been made in modelling of hydrogen assisted cracking by indirect coupling of thermal, stress-strain as well as hydrogen uptake and diffusion analyses. After a brief introduction into the subject and by revisiting various proposed cracking mechanisms, the present contribution focuses on recent developments of a numerical model based on a comparison of actual hydrogen concentrations and mechanical loads with respective hydrogen dependent material properties as crack initiation and propagation criteria. The basic procedure for numerical simulation of crack initiation and propagation is outlined and it is shown how such numerical simulations can be validated experimentally. Furthermore, it is highlighted how such a procedure has been extended to a comprehensive model for life time prediction of welded steel pipeline components and experimentally verified. Finally, it is outlined how the model can be extended to simulate cracking in heterogeneous steel microstructures on the different scales.
Das Kapitel beginnt mit einer kurzen Einführung über die Korrosion (Wechselwirkung zwischen einem Metall, einer korrosiven Umgebung und der der jeweiligen Konstruktion). Im zweiten Abschnitt werden die wichtigsten Formen der wässrigen elektrochemischen Korrosion (Flächenkorrosion, galvanische, selektive und interkristalline Korrosion sowie Loch- und Spaltkorrosion) betrachtet. Darüber hinaus wird die elektrochemische Korrosion unter mechanischer Belastung betrachtet (Spannungsrisskorrosion, wasserstoffunterstützte Rissbildung, Korrosionsermüdung), sowie Sonderformen der Korrosion (Erosion, Fretting und mikrobiologisch induzierte Korrosion). Der dritte Abschnitt befasst sich mit der chemischen und Hochtemperaturkorrosion (Oxidation, Aufkohlung, Hochtemperatur-Wasserstoffangriff, Aufschwefelung, Nitrierung, Halogenierung). Zusätzlich enthält das Kapitel Maßnahmen zur Vermeidung der Korrosion.
The chapter starts with a brief introduction about corrosion, which is defined as the interdependency between a metal, a corrosive environment, and the respective component design. The second section introduces the most important forms of aqueous electrochemical corrosion (uniform corrosion, galvanic corrosion, selective and intergranular corrosion, and finally pitting and crevice corrosion in the case of passive layer forming metals). In addition, electrochemical corrosion under applied mechanical load is introduced (stress corrosion cracking, hydrogen-assisted cracking, corrosion fatigue), as well as special forms of corrosion (erosion, fretting, and microbiologically induced corrosion). The third section of this chapter introduces (mostly dry) chemical corrosion and high-temperature corrosion (oxidation, carburization, high-temperature hydrogen attack, sulfurization, nitriding, halogenation). As in the case of electrochemical corrosion, chemical corrosion can also be superimposed by mechanical loads. Finally, general facts on the testing of corrosion are introduced.
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.
Since the mid-nineties, supermartensitic stainless steels (SMSS) have increasingly been applied to welded subsea-pipeline systems in the North Sea oil and gas fields, especially to flowlines at mild sour service conditions. However, in 2001 cracking and leaks occurred during installation and service start-up of two SMSS flowlines in the Norwegian Tune gas condensate field, welded with a new developed matching filler wire. Brittle transgranular cracking started especially at inter-run lack of fusion and propagated brittle, predominantly through the weld metal. The present paper provides a brief overview of the original failure case and respective sequence of events leading to complete replacement of the SMSS by carbon steel flowlines in 2002. Then, detailed investigations of a circumferential weld sample of the failed Tune flowline are highlighted, targeted at comparison of the failure appearance to previous investigations of this filler material type and to search for possible explanations for the brittle fracture at the crack initiation area. SEM investigations of the fracture surface revealed brittle areas only in the direction towards the top side of the weld while the major part of the investigated surface exhibited ductile fracture. As an approach to clarify, if the fracture was a consequence of hydrogen assisted cracking, five small sized specimens have been cut out of the original sample. Cracking has been introduced parallel to the original fracture surface in these specimens at respective saw cuts and bending. The results show that brittle transgranular cracking appeared only in the specimen cooled down to very low temperatures by liquid nitrogen and in the sample charged with hydrogen to an average concentration of about 15 ml/100 g. However, a fracture similar to the original surface was observed only in the hydrogenized specimen. As a further result, very similar fracture surfaces of supermartensitic stainless steel weld metals had been observed on specimens subjected to hydrogen assisted cold cracking (HACC) as well as to hydrogen assisted stress corrosion cracking (HASCC). In total, the results indicate that brittle fracture starting at the inter-run lack of fusion were not initiated by high notch tip deformation rates, but rather influenced by hydrogen, probably taken up during welding.
Hydrogen might be introduced during fabrication welding or might be taken up from an environment during sour service or cathodic protection. Thus, hydrogen assisted stress corrosion and cold cracking is still a major topic regarding the reliability of welded steel components, as for instance offshore platforms and pipelines. In order to support conclusive testing and life time evaluation of welded steel components, a numerical model for hydrogen assisted cracking has been developed, particularly taking into consideration the geometrical effects of crack propagation on the respective hydrogen distribution alongside and ahead of the crack. Numerical calculations were based on finite element analysis of the hydrogen and stress-strain distribution by using a commercially available program. The model has been verified experimentally by slow strain rate experiments of supermartensitic stainless steels which are intended to be used more extensively as materials for welded flowlines in North Sea oil and gas production. As first results of such simulations the influence of the subsurface concentration provided by different H2S saturation levels in the NACE TM 0177-96 standard solution on crack propagation and the effect of crack shape on the hydrogen distribution profile are presented in this contribution.
The Tsumeb Mine in Namibia represents one of the best-preserved mining sites in the world and is rapidly gaining cross-disciplinary interest among cultural and engineering scientists. Most of the open pit and the shaft mining equipment are still in place, including the ore processing units and the local power plant. The mining area thus deserves recognition as an industrial world heritage site, especially due to the rarity of such locations on the African continent. The Shaft #1 headgear, built in 1924, represents one of the oldest known riveted steel headgears of the Promnitz design worldwide. In contrast to similar steel structures located in the northern hemisphere, it has been exposed to a different rural semi-arid climate since it is located in the Otavi Mountain Land, characterized by semi-annual change of rainy and dry seasons. Parts of the Shaft #1 headgear have remained largely untouched for more than 70 years. Besides its outstanding heritage value, it thus also represents an interesting object for studying the composition of corrosion layers formed on mild steel surfaces when exposed to continental and industrial mining atmospheres. To find a suitable transparent corrosion prevention coating, various on-site coating samples were evaluated after 11 months of outdoor exposure, including Owatrol Oil®, which is based on natural oil and alkyd resin with strong wicking potential. The substance is frequently applied for the conservation of single components but is not yet widely used on large steel structures in the field of industrial heritage conservation. However, it represented the most stable anti-corrosion coating under the local atmospheric conditions in the on-site tests. Thus, the suitability of Owatrol Oil® as a transparent coating for corrosion protection of riveted mild steel structures in such climates was further investigated as a more recent approach for the conservation of large steel structures. Since the protective coatings are exposed to strong UV radiation in the local climate, the addition of a specific UV stabilizer mixture was also tested. For such laboratory tests, two mild steel samples were taken. The first one originated from a diagonal strut of the 1920s and the second one from a handrail mounted in the early 1960s. Using corresponding high-resolution scanning electron microscopy (HR-SEM) and energy-dispersive X-ray spectroscopy (EDX) it was found that the corrosion layers are predominantly composed of lepidocrocite and goethite. A weathering program simulating the specific environmental conditions at Tsumeb in a UV climate chamber was developed and the corrosion resistance of the mild steel surface was subsequently evaluated by potentiodynamic measurements. Such tests proved to be a fast and reliable procedure for ranking the corrosion resistance of the old mild steels. It was found that the long-term corrosion layers already provide significant protection against further corrosion in the simulated environment. However, the study also showed that this can be further improved by the application of the Owatrol Oil® as a protective coating that also seals crevices. The addition of the UV stabilizers, however, led to a significant deterioration in corrosion protection, even in comparison to that of the uncoated long-term corrosion layers on the surface. Regular overcoating seems more advisable for the long-term preservation of the Shaft #1 headgear than modifying the Owatrol Oil® Coating with the tested UV-stabilizing additives.
Hydrogen generally causes lattice distortions and phase transformations when introduced into a metallic crystal lattice. For the investigations reported in this contribution, hydrogen thermal desorption analysis has been carried out to observe the influence of hydrogen desorption on the lattice of super martensitic stainless steel during continuous heating. The lattice expansion parameter and the phase transformations have been monitored during the thermal desorption process, and the influence of hydrogen on such characteristics has been evaluated. It was found that hydrogen has a significant influence on both the lattice parameter and on the thermal expansion. However, hydrogen has no influence on phase transformation during thermal desorption. The hydrogen's desorption behavior in this process was also observed and it turned out that hydrogen desorbs in two stages, i.e., firstly diffusible hydrogen and trapped hydrogen afterward.
In situ analysis of hydrogen behaviour in stainless steels by high energy synchrotron radiation
(2011)
Hydrogen generally causes lattice distortions and phase transformations when introduced into a metal matrix. The present contribution provides a report of first in situ investigations of hydrogen effects on the lattice of the austenite and the martensite phase in a supermartensitic stainless steel using hard synchrotron radiation for respective energy dispersion diffraction measurements. Lattice distortions, such as the planar spacing expansion, have been monitored during the complete hydrogen effusion process over 24 h and are directly correlated to the dissolved hydrogen in the metal matrix. Among other results, it turned out that hydrogen at the introduced concentration level causes a reversible lattice expansion and after hydrogen effusion the lattice nearly regained its original dimensions. Hydrogen much more significantly affects the austenite phase with respect to lattice expansion than the martensite phase, but no phase transformations occur during the hydrogen desorption process.
Hydrogen interaction with residual stresses in steel studied by synchrotron X-ray diffraction
(2014)
The residual stress state in a material has an important role in the mechanism of cracking, induced or assisted by hydrogen. In this contribution, the beamline EDDI in BESSY II instrument in Berlin was used in order to investigate the influence of hydrogen upon the residual stresses state existing in a Supermartensitic stainless steel sample. The method used for investigating the residual stresses is the sinus square ψ method. This method involves the usage of high energy X-ray diffraction in order to measure the residual stress state and magnitude. It was found that hydrogen presence has a significant influence upon the magnitude of the residual stresses, as its value decreases with high hydrogen content. This effect is reversible, as hydrogen desorbs from the sample the residual stress magnitude gains its initial value before hydrogen charging.
Effect of Tensile Loading and Temperature on the Hydrogen Solubility of Steels at High Gas Pressure
(2023)
The hydrogen solubility in ferritic and martensitic steels is affected by hydrostatic stress, pressure, and temperature. In general, compressive stresses decrease but tensile stresses increase the hydrogen solubility. This important aspect must be considered when qualifying materials for high‐pressure hydrogen applications (e.g., for pipelines or tanks) by using autoclave systems. In this work, a pressure equivalent for compensating the effect of compressive stresses on the hydrogen solubility inside of closed autoclaves is proposed to achieve solubilities that are equivalent to those in pipelines and tanks subjected to tensile stresses. Moreover, it is shown that the temperature effect becomes critical at low temperatures (e.g., under cryogenic conditions for storing liquid hydrogen). Trapping of hydrogen in the microstructure can increase the hydrogen solubility with decreasing temperature, having a solubility minimum at about room temperature. To demonstrate this effect, the generalized law of the hydrogen solubility is parameterized for different steels using measured contents of gaseous hydrogen. The constant parameter sets are verified and critically discussed with respect to the high‐pressure hydrogen experiments.
Gas tungsten arc welds made on wrought magnesium AZ31 plate have
been characterized for corrosion in saline solution (3.5% NaCl). Microstructural
changes induced by the welding process resulted in different
environmental behaviour of each zone (BM-base metal, HAZ-heat affected
zone and FZ-fusion zone). The faster kinetics of corrosion in FZ and especially
HAZ are attributed to (a) the coarse microstructure, consisting of
large grains, and (b) very small amounts of ß-phase in the grain boundaries.
Also, hardness traverses have shown that these zones are weaker
than the base metal. Based on the significant effect of grain size on strength
in magnesium alloys, the weld metal grains have been systematically refined
using controlled oscillation during welding, and by adding a grain
refiner to the weld pool. Detailed microstructure analyses have been carried
out and the relationships between corrosion behaviour, mechanical
properties and microstructure (grain refinement and second-phases formation)
are highlighted.
Hydrogen-assisted cracking (HAC) or hydrogen embrittlement (HE) are the most commonly used terms to describe a time-dependant failure process, characterized by mechanical properties degradation, mainly ductility reduction, and a change in the fracture mode. With almost no direct techniques for observing atomic-scale events at crack tips in bulk specimens, HE/HAC mechanisms are deduced mainly from fractography, microscopic investigation of microstructure changes, surface-science observations, and atomistic or continuum modeling. This paper addresses to HE/HAC mechanisms into two different systems; hydride-forming and non-hydride forming materials. As a representative example of the hydride-forming systems, the discussion focuses on the hydrogen-induced second phase formation (e.g. hydrides) phenomena in titanium based alloys. Due to the large differences in the behavior of hydrogen in α and β phases of titanium, the susceptibility of titanium-based alloys to the various forms and conditions of hydrogen embrittlement can vary markedly. The microstructural changes and hydrogen-induced second phase formation due to exposure at various charging conditions, as well as the difference in hydrogen absorption/desorption behavior as a function of the prior microstructure of titanium alloys, are highlighted. In non-hydride forming materials, where as representative examples stainless steels are chosen, the paper concentrates on the qualitatively same phenomena of hydrogen-induced second phase embrittlement. The phase transitions related to hydrogen-induced cracking, the hydrogen related failure sequences and fracture modes in austenitic and supermartensitic stainless steels and a potential modeling of hydrogen-assisted cracking in these structural metallic materials are discussed.
Different characteristics of hydrogen's absorption/desorption behavior and trapping in ß-21S titanium alloy are studied by means of thermal desorption spectroscopy (TDS). Spectra analysis is supported by data from variety of other experimental techniques, such as LECO hydrogen determinator, XRD and microstructure investigations by means of optic and electronic microscopy. In contrast to many past considerations, the complex process of hydrogen evolution has been found to be significantly affected by the way hydrogen introduction into the alloy. The different trapping sites, as well as phase transformations, have to be considered as the reasons for the different hydrogen evolution behavior.
Based on its excellent combination of a high strength/weight ratio and good corrosion behavior, Ti6Al4V alloy is ranked among the most important advanced materials for a variety of industrial applications. However, in many of these technological applications, this alloy is exposed to environments which can act as sources of hydrogen, and severe problems may arise based on its susceptibility to hydrogen embrittlement. Even small hydrogen concentrations might lead to failure. Consequently, a comprehensive knowledge of hydrogen's absorption/desorption behavior and interactions between hydrogen and different microstructural features is necessary to better understand the desorption and trapping mechanisms, the types of the trap sites, and the trapped hydrogen content, in order to determine the safe service conditions of this alloy in the industry. In this paper, different characteristics of hydrogen's absorption/desorption behavior and trapping in a duplex-annealed Ti6Al4V alloy are studied by means of thermal desorption spectroscopy (TDS). Spectra analysis is supported by data from a variety of other experimental techniques, such as LECO hydrogen determinator, XRD and microstructure investigations by means of optical and electron microscopy. Hydrogen evolution is found to be a very complex process, being affected by the way hydrogen was initially introduced to the alloy, the phase transformations that may occur during the thermal analysis and the presence of potential trapping sites.
As consequence for increasing threats by IEDs (Improvised Explosive Devices) on vehicles, the blast resistance of the welded frames and bodies becomes increasingly important. Considering vehicle welds subjected to blasting, the real configurations of the joints in the structure and the position of the blast loads have to be considered. The present contribution thus focuses on a weld joint at the explosion endangered wheel well of a tactical truck. The high-strength steel welds were subsequently impacted by explosion loads within the upper range from those experienced in practical military operation to cause not only deformation, but also to investigate the ultimate fracture behaviour of the high-strength weld. The interaction between cooling time t8/5 and displacement, crack path as well as fracture surface was analysed. The analyses of the fracture surfaces revealed ductile overload failure and also the size of the dimples was influenced by the cooling time t8/5. As a prominent feature, these investigations showed that the crack path of such high-strength steel welds under blasting is less influenced by the final hardness level in the respective weld microstructures but much more affected by the hardness gradient at the fusion line and inside the Heat Affected Zone (HAZ).
The phase transformation under various cooling rates and in different HAZ regions for high-strength armour steel was analysed by dilatometry. To develop a continuous cooling transformation (CCT) diagram, the samples were heated up to a peak temperature of 1250 °C to achieve a coarse-grained microstructure and then cooled down with a cooling time t 8/5 varying from 3 to 240 s. Analysis of dilatation curves revealed the austenite decomposition process, during which transformation temperatures were determined. The results showed martensitic transformations for all welding-relevant cooling times. Furthermore, to analyse different heat-affected subzones of the weld, the peak temperature was varied between 550 and 1250 °C at a constant cooling time t 8/5 of 6 s. The simulated coarse-grained heat-affected zone (CGHAZ) and fine-grained-heat affected zone (FGHAZ) showed only martensitic transformations with transformation temperatures below 400 °C. The steel exhibited an inhomogeneous hardness with hardening in the CGHAZ and FGHAZ and softening in the intercritical and subcritical HAZ. The physically simulated microstructure was validated by a real hybrid laser-arc weld microstructure.
Materials used in military applications have to withstand multiple threats like ballistics and explosions. Thus, high-strength low alloyed (HSLA) steels are used. The main joining technique for metals is welding. Therefore, analysing the dynamic impact behaviour of high-strength welds is very important to fulfil these demands.
Investigation of welds at high strain rates has rarely been conducted in the past. To determine the dynamic impact behaviour of hybrid laser-arc welds, the Split Hopkinson Pressure Bar (SHPB) technique was used. The base material was a quenched and tempered fine-grained armour steel with yield strength of 1100 MPa.
First, a full hybrid laser-arc weld was investigated by extracting specimens consisting of weld metal and heat affected base material. The influence of two variables, the cooling time between 800 °C and 500 °C (t8/5) and strength of filler material, on the impact behaviour was studied. The cooling time t8/5 was varied by preheating to influence the microstructure in the HAZ and to analyse the effect on the hardness and dynamic compressive strength.
Subsequent analysis to detail the original Investigation was carried out by dilatometer heat treatment of specimens to create homogenous subzones of the weld. These specimens have a homogenous microstructure of HAZ and were tested by SHPB to determine the stress-strain characteristics for the different microstructures of HAZ.
The results of the weld specimen showed the effect of preheating and filler material strength on the dynamic compressive behaviour. The analysis of the different microstructures of the HAZ indicated that especially the tempered microstructure caused a reduction in dynamic compressive strength.
High temperature SSRT - ductility data from cathodic hydrogen charging of 10CrMo910 and 7CrMoVTiB1010 are implemented into a mathematical model of SCC at various operational conditions including temperatures, pH and dissolved hydrogen/oxygen contents as well as global stresses and material properties. The results show that, as a consequence of experimentally verified local acidification at initial anodic path corrosion, subsequent local hydrogen assisted cracking can be a controlling factor for SCC in high temperature water. As a particular effect at global stresses close to the yield point, operational temperatures around 270°C exhibit peak crack growth rates depending on dissolved hydrogen, oxygen, pH and global stress, which has been found to be consistent for both, experimental studies and mathematical modeling.
As an extension of recently presented stress corrosion cracking (SCC) models for pure nickel and iron in high-temperature water, the anodic path, hydrogen-assisted stress corrosion cracking of a model Cr-Ni alloy is presented, utilizing Alloy 600 (UNS 06600) mechanical properties. With the local anodic path corrosion as the crack initiation phase, the respective oxides precipitate while the crack tip solution acidify and establish local hydrogen ion reduction conditions. Depending on the applied global stress, local plastic straining then provides an active crack tip surface that transfers atomic hydrogen to the plastic zone ahead of the crack tip. Hydrogen-assisted crack propagation is then controlled by the calculated local hydrogen ion reduction charge and the plastic strain distribution ahead of the crack tip. The results show that for global stress levels close to the yield stress and at constant dissolved oxygen contents, the increasing dissolved hydrogen contents provide peak crack growth rates at hydrogen levels that decrease both with increasing temperatures and bulk pH. The peak crack growth rates also decrease with decreasing global stress as well as with increasing temperature and increasing bulk pH. The results are in accordance with published experimental investigations and operational experiences regarding the effects of dissolved hydrogen on stress corrosion rates in high-temperature waters of nuclear reactors. They are explained by the interaction between the electrochemical parameters of the respective reactions.
As a chemical metrology tool time-of-flight secondary ion mass spectrometry (ToF-SIMS) has become a very popular technique to monitor the elemental, isotopic and molecular distribution in two or three dimensions. Its reduced sampling depth, high sensitivity, great structural specificity and the direct detection of hydrogen thereby increase the emergence of ToF-SIMS for material and analytical surface science, particularly due to recent instrumental developments improving mass, depth and lateral resolution. For basic surface science, adsorption processes and surface reactivity thus can be investigated in high detail on organic as well as inorganic samples. The use of multivariate data analysis in addition can effectively assist to identify trends in the complex SIMS raw data set and define key co-variances between certain samples or mass spectra. In this contribution the essence of ToF-SIMS is illustrated by discussing two highly relevant energy applications. First, for piezoelectric electroceramics oxygen exchange active zones have been visualized to determine the impact of external field-load to the oxygen vacancy distribution between anode and cathode. As a second case study the interaction of hydrogen species with the microstructure of a duplex stainless steel was investigated. It was concluded that ToF-SIMS has a valuable essence for detailing hydrogen related degradation mechanisms.
Deuterium permeation and cracking in duplex steels as viewed by ToF-SIMS and HR-SEM with data fusion
(2016)
Better understanding of hydrogen assisted degradation and trapping mecha-nisms requires sufficient imaging techniques for respective hydrogen-microstructure interaction studies, in particular with multi-phase metallic micro-structures [1]. The present work is focusing on the elucidation of deuterium be-havior in two austenitic-ferritic duplex stainless steels (DSS) under the assumption that deuterium behaves in many ways similarly to hydrogen [2]. For case studies standard 2205 and lean 2101 DSSs were chosen due to the extensive use of these steels in industry [3]. The analyses were conducted by using a novel in-situ permeation and Time-of-Flight secondary ion mass spectrometry (ToF-SIMS) imaging technique or by ex-situ ToF-SIMS imaging following electrochemical charging experiments. Another pioneering procedure was data fusion (including chemometry) of results of powerful laterally resolved chemical analysis and high resolution structural characterization techniques .
Results for the ex-situ observations showed a different influence of deuterium loading on the two steel grades as well as different damage mechanisms in each phase. Formation of sub-surface blisters between the ferrite and austenite were obtained in both the standard and the lean DSS. In both steels, an increased deuterium concentration was observed around deformed regions such as cracks, confirming that they originate from the presence of deuterium [4]. The formation of parallel cracks was obtained only in the austenite within the standard duplex whereas in the lean duplex the highest intensity of deuterium was obtained in the austenite along the ferrite-austenite interphase.
In comparison, application of the novel in-situ permeation technique enabled to register and record the deuterium permeation through the material and the respective saturation sequence of the two phases as well as the interfaces. Faster diffusion of the deuterium was observed in the ferrite and a direct proof for deuterium enrichment at the austenite-ferrite interface has been given [1]. The integration of the specified techniques gives a better insight into the processes leading to hydrogen induced failure. These two experimental techniques provide very valuable tools for elucidation of respective metallurgical failure mechanisms that can be used for the validation of respective numerical models for hydrogen assisted cracking (HAC).
This study provides an in-depth survey of current technologies and applications for assessing the cold cracking susceptibility of welded joints. From the large variety of existent cold cracking test procedures, the most important and internationally established tests are presented and evaluated in terms of their usefulness and application limits. According to the type of loading, the test procedures are classified into self-restraint and externally loaded tests. Apart from the merely metallurgical weldability tests for determining the cracking susceptibility of base materials, filler materials and weld metals, advanced test methods are presented for evaluating the cold cracking susceptibility of welded components. A salient point brought out in this respect is the fact that the level of external loading in such component weld tests can be applied independently of the welding parameters, reproducing as realistically as possible the practical application case, i.e. the level of the restraint intensity. This study includes a summarized comparison of the cold cracking procedures. It is emphasized that highly accurate consideration and reproduction of the design-specific stiffness conditions is essential in the tests for assessing the cold cracking behaviour of welded joints. Therefore, various numerical analyses are presented in a final chapter for calculating the restraint intensity as a definitive factor affecting cold cracking.
Hot crack prevention in materials production and processing is an essential prerequisite for welded component safety. The causes of hot cracking can ultimately be attributed to the occurrence of metallurgical effects and to structural loads. More than 140 hot cracking test procedures have hitherto been developed for determining the hot cracking resistance. In principle, they are divided in self-restraint and externally loaded hot cracking tests with diverse process variants. Only some of the hot cracking tests are international standardized. Although various factors are known that encourage or prevent hot cracking, it is often not possible even with defined welding conditions to draw immediate conclusions about the hot cracking resistance of a welded component alone from a metallurgical composition of the base and filler materials. Based on an evaluation of the existing theories relating to hot cracking susceptibility assessment, this study summarizes the major hot cracking test procedures and highlights the application limits of the test procedures by presenting overviews along with explanations. It shows that weld hot cracking tests can generally be used to rank materials, welding consumables, and welding conditions. The evaluation of hot cracking test results and of their transferability among one another and to real components always requires consideration of the close relationships between metallurgy, welding process, and parameters, respectively, and prevailing restraint conditions.
Düring the past decades, weldability studies have predominantly been concentrated on the material and the welding process. In order to achieve a closer insight into the structural effects, component weldability tests have been performed by online monitoring of the reaction forces and moments. As a particular item, the effects of different strengths of the base and filier material on the reaction force and stress build up transverse to the welding direction have been studied at a specific structural restraint. As a most relevant result for practical welding it is demonstrated that the final reaction force and also the reaction stress level in the multilayer butt welds decreases with overmatch of the weld metal.