Pawel Kucharczyk

Characterization of Local Deformation in Welded Joints Using a Combined Experimental and Numerical Approach

The microstructure of welded joints differs significantly from that of the base material, what changes their mechanical properties and influences fatigue life. The aim of this work was the investigation of the local deformation field within a butt joint made of 10 mm thick structural steel S355. However, a direct sampling even of the weld metal was impossible due to small dimensions of butt joints. Therefore, the following procedure was utilized in order to manufacture big samples of the microstructure identical to that of the local weldment areas.A geometrical model of the welded structure describing the relevant areas e.g. weld metal, heat-affected zone was established. It was based on the results of the metallographic investigations, hardness mapping and electron-probe-micro-analysis of the local chemical composition. The welding process was numerically simulated using SYSWELD program to estimate the time-temperature-transition (TTT) curves for each identified area. The parameters of the heat input source were calibrated. Afterwards, the material of the defined chemical composition was heat-treated according to the TTT curves. For the validation purpose the heat-treated work pieces were evaluated in terms of microstructure and hardness distribution. Finally, the up-scaled samples of the respective bulk microstructure were manufactured and investigated in monotonic tests.

Characterization of Ductile Failure Behavior of the Ferritic Steel Using Damage Mechanics Modeling Approach

A ductile failure is characterized by pronounced plastic deformations which involve significant plastic strains. The modeling of this failure behavior requires a precise description of the material plasticity starting from the crack initiation, its propagation through the material to the final fracture. The classical theory of metal plasticity based on the von Mises or Tresca formulations assumes that the effect of hydrostatic pressure on the flow potential is insignificant. Furthermore, it postulates that the flow stress is independent of the third stress invariant of the deviatoric stress tensor. The scientific findings from last few years show, however, that these both quantities should be considered for the precise description of plasticity, especially, of the real materials [1-4].

Estimation of local properties in welded joints using samples with simulated microstructure

Purpose The purpose of this paper is to determine local properties in a coarse grain zone of 10 mm thick butt welded joints made of structural S355 and high strength S960 steels. Metallographic analyses showed that the width of the investigated zone for both S355 and S960 weldments was too small for direct sampling. Therefore, samples with reproduced microstructure were manufactured by heat treatment taking into account chemical composition and cooling conditions of weldments. Design/methodology/approach The basis for the heat treatment was the time-temperature-transition (TTT) curve, whose shape is defined by welding and cooling parameters. In this study the TTT curve was determined experimentally during welding as well as numerically simulated using SYSWELD program. The work pieces were heat-treated according to the TTT curves using thermomechanical treatment simulator and evaluated in terms of microstructure and micro hardness distribution. Finally, the secondary specimens were manufactured and investigated in monotonic tensile tests. Findings The presented approach allows for the determination of the local properties of welded joints. In this study mechanical properties (stress-strain curves) of heat affected zone (HAZ) were successfully estimated using samples with reproduced microstructure. Furthermore, it was found out that the chemical composition in the HAZ was not influenced by the welding process. Thus, the HAZ microstructure can be successfully reproduced using base material. Additionally, the paper contains recommendations for simulation of the local microstructure and suggestions for the evaluation of the obtained results. Originality/value The advantage of the proposed approach is the enlargement of the material volume with homogeneous microstructure so that different local properties like toughness, fatigue behavior, crack propagation or crashworthiness can be analyzed, what is technically infeasible for the weldments with small HAZ.

Phenomenological Modelling of Impact Toughness Transition Behaviour

The toughness transition behavior of ferritic steel results from the fact that two competing fracture mechanisms can be activated independently or progressively. Temperature, strain rate and the material ́s hardening properties are the major influences affecting the result of this competition between cleavage and ductile fracture mechanisms. An elastic visco-plastic plasticity model with stress-state dependent yielding and isotropic hardening forms the basis of a model to predict the Charpy impact toughness properties of steels with bcc crystal structure for transition behavior. A scalar damage variable is coupled into the yield potential in order to capture the effects of damage induced softening. The corresponding damage evolution law considers damage initiation criteria for both mentioned fracture mechanisms. Material parameter identifications and successful model application in terms of Charpy impact toughness tests are demonstrated.

Fatigue Properties of Laser Hybrid Weldments on Structural Steel

The behaviour of laser hybrid weldments under cyclic load was examined. The laser hybrid butt joints made of high-strength steel S690Q and structural steel EH36 (S355) were investigated in 4-point bending tests. The S-N lines were established for 15, 20 and 25 mm thick specimens. The weldments were manufactured in two variants: without a gap between the connected plates and with a narrow gap between them. The calculations were carried out in accordance with IIW and EC3 recommendations. The influence of defects on fatigue resistance was estimated in additional tests using samples with weld defects, which were determined by X-ray methods.