1. Numerical simulation of dissimilar metal welding
Z. Bézi1, Sz. Szávai1, C. Ohms 2
1 Engineering Division (BAY-ENG)
Department of Structural Integrity and Production Technologies
2 European Commission Joint Research Centre, Institute for Energy and Transport
SimDay 2015 Budapest 8 October, 2015

2. Introduction
Goal: replicate the conditions of a heterogeneous weld in a type of WWER 400 RPV nozzle and determine welding residual stress (WRS) distributions
Why to predict residual stress?
Small scale mock-up manufacturing
Finite Element Analysis – Challenges: Spatial modelling, modelling of welding heat source, material behaviour and properties, post weld treatments
Measurement method: neutron diffraction
Validation of WRS in 15H2MFA steel

3. Residual stresses
Residual stress has a significant role in many failure mechanisms: stress corrosion cracking, fatigue, and fracture…
Tensile residual stresses decrease the reliability and useful life of a component
Compressive residual stresses at the surface increase the performance
Residual stresses satisfy the equilibrium and in all components they are both tensile and compressive.
Example: compressive surface stress, imply tensile stress below surface zone, which may lead to sub-surface crack initiation
Need to understand the residual stress variations in test specimens and component

4. Why to predict weld residual stresses?
Existing components:
Residual stress is important to accurately determine intervals and detection targets for recurrent inspection (SCC, fatigue and fracture)
Assess known defects in detail to assure the margins for safe operation
Design of residual stress for new components and modification of existing welds:
Increase the useful life of a component
Post weld treatments for existing welds by mechanical or thermal methods
Optimization of welding and treatments for new components

5. Mock-up manufacturing
Getting of the Bulk materials
15H2MFA (cutting from original WWER 440 reactor sample, from Paks Nuclear Power Plant, 800x300x40)
X6CrNiTi18-10 (1.4541) (similar to 08H18N10T, 1000x300x40)
Making of the WPS
Cutting, welding and heat treatment technology
10CrMo9-10
(15H2MFA)
X6CrNiTi18-10 (1.4541)
(08H18N10T)

6. Mock-up manufacturing
Purchase of the original Russian Welding electrodes
(it was the most difficult task)
Welding of the Mock-up
Welding of the buttering layer on a preheated specimen (3±1mm)
Welding of the cushion layer with max. interpass temperature of 100°C
Heat treatment of the specimen on 670°C for 16 hours
Welding of the root weld from the root side
Welding of the filling and capping weld

7. Finite Element Analysis – Challenges
Residual stresses are such stresses which remain in a component after removal of all external loads, and are caused by
Nonhomogeneous plastic deformation due to local rapid heating/cooling and due to boundary conditions
Solidification and difference in thermal expansion
Phase transformations
Challenging factors in welding simulation:
3D modelling (bead size, 3D effects, interacting welds)
Modelling of welding heat sources
Material modelling and properties
Suitably accurate heat source model for the welding method
The processes in the arc and melted pool are not modelled
The liquid weld pool is modelled by an equivalent heat conduction model representing the welding method of interest

8. Type of Current and Polarity Type of Current and Polarity
Finite Element Analysis – Challenges
Basic information is required for the modelling:
Welding method
Welding voltage, current and travel speed
Pre-heating and interpass temperature
Number of weld beads – weld protocol/micrograph
Details specific to welding method
Convection conditions
Temperature dependent material properties
Welding Process
Current (A)
Voltage (V)
Type of Current and Polarity
Heat Input (kJ/mm)
1st layer
SMAW
24-25
DCEP
~0,61
2nd layer
25-26
~0,69
Welding Process
Current (A)
Voltage (V)
Type of Current and Polarity
Heat Input (kJ/mm)
Root weld
GTAW
50-60
12-13
DCEN
~0,1
Filling weld
SAW
28-29
DCEP
~1,16
Welding parameters of the cladding
Welding parameters

9. Finite Element Analysis
Challenges in materials properties and modelling:
Identify and quantify relevant deformation conditions: strains, cyclic loading
Conduct testing: monotonic and/or cyclic
Formulate constitutive model : type of hardening (isotropic, kinematic, mixed), temperature effects, rate effects
Required temperature dependent parameters, 20°C to 1500 °C (melting-point):
Young’s modulus
Thermal conductivity
Specific heat capacity
Thermal expansion
Measurement:
Chaboche’s parameters
JMatPro calculation:
Flow curves
CCT diagram
Hardness values
Simulation
Modelling
Heat treatment

10. Finite Element Analysis
MSC.Marc and Simufact solver
Work tasks:
Simulate the cladding process (9 beads along the plate)
Simulate the heat treatment after the cladding (670°C at a rate of 50°C/h for 16 hours)
Simulate the butt-weld process (39 passes)
3D model
Elements: 32060
Nodes: 35238
Fine mesh at heat affected zone
Convective heat transfer coefficient: 20W/m2K
FE modell

11. Section of Mock-up on diffractometer
Measuring residual stresses: neutron diffraction
Residual stress diffractometers at beam tubes HB4 and HB5 at the High Flux Reactor of the Joint Research Centre in Petten
Measurements in the welding transverse and plate normal directions have been performed in the ferritic steel section on the diffractometer at HB5.
For the welding longitudinal direction, the specimen was mounted on the diffractometer at HB4.
Section of Mock-up on diffractometer

12. Results: cladding and heat treatment
Residual stress component:
welding transverse direction HT
longitudinal direction
Temperature distribution and size of HAZ
plate normal direction

13. Results: after butt-weld
Residual stress component:
welding transverse direction
Residual stress distribution and fraction of each phase after welding
longitudinal direction
plate normal direction
Residual stresses from FE simulations at 7 mm below the upper surface

14. Distortion after dissimilar metal welding
Results:comparison of experiment and modelling
Distortion after dissimilar metal welding
Good agreement

15. Validation of WRS in 15H2MFA ferritic steel
Line A: mm from the specimen surface
Line B: mm from the specimen surface
Line C: 3.33 mm from the specimen surface
Line A
Line B
Line C
Acceptable agreement

16. Summary
3D model was utilized to predict stress fields after welding, especially the longitudinal residual stresses which are in general most harmful to the integrity of the structure among the stress components, in dissimilar steel butt-welded joints between ferritic and austenitic steels which are in essence have different thermal and mechanical properties
An acceptable agreement has been found between the predicted and the measured data that verifies the validity of the employed model
The simulation results show that reliable predictions can be achieved if detailed and careful modelling is used, especially with respect to heat source modelling and high temperature properties and material hardening behaviour

17. Further development Nozzle DN 500 simulation with phase transformation
Use the result in NDT simulation

18. Acknowledgement
This project has received funding from the European Community’s Seventh Framework Program (FP7/ ) under grant agreement n◦