NUMERICAL SIMULATION ON THE EFFECT OF HAZ SOFTENING ON STATIC TENSILE STRENGTH OF HSLA STEEL WELDS

NUMERICAL SIMULATION ON THE EFFECT OF HAZ SOFTENING ON STATIC TENSILE STRENGTH OF HSLA STEEL WELDS W. MAURER*, W. ERNST*, R. RAUCH*, S. KAPL*, R. VALLANT**, N. ENZINGER** *voestalpine Stahl GmbH, Linz, Austria **Graz University of Technology, Inst. for Materials Science and Welding, Kopernikusg. 24, 8010 Graz, Austria ABSTRACT Softening in the heat affected zone (HAZ) of welded high-strength-low-alloy (HSLA) steels joints may cause a decrease of the static tensile strength. This study investigates the influence of softening on static strength by means of a FEM analysis and gas metal arc welding experiments. Mechanical properties of different HAZ regions for the FEM simulation were determined by means of physical HAZ simulation and subsequent quasi-static tensile testing. For verification purposes welded joints were investigated by means of hardness measurement and quasi- static tensile testing. The numerical simulated mechanical behaviour of the modelled joints are in good agreement with experimental results. Based on the validated FE analysis the impact of different geometrical and material parameters on the strength of welded joints was investigated systematically and quantified by multiple linear regression methods. INTRODUCTION The substantial interest in the development of high strength low alloy (HSLA) steels is the reduction of weight for different fields of application, e.g. construction and engineering but also crane and truck industry [1]. These types of steel are characterised by high strength and toughness, good formability and weldability due to a low carbon equivalent. They are microalloyed with Ti, V and Nb and produced by a thermo-mechanical controlled process (TMCP) [2]. Therefore the resultant microstructure and mechanical properties are irreversible influenced by any subsequent thermal treatments including welding processes. Thus, mechanical properties in the heat affected zone (HAZ) may be altered significantly and the occurrence of a softened zone could be expected. The degree and extension of softening in the HAZ is primarily controlled by the chemical composition, the production parameters, the total heat input of the applied welding process and the resultant cooling rate of the weld, respectively the cooling time between 800 C and 500 C (t 8/5 ) [3]. 669

An increased cooling time t 8/5 results in an decrease of the maximum hardness in the HAZ and as well the weld metal, independently of the applied welding process e.g. gas metal arc welding or electron beam welding [4, 5]. The overall static strength of welded joints, including a soft interlayer within the HAZ, depends upon following parameters and their interactions: relative thickness, X, of the soft interlayer (ratio of width of the soft zone, w, and sheet thickness, t) ratio of sheet sample width, W, to sheet thickness, t strength decrease in the soft zone (= softening ratio, S ) local orientation of the soft zone with respect to the loading direction strength of the weld metal (= matching ratio, S ) If the X value decreases, the strength will increase and may reach the strength of the base metal. This strengthening is caused by the constraint effect. The transverse plastic flow of the soft interlayer will be held in check by the stronger base metal depending on the width of the soft interlayer. Experimental studies achieved that this constraint effect is only effective if the thickness of the soft interlayer do not exceed certain values: Wallner et al. [6] specified a criteria for the relative thickness of the soft zone in even- or overmatched joints. If the thickness of the soft zone is smaller than a quarter of the sheet thickness the strength of the joint won t underrun the tensile strength of the unaffected base metal or weld metal. According to de Meester the global strength in even- or overmatched welds benefits from constrain effect, if the width of the softened HAZ doesn t exceed the sheet thickness [4]. The tensile strength of the joint is also reduced with decreasing ratio of sheet width to sheet thickness. Satoh and Toyoda [7] distinguish between finite and infinite sample width (Fig. 1) with respect to the sheet width to sheet thickness ratio of the cross-section. In case of infinite sheet width (w inf ), a plane strain state can be assumed and the tensile strength is independent of the sheet width. An increase in weld metal strength (overmatching) assist the constraint effect and ease the strength decrease caused by a soft zone. It is possible for overmatched welds, including soft zones with small relative thickness and softening ratios bigger than 0.9, that the plastic strain and furthermore necking and fracture take place in the unaffected base metal [3, 8]. The width, respectively the relative thickness (X ), of the evenmatched or overmatched weld metal does not affect the tensile strength of a joint [8]. 670

Soft zone Base metal Relative thickness of the soft interlayer: X w t Criteria for an infinite sample width: t w X < 1 w 5 t inf Weld metal X > 1 w 5 inf w Fig. 1 Welded joint with a soft interlayer. The number of FEM studies concerning the effect of a soft zone within the HAZ on the properties of welded joints is limited in literature. Mochizuki et al. [8] investigated the strength and the deformation of welded joints by means of an analytical study to determine the influence of HAZ softening and strength mismatch. The results confirmed the experimental studies claiming that ultimate tensile strength of the joint decrease as a function of the relative thickness of the soft zone (X ). Furthermore the effect of weld metal strength on the joint strength was investigated, as mentioned in the previous paragraph. Panda et al. [9] analysed the formability of laser welded joints of advanced high strength steels with a non linear FEM- simulation. A particular attention was paid on the strain distribution in the different zones of the weld. It was found that the formability decreases due to strain localisation in the soft zone of the joint during the deformation. Hochhauser et al. investigated experimentally the influence of the heat input on the static tensile strength of welded joints, too. An increase in heat input causes a wider and softer HAZ accompanied by a softening of the weld metal [10]. Thus, the influence of the different strength determining factors cannot be quantified independently in practical welding experiments. The present study was carried out with S 700MC as base material and focuses on the determination of the ultimate tensile strength by means of a FEM analysis. The input data for the FEM analyse were generated by thermal HAZ simulations of the base metal with subsequent mechanical characterization. In addition the results of the FEM analyse were verified by gas metal arc welds, which were performed with aligned parameters. Furthermore, FE analysis in combination with a statistical evaluation was used to quantify the impact of different geometrical and material parameters (matching ratio, softening ratio, geometry and orientation of the weld) and their interactions on the strength of welded joints. 671

EXPERIMENTAL APPROACH BASE METAL The investigated steel was a TMCP steel strip (S 700MC) with 6 mm sheet thickness, microalloyed with Ti, Nb and V. The microstructure consists mainly of bainite and ferrite in delivery condition. This type of steel derives its strength from grain refinement, precipitation hardening and a less amount of transformation hardening. Chemical composition and mechanical properties can be seen in Table 1 and Table 2. Table 1 Chemical compositions of S 700MC according to EN 10149-2 (mass %) [11] C Mn Si Nb 1 Ti 1 V 1 Mo B < 0.12 < 2.10 < 0.60 < 0.09 < 0.22 < 0.20 < 0.50 < 0.005 1 Nb + Ti + V < 0.22 Table 2 Mechanical properties according to EN 10149-2 [11] yield strength [MPa] tensile strength [MPa] fracture elongation [%] 700 750-950 12 FILLER METAL A solid wire G 89 6 M Mn4Ni2CrMo for HSLA steels with 1.2 mm diameter according DIN EN ISO 16834-2007, with an yield strength exceeding that of the base metal was selected to produce a slightly overmatched weld [12]. Typical chemical composition and mechanical properties of the weld metal are shown in Table 3 and Table 4. Table 3 Chemical composition of Mn4Ni2CrMo according EN ISO 16834-A-G 89 6 M Mn4Ni2CrMo (mass %) [12] C Si Mn P S Ni Cr Mo Cu V < 0.12 0.6-0.9 1.6-2.1 < 0.015 < 0.018 1.8-2.3 0.2 0.45 0.45 0.7 < 0.3 < 0.03 If not stipulated: Ti 0,10 %, Zr 0,10 % and Al 0,12 %. Table 4 Mechanical properties according EN ISO 16834-A-G 89 6 M Mn4Ni2CrMo [12] Yield strength [MPa] Tensile strength [MPa] fracture elongation [%] > 890 940-1180 15 672

WELDING PROCESS Single pass butt welds were performed by gas metal arc welding process with five different levels of energy input (see Table 5), to establish different soft zones (strength levels, geometrical extension). Fig. 2 displays the weld preparation of the sheets. The sheets were fixed by chuck jaws and run- in and run- off sheets were added. A pulsed DC power source (Fronius TPS 5000) and automatic linear carriage was used to generate the GMA welds. Shielding gas M21 (Ar + 18% CO 2 ) according to EN 439 [13] was applied with a mass flow of 15 l/min. No. Table 5 GMA welding parameters Energy input by unit length [ kj/mm] Current [A] Voltage [V] Welding speed [mm/min] Calculated Cooling time t 8/5 1 [s] Measured Cooling time t 8/5 2 [s] 1 0.5 223 22.5 550 5.0 7.3 2 0.7 220 23.5 425 10.0 9.5 3 0.9 217 23.6 329 15.0 13.5 4 1.0 217 23.8 300 18.0 16.0 5 1.2 216 24.0 255 25.0 22.5 1 ) According to DIN EN 1011-2:2001[14] 2 ) PtRh-Pt thermocouples were dipped into the weld pool. Fig. 2 Geometry of weld preparation and welded plates. Dimensions in mm[10]. DETERMINATION OF THE PHASE TRANSFORMATION Phase transformation points A C1 and A C3 of the base metal were determined by means of dilatometry. Tubular specimens (external diameter of 4mm internal diameter of 3 mm and 10 mm length) were heated up with a heating rate from 575 K/s with a Bähr Dil 805 A/D dilatometer. 673

PHYSICAL HAZ SIMULATION Physical HAZ simulation (Fig. 3) was used to assess the mechanical properties of the different zones of the HAZ adjacent to the weld. Therefore, specimens of the base material (6 x 10 x 150 mm) were heated by conduction and cooled by air and water to simulate the different time temperature cycles that appear in the HAZ of welds. The constant heated area with the same mechanical properties was extended over approximately 30 mm in the centre of the sample. The temperature of the sample was controlled with a NiCr-Ni thermal element, which was attached on the surface. Table Table 6 lists the parameters (peak temperature, cooling time t 8/5 ) of the physically simulated time temperature cycles. In case of peak temperatures below 800 C, cooling rates ( C/s) to room temperature were assumed to be equivalent to that of time temperature cycles with peak temperatures higher than 800 C. Heating rate was 600 K/s for all peak temperatures. Fig. 3 Physical HAZ simulation. Table 6 Peak temperature and cooling time t 8/5 of the physical HAZ simulation Cooling time t 8/5 [s] Heat affected zone Peak temperature [ C] 5 10 15 20 25 Coarse grained HAZ 1300 x x x x x Coarse grained HAZ 1250 x x Coarse grained HAZ 1150 x x x x x Fine grained HAZ 1050 x x Fine grained HAZ 950 x x x x x Fine grained HAZ 850 x x Fine grained HAZ 800 x x Intercritically HAZ 750 x x Intercritically HAZ 700 x x Subcritically HAZ 600 x x Subcritically HAZ 400 x x 674

CHARACTERISATION OF THE WELDS AND THE SAMPLES OF THE PHYSICAL HAZ SIMULATION The polished cross sections of the welds were etched with Nital and investigated by means of light optical microscopy (LOM). The hardness distribution over the weld was determined according the Vickers measurement method [15] by means of a hardness mapping. In case of the samples from physical HAZ simulation average hardness value was measured at cross sections from the center. The mechanical properties and as well the flow curves of the welded joints transverse to the weld line, of the fused and diluted weld metal and of the samples from the physical HAZ simulation were assessed by tensile tests. Flat bar specimens, transverse to weld direction were machined from the welded joints. The specimens were grounded on the root side and milled from the top side to a sheet thickness of 5 mm. To investigate the influence of specimen width, the testing and head width was varied according to Table 7. The dimensions and measurement length are pictured in Fig. 4. Special flat bar specimen, which was developed for weld metal testing, were machined from the weld metal along the weld line, see Fig. 5. Another special flat bar specimen type was used for the samples of the physical HAZ simulation due to their limited dimensions. The measurement length in this case was reduced to 15 mm and the width within the measurement length was reduced to 6 mm. Fig. 4 Flat bar specimen transverse to the weld. Table 7 Flat bar specimen widths Specimen Width in the gauge length b [mm] Head width B [mm] A 6 20 B 12 20 C 30 40 D 50 70 E 100 120 675

Fig. 5 Special flat bar specimen for the fused weld metal. Fig. 6 Special flat bar specimen for testing the physical HAZ simulated base metal. FEM-MODEL AND NUMERICAL SIMULATION Modelling of the welded joints A numerical analysis was performed to investigate the effect of geometrical and mechanical parameters on the tensile strength of the welded joints under tension transverse to the weld line. ABAQUS 6.10 was used for the three- dimensional large- deformation finiteelement (FEM) calculation. The joints were modelled by patching distinctive zones with different assigned mechanical properties (flow curves) of the HAZ and the weld, see Fig. 7 and Table 8. The geometrical extension of each zone and the assignment of the mechanical properties were done by means of LOM and hardness mapping. The subcritical HAZ was only implemented in the FEM model for t 8/5 times of 5s and 25s, due to lack of data. Therefore at cooling times between 10 and 20 s material data of the base metal was assigned to this area. The mechanical properties from the fused and diluted weld metal were implemented in the FEM model too. The tensile force in the simulation is applied perpendicular to the weld. Symmetry of the weld was assumed to minimize the calculation time. 676

Table 8 Different zones of HAZ defined by peak temperature and cooling time Cooling time t 8/5 [s] Simulation zone Peak temperature [ C] 5 10 15 20 25 Coarse grained HAZ 1250 x x x x x Fine grained HAZ 950 x x x x x Subcritically HAZ 750 x x Base material 20 x x x x x Abbr. CGHAZ FGHAZ SCHAZ BM Simulation zone Weld metal Coarse grained HAZ Fine grained HAZ Subcritically HAZ Base metal Fig. 7 Symmetrical FEM model of the selected welding zones. Simplified model for statistical investigations Further investigations on a simplified model were done to quantify systematically the impact of different determining geometry and materials parameters on static strength of weld joints including a soft zone, see Fig. 8. Softening ratio: H S H BM Matching ratio: H S H BM Relative thickness: w t X Relative thickness: X Fig. 8 Geometrically simplified FEM model for the systematic investigation of the influencing variables. w t 677

Table 9 lists the ranges of the investigated parameters. The mechanical properties of the soft zone with different softening ratios correspond to thermal weld cycles with a peak temperature of 950 C and cooling times t 8/5 from 5s and 25s and were determined by means of the physical HAZ simulation. The investigated range of relative thickness covers values observed in real gas metal arc welds (0.3 to 0.6) as well extended values up to 7. Flow curves for the different matching ratio S values were determined by multiplying the measured weld metal flow curve out of the welding experiment with t8/5 of 10 s. from the physical HAZ simulation (evenmatched, matching ratio value 1) with a factor. The varied relative thickness is considering the effect that the weld metal width is also influenced by the heat input. The values were taken from experimental welds with low, middle and high energy input (cooling time t 8/5 5s, 15s and 25s). The influence of the parameters on the relative tensile strength was quantified by means of a multiple linear regression analysis. Table 9 Simulation parameter for the statistical investigation RESULTS AND DISCUSSION HARDNESS AND METALLOGRAPHIC OF THE WELDS Fig. 9 and Fig. 10 display the hardness mapping of welds with different cooling times t 8/5. Increased heat input increases the cooling time t 8/5 and raises the extension of the heat af- 678

fected zone. The weld metal and the subcritically HAZ show slightly more increased hardness values than the base metal, whereas softening occurs in the intercritical, fine and the coarse grained HAZ. Fig. 9 Hardness mapping, measured cooling time t 8/5 = 7.3 s. Fig. 10 Hardness mapping, measured cooling time t 8/5 = 16 s. The hardness of the weld metal and of the softened HAZ slightly decreases as a function of the cooling time t 8/5 (Fig. 11). The used filler metal was usually designated for base materials with a yield strength higher than 890 MPa. Therefore significant overmatching was expected. Nevertheless, due to the dilution of the filler metal with the base metal only slightly overmatched and evenmatched welds were generated. Fig. 11 Weld metal and softened HAZ hardness as a function of the measured cooling time t 8/5. 679

Fig. 12 shows the relation between the measured cooling time t 8/5 and the relative thickness of the soft zone. The extension of the softened HAZ raises with increased cooling time t 8/5. The increase of the standard deviations can be attributed to the additional change of geometrical shape of the soft zone with increased heat input. The width was averaged from the bottom and top hardness tracks on both sides of the weld. Fig. 12 Relative thickness of the soft zone as a function of the measured cooling time t 8/5. DILATOMETRY AND PHYSICAL HAZ SIMULATION The transformation temperatures are quoted in Table 10. Table 10 Determination of the phase transformation Phase transformation point Temperature [ C] A C1 810 A C3 920 Fig. 13 displays the hardness of the physical simulated HAZ samples as a function of the peak temperature and the cooling time t 8/5. Between 650 C and the beginning of the phase transformation a slight hardening can be observed, which could be assigned to precipitation hardening. 680

Fig. 13 Hardness as a function of the peak temperature and the cooling time t 8/5. If the peak temperature exceeds the A c1 -temperature further softening is observed. The hardness values show no significant sensitivity on the cooling time t 8/5 in the temperature range up to 1000 C (intercritical and fine grained HAZ). At peak temperatures between 1000 and 1350 C (coarse grain HAZ), low cooling times led to a higher hardness again. This effect is influenced by the established austenite grain size and the chemical composition of the steel. The hardenability increases with raising austenite grain size, because of the reduced number of nucleation sites for ferrite and pearlite, with the result that these transformations are retarded [16]. The hardness values of the physical HAZ Simulation (Fig. 13) at different peak temperatures and cooling times t 8/5 correlates well with the measured values of the HAZ hardness of the welds (Fig. 9, Fig. 10 and Fig. 11), exceptional in the coarse grained HAZ. The hardness increase at low cooling times t 8/5 in the coarse grained HAZ can only observed at samples from the physical HAZ simulation. This could be attributed to the prevented grain coarsening in the HAZ of the welds due to the adjacent microstructure. The smaller grain size may be the reason for the reduced hardenability. Fig. 14 displays exemplary flow curves of the different parts of a weld with a cooling time t 8/5 =5 s. The flow curves from the base metal and the subcritical HAZ were modified through smoothing the peak of the upper and lower yield strength to enable implicit calculation method in ABAQUS. It is assumed that this modification hasn t an influence on the determination of the ultimate tensile strength by FEM simulation. 681

Fig. 14 Flow curves of the different zones including HAZ of the weld with 5s cooling time t 8/5. COMPARISON BETWEEN WELDING EXPERIMENTS AND FEM SIMULATION Fig. 15 pictures the relation of the relative thickness of the soft zone and the ratio between sheet thickness to sample width on the static strength of welded joints. The numerical simulated mechanical behaviour of the modelled joints was in good accordance with experimentally measured results. A small soft zone width has a beneficial effect on the ultimate tensile strength of the welded joint. The ultimate tensile strength rises as a function of a decreased sheet thickness to sample width ratio as a result of the appeared plane strain state at increased sheet widths. 682

Fig. 15 Ultimate tensile strength as a function of the relative thickness X and the ratio of sheet thickness to sample width. Fig. 16 shows a sectional view of the three-dimensional diagram in Fig. 15 at three different t 8/5 cooling times respectively relative thicknesses of the soft zone. These results also agree with the findings from Satoh and Toyoda [7] in concern of beneficial influence of increase sample width to the tensile strength of welds with a soft interlayer. Fig. 16 Comparison of the ultimate tensile strength between the experimental welds and the simulation as a function of plate thickness to testing width ratio. 683

The curve progression of the experimental data show drops at low sheet thickness to sample width ratios at short cooling times t 8/5 (5 and 10s) in comparison to the simulated values (Fig. 16). This effect is caused by a change of the tensile testing machine due to the raised required forces in case of increased sample widths. Fracture and necking at GMAW welds took place in the softened HAZ independently of the heat input and the sample width. Fig. 17 Comparison of the FEM model with and without a subcritical HAZ. To verify the influence of the observed hardness peak in the subcritical HAZ on the constraint effect and the ultimate tensile strength of the joints, a comparative simulation was done. The calculation with and without a subcritical HAZ with a cooling time t 8/5 =25s showed no significant differences in the tensile strength (Fig. 17). The good agreement between the simulated and experimental data can be attributed to the assessment of the material properties, especially the hardening behaviour, of the different HAZ regions by means of the physical HAZ simulation. This method overcomes iterative and inverse calculations to determine the mechanical properties of the HAZ as presented elsewhere [9] in the past. SYSTEMATICAL FE ANALYSIS AND STATISTICAL INVESTIGATIONS FE analysis in combination with a statistical evaluation of the results was used to quantify the impact of different geometrical and material parameters on the macroscopic quasi static behaviour. The systematical FE analysis should afford a quantification of the impact of the different influencing factors (Fig. 18 to Fig. 22) and their interactions on the quasi static strength of welded joints. 684

The influence of the softening ratio on relative tensile strength is pictured in Fig. 18. The relative ultimative tensile strength increases linearly as a function of increasing strength level of the HAZ. Fig. 18 Influence of the softening ratio on the relative tensile strength, X = 0.4, S = 1.0, X = 0.66, A = 90. Fig. 19 displays the influence of the relative soft zone thickness and different softening ratios on the relative tensile strength. A significant drop in the relative tensile strength arises as result of an increased soft zone width. The degree of the decrease is more pronounced at a lower strength level of the soft zone. Complete loss of the constraint effect can be adopted at values greater than 7 of the relative thickness. The strength decreases even at very small relative thicknesses of the soft zone (X <0.25) which disagrees with the experimentally assessed criteria of Wallner et al. [6] and the experimental results from Hochhauser et al. [10]. This difference can be attributed to exact evenmatching of the weld metal strength and the assumed extensive strength decrease of the soft zone even at low relative thicknesses of the soft zone in the present FE analysis. Furthermore scatter is inherent to experimental data. 685

Fig. 19 Influence of the relative width on the relative tensile strength, S = 1.0, X = 0.66, A = 90. The use of filler metal with increased strength levels results in an increase of the matching ratio and has a beneficial effect on the relative tensile strength (Fig. 20). Overmatching of the welds may eliminate the drop of strength as consequence of HAZ softening. Fig. 20 Influence of the matching ratio on the relative tensile strength, X = 0.66, A = 90. The relative weld metal width hasn t an influence on the relative tensile strength in the investigated range (X =0.66 till 1.66) (Fig. 21). This statement correlates well with the finding in the publication from Mochizuki et al., who affirms that tensile strength hardly 686

change at this values. At lower values of X the joint strength becomes significant larger and saturates around X >0.5. [8] Fig. 21 Influence of the relative width on the relative tensile strength, A = 90. Fig. 22 shows the influence of bevel angel A on the static relative tensile strength. There is hardly any influence observed. In case of a significant softening and enlarged relative soft zone width a slightly decline of the relative tensile strength with increasing angel of bevel is observed. Fig. 22 Influence of the angel of bevel on the relative tensile strength, S =1.0, X =0.66. 687

By means of different models of multiple regressions analysis, empirical formulas were achieved to determine the relative tensile strength of welded joints in consideration of the described parameters. Linear: rel. UTS 0.4854 0.3697 S 0.037 X 0.1737 S 0.0037 X 0. 0001 A Linear + quadratic: rel. UTS 0.8018 2.7101 S 0.2614 S 2 0.0027 X 1.3606 S 0.0027 X 2 2 0.0649 X 0.0112 X 0.0006 A 0.000003 A 2 2 0.7485 S Linear + Quadratic with interactions: rel. UTS 0.2674 0.3668 X 0.0137 X 1.0835 S 0.3791 S 0.366 S X 0.0014 S A 0.005 X X 0.0003 X A 0.0018 S X 0,0011 S A At the linear quadratic regression analysis with interactions, only factors and there combinations with a significant influence were considered. The coefficient of determination of the different regression models are listed in Table 11. Fig. 23 pictures the forecast values and the residual values of the relative tensile strength. Table 11 Regression models with the coefficient of determination Regression model Coefficient of determination R² Linear 0.8602 Linear + Quadratic 0.8888 Linear + Quadratic with interactions 0.9782 2 2 Fig. 23 Forecast and residual values of the relative ultimate tensile strength. 688

Pareto analysis (quantification of the influence factors) based on the multiple regression, confirmed the findings from the systematically study. The ultimate tensile strength is most significantly influenced by the relative thickness of the soft zone X, followed by the softening ratio and the matching ratio. The influence of the angle of bevel as well the relative thickness of the weld metal can be neglected. CONCLUSION & OUTLOOK In this study a FEM model of gas metal arc weld was accomplished and implemented with the data of the physical HAZ simulation. The determined ultimate tensile strength from the FEM simulation and the tensile strength of the welds correlate well. The relative thickness of the soft zone X has a distinctive greater influence on the static strength than the ratio plate thickness to plate width. An increasing sample width to thickness ratio at constant X value causes a rise of the tensile strength. The strength of the investigated welds with different cooling times t 8/5 (from 5 to 25s) doesn t drop below the specified tensile strength of the base metal. The performed systematical FE analysis revealed that a separated consideration is necessary to determine the influence of each parameter and to eliminate interaction among each other. The soft zone width displays the most significant drop of the static tensile strength. The softening ratio has also an influence on the static tensile strength of welded joints, especially in combination with the width of the soft zone. An increased matching ratio has also a beneficial effect on the joint strength and may inhibit the loss of strength through the HAZ softening. The FE simulation showed that the influence of the width of the weld metal and the angel of bevel on the ultimate tensile strength of welded joints can be neglected. Further FE analysis will be carried out to investigate the impact of the different geometrical and material parameters on the yield strength and the uniform elongation of welded joints. Another point of view will be the evaluation of the different influencing factor and there interactions on the tensile strength by means of a Pareto diagram. ACKNOWLEDGMENT The K-Project Network of Excellence for Joining Technolgoies JOIN4+ is fostered in the frame of COMET - Competence Centers for Excellent Technologies by BMVIT, BMWFJ, FFG, Land Oberösterreich, Land Steiermak, SFG and ZIT. The programme COMET is handled by FFG. 689

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