Welding simulation of a test cover for hydropower plant was made due to very large dimensions of the cover. The main aim was to predict distortion after welding in order to avoid machining the cover. Welding process was simulated with the Sysweld program to keep distortion in desired limits. Various welding sequences and clamping conditions were calculated to reduce the distortion. Calculation of microstructure constituents in virtual complex geometry of joints was also analyzed.Simulacija zavarivanja testnog pokrova hidroelektrane provedena je zbog velikih dimenzija ispitne prevlake. Osnovni je cilj predvidjeti izobličenje nakon zavarivanja. Radi postizanja veličine izobličenja u željenim granicama proces zavarivanja je simuliran programom Sysweld. Različiti tijekovi zavarivanja i uvjeta spajanja prorač unati su radi smanjenja izobličenja. Određivanje mikrostrukturnih konstituenata u virtualnoj komleksnoj geometriji spojeva je također provedeno

J. Medved

P. Mrvar

S. Kastelic

English

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S. KASTELIC, J. MEDVED, P. MRVARPREDICTION OF NUMERICAL DISTORTION AFTER WELDINGWITH VARIOUS WELDING SEQUENCES AND CLAMPINGSReceived – Prispjelo: 2009-11-23Accepted – Prihva}eno: 2010-01-15Original Scientific Paper – Izvorni znanstveni radINTRODUCTIONWelding is an important process and has significantrole in industry, especially in automotive, marine andenergy industries.Knowledge of all the properties and welding param-eters enables prediction of final distortion. Accurate pre-diction of distortion is important when distortion ofsome unique, big parts has to be predicted. In order tokeep deformation in desirable limits, changes of weld-ing parameters, such as welding sequence and clampingof the welded parts, may be needed. If change of weld-ing parameters for a complex part, with big number ofbeads or multipass welding is made, then it is not easy topredict distortion after the welding 1, 2.In order to obtain all the needed data with numericalcalculation, all the main physical effects that accrue inwelding must be taken in account. All results were cal-culated with the modified heat convection equation (1).It enables to perform non-linear computations with allthe material properties that depend on temperature,phase and material transformations, fractions of chemi-cal elements and other accompanying variables 3-5.Numerical simulation was used to predict final dis-tortion after welding a test cover for hydro-power plant.Test cover is shown in Figure 1. Its diameter is 5,5 m andflange on the cover is 120 mm thick. Making the cover isnot issue of this paper. Cover was welded and then ma-chined in workshop to desirable dimensions. Problemoccurred with transport. These test cover will be used ina reversible hydro-power plant that is located high in theAlps – about 2000 m above the sea level. Transport of5,5 m diameter test cover by road is not possible becausetunnels are not big enough. Transport with helicopter isalso not possible because the cover is too heavy at 2000m above the sea level. So test cover must be cut to twohalves for transport by road and then welded together atthe hydro-power plant place.P CTtP T Li iiiiij( ) 		 				 ( )T A Qiji j (1)P phase proportionT temperaturet timei, j phases mass densityC specific heat thermal conductivityQ heat sourcesLij(T) latent heat of i›j transformationAij proportion of phase i transformed to j in time unitThe goal of this numerical simulation was to predictdistortion after welding and to determine welding se-quences to keep distortion in tolerances. Thus weldingMETALURGIJA 49 (2010) 4, 301-305 301Welding simulation of a test cover for hydropower plant was made due to very large dimensions of the cover.The main aim was to predict distortion after welding in order to avoid machining the cover. Welding processwas simulated with the Sysweld program to keep distortion in desired limits. Various welding sequences andclamping conditions were calculated to reduce the distortion. Calculation of microstructure constituents in vir-tual complex geometry of joints was also analyzed.Key words: welding simulation, finite element method, multipass welding, distortion predictionNumeri~ko predvi|anje izobli~enja nakon zavarivanja s razli~nim slijedom zavarivanja i spajanja.Simulacija zavarivanja testnog pokrova hidroelektrane provedena je zbog velikih dimenzija ispitne prevlake.Osnovni je cilj predvidjeti izobli~enje nakon zavarivanja. Radi postizanja veli~ine izobli~enja u `eljenim granica-ma proces zavarivanja je simuliran programom Sysweld. Razli~iti tijekovi zavarivanja i uvjeta spajanja pro-ra~unati su radi smanjenja izobli~enja. Odre|ivanje mikrostrukturnih konstituenata u virtualnoj komleksnojgeometriji spojeva je tako|er provedeno.Klju~ne rije~i: simulacija zavarivanja, metoda kona~nih elementa, zavarivanje u vi{e slojeva, predvi|anje izo-bli~enjaISSN 0543-5846METABK 49(4) 301-305 (2010)UDC – UDK 669.05:621.791.052:608.4=111S. Kastelic, J. Medved, P. Mrvar, Faculty of Natural sciences and engi-neering, University of Ljubljana, Ljubljana, Sloveniawill be performed without machining after the weldingprocess.PREPARING FEM MESH FORCALCULATIONSFinite elements mesh of test cover with Visual Meshprogram was prepared for this calculation. The finite el-ements mesh for calculation is shown in Figure 2. Onlyhalf of the test cover was meshed because symmetry wastaken into account.Two simulations with different welding sequencesand clamping conditions were made. The sequence forthe first simulation started with welding on the flange.First 17 beads were made from the top of flange andcontinued with second 17 beads from the lower side offlange. Then 26 beads were made on the upper side offlange and at the end the rest of 26 beads on the lowerside of flange. Together there were 86 beads altogetheron flange. After flange was welded on the cover, weld-ing of cover itself started, at first with six beads on thetop and then with six beads on the lower side of cover.Welding sequence on flange is presented in Figure 3. Incalculation with the first sequence, the clamping wasminimal, it was without reinforcement plates, as shownin Figure 2. These plates were taken into account in thesecond calculation.In the second simulation, also welding sequence waschanged. Welding started on the upper side of coverwith 6 beads and continued with 17 beds on the upperside of flange. Afterwards, 6 beads were welded onto thelower side of cover and welding continued with 17beads on the lower side of flange. Then welding contin-ued on the upper side of flange with 26 beads and fin-ished with 26 beads on lower side of flange. Clampingwas also changed. In the second simulation, the clamp-ing was stiffer and reinforcement plates, as shown inFigure 2, were included into the calculation.DEFINING HEAT INPUTDefining the heat input is based on actual weldingparameters that are presented in Table 1. These parame-ters are also important for preparation of mesh. Volumeof deposited material for each bead is determined withelectrode size and welding speed. Energy input is de-fined with welding current, voltage and welding speed.Defined heat input is based on the size of test coverand on welding parameters. The method, applied in thisparticular case, is called “Macro weld deposit methodol-ogy”. Heat is transferred into weld instantaneously inone or several macro steps. Real weld trajectory is di-vided into several macro sections. It was included intothe structure before the beginning of computation andomitted after definition of the macro time steps. En-ergy/length ratio that is transferred into the structure isthe same as in actual process, but it is taking place in an-other time frame.Heat input for each bead on flange was defined inone single macro step. Temperature distribution duringthe cooling after the last bead has been welded ontoflange is presented in Figure 4.302 METALURGIJA 49 (2010) 4, 301-305S. KASTELIC et al.: PREDICTION OF NUMERICAL DISTORTION AFTER WELDING WITH VARIOUS WELDING...Figure 1 CAD model of test coverFigure 2 FEM mesh of the test coverFigure 3 Welding sequence onflangeTable 1 Welding parametersFlange CoverElectrode EVB 50 EVB 50Current type DC / + DC / +Electrode size 3,25 / 4 / 5 3,25 / 4Current110-130 / 140-160 /180-200 A110-130 / 140-150AVoltage 24-26 / 25-27 / 26-28 V 24-26 / 25-27 VWelding speed15-20 / 20-25 / 25-30cm/min12-15 / 15-18cm/minHeat input on the cover was defined for each beadwith ten macro steps. It meant that weld on the coverwas divided into ten segments.Heat was then defined for each segment separatelywith time delay between single segments. Time delaybetween segments depended on the length of segmentand on the welding speed. Temperature distributionduring welding the first bead onto the cover is shown inFigure 5.DEFININING MECHANICAL PROPERTIESBase material of the cover is St355 steel with chemi-cal composition presented in Table 2. In order to obtainreliable numerical results, precise thermal and materialproperties of the used material must be taken in account.All these properties must be measured as functions oftemperature and phases. Yield stress, thermal strains,Young’s modulus, Poisson ratio, strain hardening, den-sity, thermal conductivity and latent heat must be knownfor quality welding. Some of these properties are pre-sented in graphs in Figures 6a to 6f.Digitalized CCT diagram is needed for calculation ofmicrostructural constituents. Diagram is presented inFigure 7.RESULTSWith all these data several results for deformation af-ter welding can be obtained. In our case, deformation af-ter welding was the main goal. Next to deformation,METALURGIJA 49 (2010) 4, 301-305 303S. KASTELIC et al.: PREDICTION OF NUMERICAL DISTORTION AFTER WELDING WITH VARIOUS WELDING...Figure 4 Temperature distribution after welding the lastbead on flangeFigure 5 Temperature distribution during the first bead oncoverTable 2 Chemical composition of St355 steelElement C Si Mn P S Al N Cr Cu NiComposition in wt% 0,18 0,47 1,24 0,029 0,029 0,024 0,0085 0,10 0,17 0,06Figure 6a Density Figure 6b Thermal conductivity Figure 6c Young’s modulusFigure 6d Latent heat Figure 6e Yield stress Figure 6f Strain hardeningvery important parameters are also stresses andmicrostructure in the welding area after the welding.Deformation of test cover after the welding with thesecond sequence in the flange area was less than 2 mm.The deformed shape of the cover is presented in Figure8. In this picture wireframe of cover before welding andalso the deformed shape afterwards are presented. Thedeformed shape was multiplied by 20 so that deformedshape is more pronounced.Effect of various welding sequences and clampingconditions is presented in Figures 9a and 9b. Maximumdeformation after welding with the first sequence was4,3 mm. Deformation of test cover on the X-axis afterwelding is shown in Figure 9a.Welding with the second sequence resulted insmaller deformation. The deformation on the X-axis,shown in Figure 9b, is smaller and it is less than 2 mm.The deformed shape in Figures 8 and 9 is multiplied by20.Next to deformation also distribution of stresses inheat affected zone was calculated. It became obviousthat there was compressive stress in the middle of flangewhere first beads were weld onto flange, Figures 10aand 10b. The highest tensile stress was found under thesurface of flange and in the area where last beads werewelded.The results that are presented in Figures 11a, 11b,12a, and 12b show the microstructure of the heat af-fected zone after welding. Figures 11a and 11b presentdistribution of bainite in the welding area. The amountof bainite in the beads was quite high (90 vol. %) sincepreheating of area was 150 °C. Increased amount ofmartensitic phase, being between 10 to 20 vol. %, wasfound on the interface between base material and304 METALURGIJA 49 (2010) 4, 301-305S. KASTELIC et al.: PREDICTION OF NUMERICAL DISTORTION AFTER WELDING WITH VARIOUS WELDING...Figure 7 CCT diagram of St355 steelFigure 8 Deformation after welding with the first sequen-ceFigure 9 Deformation in the X-axis after welding, a) firstsequence, b) second sequenceFigure 10 Stresses after welding with first sequence (a), and second sequence (b)Figure 11 Bainite distribution after welding with first sequence (a), and second sequence (b)Figure 12 Martensite distribution after welding with first sequence (a), and second sequence (b)welded-on beads. This was result of higher cooling ratesat the interface with the base material. Distribution ofmartensite is presented in Figures 12a and 12b.CONCLUSIONIn order to predict distortion after welding the cover,two simulations were made with various welding se-quences and clamping conditions. Deformation with thesecond simulation was smaller. In order to reduce defor-mation further, another calculation with changed weld-ing sequence could be made, but obtained results weresatisfactory for now. Peak values of stresses were rela-tively high for this material, but these peaks referred tovery small areas. Also these values should be moderatedwith some simple trial welding under similar conditions.Trial welding should be simulated too that a comparisoncould be made whether these stresses would cause somecracks or not. Absolute values of stresses in the secondcase were slightly higher since clamping in the secondcase was more rigid, because the reinforcement plateswere taken into consideration.Amount of martensitic phase on the interface couldbe reduced with higher preheating temperature, buthigher temperature could be hardly reached becauseheat capacity of cover was big. Also the possibility ofcracks was relatively small because the areas withhigher fraction of martensite did not appear on the samespots as the areas with high stresses.REFERENCES1 D. Deng, H. Murakawa, W. Liang: Comput. Methods Appl.Mech. Engrg., 196 (2007), 4613–46272 T. Schenk, I. M. Richardson, M. Kraska, S. Ohnimus: Com-putational Materials Science, 45 (2009), 999–10053 ESI Group: Sysweld reference manual, digital versionSYSWELD 2008.14 F. Boitout, D. Dry, P. Mourgue, H. Porzner, Y. Gooroo-churn: Transient Simulation of Welding Processes - Ther-mal, Metallurgical and Structural Model, Sysweld v20045 F. Boitout, D. Dry, P. Mourgue, H. Porzner, Y. Gooroo-churn: Distortion control for large maritime and automotivestructures, Sysweld v2004Note: The responsible person for English language is prof. dr. A. Paulin.METALURGIJA 49 (2010) 4, 301-305 305S. KASTELIC et al.: PREDICTION OF NUMERICAL DISTORTION AFTER WELDING WITH VARIOUS WELDING...

Numeričko predviđanje izobličenja nakon zavarivanja s različnim slijedom zavarivanja i spajanja

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