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Residual stresses are crucial when assessing the performance of welded components. The present work deals with the possibilities of transferring the real-life boundary conditions of welding, which influence the residual stress, into the laboratory. The possibilities of a test system specifically developed for this purpose with a maximum load capacity of 2 MN are shown. Due to the structural design, global process, geometry and material-dependent stresses are induced, which can be simulated and quantified within the system. Additionally, X-ray diffraction can be applied to determine the resulting local residual stress distribution precisely with high spatial resolution. Examples are presented how the conditions to be found during production are simulated in the laboratory. It is shown how welding residual stresses in high-strength steels are affected by the heat control. It was possible to clarify why elevated working temperatures significantly increase the tensile residual stresses in the heat affected zone (HAZ). The effect of a heat treatment applied under mechanical stress resulting from welding is demonstrated by the example of a creep resistant steel. Reheat cracking is significantly increased in this case compared to small scale laboratory based tests.
Additive Manufacturing (AM) through the Selective Laser Melting (SLM) route offers ample scope for producing geometrically complex parts compared to the conventional subtractive manufacturing strategies. Nevertheless, the residual stresses which develop during the fabrication can limit application of the SLM components by reducing the load bearing capacity and by inducing unwanted distortion, depending on the boundary conditions specified during manufacturing. The present study aims at characterizing the residual stress states in the SLM parts using different diffraction methods. The material used is the nickel based superalloy Inconel 718. Microstructure as well as the surface and bulk residual stresses were characterized. For the residual stress analysis, X-ray, synchrotron and neutron diffraction methods were used. The measurements were performed at BAM, at the EDDI beamline of -BESSY II synchrotronand the E3 line -BER II neutron reactor- of the Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. The results reveal significant differences in the residual stress states for the different characterization techniques employed, which indicates the dependence of the residual state on the penetration depth in the sample. For the surface residual stresses, longitudinal and transverse stress components from X-ray and synchrotron agree well and the obtained values were around the yield strength of the material. Furthermore, synchrotron mapping disclosed gradients along the width and length of the sample for the longitudinal and transverse stress components. On the other hand, lower residual stresses were found in the bulk of the material measured using neutron diffraction. The longitudinal component was tensile and decreased towards the boundary of the sample. In contrast, the normal component was nearly constant and compressive in nature. The transversal component was almost negligible. The results indicate that a stress re-distribution takes place during the deposition of the consecutive layers. Further investigations are planned to study the phenomenon in detail.
Die Verwendung von LTT-Zusatzwerkstoffen stellt einen alternativen Ansatz zu den sonst üblichen Verfahren der Schweißnahtnachbehandlung zur Schwingfestigkeitserhöhung geschweißter Stahlkonstruktionen dar. Der wesentliche Wirkmechanismus beruht auf der Beeinflussung des Eigenspannungszustands durch die niedrige Martensitstarttemperatur bereits während des Schweißens. Dadurch werden die Druckeigenspannungen aus der behinderten Volumenausdehnung infolge Phasenumwandlung voll wirksam. Weiter weist die Schweißnaht eine hohe Härte auf, die die Schwingrissbildung verzögern kann, allerdings auch zu einer niedrigen Kerbschlagarbeit führt.
Im ersten Teil dieses Beitrages werden grundlegende Untersuchungen zur schweißtechnischen Verarbeitung eines LTT-Zusatzwerkstoffes präsentiert. Dies beinhaltet zunächst die Sicherstellung der Schweißbarkeit sowie die Charakterisierung der Schweißgefüge und deren Härte mit dem Ziel, reproduzierbare Verbindungseigenschaften für die nachfolgende Schwingfestigkeitsprüfung herzustellen.
Der zweite Teil des Beitrages beschäftigt sich mit den dazugehörigen Schwingfestigkeitsuntersuchungen an den Stählen S355J2 und S960Q unter Verwendung konventioneller Zusatzwerkstoffe im Vergleich zum LTT-Zusatz. Das verwendete Schweißdetail ist eine Verbindungsschweißung am Stumpfstoß unter Variation der Schweißnahtausführung (DY-Naht und V-Naht). Weitergehend wird eine bauteilähnliche Probe (die Längssteife) untersucht, bei der der LTT-Zusatzwerkstoff als zusätzliche Schweißlage aufgebracht wurde. Neben der Schwingfestigkeit werden die Schweißnähte hinsichtlich der Eigenspannungen, der Eigenspannungsstabilität im Schwingversuch sowie metallo-grafisch charakterisiert. Die Ergebnisse zeigen, dass LTT-Zusatzwerkstoffe unter Einhaltung wesentlicher Konstruktionsrichtlinien zu einem deutlichen Anstieg der Schwingfestigkeit führen. Dadurch kann das Leichtbaupotential hochfester Stahlgüten genutzt werden.
Submerged arc welded (SAW) components of creep-resistant low-alloyed Cr-Mo-V steels are used for thick-walled heavy petrochemical reactors (wall-thickness up to 475 mm) as well as employed in construction of modern high-efficient fossil fired power plants. These large components are accompanied by significant restraints during welding fabrication, especially at positions of different thicknesses like welding of nozzles. As a result, residual stresses occur, playing a dominant role concerning so-called stress relief cracking (SRC) typically during post weld heat treatment (PWHT). Besides specific metallurgical factors (like secondary hardening due to reprecipitation), high tensile residual stresses are a considerable influence factor on SRC.
For the assessment of SRC susceptibility of certain materials mostly mechanical tests are applied which are isolated from the welding process. Conclusions regarding the influence of mechanical factors are rare so far. The present research follows an approach to reproduce loads, which occur during welding of real thick-walled components scaled to laboratory conditions by using Tests designed on different measures. A large-scale slit specimen giving a high restraint in 3 dimensions by high stiffness was compared to a medium-scale multi-pass welding U-Profile specimen showing a high degree of restraint in longitudinal direction and a small-scale TIG-remelted specimen. The small-scale specimens were additionally subjected to mechanical bending to induce loads that are found during fabrication on the real-scale in heavy components. Results show for all three cases comparable high tensile residual stresses up to yield strength with high gradients in the weld metal and the heat affected zone. Those high tensile stresses can be significant for cracking during further PWHT.
Die Verwendung von LTT-Zusatzwerkstoffen stellt einen alternativen Ansatz zu den sonst üblichen Verfahren der Schweißnahtnachbehandlung zur Schwingfestigkeitserhöhung geschweißter Stahlkonstruktionen dar. Der wesentliche Wirkmechanismus beruht auf der Beeinflussung des Eigenspannungszustands durch die niedrige Martensitstarttemperatur bereits während des Schweißens. Dadurch werden die Druckeigenspannungen aus der behinderten Volumenausdehnung infolge Phasenumwandlung voll wirksam. Weiter weist die Schweißnaht eine hohe Härte auf, die die Schwingrissbildung verzögern kann, allerdings auch zu einer niedrigen Kerbschlagarbeit führt.
Im ersten Teil dieses Beitrages werden grundlegende Untersuchungen zur schweißtechnischen Verarbeitung eines LTT-Zusatzwerkstoffes präsentiert. Dies beinhaltet zunächst die Sicherstellung der Schweißbarkeit sowie die Charakterisierung der Schweißgefüge und deren Härte mit dem Ziel, reproduzierbare Verbindungseigenschaften für die nachfolgende Schwingfestigkeitsprüfung herzustellen. Der zweite Teil des Beitrages beschäftigt sich mit den dazugehörigen Schwingfestigkeitsuntersuchungen an den Stählen S355J2 und S960Q unter Verwendung konventioneller Zusatzwerkstoffe im Vergleich zum LTT-Zusatz.
Das verwendete Schweißdetail ist eine Verbindungsschweißung am Stumpfstoß unter Variation der Schweißnahtausführung (DY-Naht und V-Naht). Weitergehend wird eine bauteilähnliche Probe (die Längssteife) untersucht, bei der der LTT-Zusatzwerkstoff als zusätzliche Schweißlage aufgebracht wurde. Neben der Schwingfestigkeit werden die Schweißnähte hinsichtlich der Eigenspannungen, der Eigenspannungsstabilität im Schwingversuch sowie metallografisch charakterisiert. Die Ergebnisse zeigen, dass LTT-Zusatzwerkstoffe unter Einhaltung wesentlicher Konstruktionsrichtlinien zu einem deutlichen Anstieg der Schwingfestigkeit führen. Dadurch kann das Leichtbaupotential hochfester Stahlgüten genutzt werden.
Submerged arc welded (SAW) components of creep-resistant low-alloyed Cr-Mo-V steels are used for thick-walled heavy petrochemical reactors (wall-thickness up to 475 mm) as well as employed in construction of modern high-efficient fossil fired power plants. These large components are accompanied by significant restraints during welding fabrication, especially at positions of different thicknesses like welding of nozzles. As a result, residual stresses occur, playing a domi-nant role concerning so-called stress relief cracking (SRC) typically during post weld heat treat-ment (PWHT). Besides specific metallurgical factors (like secondary hardening due to re-precipitation), high tensile residual stresses are a considerable influence factor on SRC. For the assessment of SRC susceptibility of certain materials mostly mechanical tests are applied which are isolated from the welding process. Conclusions regarding the influence of mechanical factors are rare so far. The present research follows an approach to reproduce loads, which occur during welding of real thick-walled components scaled to laboratory conditions by using tests designed on different measures. A large-scale slit specimen giving a high restraint in 3 dimensions by high stiffness was compared to a medium-scale multi-pass welding U-profile specimen showing a high degree of restraint in longitudinal direction and a small-scale TIG-re-melted specimen. The small-scale specimens were additionally subjected to mechanical bending to induce loads that are found during fabrication on the real-scale in heavy components. Results show for all three cases compa-rable high tensile residual stresses up to yield strength with high gradients in the weld metal and the heat affected zone. Those high tensile stresses can be significant for cracking during further PWHT.
Compared to conventional welding consumables using low transformation temperature (LTT) filler materials is an innovative method to mitigate tensile residual stresses due to delayed martensite transformation of the weld. For the effective usage of LTT filler materials, a deeper understanding of the complex processes that lead to the final residual stress state during multipass welding is necessary. Transformation kinetics and the strain evolution of multi-pass welds during welding were investigated in situ at the beamline HEMS@PETRAIII, Germany. Compared to conventional welds, the total strain was reduced and compression strain was achieved when using LTT filler materials. For an optimal use of the LTT effect in the root of multi-pass welds, the alloying concept must be adapted taking care of dilution.
Additive Manufacturing (AM) through the Selective Laser Melting (SLM) route offers ample scope for producing geometrically complex parts compared to the conventional subtractive manufacturing strategies. Nevertheless, the residual stresses which develop during the fabrication can limit application of the SLM components by reducing the load bearing capacity and by inducing unwanted distortion, depending on the boundary conditions specified during manufacturing.
The present study aims at characterizing the residual stress states in the SLM parts using different diffraction methods. The material used is the nickel based superalloy Inconel 718. Microstructure as well as the surface and bulk residual stresses were characterized. For the residual stress analysis, X-ray, synchrotron and neutron diffraction methods were used. The measurements were performed at BAM, at the EDDI beamline of -BESSY II synchrotron- and the E3 line -BER II neutron reactor- of the Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin.
The results reveal significant differences in the residual stress states for the different characterization techniques employed, which indicates a dependence of the residual state on the penetration depth in the sample. For the surface residual stresses, longitudinal and transverse stress components from both X-ray and synchrotron agree well and the obtained values were around the yield strength of the material. Furthermore, synchrotron mapping disclosed gradients along the width and length of the sample for the longitudinal and transverse stress components. On the other hand, lower residual stresses were found in the bulk of the material measured using neutron diffraction. The longitudinal component was tensile and decreased towards the boundary of the sample. In contrast, the normal component was nearly constant and compressive in nature. The transversal component was almost negligible. The results indicate that a stress re-distribution takes place during the deposition of the consecutive layers. Further investigations are planned to study the phenomenon in detail.
The residual stress distribution in IN718 elongated prisms produced by Selective Laser Melting was studied by means of neutron (bulk) and laboratory X-ray (surface) diffraction.
Two deposition hatch lengths were considered. A horizontal plane near the top surface (perpendicular to the building direction) and a vertical plane near the lateral surface (parallel to the building direction) were investigated. Samples both in as-built (AB) condition and removed (RE) from the base plate were characterized.
While surface stress fields seem constant for AB condition, X-ray diffraction shows stress gradients along the hatch direction in the RE condition. The stress profiles correlate with the distortion maps obtained by tactile probe measurements.
Neutron diffraction shows bulk stress gradients for all principal components along the main sample directions. We correlate the observed stress patterns with the hatch length, i.e. with its effect on temperature gradients and heat flow. The bulk stress gradients partially disappear after removal from the baseplate.
Influence of support configurations on the characteristics of selective laser-melted Inconel 718
(2018)
Samples fabricated using two different support configurations by following identical scan strategies during selective laser melting of superalloy Inconel 718 were characterized in this study. Characterization methods included optical microscopy, electron back-scattered diffraction and x-ray diffraction residual stress measurement. For the scan strategy considered, microstructure and residual stress development in the samples were influenced by the support structures. However, crystallographic texture intensity and the texture components formed within the core part of the samples were almost independent of the support. The formation of finer grains closer to the support as well as within the columnar grain boundaries resulted in randomization and texture intensity reduction by nearly half for the sample built on a lattice support. Heat transfer rates dictated by the support configurations in addition to the scan strategy influenced the microstructure and residual stress development in selective laser-melted Inconel 718 samples.