Solidification Cracking Assessment of LTT Filler Materials by Means of Varestraint Testing and µCT

Investigations of the weldability of metals often deal with hot cracking, as one of the most dreaded imperfections during weld fabrication. The hot cracking investigations presented in this paper were carried out as part of a study on the development of low transformation temperature (LTT) weld filler materials. These alloys allow to mitigate tensile residual stresses that usually arise during welding using conventional weld filler materials. By this means, higher fatigue strength and higher lifetimes of the weld can be achieved. However, LTT weld filler materials are for example, high-alloyed Cr/Ni steels that are susceptible to the formation of hot cracks. To assess hot cracking, we applied the standardized modified varestraint transvarestraint hot cracking test (MVT), which is well appropriate to evaluate different base or filler materials with regard to their hot cracking susceptibility. In order to consider the complete material volume for the assessment of hot cracking, we additionally applied microfocus X-ray computer tomography (µCT). It is shown that by a suitable selection of welding and MVT parameter the analysis of the complete 3D hot crack network can provide additional information with regard to the hot cracking model following Prokhorov. It is now possible to determine easy accessible substitute values (e.g., maximum crack depth) for the extent of the Brittleness Temperature Range (BTR) and the minimum critical strain P m i n

Surface- and volume-based investigation on influences of different Varestraint testing parameters and chemical compositions on solidification cracking in LTT filler metals

The subject of this study is how, and to what extent, Varestraint/Transvarestraint test results are influenced by both testing parameters and characteristics of evaluation methods. Several different high-alloyed martensitic LTT (low transformation temperature) filler materials, CrNi and CrMn type, were selected for examination due to their rather distinctive solidification cracking behaviour, which aroused interest after previous studies. First, the effects of different process parameter sets on the solidification cracking response were measured using standard approaches. Subsequently, microfocus X-ray computer tomography (μCT) scans were performed on the specimens. The results consistently show sub-surface cracking to significant yet varying extents. Different primary solidification types were found using wavelength dispersive X-ray (WDX) analysis conducted on filler metals with varying Cr/Ni equivalent ratios. This aspect is regarded as the main difference between the CrNiand CrMn-type materials in matters of cracking characteristics. Results show that when it comes to testing of modern highperformance alloys, one set of standard Varestraint testing parameters might not be equally suitable for all materials. Also, to properly accommodate different solidification types, sub-surface cracking has to be taken into account

Residual Stress Influence on the Flexural Buckling of Welded I-Girders

Abstract. The nonlinear analysis became a common tool to precisely assess the load-bearing behavior of steel beam and column members. The failure level is significantly influenced by different types of imperfections, among geometric also structural imperfections (residual stresses). Here are still gaps in the knowledge. Nowadays, 3-D welding simulation developed to a level where it could provide reliable estimation of weld-induced distortion and residual stresses. Nevertheless, modelling and computational effort are still in a less practicable range. In this study a simplified procedure to implement residual welding stresses in continuous large scale members is proposed and the influence on the ultimate limit state of slender members in compression is evaluated for two common structural steel grades. The results showed significant improvements in the utilization of load bearing capacity compared with simplified design methods. The comparatively general approach in this study offers potential for future optimization. Introduction Welded I-girders are used in different applications in steel construction (e.g. industrial buildings, bridges) due to either dimensions and/or efficiency through customizable plate thicknesses, shapes and/or materials. Many standards, including Eurocode 3 (EC 3) permit the use of non-linear finite element analysis (FEA) for the design of steel structures. The development in this field allows performing "experiments" in computing software instead of the laboratory or expensive in-situ experiments. Still, the implementation of imperfections (including residual stresses due to weld manufacturing) remains a major task in the performance of such analysis. Unlike geometrical imperfections, residual stresses (RS) are not standardized. For that reason, more or less founded simplified distribution functions as proposed by the Swedish design code BSK 99 [1] are used. Limitations on the applicability of this model are not reported except for the plate thicknesses which should not exceed 40 mm. However, the influence of RS seemed somewhat overestimated for many cases comparing conventional structural steel S355 and S690QL. As in the EC 3, direct proportionality of the tensile and compressive RS (the latter are the main interest in this study) and the yield strength is assumed. An opposite effect was recently noticed i

Formation of welding residual stresses in low transformation temperature (LTT) materials Tensões residuais de soldagem em materias de baixa temperatura de transformação (BTT)

For the safety and cost efficiency of welded high-strength steel structures, precise knowledge of the level and distribution of welding- and cooling-specific stresses and residual stresses is essential, since they exert a decisive influence on strength, crack resistance, and finally on the bearable service load. This paper presents innovative filler materials, of which the phase transformation temperature was deliberately adjusted via the chemical composition. The transformation behaviour of these martensitic Low Transformation Temperature (LTT-) filler materials shows direct effects on the local residual stresses in the weld and the HAZ. These effects can purposefully be exploited to counteract the thermally induced shrinkage of the material and to produce significant compressive residual stresses in the weld. Comparative welding experiments were carried out on 690 MPa high-strength base materials using various LTT-filler materials. High energy synchrotron radiation was used for residual stress measurement. Particularly the use of high energy synchrotron radiation makes it possible to detect the residual stress condition fast without destruction of material. Thereby, residual stress depth gradients can be determined simultaneously without removing material. In steel, gradients of up to 150 µm can be resolved in such a way. Furthermore, the application of high energy radiation permits determination of residual stresses of any available residual austenite contents. Results show significant dependence of transformation temperatures on the resulting residual stress level and distribution.<br>Para a segurança e eficiência do custo de estruturas soldadas de aço de alta resistência, um conhecimento preciso do nível e distribuição das tensões residuais de soldagem é essencial pois estas exercem uma influência decisiva na resistência à fissuração e na carga suportada em serviço. Este artigo apresenta metais de adição inovativos nos quais a temperatura de transformação foi deliberadamente ajustada pela composição química. A transformação destes metais de adição martensíticos causa um efeito direto nas tensões residuais nas zonas fundida e afetada pelo calor (ZAC). Estes efeitos são explorados para contrabalancear a contração térmica do material e produzir tensões residuais compressivas na solda. Testes comparativos de soldagem foram feitos em um metal base de alta resistência de 690 MPa usando diferentes metais de adição de BTT. Radiação sincrótona de alta energia foi usada para medir as tensões residuais. O uso desta radiação permite medir as tensões rapidamente e de forma não destrutiva. Os resultados mostram uma dependência significativa da temperatura de transformação no nível e distribuição das tensões residuais resultantes

In situ Analyse der Phasenumwandlungskinetik während des Schwei ens

Kurzfassung Zugeigenspannungen, wie sie beim Schweißprozess durch inhomogene Temperaturverteilungen und Schrumpfungen hervorgerufen werden, können die Lebensdauer geschweißter Verbindungen signifikant herabsetzen. Eine neue und außerordentlich attraktive Methode, um Druckeigenspannungen bereits während des Schweißens gezielt einzustellen, gelingt mit sogenannten LTT (Low Transformation Temperature)-Legierungen. LTT-Legierungen weisen eine martensitische Phasenumwandlung bei relativ niedrigen Temperaturen auf, wobei die damit verbundene Volumenexpansion zu einer Reduktion der Schrumpfeigenspannungen bzw. Erzeugung von Druckeigenspannungen führt. Zum direkten Nachweis der Phasenumwandlungen und der damit verbundenen resultierenden Schweißeigenspannungen wurden erstmals In-situ-Schweißexperimente unter Nutzung hoch energetischer, polychromatischer Synchrotronstrahlung (Weißstrahl) realisiert, um die Umwandlungskinetik während eines realen Schweißprozesses und die daraus resultierenden Schweißeigenspannungen zu analysieren. Es wird gezeigt, dass mit LTT-Legierungen signifikante Druckeigenspannungen in der Schweißnaht erreicht werden.</jats:p

In situ analysis of the strain evolution during welding using low transformation temperature filler materials

Compared to conventional welding consumables using low transformation temperature (LTT)filler materials is an innovative method to mitigate tensile residual stresses due to delayedmartensite transformation of the weld. For the effective usage of LTT filler materials, a deeperunderstanding of the complex processes that lead to the final residual stress state during multipasswelding is necessary. Transformation kinetics and the strain evolution of multi-pass weldsduring welding were investigated in situ at the beamline HEMS@PETRAIII, Germany. Comparedto conventional welds, the total strain was reduced and compression strain was achieved whenusing LTT filler materials. For an optimal use of the LTT effect in the root of multi-pass welds, thealloying concept must be adapted taking care of dilution

Influence of Heat Control on Properties and Residual Stresses of Additive-Welded High-Strength Steel Components

Advanced high-performance filler metals for wire arc additive manufacturing (WAAM) exist on the market already. Nevertheless, these high-strength steels are not yet widely used in industrial applications due to limited knowledge of cold-cracking susceptibility, welding residual stresses, and therefore sufficient safety in terms of manufacturing and operation. High residual stresses promote cold-cracking risk, especially in the welding of high-strength steels, as the result of a complex interaction between the applied material, process conditions, and component design. The focus of the present investigation was the determination of the influence of the process parameters on the ∆t8/5 cooling time, mechanical properties, and residual stresses to correlate, for the first time, heat control, cooling conditions, and residual stress for WAAM of high-strength filler materials. This contributed to the knowledge regarding the safe avoidance of cold cracking. In addition to a thermophysical simulation using a dilatometer of different high-strength steels with subsequent tensile testing, reference WAAM specimens (open hollow cuboids) were welded while utilizing a high-strength filler metal (ultimate tensile strength > 790 MPa). The heat control was varied by means of the heat input and interlayer temperature such that the ∆t8/5 cooling times corresponded to the recommended processing range (approx. 5 s to 20 s). For the heat input, significant effects were exhibited, in particular on the local residual stresses in the component. Welding with an excessive heat input or deposition rate may lead to low cooling rates, and hence to unfavorable microstructure and component properties, but at the same time, is intended to result in lower tensile residual stress levels. Such complex interactions must ultimately be clarified to provide users with easily applicable processing recommendations and standard specifications for an economical WAAM of high-strength steels. These investigations demonstrated a major influence of the heat input on both the cooling conditions and the residual stresses of components manufactured with WAAM using high-strength filler materials. A higher heat input led to longer cooling times (∆t8/5) and approx. 200 MPa lower residual stresses in the surface of the top layer