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Based on the state-of-the-art research and regulations, the application of the fracture mechanics master curve (MC) concept to ferritic ductile cast iron (DCI) is being investigated in a joint research project between MPA Stuttgart and BAM Berlin. This paper outlines the research approach, the planned investigations and first results of the experimental program.
For the hydrogen-based energy economy of tomorrow, the construction of the necessary infrastructure will play a central role. Most materials used to date, such as welded steels, can be prone to hydrogen embrittlement under certain conditions. This includes the classic delayed cold cracking during welding as well as degradation phenomena during service of components in hydrogen-containing environment. For the evaluation of any hydrogen effect, for example, on the mechanical properties of a welded metallic material, the hydrogen content must be precisely determined. In the case of weld seams, the carrier gas hot extraction (CGHE) according to ISO 3690 is meanwhile state-of-the-art. CGHE is based on accelerated hydrogen degassing due to the thermal activation of hydrogen at elevated temperatures. In addition to the quantification of hydrogen, thermal desorption analysis (TDA) with varying heating rates can be used to determine and evaluate the hydrogen trapping at microstructural defects in the material. For both techniques, experimental and metrological influences must be considered, which have a major effect on the result. For example, ISO 3690 suggests different sample geometries and minimum extraction times for CGHE. This study summarizes the results and experiences of numerous investigations at the Federal Institute for Materials Research and Testing (BAM) with different sample temperatures and geometries (ISO 3690 type B and cylindrical TDA samples) regarding the influence of the sample surface (polished/welded), measurement accuracy depending on the sample volume and the insufficient monitoring of the effect of PI control on the extraction temperature. A deviating extraction temperature from the target temperature can significantly falsify the measurement results. Based on the results, methods are shown which allow the desired extraction temperature to be reached quickly without physically interfering with the measuring equipment. This serves to significantly improve the reliability of the hydrogen measurement through increased signal stability and accelerated hydrogen desorption. In general, an independent temperature measurement with dummy samples is recommended for the heating procedure of choice to exclude possible undesired temperature influences before the measurement. The methods described can be transferred directly to industrial applications
In this contribution, we present a physically motivated heat source model for the numerical modeling of laser beam welding processes. Since the calibration of existing heat source models, such as the conic or Goldak model, is difficult, the representation of the heat source using so-called Lamé curves has been established, relying on prior Computational Fluid Dynamics (CFD) simulations.
Lamé curves, which describe the melting isotherm, are used in a subsequent finite-element (FE) simulation to define a moving Dirichlet boundary condition, which prescribes a constant temperature in the melt pool. As an alternative to this approach, we developed a physically motivated heat source model, which prescribes the heat input as a body load directly. The new model also relies on prior CFD simulations to identify the melting isotherm. We demonstrate numerical results of the new heat source model on boundary-value problems from the field of laser beam welding and compare it with the prior CFD simulation and the results of the Lamé curve model and experimental data.
Electromagnetic stirring is known to promote material flow, reduce porosity, uniform elements distribution, and refine grain in laser beam welding (LBW), which enhances the applicability of LBW in various industries. In this study, a phase-field model of dendrite growth in AA5754 Al alloy electromagnetic stirring laser beam welding was established. The model considered the thermal electromagnetic Lorentz force resulting from the interaction between the electric field generated by the Seebeck effect and the magnetic field, as well as the temperature gradient and solidification rate of the solidification interface obtained from the computational fluid dynamics electromagnetic stirring LBW model. The variation rules of dendrite growth with different magnetic parameters and effects are analyzed. Comprehensively, the magnetic field promotes the solidification rate, thus promoting interfacial instability and a large magnetic flux density leads to a faster interface instability. The solidification rate as well as the temperature gradient affect the growth rate, and the accelerated growth caused by the so lidification rate with a high frequency and a large magnetic flux density effectively inhibits the slow growth caused by the temperature gradient. The thermal electromagnetic Lorentz force is the main factor for the branch increment at low frequencies, while both thermal electromagnetic Lorentz force and temperature gradient in crease the number of branches at high frequencies. The calculated average branch numbers considering various factors in the stable stage under different magnetic parameters were consistent with the results of the scanning electron microscope tests.
Next to chemical composition, metallurgy and welding parameters, the intensity of restraint is one of the variables influencing solidification cracking. Tests like the Houldcroft Test, rate the hot cracking susceptibility indirectly on the amount of restraint the welding can endure without cracking. Modern lightweight steel construction welds can be subject to a larger amount of restraint due to their high-strength nature compared to classical use cases. By varying the plate thickness of Controlled Thermal Severity (CTS) tests produced out of S1100 QL, it was possible to vary the intensity of restraint on fillet welds at a high level. Testing was conducted with four different filler wires for Metal Active Gas (MAG) welding, including three solid and one metal-cored wire. In addition, two sets of welding parameters were tested. The first set with high heat input and high welding speed was shown to be more prone to solidification cracking compared to the second set with lower heat input and welding speed. The results show an increase in solidification cracking with increasing restraint severity.
This article focuses on the selective placement of additional LTT layers to generate compressive residual stresses in fatigue-critical areas of conventional weld joints. This enables an economical solution without effecting the integrity of welded joints. For this, longitudinal stiffeners made of high-strength steel were gas metal arc welded using conventional welding consumable in the first layer. Afterwards, a chromium-nickel alloyed LTT welding consumable was deposit on front sides of the stiffeners. By varying the welding parameters, different weld geometries of the LTT filler metal could be analyzed. The effects of additional LTT layers were investigated with regards to residual stresses, microstructure and strength. X-ray residual stresses measurements show that the residual stresses at the failure critical weld toe are significantly reduced by using additional LTT layers. While the conventional weld is characterized by tensile residual stresses, compressive residual stresses can be detected at the LTT weld. The level of residual stresses is influenced by the geometry of the LTT layer. Additional LTT layers with a high offset to the conventional weld generate more compressive residual stress in the HAZ than with a low offset. Therefore, the weld geometry has a considerable impact on the residual stress profile.
Influence of machining on residual stresses in additive manufactured high-strength steel components
(2024)
This study focuses on the effect of machining on the residual stresses and distortion of WAAM specimens. Defined specimens were welded fully automatically with a special WAAM solid wire (yield strength >820 MPa) with different geometric designs. The residual stresses state before and after cutting of the AM structure from the substrate plate were analyzed by means of X-ray diffraction on the specimen surface and complementary by 3d deformation analyses using photogrammetry. The results reveal significant influences of the geometry on the relaxation and redistribution of residual stresses.
MVT testing was conducted in Transvarestraint mode in multiple welding conditions and under a range of bending speeds. The materials observed were high strength filler metals used in GTAW namely DIN EN ISO 16834-A G 69 4 M21 Mn3Ni1CrMo (G69), 16834-A G 89 6 M21 Mn4Ni2CrMo (G89 6), 16834-A G 89 5 M21 Mn4Ni2,5CrMo (G89 5) and a filler wire 18276-A T 89 4 ZMn2NiCrMo M M21 1 H5 (T89). For evaluation light optical microscope pictures were used. Pixels containing cracks were manually segmented using a self-written program. Out of the segmented images data including crack length, position and area can be calculated for every crack. The results show dependencies of solidification cracking on the test parameters.
This presentation describes the usability of the hollow tensile specimen technique for in-situ material testing in a hydrogen atmosphere.
In addition, the presentation provides an outlook on the methodology for investigating the suitability of pipeline steels and their weld seams for hydrogen operation.
The weldability of stainless steels is largely controlled by the chemical composition, and alloys with ferritic or ferritic-austenitic solidification show the highest resistance to hot cracking. As the resulting phase balance also
affects the final properties, it may be beneficial to both foresee and measure the weld metal ferrite content. The WRC ’92 constitution diagram is currently the most accurate prediction tool available, but it does not take the cooling rate into consideration and the precision may be less accurate for stainless steels with high ferrite numbers (FN). This study aims to validate the reliability of the WRC ’92 diagram for weld metals with FN > 50. The chemical composition was altered through gas tungsten arc welding (GTAW) of UNS S32205 with ER347 filler wire that had been PVD-coated with either niobium (Nb), copper (Cu), nickel (Ni), manganese (Mn), carbon (C), or silicon (Si). The actual ferrite content was assessed using image analysis, FERITSCOPE® and X-ray diffraction (XRD). While the WRC ’92 diagram predictions were deemed accurate to acceptable for Ni, Si, and Mn, notable deviations were observed for Nb, Cu, and C. The FeriteScope exhibited a consistent trend with image analysis, though the values were higher, and scatter was wider, and the conversion factor is open for discussion. The lowest accuracy and largest spread were obtained using non-contact XRD, rendering it unsuitable for ferrite measurements of welds.