Thomas Nitschke-Pagel

Fatigue Behaviour of Welded High-Strength Steels after High Frequency Mechanical Post-Weld Treatments

Investigations have been carried out regarding the fatigue strength of welded details improved by high frequency treatment methods. These methods increase the fatigue strength by cold forming of the surface, so that the weld toe is smoothened, the surface hardness is increased and compressive residual stresses are present up to a depth of 1 to 1.5 mm. In this paper, the surface residual stresses at the weld toe are investigated before and after different loading conditions and for different steel grades. It is shown that only high tensile fatigue loading can lead to a slight residual stress relaxation for low-strength steels. The fatigue crack behaviour is analysed in more detail. The crack propagation rates with and without surface treatment are investigated, using defined lines of rest. The study shows that crack propagation in the edge layers is reduced. Several cracks may start in the UIT-treated zone but will not propagate further, until one final crack, often close to the edge of the specimens, will lead to failure. The results of fatigue tests for butt welds and longitudinal stiffeners improved by high frequency hammer peening are presented. The fatigue strength is seen to be doubled. For high-strength steels, the improvement at different load levels is identical, but for lower-strength steels, high stress ranges lead to reduced improvement. This fact results in flatter SN-curves and can be explained by the lower maximum of residual stresses and residual stress relaxation.

Residual Stresses in Welded Joints – Sources and Consequences

Residual stresses in welded joints are a consequence of hindered shrinkage of the heated zones and in materials like steels where phase transformations occur of the combined effect of hindered volume expansion. With these two elementary processes any residual stress distribution can be described conclusively if the cooling conditions and the material properties like the transformation behaviour are known. The grade of restraint of the weld seam is the most important boundary condition which is responsible for the opportunity that high residual stresses may occur. The grade of restraint is given by the ratio between the heated zones and the apparent colder material and is not primarily depending on the size of a component. Every way to reduce the grade of restraint offers principally the possibility of an increasing distortion. Many experiments show, that a certain control of the welding parameters which should be carefully adjusted on the properties of each material enables a combination of favourable strength and toughness properties with acceptable residual stresses.

Mechanisms of Residual Stress Relaxation and Redistribution in Welded High-Strength Steel Specimens under Mechanical Loading

In this article, the relaxation of surface and near-surface welding residual stress fields in S690QL under static and cyclic loadings has been studied. The influence of local mechanical properties in the weld metal, heat-affected zone and in the base metal upon relaxation and redistribution of residual stress were considered. Welded specimens were loaded statically and cyclically. Under static loading, the variation of the residual stress field under tension and compression was investigated and it was observed that, by increasing the load, residual stress relaxes continuously. However, the relaxation under compression sets in with delay, since the tensile residual stresses should be first overcome. Under cyclic loading, with or without mean stress, the behaviour of the residual stresses depends on the initial residual stress, local yield strength and maximum applied load. If the von Mises stress, which is a function of residual stress, load and mean stress, exceeds the local yield strength, plastic deformation and thus a relaxation occurs. In the case of relaxation, the first load cycle is of importance, in which a considerable amount of the initial residual stress field is relaxed. After the first relaxation, no significant variation of the residual stress field in the fatigue crack free phase could be observed. When no residual stress relaxation in the first load cycle occurred, the number of load cycles did not change the residual stress field. The specimens were loaded until failure or maximum 2×106 cycles.

Residual Stress Calculations And Measurements — Review And Assessment Of The Iiw Round Robin Results

A Round Robin Programme was established by a working group of Commission X in 1997 in order to evaluate the possibilities of residual stress and distortion prediction (RSDP) in welded structures and to validate and benchmark prediction codes based on finite element simulation of the welding process. Calculations on residual tresses have been carried out on an austenitic steel plate with well-defined material and geometrical data, welding conditions and with the proposal to use the kinematic hardening model. In addition, a second programme for measurements of residual stresses in this plate has been started in 2003. Measurements have been taken on three austenitic steel plates using the X-ray measuring method as well as neutron diffraction methods and different kinds and variations of the hole drilling method. Although a rough agreement between the measured results and the calculated ones was found in principle, distinct differences have been observed in detail — especially for the longitudinal residual stresses. The calculations indicated nearly constant tensile stresses of the order of the initial yield strength in the weld seam and in the adjacent heat affected zone (HAZ). The measurements however revealed stress maxima in the HAZ with values clearly higher than the initial yield strength. Therefore additional calculations have been carried out with different hardening models in order to investigate their influence on the residual stress results. In fact, calculations performed with the isotropic hardening model could approve these stress maxima quite exactly. In further detailed investigations, it could be shown that the reason for the stress maxima is the strain hardening in the HAZ due to thermal stresses during the welding process. Finally basic considerations could explain that the kinematic hardening model, including the Bauschinger effect, cannot account for this hardening effect. Literature indicates that the Bauschinger effect is at least not fully effective at high working temperatures of austenitic steels. Altogether, the calculation programme has shown that a rough estimation of the residual stress state after welding is possible without any special distinction of the hardening model to be used. For a more precise and detailed information, the chosen hardening model is of significant relevance.

Stability and Relaxation of Welding Residual Stresses

Residual stress relaxation of butt-welded small scale steel specimens under static and cyclic mechanical loading was investigated. The experiments were carried out on different types of steel with yield strengths between 300 and 1200MPa. The x-ray and neutron diffraction techniques were applied for the residual stresses analysis. The maximum values of initial residual stresses were measured at the weld bead centerline and were not as frequently assumed as high as the yield strength of the material. From fatigue point of view the internal stresses at the weld toe are of importance. It was observed that only a fraction of maximum residual stress accommodated in the weld bead centerline, is available at the weld toe. Under static tensile and compressive loading by increasing the load level the residual stress field relaxes continuously. The relaxation sets in with delay under compression since the tensile residual stresses should be first overcome. Under cyclic loading, once the first relaxation takes place further cyclic relaxation is either not considerable or continues moderately depending on loading conditions. In high cycle fatigue the residual stresses are stable until 2x106 cycles. In low cycle fatigue however the variation of the residual stresses continues until failure. This variation is partly related to crack initiation and propagation. The von Misses failure criterion with the local yield strength as material resistance against plastic deformation was able to describe the relaxation of surface welding residual stresses in low cycle fatigue.

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.

Residual Stress Relaxation of Quasi-Statically and Cyclically-Loaded Steel Welds

Weld fatigue strength is currently the bottleneck to designing high performance and lightweight welded structures using advanced materials. In addition to loading conditions, environmental aspects, geometrical features and defects, it has been proven that studying the influence of residual stresses on fatigue performance is indispensible. The extent of the influence is, however, a matter of discussion. In this work, residual stress behaviour in welded S355J2G3 and S1100QL steel specimens under quasi-static and cyclic loading were studied and the correlation between the relaxation of residual stress field and mechanical properties was studied. Residual stress measurements were performed using the X-ray diffraction technique. The relaxation behaviour in S355J2G3 at weld metal, weld toe and in the base metal could be described by Von Mises criteria. This was not the case for S1100QL. In the case of relaxation, it was observed that if the Von Mises stress, as a function of residual, load and mean stresses, exceeds the monotonic yield strength of the base metal, relaxation takes place. Otherwise, the internal elastic stresses remain stable regardless of the number of loading cycles and could contribute to fatigue damage.

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.

Welding Residual Stresses in Tubular Joints

In spite of an increased awareness of welding residual stress threat to structural integrity, the extent of its influence on fatigue especially under multiaxial loading is still unclear and is a matter of debate. One important reason for this lack of clarities is that the determination of the initial welding residual stress field in welded structures even at the fatigue crack initiation sites is difficult and requires complementary instruments. Since the fatigue crack initiation in sound welds almost always occurs on the surface, the determination of surface residual stresses could increase the awareness of the extent of their threat to the structural safety. In this paper the development of residual stresses in different TIG-welded tubular specimens out of S355J2H and S690QL steel is studied and compared. The mechanisms of the development of residual stresses based on heat input and cooling rate are discussed. The welding parameters and thus heat inputs are varied and the mechanisms leading to different residual stress states are investigated. X-ray method was used for residual stress state characterization.