Die Gefahr der Versprödung von Stählen beim Schweißen in kaltumgeformten Bereichen führt zu vielen Fertigungsrestriktionen bzw. zu hohen Fertigungskosten. Der Grund der Versprödung liegt in der ...Grobkornbildung der Rekristallisationszone. In diesem Beitrag wurden drei hochfeste Stähle hinsichtlich ihrer Versprödungsneigung bei unterschiedlichen Umformgraden über ein Temperaturspektrum untersucht. Ziel war es, die Temperaturen während des Schweißens in der Rekristallisationszone zu simulieren und damit das Kornwachstum abzuschätzen. Dazu wurden die Werkstoffe auf maximale Dehnungen bis kurz vor der Gleichmaßdehnung gelängt. Die anschließende Wärmebehandlung erfolgte zwischen 500 °C und 800 °C mit verschiedenen Haltezeiten. Über die gesamte Versuchsreihe konnte kein nennenswerter Zusammenhang zwischen Kornwachstum, Umformgrad, Glühdauer und Glühtemperatur festgestellt werden. Die Ergebnisse deuten darauf hin, dass beim Längen hochfester Stähle der kritische Umformgrad nicht erreicht werden kann und somit keine Grobkornbildung in der Rekristallisationszone stattfindet.
Translation
Grain size development of high‐strength steels after cold forming and heat treatment
The risk of embrittlement of steels during welding in cold‐formed areas leads to many production restrictions or high production costs. The reason for embrittlement is the coarse grain formation in the recrystallization zone. In this paper, three high‐strength steels were investigated with respect to their tendency to embrittlement at different degrees of deformation over a temperature spectrum. The aim was to simulate the temperatures during welding in the recrystallization zone and thus to estimate the grain growth. For this purpose, the materials were elongated to maximum elongations up to just before the uniform elongation. The subsequent heat treatment was carried out between 500 °C and 800 °C with different holding times. Over the entire series of tests, no significant correlation was found between grain growth, the degree of deformation, the annealing time and annealing temperature. The results indicate that the critical degree of deformation cannot be reached during the elongation of high‐strength steels, and thus no coarse grain formation takes place in the recrystallization zone.
The temperature- and phase-dependent yield strength plays a decisive role in the precise prediction of residual stresses in structural steel weldments. Since it is very expensive and time-consuming ...to directly measure the yield strength of a given steel by experimental method, numerical simulation has commonly been performed using the available yield strength of the same steel grades in scientific literature and material database or using the evaluated data based on the empirical formulations in standards/codes for the given steel. Nevertheless, the yield strengths of the same steel grades are usually different mainly due to the disparity in chemical composition and grain size. Furthermore, there are considerable differences in the yield strength at elevated temperatures obtained from the predictive functions for the given steel. Therefore, it is necessary to clarify the influence of the variation in temperature- and phase-dependent yield strength obtained from different methods on the predicted welding residual stresses in numerical simulation. In this study, numerical sensitivity analyses have been conducted to clarify the effect of yield strength on the predicted residual stresses in ultra-high strength steel S960 weldments. Furthermore, tensile tests and welding experiments were carried out for comparison and validation. Based on the current work, the effect of temperature- and phase-dependent yield strength on the calculated welding residual stresses is clarified. Besides, the reliability of the proposed functions to evaluate yield strength based on the tensile test results mainly from mild steels and conventional high strength steels is assessed for the recently developed ultra-high strength steel S960. Finally, guidance on how to economically determine the reliable temperature- and phase-dependent yield strength for S960 steel in numerical welding simulation is provided.
Welding-induced residual stresses (WIRS) can have influence on the performance and service life of the welded structures. The present study experimentally and numerically investigates the reason and ...mechanism behind the generation and distribution of WIRS concretely. The results show that the thermal shrinkage process and phase transformation process are the main sources generating welding-induced longitudinal residual stress (σLRS). Nevertheless, except for these two processes, the quenching process would be another source producing transverse residual stress (σTRS). In the current work, it is found that the maximum σLRS generally reaches the yield strength (σy) of base metal (BM) at ambient temperature (TRT) for both transforming and non-transforming materials but located at different areas. In the single-pass weldments without external restraint, σTRS profile on the top surface is usually always M-shaped for the non-transforming materials but W-shaped instead for the transforming materials. The distribution mechanism behind the M-shaped or W-shaped σTRS profile is systematically clarified here.
From the viewpoint of mechanics, weld cracking tends to occur if the induced tensile stress surpasses a certain value for the particular materials and the welding processes. Welding residual stresses ...(WRS) can be profoundly affected by the restraint conditions of the welded structures. For estimating the tendency of weld cracking, the small-scale H-type slit joints have been widely used for cracking tests. However, it is still hard to decide whether the real large-scale component can also be welded without cracking even though the tested weld cracking specimens on the laboratory scale can be welded without cracking. In this study, the intensity of restraint which quantitatively indicates how much a joint is restrained is used. The influence of restraint condition (intensity of restraint) on WRS is systematically investigated using both the numerical simulation and the experimental method. The achievement obtained in the current work is very beneficial to design effective H-type self-restrained cracking test specimens for evaluating the sensitivity of the material and the welding procedures for weld cracking in the real large-scale components.
Shot peening is a mechanical surface treatment to improve the fatigue strength of metallic components. Similarities exist between regular shot peening and conventional industrial clean blasting. ...However, the main difference between these two processes is the peening media used and the lack of control and documentation of peening parameters. The clean blasting process is not yet qualified to optimize fatigue enhancement, although it holds a similar potential to regular shot peening. Clean blasting is frequently applied to welded components, with the purpose of surface preparation for application of corrosion protection. This article presents the results of regular shot peened double V-groove (DV) butt welds made from construction steels S355N and S960QL, as well as the high strength aluminum alloy Al-6082. The peening parameters are varied widely. Furthermore, the effect of coverage and intensity is investigated to test the robustness of the peening processes. The data is completed with industrially clean blasted welds, representing typical workshop conditions. The overall objective of this work is to derive minimum peening parameters that still allow significant fatigue strength benefits. The presented data show a high robustness of the fatigue results to peening parameters.
Thermal and mechanical cutting processes are commonly applied in manufacturing industries for making a specific size and shape of samples. The material properties of the heat-affected metal at the ...cut edge might be different from those of the parent metal. Hardness as a very important parameter for the assessment of material properties has been widely used in the design procedure. The Vickers hardness test is a versatile method to measure material hardness in practice. In this study, a comparison of cut-edge hardness between the thermal and mechanical cutting processes was performed on the structural steels S355 and S1100 by using the Vickers hardness test method with a small angle at cut edges to widen the heat-affected zone (HAZ). Based on the measurements, the influence of the Vickers test load on the magnitude and distribution of cut-edge hardness for laser-cut samples of steels S355 and S1100 was clarified. Furthermore, the effects of cutting processes on the hardness were illustrated. Finally, a comparison between the distribution characteristics of the cut-edge hardness of steels S355 and S1100 was made. For structural steel, it is recommended to apply the Vickers hardness testing method with a test load of 1 kpf and with a small angle at cut edges to widen the HAZ and to get reasonable results. The hardness distribution and the maximum for laser- and plasma-cut edges are similar, depending on the material, with a maximum of 450 HV1 for S355N and of 550 HV1 for S1100M.
•The mechanism of the strain hardening recovery induced by solid-state phase transformation is clarified in detail.•The influence of the strain hardening recovery induced by solid-state phase ...transformation on welding residual stresses is investigated by methods of both experimental method and numerical simulation.•The effect of strain hardening model on welding residual stresses in S355 steel weldments is studied systematically.
A series of numerical simulations were performed to examine the variability in welding residual stresses (WRS) for different material models. An experiment was carried out to measure thermal cycles and WRS in the ferritic steel S355 weldment for validation. Since solid-state phase transformation (SSPT) affects the dislocation density and thereby also the strain hardening, strain hardening recovery caused by SSPT should be considered for the accurate prediction of WRS, especially in single-pass welds. The mixed hardening model provides the more accurate prediction of WRS. It is recommended to consider SSPT regardless of strain hardening in numerical simulation of WRS for simplification but still with acceptable accuracy.
Fatigue-loaded welded constructions are mainly designed according to the experimental results of low-strength steels, although great knowledge about the properties of modern high-strength steels is ...available. Due to the increasing notch sensitivity with increasing yield strength, the applicable load amplitudes are mostly independent of the steel grade. This paper presents various options for exploiting the potential of high-strength steels subjected to cyclic loads. Therefore, different post-weld treatments are investigated. The calculated fatigue strength is thereby based on IIW guidelines. The results indicate a high potential for an increase in fatigue strength up to a calculated FAT class with a variable slope of FAT392. This is considerably higher than the currently valid base metal level of FAT160.
The Goldak’s double-ellipsoid volumetric heat source model is commonly used for arc welding in numerical simulation. Usually, heat source geometric parameters and arc efficiency need to be calibrated ...in thermal analysis until a good agreement with experiments is achieved. However, the influence of individual heat source geometric parameters and arc efficiency on welding temperature field, residual stress, and distortion is still unclear. In this study, this influence has been systematically investigated in a thin-plate full-penetration weld by methods of both numerical simulation and experimental method. The predicted and measured results indicate that the location of HAZ boundary and the Δ
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time can only be influenced by arc efficiency (significantly) and heat source width (slightly) in thin-plate full-penetration welds. Furthermore, the variation of arc efficiency can significantly affect welding residual stress and distortion, while the reasonable change of heat source geometric parameters has a slight effect. In this study, a calibration procedure of heat input model in FE simulation of thin-plate full-penetration welds is provided.
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