Materials Science Forum Vols. 768-769

Abstract: The developments in the field of residual stress determination during the last decades have contributed to a better understanding of the origins and sources of residual stresses in different engineering disciplines. The many investigations concerning the behavior of residual stresses under mechanical loading have also provided a solid foundation to clarify the important aspects of residual stresses and fatigue. The question that arises now is if this available body of knowledge is being used effectively in the field of welding technology to design and construct structures with better fatigue performances. In this paper the necessity of the development of the concept residual stress engineering for welds in which wanted residual stress states are tailored for specific cases by appropriate means will be discussed. The possibilities of the quantitative consideration of the benefits in the fatigue design codes will be presented in a practical example.

Abstract: Welding residual stress is of major concern for structural integrity assessment in industrial components. Shear and volume strains resulting from the austenite-martensite-transformation affect the development of residual stress during welding. Controlling the phase transformation allows adjustment of the welding residual stress. Low transformation temperature (LTT) weld filler materials exhibiting reduced MS-temperatures allow postponing the phase transformation. The associated strain arising from the delayed transformation compensates for the thermal contraction strains and as such may reduce tensile or even introduce compressive residual stress. In this article we discuss the tri-axial residual stress distribution in 15 mm S690Q steel plates joined with LTT filler materials with 10 wt% Cr and a Ni-content that varies from 8 to 12 wt%. Using complementary synchrotron X-ray and neutron diffraction stress analysis the macroscopic residual stress was derived from the phase specific lattice strain and phase fraction of martensite and retained austenite, respectively. The local phase specific unstrained lattice parameters were determined using stress relieved combs. The investigation revealed increasing phase fraction of retained austenite with increasing Ni-content. Further, independent of the Ni-content in each weld in the fusion zone, significant compressive residual stresses were found in the longitudinal direction, which are balanced by tensile residual stresses in the heat affected zone (HAZ). In the weld transverse and normal direction the stress distribution is qualitatively similar but less in magnitude. The increased amount of retained austenite reduces the compressive stress arising from shear and volume strains during the delayed phase transformation and therefore no significant increase in compression was observed for decreasing M_S-temperatures.

Abstract: CA6NM and UNS S41500 martensitic stainless steels are widely used for manufacturing and repair of hydraulic turbine runners. They offer good mechanical properties and superior cavitation resistance when compared to mild steels. They are also relatively easy to weld. However, when welded homogeneously, they require a post-weld heat treatment (PWHT) in order to temper the as-welded martensite. This PWHT is also beneficial for residual stresses reduction as it effectively lowers the stress peaks. To avoid this PWHT, austenitic filler metals are often used for repair. But omitting PWHT inevitably leaves weld-induced residual stresses in the assembly. In order to better understand the impact of the weld filler metal choice on the importance of residual stress, an experimental study has been conducted on three different filler alloys. The chosen alloys were: • 410NiMo, a martensitic grade having the same composition as the base metal (13%Cr-4%Ni-0.5%Mo) ; • 309L, an austenitic grade widely used for repair (24%Cr-13%Ni) ; • A proprietary low transformation temperature (LTT) martensitic grade (13%Cr-6%Ni). This paper compares residual stresses in the as-welded condition on welds of UNS S41500 (13%Cr-4%Ni) made using these filler metals. Residual stresses were measured using the contour method. Microstructural analysis was performed to identify the phases in the weld and the heat-affected zones (HAZ). Microhardness maps were done to see the hardness distribution of each weldment.

Abstract: Residual stresses may affect the behavior of welded steels under fatigue loading. However, for design of welded structures the height and distribution of residual stresses from welding are often not known so that tensile residual stresses in the order of the yield strength are conservatively assumed. Here presented results focus on the influence of residual stresses on the fatigue strength of longitudinal stiffeners made from a mild steel S355NL and a high strength steel S960QL. The initial residual stress conditions were measured using X-ray and neutron diffraction. In order to characterize the influence of residual stresses on the fatigue strength, specimens were tested in the as-welded condition and after a stress relieving heat treatment. The fatigue testing was conducted under alternating constant amplitude loading with a stress ratio of R=-1.

Abstract: Temperature-induced strain with, at the same time, reduced formability is, among other things, responsible for crack development in the range of high temperatures. For a more detailed examination of these so-called hot cracks, experimental measurements of the strain during the welding process have been carried out using neutron diffraction. The measurement of strain is important since it exerts decisive influence on the development of cracks.

Abstract: This paper presents the numerical analysis of phase proportions and residual stresses in an autogenous beam edge weld. The thin beam was welded running a heat source along its longer edge using a TIG process. There is no addition of any material so the focus of modelling the process could be concentrated on the thermal analysis and the phase transformations. Temperature dependent material properties and a continuous cooling transformation (CCT) diagram of the base material were provided. The simulations took into account metallurgical effects and used a Goldak-type heat source. Simulations with and without phase transformations were carried out, in order to analyse the effect on the predicted residual stress.

Abstract: Innovative low transformation temperature (LTT) welding filler materials are featuring a characteristic chemical composition which favors the formation of martensite at comparatively low temperatures. This permits deliberate adjustment of welding residual stresses. Even though numerous investigations can be found in the literature on this issue, they provide only little insight into the interaction between phase transformation and resulting welding residual stresses. For this purpose, a component weld test was performed in a special large-scale testing facility. The results illustrate that the desired residual stress control by using LTT alloys is actually feasible. With increasing shrinkage restraint, however, higher tensile residual stresses are formed in transverse direction of the weld. By contrast, the residual stress level in longitudinal weld direction is nearly independent of the restraint conditions. On-line stress analysis revealed that the amount of stress reduction during cooling of the individual weld runs is dependent on the weld volume undergoing phase transformation. Overall, evidence was furnished that the approach of residual stress engineering by LTT alloys is suitable even in the case of large-scale multilayer welding.

Abstract: It is well known that fatigue strength of welded joints does not depend on steel strength. Better fatigue strength of welded joints, e.g. longer life time of fatigue loaded weld structures, can be achieved with a smooth transition between the weld and the base material to minimize stress concentration. It has also been recognized that residual stresses play a critical role in the fatigue behaviour of welds. In the last decade an extensive research has been performed in order to increase the fatigue strength of high strength steel weldments. The martensite and bainite transformation start temperatures of weld metals have been shown to have a large effect on fatigue life time of high strength steel welds. This is of particular importance if the full potential of high strength steels is to be used in fatigue loaded constructions. A detailed investigation of the effect of phase transformation temperature on residual stress distribution in the vicinity high strength steel welds and its effect on fatigue life time has been performed. The transformation temperature of the weld metal was varied by changing the chemical composition of the filler material. Residual stress distributions have been measured by neutron as well as by X-ray diffraction and fatigue tests have been performed on the fillet welds. A strong effect of weld metal phase transformation temperature on residual stress level was observed. Fatigue strength increased approximately three times when an optimised low transformation temperature filler material was used in comparison to the application of conventional filler material.

Abstract: Cold formed and precipitation hardened aluminium alloys welded with arc welding and electron beam welding processes have been investigated to analyse the local deformation behaviour under static and cyclic loads. Therefore lateral strain measurements with different techniques have been carried out in the surrounding of the weld seam. The strain measurements were combined with micro hardness measurements in order to determine loading induced local softening and hardening. These investigations were combined with X-ray diffraction experiments which should provide information about the residual stress condition and additionally about load induced changes of the hardening condition by the analysis of the FWHM-values of the diffraction lines. Additionally in-situ diffraction experiments using synchrotron radiation where carried out to analyse the deformation behaviour of particular welds under varying static loads. The paper gives an overview about the experimental procedures and important results and their consequences for the evaluation of such aluminium welds.

Abstract: Experimental results in the literature show that there are two flow areas of material during the friction stir welding (FSW) process [1]; namely the “pin-driven flow” and the “shoulder-driven flow”. These areas should completely join together to create a weld with no defect. First, in order to numerically predict the local distribution of flow stress around the pin as well as the temperature, strain, and strain rate fields during FSW, a two-dimensional steady-state Eulerian multiphysics finite element model has been employed in this work for aluminum alloy 6061using the COMSOL software. In this model, the non-Newtonian flow mode of computational fluid dynamics (CFD) module, general heat transfer mode of the heat transfer module, and the plain stress mode of the structural mechanics module of the software have been coupled. Slip/stick condition between the tool and workpiece, frictional and deformation heat sources, the convectional heat transfer in the workpiece, the solid mechanics-based viscosity definition, the temperature-dependent physical properties and the Zener-Hollomon- based thermo-visco-plastic mechanical properties with a cut-off temperature of 582oC were considered. Next, the thermal history during the process predicted by the model was used as input for an elasto-visco-plastic analysis to estimate the local residual stresses distribution due to the workpiece thermal expansion effect. Finally, the predicted longitudinal and transverse residual stresses were verified by comparing to experimental data.