Jonas Hensel

On the effects of austenite phase transformation on welding residual stresses in non-load carrying longitudinal welds

Residual stresses affect the fatigue strength of welded structures and components. A common sample type used for studies on residual stress effects is the fillet-welded longitudinal gusset. This sample type shows in fatigue tests significant residual stress effects. But the mechanisms of residual stress generation are not clarified for this sample type yet. High tensile residual stresses in the surface layer near the weld toe could not be proven by experimental methods but are generally assumed. Here presented are results from experimental and numerical investigations on welding residual stress generation. Specimens with single and multilayer fillet welds have been produced as well as simplified curved multilayer deposition welds studying residual stress build-up. Temperature measurements have been conducted during welding examining the influence of austenite phase transformation. Residual stresses have been determined by means of X-ray diffraction at the surface as well as by neutron diffraction over specimen thickness. Further, the mechanisms of residual stress build-up have been evaluated by finite element calculations. It could be shown that the austenite phase transformation has significant effect on the residual stresses near the weld toe also for this sample type.

Effects of residual stresses and compressive mean stresses on the fatigue strength of longitudinal fillet-welded gussets

Results from fatigue testing of small-scale specimens are widely used to study residual stress effects on fatigue of welded structures. It was observed from a literature that welding distortion may cover residual stress effects due to high-bending stresses from clamping. Here, presented are fatigue test results and results from residual stress measurements from welded longitudinal stiffeners in different residual stress conditions. The axial welding distortion was corrected for all samples by straightening reducing effects from clamping and making residual stress effects visible. It was found that the fatigue strength depends strongly on the stabilized residual stresses, especially at high numbers of load cycles. It could be shown that residual stresses at the weld toe either were far below the yield strength or were degraded mainly at the first load cycle but still have major effects. This was investigated at two stress ratios of R = −1 and R = −3.

Engineering model for the quantitative consideration of residual stresses in fatigue design of welded components

Residual stresses are one of the major factors influencing the fatigue strength of welded components. However, the current IIW bonus factor concept for the mean stress correction is limited to a qualitative evaluation of residual stress effects. By combining residual stress measurements and fatigue testing, the authors derive an improved bonus factor concept that considers residual stresses quantitatively. The proposed concept considers the combined effect of load mean stresses and cyclically stabilized residual stresses. It is pointed out that the yield strength is not a capable measure to determine whether residual stresses have “low” or “extreme” impact on the fatigue strength of welded steels. It is rather recommended to evaluate residual stress effects based on the effective stress ratio reflecting local loading conditions.

Residual Stress Relaxation in Welded Steel Joints – an Experimentally-based Model

Residual stresses may affect the fatigue strength of welded components significantly. Structural design concepts for fatigue loaded welds do not account for real residual stress conditions but rather generally estimate high tensile residual stresses. The assumption of high tensile residual stresses in current engineering practice is resulting in over-conservative designs. The consideration of real residual stress conditions in the design process is one of the major objectives in current research on structural engineering. In order to achieve this objective, one must be able to describe the residual stress generation due to manufacturing and the relaxation of residual stresses during component life time. However, nowadays it is not practical to describe the relaxation process by means of numerical or analytical methods. This work describes an experimentally-based model for the estimation of the stabilized residual stresses in welded steels. The model is capable of describing residual stress relaxation depending on the initial residual stresses, the load magnitudes and the material strength. The model is based on XRD residual stress measurements during the fatigue life of typical welded joints. The samples used here are longitudinal fillet welded gussets made from low-carbon highstrength construction steels S355NL (yield strength 360 MPa) and S960QL (yield strength 960 MPa). Finally, the model is extended to butt welded joints using experimental data from the literature. Introduction Residual stress effects in fatigue design of welded steels. The fatigue strength of welded components is of major concern for component safety and durability. In engineering practice fatigue strength is mainly examined by means of the nominal stress approach. Speaking generally this method compares expected fatigue loads in the net section of the weld details to a reference design S-N curve [1, 2]. The reference S-N curves are provided by technical standards or design recommendations and are specific to certain weld details such as un-welded base metal, butt welds, cruciform joints or longitudinal fillet welded gussets. Each design S-N curves relates the expected fatigue load to a specific number of load cycles until this load can be born by the component without Fig. 1 Effect of tensile mean stresses on the design fatigue strength for welded steels in dependence of the residual stress conditions [1] -1,00 -0,75 -0,50 -0,25 0,00 0,25 0,50 1,0 1,1 1,2 1,3 1,4 1,5 1,6 B on us fa ct or f( R ) Nominal stress ratio R Low tensile residual stresses Medium tensile residual stresses High tensile residual stresses

Residual Stresses and Fatigue Behavior of High Strength Structural Steels with Fillet Welded Longitudinal Stiffeners

Abstract Residual stresses may affect the behavior of welded steels under fatigue loading. However, for the design of welded structures the level 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. Stress relief annealing is generally suspected to enhance the fatigue strength. The results presented here focus on the influence of residual stresses and thermal stress relief on the fatigue strength of longitudinal stiffeners made of 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. Fatigue testing was conducted under constant amplitude loading with a stress ratio of R = −1 using samples with high and low stress concentration. It was shown that the annealing process influenced not only the welding residual stresses but also the welding distortion. It was found that welding distortion can be increased by thermal stress relief which again affects the fatigue strength.

Effects of Residual Stresses on the Fatigue Performance of Welded Steels with Longitudinal Stiffeners

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.

Untersuchungen zur verlässlichen Messung der Härte nach dem UCI – Verfahren (Ultrasonic Contact Impedance)

Kurzfassung Bei der Herstellung von Stahltragwerken nach DIN EN 1090 sowie bei der allgemeinen Materialprüfung ist die Messung der Härte Bestandteil der normgerechten Bauteilprüfung. Dabei wird zumeist die konventionelle stationäre Vickershärteprüfung gefordert. Beispielsweise werden bei der Erzeugung von thermischen Schnittkanten obere Grenzwerte für die Härte vorgeschrieben, die es folglich zu überprüfen gilt. Die Ermittlung der tatsächlich vorliegenden Härte an derartigen technischen Oberflächen stellt sich in der Praxis jedoch als schwierig dar, da die hohen Härten verfahrensbedingt nur in dünnen Schichten vorliegen und die zugänglichen Oberflächen der Schnittkanten Schneidriefen aufweisen. Die umständliche Verfahrensprüfung und die normgerechte Härtemessung an metallografischen Schliffen sind selbstverständlich möglich, aufgrund des hohen experimentellen Aufwands jedoch nicht immer praxistauglich. Im Zusammenhang mit der rechtlichen Produkthaftung, die für jedes gefertigte Bauteil gewährt werden muss, besteht unmittelbarer Bedarf an einem Verfahren zur verlässlichen Ermittlung der Härte, auch an nicht normgerecht vorbereiteten Oberflächen.

Metallurgical investigation of electron beam welded duplex stainless steel X2CrNiMoN22-5-3 with plasma nitrided weld edge surfaces

Abstract Duplex stainless steel is common in thick-walled components such as longitudinal welded pipes in the oil and gas industries and as parts of various machines in the chemical and food industries. Electron beam welding is a very suitable method for welding such components. Due to the high power density of the electron beam combined with the decreased evaporation temperature in a vacuum atmosphere, steel with a sheet thickness of up to 150 mm can be welded in one pass. In the case of the electron beam welding of duplex stainless steels, vacuum atmosphere in the working chamber causes a nitrogen effusion from the weld pool. The microstructure of the resulting weld is characterized by an unacceptable high ferrite content, which is the main reason for both low impact toughness and low pitting corrosion resistance. This work focuses on the influence of nitrided weld edge surfaces on metallurgical properties of the resulting welded joint. The aim here is to investigate the effect of increased nitrogen content at the weld edges on nitrogen loss during EB-welding. The welds produced within the experimental work were characterized by means of microstructural analysis and the use of optical and energy dispersive X-ray spectroscopy. It was shown that nitrided weld edge surfaces can compensate nitrogen loss from the weld pool and decrease the ferrite content in the resulting weld.

Analysis of Residual Stress State in Deep-Rolled HT-Bolts

Results of residual stress measurements of HT-bolts gained by neutron diffraction at HZB will be presented. The in-depth residual stress of different conditions of “rolled-before heat-treatment and galvanized” M24 HT-bolts made from 33MnCrB5-2 will be shown: Asmanufactured, pre-stressed and fatigue loaded. Additionally, results of an unloaded, “rolled-after heat-treatment and hot-dipped” galvanized M36 HT-bolt will be presented. It will be shown that the manufacturing sequence “rolled-before heat-treatment and galvanized” can develop compressive residual stresses due to heat-treatment close to the surface and that “rolled-after heat-treatment and hot-dipped” shows a contraire residual stress path as “rolled-before heattreatment and galvanized” with a maximum below the surface. Introduction Bolted joints are generally the most frequently used joining elements in mechanical and plant engineering. High-tensile (HT) bolts with large diameters (M30 up to M72) are heavily used in wind power plants both onshore and offshore. Bolted joints are made from hardened steels (for instance 33MnCrB5-2) and show high stress concentration in the thread. One of the fatigue strength governing parameters is the production method used to manufacture those threads, shown in Fig. 1. During the cold-rolling process compressive residual stresses are generated at the root of the thread in combination with strain hardening. The residual stress state can be significantly influenced by heat-treatment or loading. The in-depth residual stresses of bolts in different conditions of “rolled before heat-treatment” and galvanized HT-bolts M24x130 10.9 tZn made from 33MnCrB5-2 have been investigated by neutron diffraction: As-manufactured, pre-stressed in tension and fatigue loaded. Additionally, an unloaded, “rolled after heat-treatment and hot-dipped galvanized” M36x235 10.9 tZn HT bolt has been examined. The mechanical properties are listed in Tab. 1. The as-manufactured sample represents the residual stress condition induced by the manufacturing process. The pre-stressed condition reflects the in-situ stress state of a mounted bolt under pre-tension (about 70% of Rp0.2). This was achieved by mounting the bolt in an artificial restraint device. The pre-loaded bolt was pre-stressed and fatigue loaded at TU Braunschweig in the finite life regime until N = 100.000 load cycles which is assumed to correspond to a cyclically stabilized state after residual stress relaxation. The residual stress history can be used to further interpret given fatigue test results. Residual Stresses 2018 – ECRS-10 Materials Research Forum LLC Materials Research Proceedings 6 (2018) 209-214 doi: http://dx.doi.org/10.21741/9781945291890-33 210 Table 1 Mechanical properties Rp0,2 Rm E ν θ0 [MPa] [MPa] [MPa] [rad] M24 969 1081 198400 0,28 77,882 M36 912 105