The amount and the geometrical extension of softening effects in the heat affected zone (HAZ) of steel welds have a significant influence on the static and fatigue strength properties in transverse direction of the entire welds. This is basically defined by the hardness loss with respect to base material and the ratio of Soft Zone width to sheet thickness. Especially steels with bainitic and martensitic phase matrices are sensitive to tempering effects.
The Soft Zone can also be found in conventional quenched and tempered steels. The softening effects of the latter have been investigated in detail in the past for a yield strength up to 600MPa. Nevertheless, no results are available for new thermo-mechanically produced and precipitation hardened high strength steels (e.g. ALFORM-M) with yield strength between higher than 700MPa. In addition the influence of such Soft Zones on the toughness and fatigue strength of welded components is not known yet.
In this project the effect of the Soft Zone on the joint behaviour will be investigated and characterised experimentally and numerically. From the newly gained knowledge criteria for limitations of the soft zone in components will be developed.
Welding experiments: Various geometries and width of HAZ and soft zones will be produced by using different welding procedures and process parameters for different sheet thicknesses. Microstructure and mechanical properties of the welds will be investigated with optical and electron microscopy as well as by tensile, charpy, hardness and fatigue tests. Numerical simulation will be utilized to support experimental work.
Microcstructure Modelling: In a first step process dependant temperature cycles will be calculated and simultaneously a model will predict the microstructure development in the cross section. These results have to be evaluated according to the hardness representing a measurement of the Soft Zone. After validating this tool it can be applied to evaluate the geometry and the microstructure of the Soft Zones. To validate the simulation, the calculated hardness and strength loss will be compared with the experiments.
Thermo-Physical Simulation: It is essential to examine and know the different zones in the heat affected zone (HAZ). By using the GLEEBLE simulator and the HAZ simulator Soft Zones of different dimensions and properties will be produced by various temperature-time cycles on a representative volume. Subsequently the GLEEBLE specimens can be tested to evaluate the mechanical behaviour of the Soft Zones. Therewith the GLEEBLE physical simulation can provide the relevant data for the numerical simulations, like Stress-Strain Diagram, Strain Hardening Exponent, Fatigue endurance curve (Woehler). The properties of the Soft Zone are the basis for the following numerical simulation.
Finite Elemente Simulation: Using the thermo-physical analysis of different layers of the HAZ an exact analysis for the Soft Zone is not yet possible. There are still further parameters which influence the properties of the component due to the formation of the Soft Zone, like joint geometry, load direction, loading rate, size and properties of the Soft Zone. Firstly, the focus of the FEM-Simulation will be the evaluation of the critical size of the Soft Zone, i.e. the ratio of Soft Zone to plate thickness concerning to static strength and fatigue strength (low cycle and high cycle). The FEM simulation to describe complex geometries of Soft Zones will be in 2D and 3D. . The next steps for the FEM Simulation will be the implementation of damage parameters to predict the service life of the component
Verification: Experimental investigations of the hardness loss in HAZ of the welds and the effects of the Soft Zone on the static strength, toughness and fatigue strength for new thermo-mechanically produced high-strength steel sheets with ferritic-bainitic and martensitic microstructure, will be done. These data are used to verify the calculations which are an essential part of this project.
This study should lead to a definition of an acceptable amount and maximum geometrical extension of the Soft Zone in the HAZ, including the related welding procedures and parameters, to ensure the required properties of the welds in the new high strength steel sheets. With the help of the numerical simulation it should be possible to predict the damage and failure behaviour due to the appearance of the Soft Zone. In addition the influence of different welding procedures and welding parameters on the formation of the Soft Zone will be calculated to minimize the Soft Zone size. The adjustment of the program for different steel types should be possible.
1) Welding experiment using MAG (Metal Active Gas) and HFI procedures (High Frequency Induction), including post weld heat treatment (different grove preparation, welding processes, materials) by company partners.
2) Systematic investigation of properties (metallography, hardness, tensile, impact and fatigue tests, fracture surface analysis), partially by company and scientific partners.
3) PhD-Thesis 1 (company partner), empirical model: Numerical Simulation of static and dynamic properties of soft-zone containing welded plates and components (working title). The experimental focus of this work is the physical simulation and the subsequent static and dynamic testing of specimens and components-like containing different Soft Zones. Therefore a HAZ simulator can be utilized as well as equipment for static and dynamic testing, for to generate Stress-Strain curves and strain-hardening-coefficients of Soft Zones for different geometric sizes and different thermal histories (--curves as a function of applied welding cycles). The simulation of the static behaviour can be performed by applying SYSWELD and ABAQUS software, e.g. Furthermore Woehler fatigue curves will be tested and measured (-N=f(T-t)). Subsequent a prediction of the life cycle for different components should be possible by application of the FEMFAT software, e.g..
4) PhD-Thesis 2 (scientific partner), physical model: Simulation of microstructural development in the Soft Zone during welding (Changes of grain boundaries, precipitations and dislocation structures in the Soft Zone and adjacent zones of loaded specimens and components). The experimental focus of this work is the overall microstructural characterization of the Soft Zone using special electron-microscopic investigation methods, like SEM, TEM, EBSD, WDX and EDXS. The prediction of the static and dynamic behaviour will be simulated using the software packages MATCAD (kinetic of precipitations and grain structures and dislocations).
Due to this material characterizations as base for the empirical and physical modelling it will be possible to predict the Soft Zones of welded plates and components and its influences on the static and dynamic properties.