Welding Simulation and calculation of residual stresses. Dr.-Ing. Tobias Loose

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1 Welding Simulation and calculation of residual stresses Dr.-Ing. Tobias Loose

2 Causality 1. Welding torch 2. Temperature field 3. Elongation - shrinking Mechanical response of component Residual stresses and distortion 2

3 Residual stress - Distortion Residual stress Large distortion Risk of process failure High plastic strain Risks of in service failure Distortion free shrinking soft structure not clamped.. Optimum.. shrinking disabled stiff structure fully clamped 3

4 Why Welding Simulation? After welding, the material does not behave like bevor. Specimem from Rhein Bridge Breisach St 37 from 1962 Spot weld bending test according to Steidl 4

5 Results of Welding Simulation Temperatur evaluation during welding Microstructure Hardness Residual stesses and strains Remaining plasticity - ductility Yield stress in weld and HAZ strainrate during welding Mikroschliff von: 55 % Ms Phase-proportion Ms 0% 50 % The evaluation of these results enables estimations - overheating of workpiece - quality of weld - crack - fatique - ultimate load and - behaviour under service load 5

6 heating Most important: Micostructure α krz Ferrit- Perlit (Base Material) cooling Austenit γ kfz increasing cooling rate α krz trz Ferrit / Perlit Bainit Martensit Strength depends on microstructure 6

7 Thermal strain in % Most important: Micostructure α krz γ kfz phase-transformation strain thermal strain Temperature in C 7

8 Most important: Microstructure CCT and IT - description of phase transformation Phase Transformation Calibration Manager automatic calibration of CCT and IT Diagrams for SYSWELD 8

9 Most important: material properties Yield Re Yield Re in N/mm² Material properties as a function of temperature and microstructure Temperature in C 9

10 Most important: reset of plastic strains without reset cumulated plastic strain When material is molten or in case of phase transformation Austenit - Martensit the plastic strains has to be set back to zero. with reset 10

11 Most important: material properties Material Database Manager Thermal material properties Thermal konduktivity Density Specific heat or Enthalpy Mechanical material properties E-Modul Yield Slope (strain hardening) Thermal strain Poisson Coefficient Description of phase transformation IT and CCT Material Database Manager to manage complex material data easily for Welding Simulation with SYSWELD 11

12 Available material data For many materials data for welding simulation with SYSWELD are available: DP-W-600 TRIP700Z DC04 S _Grade_70 X80TA1050 Nirosta_H400 X20Cr13 X5CrNi1810 CF35 316L 16MnCr5 100Cr6 INCONEL718 INCONEL Alloy 82 MONEL400 Ecodal_608 AlMgSi-Wire-AlMgSi AlMgMn-Wire-AlMgSi AlMgMn-Wire-AlMgMn 12

13 Setup of Welding Simulation Method of Finite Elements FEM Description of geometry of component - CAD Divide in Finte Elements Meshing Welding Definition of heat source Material Material properties Boundary Conditions Heat transfer Clamps Loads 13

14 Heat Source The evaluation of heat input is not simulated. The equivalent heat source is the thermal load input for welding simualtion and has to cover all physical effects around the weld pool. Calibration of heat source according to microsection Automatic calibration of heat source according to estimated weld pool and imposed energy per unit length of weld with SYSWELD 14

15 Validation Investigations in the evolution of residual stresses New 3D-calculations of residual stressis consistent with measured results of the IIW round robin programme Dr.-Ing. Tobias Loose Dipl.-Ing. Jens Sakkiettibutra Prof. Dr.-Ing. Helmut Wohlfahrt Specimem made of steel 316L 15

16 Bauschinger effect The plastic material behaviour of austenitic steels at room temperature can be described by a combination of isotropic and kinematic hardening in equal percentages [5]. The influence of the Bauschinger effect decreases for plastic deformations of austenitic steels under elevated temperatures (e.g. 480 C) [6]. 5] [6] T. Manninen et al.: Large-strain Bauschinger effect in austenitic stainless steel sheet, Materials Science and Engineering A 499 (2009) pp M.C. Mataya and M.J. Carr: The Bauschinger Effect in a Nitrogen-strengthened Austenitic Stainless Steel, Materials Science and Engineering 57(1983) pp

17 Comparison of measured and with isotropic hardening calculated residual stresses Longitudinal residual stresses measured stresses at 128 mm distance to the end of the weld seam 500 measured stresses at mm distance to the end of the weld seam 300 measured stresses at 106 mm distance to the end of the weld seam 200 with isotropic hardening calculated stresses at 90 mm distance to the end of the weld seam Transversal residual stresses with isotropic hardening calculated stresses at 90 mm distance to the end of the weld seam measured stresses at 128 mm distance to the end of the weld seam 100 measured stresses at mm distance to the end of the weld seam distance to weld center [mm] The magnitudes of the measured and with isotropic hardening calculated peaks fit together, as well residual stresses [MPa] residual stresses [MPa] 400 The graphs of the with isotropic hardening calculated stresses show the typical stress peaks in the HAZ as well as the calculated. measured stresses at 106 mm distance to the end of the weld seam distance to weld center [mm] 17

18 Stress distribution on the surface (isotr. hardening) Welding direction Welding direction Stress development The stress development is dependent on the geometry. The von Mises stresses will be investigated to illustrate to different partly opposed influences 18

19 Stress development The magnitudes of the measured and with isotropic hardening calculated peaks fit together, as well. Temperature (2. layer) 1500 before welding (3000 s) 1250 temperature [ C] max. Temperature (3269 s) 1000 at the beginning of the cooling phase (3301 s) Yield strength (2. layer) , distance to weld center [mm] yield strength [MPa] The magnitudes of the measured and with isotropic hardening calculated peaks fit together, as well. 250, , s (before welding) 150, s (max. Temperature) 100, s (at the beginning of the cooling phase) s (after cooling) 50,000 0, distance to weld center [mm] 19

20 Stress development Work hardening during heating occurs as a consequence of plastic deformation where the highest stresses and the lowest yield strength values are, that is in the HAZ. Von Mises stresses (2. layer) 500 before welding (3000 s) max. Temperature (3269) The von Mises stresses are limited by the temperature and hardening dependent yield strength. Reach a maximum in the work hardened HAZ stresses [MPa] 400 at the beginning of the cooling phase (3301 s) after cooling (15000 s) distance to weld center [mm] 20

21 Stress development Longitudinal stresses occur - in the HAZ due to its extension during heating and its shrinkage during cooling. They can reach magnitudes higher than the original yield strength due to work hardening in the HAZ, - in the weld due to the hindered shrinkage of the pool. Transversal stresses occur - due the same reasons as longitudinal stresses, - but have lower magnitudes than the longitudinal stresses due to a lower restraint in the transverse direction. 21

22 Validation of calculated residual stresses (low alloyed steel: S355) S355 E = 5,83 kj/cm v = 1,66 mm/s Measured distortion: w = 0,34 mm Calculated distortion: w = 0,32 mm Test: Dr. Nitschke-Pagel, Simulation: Dr. Loose 22

23 Bead on plate Dr. Nitschke-Pagel (1985) Temperaturfield Microsection Simulation S355 E = 5,8 kj/cm v = 1,66 mm/s no pre heating t = 9,2 mm 1 Weld 23

24 Bead on plate Dr. Nitschke-Pagel (1985) 24

25 Bead on plate Dr. Nitschke-Pagel (1985) S355 E = 5,8 kj/cm v = 1,66 mm/s no pre heating t = 9,2 mm 1 Weld 25

26 Bead on plate Dr. Nitschke-Pagel (1985) Pre heating 300 C E = 5,8 kj/cm v = 1,66 mm/s pre heating Spannung in N/mm² S C t = 9,2 mm 1 Weld 26

27 Microstructure after welding S235 FerritPerlit S355 FerritPerlit Bainit Martensit Bainit Martensit 27

28 Yield stress after welding S235 S355 Abhängig von Gefüge und von der Verfestigung 28

29 T-Joint Sakkiettibutra (2007) Temperature field simulation and reality Weld pool HAZ Simulation with consideration of Tack welds Filler material Contact 29

30 Longitudinal stress - evolution

31 Transversal stress - evolution

32 For Welding simulation I use SYSWELD Solver because: all important physical effects are taken into account calculation of microstructure and hardness is possible validable and good results easy material data managment available data for mainly important Materials no limitation on geometry no limitation on weld process DMP-solver alvailable to manage large models easy setup of project files