Papers by Author: M.J.M. Hermans

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Authors: R.K. Dutta, R.M. Huizenga, M. Amirthalingam, H. Gao, A. King, M.J.M. Hermans, I.M. Richardson
Abstract: The effect of plastic deformation of austenite at elevated temperatures on the kinetics ofphase transformations during continuous cooling was studied in a high strength quenched and tem-pered structural steel S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt.%)) by means of in-situsynchrotron diffraction. The steel was heated to 900 C (above Ac3) in the austenite region and elon-gated by 6% followed by quenching to room temperature. Time-temperature-load resolved 2D syn-chrotron diffraction patterns were recorded and used to calculate the local d-spacings between latticeplanes. The plane specific diffraction elastic constants of austenite at 900 C in the steel were deter-mined from the local d-spacings. The effect of the deformation of austenite on the phase transforma-tion kinetics was studied. The evolution of lattice parameters and the phase fraction of the bcc phasesduring the quenching process were calculated.The calculated plane specific elastic constants of austenite at 900 C varied between 32 GPa to140 GPa for the different fhklg reflections of austenite. The deformation of austenite at 900 C re-sulted in the formation of a mixture of 38 % bainite, 59 % martensite and 3 % retained austenite afterquenching to room temperature. Without hot deformation, austenite transformed to 9 % bainite and88 % martensite with 3 % retained austenite. The presence of the bainitic and the martensitic phaseswas observed fromthe change in the slopes of the lattice parameters of the bcc phase during quenchingand confirmed by microscopy.
Authors: A.M.A. Pazooki, M.J.M. Hermans, I.M. Richardson
Abstract: Welding residual stress and subsequent distortions are a common problem during fabrication. Several techniques for controlling residual stress and distortion during welding, amongst which transient thermal tensioning is one of the more promising methods. Thermal tensioning can be achieved by two side heaters moving along with the weld heat source. The concept of transient thermal tensioning induces a redistribution of the stress field in the welded plate by altering the thermal field. In this paper, the results are presented on the mitigation of out-of-plane deformation during gas tungsten arc butt welding of 2 mm thick AISI-316L stainless steel. Acetylene-air burners are applied as side heaters. The thermal tensioning process induces a number of additional process parameters including the intensity, the size and the location of the heaters. A thermo-mechanical (TM) finite element model is used to cope with the complexity of the process. The model provides a better understanding of the development of the stress field, both in conventional and transient thermal tensioned welds and of the mechanisms which mitigate distortion. The model is validated by temperature, stress and distortion measurements. It appears that the redistribution of stresses and the introduction of tensile stresses by side heating is the main mechanism responsible for the reduction of distortion.
Authors: E.M. van der Aa, M.J.M. Hermans, I.M. Richardson, N.M. van der Pers, Rob Delhez
Abstract: It has been shown that the introduction of a trailing heat sink following a welding heat source at a short distance can reduce or even eliminate the buckling deformation of welded thin plate structures. To increase the insight into the stress development during welding with a trailing heat sink, residual stress measurements were carried out by means of laboratory X-ray and synchrotron X-ray diffraction techniques. Both the laboratory X-ray and the synchrotron X-ray experiments show a difference between the residual stress distributions for the cooled and the uncooled samples. The longitudinal stresses show a reduction in the compressive stress level towards the plate edges, to a level below the critical buckling stress. The transverse stresses are drastically reduced when the active cooling is applied.
Authors: M.H.E. Janssen, M.J.M. Hermans, M. Janssen, I.M. Richardson
Abstract: Advance high strength steels (AHSS), like dual phase (DP) and transformation induced plasticity (TRIP) steels, offer high strength and toughness combined with excellent uniform elongation. However, the higher alloying content of these steels limit their weldability and the thermal cycle of welding processes destroys the carefully designed microstructure. This will result in inferior mechanical properties of the joint. Therefore, joining processes with a low heat input, like brazing, are recommendable. Data regarding mechanical properties of joints in DP and TRIP steel is limited, especially for brazed joints. Results with respect to the fatigue lifetime of laser brazed butt joints are presented. In DP and TRIP steel, crack initiation takes place at the braze toe. In DP steel the crack propagates through the base metal. In TRIP steel, however, the crack may either follow the interface or may continue through the steel depending on the maximum stress level. The different failure mechanisms are explained on the basis of process conditions, the microstructure and the stress state.
Authors: M. Amirthalingam, M.J.M. Hermans, I.M. Richardson
Abstract: In this work, the microstructural evolution of aluminum containing commercial grade TRIP steels during gas tungsten arc (GTA), Laser beam (LB) and resistance spot (RS) welding have been studied. Microstructural analysis was carried out using optical and scanning electron microscopy. Results show that fusion zones of welded TRIP steels contain complex inclusions with similar size distribution. The energy dispersive spectroscopy analysis of inclusions indicated that these inclusions are primarily oxides of aluminum with epitaxial enrichment of manganese and phosphorous. The fusion lines of GTA and LB welded aluminum containing TRIP steel contain a zone of polygonal ferrite with a size of about 200 m and 50 m respectively. It is found that aluminum partitioned from the liquid weld to the solidified delta ferrite in the fusion line causing enrichment and resulting in ferrite stabilisation. This ferrite zone was not found in the case of resistance spot welded samples due to faster cooling rates.
Authors: Yoshinori Hirata, K. Tsujimura, B.Y.B. Yudodibroto, M.J.M. Hermans, I.M. Richardson
Abstract: In Gas shielded Metal Arc Welding process, the molten drop at the electrode wire tip is detached and transferred into the weld pool by various driving forces, which have been discussed mainly focusing on gravity force, electro-magnetic force, surface tension force and plasma drag force. In this paper the oscillation phenomena of metal drop and their numerical model are described. The numerical model developed is an axial symmetrical 2D model which enables to calculate and visualize time-change of the drop shape and the flow in the drop using VOF-CSF method. The validity of the model is verified through the comparison between calculations and experiments using waterdrop.  It is shown that the natural frequency of the pendant drop decreases with increase of the drop size. And the numerical simulation predicts that it is possible to detach the molten drop from the electrode wire by exciting the forced oscillation around the natural frequency with pulsed current.
Authors: A.M.Akbari Pazooki, M.J.M. Hermans, I.M. Richardson
Abstract: Dual phase steel consists of martensite embedded in a ferrite matrix. The material experiences high heating and cooling rates during welding, which alter the microstructure significantly. In this work the effects of solid state phase transformations on the prediction of residual stresses and distortion during welding of DP600 steel is investigated. Phase fractions have been calculated implicitly using continuous cooling time (CCT) diagrams. The results of the model are compared with experimental measurements for bead-on-plate welds made on DP600 sheets. It is found that the volume changes and the increase of the strength due to the martensitic transformation have both a significant effect on the residual stress and distortion level although in opposite directions. Martensitic phase transformations in DP600 steel tend to reduce tensile residual stresses in the weld metal.
Authors: H. Gao, R.M. Huizenga, R.K. Dutta, M. Amirthalingam, M.J.M. Hermans, T. Buslaps, I.M. Richardson
Abstract: Six pass welds were made on a 16 mm thick high strength quenched and tempered structural steel plate (S690QL1, Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt.%)). Depth resolved measurements in two orthogonal directions were carried out using energy dispersive synchrotron X-ray diffraction at the ID15 beamline of the European Synchrotron Radiation Facility. The strains were calculated from the shift in the local d-spacing for four bcc planes ({200}, {211}, {220}, {310}). The planar stresses were calculated from the biaxial Hooke’s law, using the diffraction elastic constants of the individual planes. A two dimensional cross-sectional residual stress map with a depth resolution of 2 mm was obtained. Transverse compressive stresses were found at the weld toes and root. Transverse tensile stresses were present in the middle of the plate. Longitudinal tensile stresses concentrated along the fusion line. This work describes the procedures to obtain the depth resolved residual stress map and the generated results provide necessary information to validate thermal mechanical finite element model of multi-pass welding.
Authors: R.K. Dutta, R.M. Huizenga, M. Amirthalingam, H. Gao, A. King, M.J.M. Hermans, I.M. Richardson
Abstract: In-situ phase transformation behaviour of a high strength (830 MPa yield stress) quenched and tempered S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt. %)) structural steel, during continuous cooling under different mechanical loading conditions to promote the bainitic transformation, was studied. Time-temperature-load resolved 2D synchrotron diffraction patterns were recorded and used to calculate the transformation strains. The temperature dependent elastic constants of ferrite in the steel were also determined using \textit{in-situ} tensile tests at different temperatures in a synchrotron X-ray diffractometer. The transformation strains were calculated under different loading conditions.The elastic constants were calculated from the lattice parameters at 25 °C, 200 °C, 300 °C, 400 °C, 500 °C and 600 °C. The elastic constants varied from 202 GPa at 25 °C to 143 GPa at 600 °C. The variation in lattice plane strains during phase transformation under small external loads were calculated. Bulk measurement techniques such as dilatation experiments give the averaged transformation strains. However, in-situ synchrotron measurements performed in this work describe the transformation strains of the individual transforming phases and the strains arising due to possible variant selection.
Authors: M.J.M. Hermans, B.Y.B. Yudodibroto, Yoshinori Hirata, G. den Ouden, I.M. Richardson
Abstract: This paper gives an historic overview and new developments of research activities in the field of the oscillatory behaviour of liquid metal in arc welding. Early work focused on the oscillation behaviour of the weld pool in Gas Tungsten Arc Welding (GTAW). Agitated weld pools exhibit specific modes of oscillation, the frequency of which can be measured from the arc voltage data and is conditioned by the geometry of the weld pool and the properties of the liquid metal. Of technological interest is the alteration of the oscillation behaviour for partially and fully penetrated situations, which can be used for penetration control during welding. A logical extension of the research activities was related to the influence of filler wire addition on the oscillation behaviour. An intermediate step towards the description of Gas Metal Arc Welding (GMAW), is the situation of GTAW with cold filler wire supply. It was found that both the liquid weld pool and the pendant liquid droplet at the tip of the filler wire experience an oscillation, which obscures the influence of the individual contributions of both liquid masses on the voltage data. It was shown that online penetration control is still possible, provided that the metal is transferred in an uninterrupted way, i.e. the filler wire flows smoothly into the weld pool. For GMAW, in which detached droplets collide with the weld pool surface, the difficulties are even more prominent. Recent work is related to this issue. Monitoring of the phenomena occurring at the weld pool and the pendant droplet become problematic by means of the voltage data. Observations by means of high-speed video imaging will be discussed. Apart from the experimental studies, efforts are undertaken in numerical simulations of the processes. A good correlation is obtained between experimental data and the results of the numerical models.
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