TY - JOUR A1 - Zhang, X.-X. A1 - Würth, Christian A1 - Zhao, L. A1 - Resch-Genger, Ute A1 - Ernsting, N.P. T1 - Femtosecond broadband fluorescence upconversion spectroscopy: improved setup and photometric correction N2 - A setup for fluorescence upconversion spectroscopy (FLUPS) is described which has 80 fs temporal response (fwhm) for emission in the spectral range 425–750 nm. Broadband phase matching is achieved with tilted gate pulses at 1340 nm. Background from harmonics of the gate pulse is removed and sensitivity increased compared to previous designs. Photometric calibration of the upconversion process is performed with a set of fluorescent dyes. For Coumarin 153 in methanol the peak position, bandwidth, and asymmetry depending on delay time are reported. KW - Calibration KW - Fluorescence spectroscopy KW - Fluorescence KW - Upconversion KW - Standard KW - Spectral responsivity KW - Time-resolved fluorescence KW - Quality assurance KW - Coumarin 153 KW - Solvation dynamics PY - 2011 U6 - https://doi.org/10.1063/1.3597674 SN - 0034-6748 SN - 1089-7623 VL - 82 SP - 063108-1 EP - 063108-8 PB - American Institute of Physics CY - Melville, NY AN - OPUS4-23886 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Zhang, G. A1 - Österle, Werner A1 - Jim, B. A1 - Häusler, Ines A1 - Hesse, Rene A1 - Wetzel, B. T1 - The role of surface topography in the evolving microstructure and functionality of tribofilms of an epoxy-based nanocomposite N2 - The topographic effect of steel counterface, finished by mechanical grinding with Ra ranging from 0.01 to 0.95 µm, on the structure and functionality of the tribofilm of a hybrid nanocomposite, i.e. epoxy matrix filled with monodisperse silica nanoparticles, carbon fibers and graphite, was systematically investigated. The nanostructure of the tribofilm was comprehensively characterized by using combined focused ion beam and transmission electron microscope analyses. It was identified that oxidation of the steel surface, release, compaction and tribosintering of silica nanoparticles and deposition of an epoxy-like degradation product as well as fragmentation of carbon fibers are main mechanisms determining the structure and functionality of the tribofilm. The size of roughness grooves determines the type and size class of wear particles to be trapped at the surface. An optimum groove size leading to a maximum of surface coverage with a nanostructured tribofilm formed mainly from released silica nanoparticles was identified. KW - hybrid nanocomposite KW - tribological performance KW - topographic effect KW - tribofilm KW - nanostructure PY - 2016 U6 - https://doi.org/10.1016/j.wear.2016.06.012 VL - 364-365 SP - 48 EP - 56 PB - Elsevier B.V. AN - OPUS4-37937 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Qi, H. A1 - Zhang, G. A1 - Wetzel, B. A1 - Wang, T. A1 - Wang, Q. A1 - Österle, Werner T1 - Exploring the influence of counterpart materials on tribological behaviors of epoxy composites N2 - The dependence of the friction and wear of epoxy (EP) composites materials on counterpart materials, such as standard bearing steel, medium carbon steel and chrome-plating (Cr), was investigated. The conventional composite filled with short carbon fiber (SCF) and graphite shows the highest tribological performance when rubbing against Cr, whereas, the hybrid nanocomposite (EP filled with SCF, graphite and silica nanoparticles) exhibits the lowest friction and wear when sliding against the standard bearing steel. The role of nanoparticles in the tribological performance is distinctly different when sliding against with various counterpart materials. It is demonstrated that counterpart materials exert an important influence on material transfer, tribo-oxidation and mechanical mixing of wear products, resulting in the different formation mechanisms of transfer film. KW - Counterpart materials KW - Transfer film KW - Epoxy composites KW - Nanoparticles PY - 2016 U6 - https://doi.org/10.1016/j.triboint.2016.08.015 VL - 103 SP - 566 EP - 573 PB - Elsevier Ltd. AN - OPUS4-37941 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Zhao, F. A1 - Li, G. A1 - Österle, Werner A1 - Häusler, Ines A1 - Zhang, G. A1 - Wang, T. A1 - Wang, Q. T1 - Tribological investigations of glass fiber reinforced epoxy composites under oil lubrication conditions N2 - The tribological performance of short glass fibers (SGF),solid lubricants and silica nanoparticles filled epoxy (EP) composites was investigated under oil lubrication conditions. It is demonstrated that the addition of SGF greatly reduces the friction and wear of EP. However, further addition of solid lubricants and silica nanoparticles does not change obviously the friction and wear. It is identified that the high tribological performance of SGF reinforced EP is related to the high load carrying capacity and abrasion resistance of SGF. The nanostructure of the tribofilm was comprehensively characterized. It is deemed that the tribofilm plays an important role in the tribological performance by avoiding the direct rubbing of the sliding pairs exposed to boundary and mixed lubrication conditions. KW - Reinforced epoxy composites KW - Short glass fiber KW - Oil lubrication KW - Tribofilm PY - 2016 U6 - https://doi.org/10.1016/j.triboint.2016.07.002 SN - 0301-679X VL - 103 SP - 208 EP - 217 PB - Elsevier Ltd. AN - OPUS4-38145 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Weller, Andreas A1 - Zhang, Z. A1 - Slater, L. A1 - Kruschwitz, Sabine A1 - Halisch, M. T1 - Induced polarization and pore radius — A discussion N2 - Permeability estimation from induced polarization (IP) measurements is based on a fundamental premise that the characteristic relaxation time τ is related to the effective hydraulic radius reff controlling fluid flow. The approach requires a reliable estimate of the diffusion coefficient of the ions in the electrical double layer. Others have assumed a value for the diffusion coefficient, or postulated different values for clay versus clay-free rocks. We have examined the link between a widely used single estimate of τ and reff for an extensive database of sandstone samples, in which mercury porosimetry data confirm that reff is reliably determined from a modification of the Hagen-Poiseuille equation assuming that the electrical tortuosity is equal to the hydraulic tortuosity. Our database does not support the existence of one or two distinct representative diffusion coefficients but instead demonstrates strong evidence for six orders of magnitude of variation in an apparent Diffusion coefficient that is well-correlated with reff and the specific surface area per unit pore volume Spor. Two scenarios can explain our findings: (1) the length scale defined by τ is not equal to reff and is likely much longer due to the control of pore-surface roughness or (2) the range of diffusion coefficients is large and likely determined by the relative proportions of the different minerals (e.g., silica and clays) making up the rock. In either case, the estimation of reff (and hence permeability) is inherently uncertain from a single characteristic IP relaxation time as considered in this study. KW - Spectral induced polarisation KW - Pore radius KW - Permeability KW - Specific surface KW - Mercury intrusion PY - 2016 U6 - https://doi.org/10.1190/GEO2016-0135.1 SN - 0016-8033 SN - 1942-2156 VL - 81 IS - 5 SP - D519 EP - D526 AN - OPUS4-38393 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Österle, Werner A1 - Dmitriev, A.I. A1 - Gradt, Thomas A1 - Häusler, Ines A1 - Hammouri, Basem A1 - Morales Guzman, Pablo Israel A1 - Wetzel, B. A1 - Yigit, D. A1 - Zhang, G. T1 - Exploring the beneficial role of tribofilms formed from an epoxy-based hybrid nanocomposite N2 - The composition and nanostructure of a beneficial tribofilm formed during sliding of a hybrid nanocomposite against steel were characterized comprehensively. A similar nanostructure was produced by high energy ball milling of the three identified tribofilm constituents: silica, hematite and graphite. By supplying powders to a pin-on-disc test it has been shown that neither silica, nor hematite, nor a mixture of both provide the low coefficient of friction (COF) observed for the hybrid composite. Only if graphite was blended with the oxides, the low COF was obtained. Thus, a film of finely dispersed stable inorganic wear products containing 15 vol% graphite provides low friction and wear in the considered case. KW - TEM KW - Nanocomposite KW - Tribofilm KW - Ball milling KW - Pin-on-disc test PY - 2015 U6 - https://doi.org/10.1016/j.triboint.2015.03.006 SN - 0301-679X VL - 88 SP - 126 EP - 134 PB - Butterworth-Heinemann CY - Oxford AN - OPUS4-33035 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Österle, Werner A1 - Giovannozzi, A. A1 - Gradt, Thomas A1 - Häusler, Ines A1 - Rossi, A. A1 - Wetzel, B. A1 - Zhang, G. A1 - Dmitriev, A.I. T1 - Exploring the potential of Raman spectroscopy for the identification of silicone oil residue and wear scar characterization for the assessment of tribofilm functionality N2 - We applied a combination of Raman spectroscopy (RS) and cross-sectional transmission electron microscopy (X-TEM) to identify silicone oil residues and tribofilms at steel disc surfaces after tribological testing. Neither chemical cleaning nor mechanical removal of a 50 µm thick surface layer produced a surface without any silicone residue. Nevertheless, long-term tribological properties are not affected due to silicone degradation which has been proved by Raman spectroscopy. Excellent anti-wear and anti-friction properties of a nanocomposite at severe stressing conditions correlated with the formation of a silica-based tribofilm containing amorphous and graphite-like carbon nanoparticles. Since reliable carbon quantification by analytical TEM is difficult, RS is a useful complementary method for carbon identification at wear scars. KW - Raman spectroscopy KW - Cross-sectional TEM KW - Silicone oil residue KW - Tribofilm PY - 2015 U6 - https://doi.org/10.1016/j.triboint.2015.04.046 SN - 0301-679X VL - 90 SP - 481 EP - 490 PB - Butterworth-Heinemann CY - Oxford AN - OPUS4-33421 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Zhang, G. A1 - Wetzel, B. A1 - Jim, B. A1 - Österle, Werner T1 - Impact of counterface topography on the formation mechanisms of nanostructured tribofilm of PEEK hybrid nanocomposites N2 - The effect of steel counterface topography on the formation mechanisms of nanostructured tribofilms of polyetheretherketone (PEEK) hybrid nanocomposites was studied. Three types of surface finishes with mean roughness Ra ranging from nano- to micro-scale were investigated. Tribo-sintering of nanopartides, oxidation of counterface steel and compaction of wear debris are identified to be competing factors dominating the formation and function of the tribofilms. Counterface topography played an important role on the competing factors, and thereby influenced significantly the final structure, the load-carrying capability and the lubrication performance of the tribofilms. It was disclosed that a thin tribofilm, which mainly consists of silica nanoparticles and which forms on the counterface with a submicron roughness, benefits best the tribological performance of the composites KW - Surface topography KW - Nanocomposite KW - Tribology KW - Tribofilm PY - 2015 U6 - https://doi.org/10.1016/j.triboint.2014.11.015 SN - 0301-679X VL - 83 SP - 156 EP - 165 PB - Butterworth-Heinemann CY - Oxford AN - OPUS4-34612 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Zerbst, Uwe A1 - Ainsworth, R. A. A1 - Beier, H.T. A1 - Pisarski, H. A1 - Zhang, Z. L. A1 - Nikbin, K. A1 - Nitschke-Pagel, T. A1 - Münstermann, S. A1 - Kucharczyk, P. A1 - Klingbeil, Dietmar T1 - Review on fracture and crack propagation in weldments - A fracture mechanics perspective N2 - Welding is one of the most common methods in industrial practice for joining components. Its main advantages are high speed in manufacturing combined with low costs and, usually, a high degree of flexibility, integrity and reliability. Nevertheless, welding is a highly complex metallurgical process and, therefore, weldments are susceptible to material discontinuities, flaws and residual stresses which may lead to structural failure and life time reduction. As a consequence weldments are an important field of fracture mechanics methods although its application is more complex than for homogeneous or non-welded structures. The aim of the paper is to provide an overview on the current state of fracture mechanics application to weldments. It starts by discussing the specific features which any fracture mechanics analysis of weldments has to take into account. Then, the experimental determination of fracture toughness, fatigue crack propagation and tensile properties of weldments is addressed. Finally, the analytical determination of the crack driving force in components and structural integrity assessment approaches for weldments are presented. KW - Weldments KW - Fracture mechanics KW - Fracture toughness KW - Fatigue crack propagation KW - Residual stresses KW - Strength mismatch PY - 2014 U6 - https://doi.org/10.1016/j.engfracmech.2014.05.012 SN - 0013-7944 SN - 1873-7315 VL - 132 SP - 200 EP - 276 PB - Elsevier Science CY - Kidlington AN - OPUS4-32819 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - THES A1 - Zhang, Lei T1 - Microstructure-property relationship in microalloyed high-strength steel welds N2 - Hochfeste Stähle sind bevorzugte Werkstoffe für die Herstellung von sicheren und zuverlässigen Strukturen in der Industrie. Dabei ist das Schmelzschweißen die Hauptverbindungstechnologie für diese Werkstoffgruppe. Während der Entwicklung der hochfesten niedriglegierten (engl. HSLA) Stähle wurden in der Vergangenheit unterschiedliche Legierungskonzepte mit fortgeschrittenen Herstellungstechniken kombiniert und umfassend untersucht. Jedoch befassten sich nur wenige Studien damit, wie die unterschiedliche Zusammensetzung der Legierungen die Eigenschaften der hochfesten Schweißverbindungen dieser Stähle beeinflussen, selbst im Fall begrenzter Gehalte von Mikrolegierungselementen. In der Schweißpraxis dieser hochfesten Stähle sind die Herausforderungen an die sich ausbildenden Mikrostruktur und den resultierenden mechanischen Eigenschaften von sehr großem Interesse. Diesbezüglich liegt der Hauptfokus des Interesses beim Einfluss der Mikrolegierungselemente auf die Phasenumwandlung sowie die resultierende Performance der Schweißverbindung selbst. Geringes Erweichen (Softening) der Wärmeeinflusszone (WEZ) sowie ein begrenztes Austenitkörner-Wachstum sind dabei erwünschte Eigenschaften der Schweißnaht, jedoch liegt das Hauptaugenmerk auf der Sicherstellung hervorragender Zähigkeits- und Zugeigenschaften. Zum Erreichen dieses Zieles werden Mikrolegierungselemente wie Ti, Nb oder V bewusst zu diesen modernen hochfesten Stählen zulegiert. Der Fokus der vorliegenden Arbeit ist das Verständnis, wie die mechanischen Eigenschaften der Verbindungen dieser HSLA-Stähle von Unterschieden in den jeweiligen Legierungskonzepten abhängen, die durch moderne Schweißprozesse gefügt werden. Zunächst wurden dazu drei unterschiedlich mikrolegierte (Nb, Ti und Ti+V Zugabe) Stähle vom Typ S690QL untersucht. Lichtmikroskopische Untersuchungen bestätigten dabei, dass eine ähnliche Zusammensetzung aus angelassenem Bainit und Martensit in allen drei Grundwerkstoffen vorherrschte und unterschiedlich stark vergröberte Ausscheidungen der Mikrolegierungselemente beobachtet wurden. Diese Ausscheidungen wurden weiterführend mittels thermodynamischer Softwareberechnung analysiert und durch Transmissions-Elektronen-Mikroskopie (TEM) identifiziert. Die Ergebnisse der mechanischen Werkstoffprüfung zeigten, dass alle drei Stähle oberhalb der nach Norm geforderten Zähigkeits- und Zugfestigkeitswerte lagen, jedoch Unterschiede im Dehnungsverhalten aufwiesen. Die drei Stähle wurden dann anschließend unter Verwendung des gleichen Schweißzusatzes geschweißt. Dabei wurde das abgeschmolzene Schweißgut durch die Mikrolegierungselemente aus dem Grundwerkstoff infolge der hohen Aufmischung beeinflusst. Die erhöhte Aufmischung bildet dabei ein wesentliches Merkmal der verwendeten modifizierten Sprühlichtbogentechnik. Als Ergebnis zeigte der Nb-mikrolegierte Stahl eine genügend hohe Aufnahme von Legierungselementen aus dem Grundwerkstoff in das Schweißgut, um dessen Mikrostruktur im Fall steigender Abkühlraten von nadeligem Ferrit hin zu Bainit zu verändern. Dieses wiederum reduzierte die Zähigkeitseigenschaften des Schweißgutes dieses Nb-legierten Stahls. Dieses Verhalten wurde in den beiden anderen Stählen nicht beobachtet. Ein zweiter Hauptpunkt dieser Arbeit war die Ausbildung der Mikrostruktur in der Feinkorn- und Grobkorn-WEZ und deren Zähigkeitseigenschaften mit den sich verändernden Schweißparametern. Zu diesem Zweck wurden definierte Werkstoffzustände physikalisch simuliert, um die resultierende Mikrostruktur sowie das Austenitkorn-Wachstum zu charakterisieren. Die Mikrolegierungselemente bildeten dabei einen wesentlichen Faktor zur Begrenzung des Austenitkörner-Wachstums. Das Ausmaß der Austenit-Vergröberung in der WEZ war dabei stark abhängig von der Art und dem Volumenanteil der unterschiedlichen Ausscheidungen infolge der unterschiedlichen Mikrolegierung. Von allen dreiStählen zeigte die WEZ des Ti-legierten Grundwerkstoffes das geringste Kornwachstum als Folge des ausreichenden Umfangs von stabilen Ti-Ausscheidungen. Die Ausbildung von nadeligem Ferrit im Korn wurde dabei durch die Ti-Ausscheidungen unterstützt, da diese als bevorzugte Stellen der Nukleation des Ferrits dienten. Die Zähigkeit der WEZ erhöhte sich dabei infolge der Großwinkelgrenzen der feinen Ferrit-Platten. Aufgrund des kombinierten Effektes von Nb und Mo, welcher sich in der bevorzugten Ausbildung von unterem Bainit äußert, konnte die WEZ-Zähigkeit bei hohen Abkühlraten weiter verbessert werden. Im Fall eines größeren Wärmeeintrags bildete sich jedoch bevorzugt oberer Bainit, welcher wiederum die Zähigkeit reduzierte. Der abschließende experimentelle Teil der Arbeit konzentrierte sich auf das Verständnis der Mechanismen, die in bestimmten Fällen zur Erweichung (oder Softening) der WEZ führen. Dieses Erweichen äußerte sich in den unterschiedlichen Zugeigenschaften der geschweißten Verbindungen der Stähle. Dabei war die Bruchlage entweder in der erweichten WEZ oder im Grundwerkstoff, abhängig von den Schweißparametern sowie der Art des geschweißten Stahls. Im Ti-legierten Stahl führte dabei ein erhöhter Wärmeeintrag zur Vergrößerung der Erweichungszone. Dieses führte zu einer signifikanten Abnahme der Härte und anschließend zum Versagen in dieser erweichten Zone im Zugversuch. Die Veränderung der Bruchlage hin zum Grundwerkstoff wurde durch die Begrenzung des Wärmeintrags erreicht. Dieses Verhalten wurde nicht in den beiden anderen Stählen beobachtet. Dieses Verhalten zeigt, dass bereits kleine Unterschiede im Gehalt der Mikrolegierungselemente der Stähle zu großen Variationen in den Zugeigenschaften führten. Für alle drei Stähle, zeigten die Ti-enthaltenden Schweißverbindungen das am deutlichsten ausgeprägte Softening, gefolgt von den Ti+V-enthaltenden Schweißungen und schließlich den Nb-enthaltenden Schweißverbindungen. Das unterschiedliche Softening konnte dabei auf zwei Prozesse bezogen werden, die auch über zusätzliche Dilatometrie-Experimente gestützt wurden: die Phasenumwandlung und das Anlassverhalten. Im Ti-legiertem Stahl lag nach der Phasenumwandlung großformatiger Ferrit als Konsequenz der ursprünglich großen Austenitkörner vor. Dieses führte zu einer abgesenkten Härte dieses Stahls. Weiterhin resultierte die geringere Anlassbeständigkeit des Ti-legierten Stahls (gegenüber dem Nb-legierten Stahl) zu einem weiteren Softening der erweichten WEZ. Deswegen erwies sich diese Kombination aus Legierungszusammensetzung und Schweißwärmeeintrag als kritisch, gestützt durch die Experimente am gleichen S690QL Stahl. Die vorliegende Arbeit hebt den wesentlichen Einfluss der Mikrolegierungselemente auf die Schweißmikrostrukturen und die mechanischen Eigenschaften der Schweißverbindungen hervor. Die Kenntnis dieser empfindlichen Balance zwischen Legierungskonzept des entsprechenden Stahls und geeigneten Schweißparametern ist als kritisch für das fertige Produkt anzusehen. Dazu stellt diese Arbeit spezifische Empfehlungen und Ergebnisse zur Verfügung, um die korrekte Schweißpraxis zu gewährleisten als auch für die Zusammensetzung mikrolegierter hochfester Stähle. N2 - High-strength steels are favoured materials in the industry for production of safe and sustainable structures. The main technology used for joining the components of such steel is fusion welding. Steel alloy design concepts combined with advanced processing technologies have been extensively investigated during the development of High-Strength Low-Alloy (HSLA) steels. However, very few studies have addressed the issue of how various alloy designs, even with limited microalloy addition, can influence the properties of high-strength steel welds. In high-strength steel welding practices, the challenges regarding microstructure evolution and the resulting mechanical properties variation, are of great interest. The main focus is the debate regarding the role of microalloy elements on phase transformation and weld performance. Limited Heat Affected Zone (HAZ) softening and limited austenite grain coarsening are significant design essentials, but the primary goal is to ensure excellent toughness and tensile properties in the steel weld. To achieve this purpose, microalloy elements such as Ti, Nb, or V were intentionally added to modern high-strength steels. The focus of this work was to understand the mechanical properties of HSLA steels resulting from differences in alloy design after joining by modern welding processes. To begin, three microalloyed S690QL steels (Nb, Ti, and Ti+V addition) were investigated. Optical microscopy confirmed that similar mixtures of tempered bainite and martensite predominated the parent microstructure in the three steels, different types of coarse microalloy precipitates were also visible. These precipitates were analysed by using a thermodynamic-based software and then identified by Transmission Electron Microscopy (TEM). Results of mechanical testing revealed that all three steels performed above the standard toughness and tensile strength values, but with varied yielding phenomena. During the welding operation, each of the three steels was joined by using the same filler material. The fused weld metal was influenced by the high dilution of microalloyed elements in the base metal, this was significantly pronounced during the modified spray arc welding technique. As a result, the Nb-containing steel exhibited sufficient amounts of alloy pick-up to transition the microstructure in the weld metal from acicular ferrite to bainite as cooling rate was increased, leading to reduced toughness. This was not observed with the other two steels. A second focus was made on the microstructure Evolution and toughness properties of the coarse and fine grained HAZ as welding parameters changed. In order to characterise the microstructure and austenite grain growth behaviour, physical simulations were conducted. The microalloy precipitates were found to be a dominant factor restricting the austenite grain coarsening. The extent of Austenite coarsening in the HAZ is closely related to the type and volume fraction of each microalloy precipitate. Among the three steels, the Ti-containing HAZ exhibited the smallest extent of grain growth due to the sufficient amount of stable Ti-rich precipitates. Microalloy Addition also markedly influenced the subsequent phase transformation in the HAZ. The formation of intragranular acicular ferrite was promoted by Ti-rich precipitate, acting as favourable nucleation sites of ferrite. This structure enhanced the HAZ toughness owing to fine, high-angle boundaries of ferrite plates. The synergistic effect of Nb and Mo elements was beneficial to improve the HAZ toughness at fast cooling rates by promoting fine lower bainite formation. At high heat input, large upper bainite was formed which caused reduced toughness. The final set of experimental work was concentrated on understanding the HAZ softening mechanisms that influenced variations in the tensile properties of the welded joints. The tensile failure in the softened HAZ or base material depended on the welding parameters and the type of steel being joined. In Ti-containing steel, increased heat Input extended the softened zone width, which caused a significant decrease in hardness and then resulted in failure in this area. Therefore, limited heat Input was used to shift failure position to base material. But this was not observed in the other two steels. Hence, small differences in microalloy addition exhibited large variation in tensile properties. Among the three steels, Ti-containing welds were found to have the most pronounced softening, followed by Ti+V-containing welds and finally Nb-containing welds. This varied softening phenomenon was related to two significant processes supported by the results of additional dilatometry simulation: phase transformation and tempering behaviour. In the Ti-containing steel, the phase Transformation product ferrite was large-sized, as a consequence of initial large austenite grains. This led to the decreased corresponding hardness of the Ti-containing steel. Furthermore, lower tempering resistance in Ti-containing steel as compared to Nb-containing steel, resulted in additional softening effect in the softened HAZ. Therefore, steel alloy identification and heat Input during welding were critical, proven by the experimentation within the same S690QL steel grade. This work emphasised the influence of microalloy elements on weld microstructure and mechanical properties in welded joints. Knowledge of this delicate balance between steel alloy design and appropriate welding parameters is critical for the end product. Thus, this work provides specific recommendations and results to ensure proper welding practice and steel design of microalloyed high-strength steels. T3 - BAM Dissertationsreihe - 155 KW - Microalloyed steel KW - Weld microstructure KW - HAZ softening KW - Mechanical properties PY - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-391574 SN - 978-3-9818270-4-0 SN - 1613-4249 VL - 155 SP - 1 EP - 183 PB - Bundesanstalt für Materialforschung und -prüfung (BAM) CY - Berlin AN - OPUS4-39157 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -