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Svetssimulering – utmaningar och möjligheter
Jens Gunnars, Inspecta Technology AB
Fogningsdagarna 2015, Svetskommissionen, Sundsvall 16-17 april 2015
1
2015-04-16
Welding simulation – challenges and possibilities
Jens Gunnars, Inspecta Technology AB
Fogningsdagarna 2015, Svetskommissionen, Sundsvall 16-17 april 2015
2
2015-04-16
Outline
3
•
Effects from Welding and Residual Stresses
•
Residual Stress Modeling
•
Measurement Methods
•
Validations
•
Weld Residual Stress Design
•
Examples
•
Conclusions
2015-04-16
Effects from welding and residual stresses
• Welding induces deformation and residual stresses
• Deformation and warping may influence function and
appearance of components
• Residual stress has a significant role for component
life time, cracking and ageing
o Stress corrosion cracking, fatigue, fracture, creep,
impact damage, …, …
o Tensile residual stresses decrease the reliability and
useful life of a component
o Compressive residual stresses increase the reliability
What is residual stress?
• Residual stress are caused by non-uniform plastic deformation
due mechanical loading or local rapid heating/cooling, phase
changes, solidification and difference in expansion, …
• Most manufacturing processes generate residual stresses at
different scales:
−
−
−
−
−
Casting, rolling, bending, forging
Turning, milling, grinding
Welding, soldering
Peening, burnishing
Surface hardening, nitriding, …, …
• Residual stress don’t leave any outward sign. Residual stress
remain after removal of all external loads.
• They are self balanced within a component, and in all
components they are both tensile and compressive
• They have large influence on reliability => we need to
understand residual stress distributions in components
Why predict weld residual stresses
by simulation?
• Not possible to measure non-destructively residual
stresses in typical components
• Assessment of components in operation
− Residual stress is important to accurately determine intervals and
detection targets for inspections
− Assess found defects in detail to assure
safe operation
• Possibility to design for beneficial residual
stresses in new components
− Improve the tolerance to surface damage
− Increase the useful life of a component
− Design and optimization of welding and treatments in order to
reduce or create compressive stress
300
250
200
as-welded
150
100
after pressure test
50
In operation
0
Pf=0
Pf=0.84
Pf=1.23
-50
0
0.2
0.4
0.6
u/t
0.8
1
Residual Stress Modelling
2015-04-16
Weld Residual Stress Finite Element Analysis
Residual stress are caused by
− Nonhomogeneous plastic deformation due to local
rapid heating/cooling, and due to applied mechanical
loading/BCs, and
− Solidification and difference in thermal expansion
− Phase transformations
− …
Factors in welding simulation:
• Spatial geometry and modeling
(weld geometry, bead size, interacting welds, 3D effects, …)
• Modeling of welding heat sources
• Material modeling and properties
• Post weld treatments designed to reduce or
modify the stress field
2015-04-16
Modeling of welding heat source
• It is important to have suitably accurate
heat source model for the welding method
− Residual stresses occur as a consequence of material
heating and cooling during welding
− We do not model the processes in the arc (plasma) and
melted pool (flow in liquid).
− The liquid weld pool is modeled by an equivalent heat
conduction model representing the welding method
analyzed
− A heat source calibration procedure is required when
using a 3D analytical Rosenthal type travelling heat source
in a 2D FE model
• Basic information required for the modelling:
− WPS:
• Welding method, details specific to welding method
• Welding voltage, current and travel speed
• Pre-heating and interpass temperature
• Number of weld beads – weld protocol/micrograph
− Convection conditions
− Temperature dependent thermal properties
[ IR picture of weld pool, Visual analysis
of welding processes, Ogawa 2012 ]
Material modeling is challenging in welding simulation
Identify and quantify relevant deformation conditions:
§ Large strains, >5%
§ Cyclic loading, 1 - 10 major plastic cycles
§ All physical and mechanical properties as a function of
the full temperature range, 20 C to 1500 C
Conduct testing:
§
§
§
§
§
Need material data for parent and weld materials
Monotonic and effect of cyclic loading
Relevant strain ranges and temperatures
Biaxal compression and tension
Specimen heat treatment (annealed conditions)
Formulate constitutive model
§
§
§
§
§
Type of hardening (isotropic, kinematic, mixed, …)
Temperature effects
Rate effects,
Annealing, recovery and saturation
Phase changes
Measurement of Residual Stress
2015-04-16
Methods for measuring residual stresses
Destructive methods – relaxation techniques:
• Hole drilling and measurement by strain gauge
• Incremental Deep Hole Drilling (IDHD) technique
• Contour method
• Slitting, Layer removal
o Uncertainty in all methods is 20% in challenging cases and
assumptions for evaluation
Non-destructive methods – diffraction techniques:
• Neutron diffraction
• X-ray diffraction (standard, synchrotron high energy)
• Research: Ultrasonic methods, magnetic methods
Slitting;
Circumferential mean
stress estimate
 1
1 

σ = Et 
−
D 1 
 D0
Validation
– International Blind Round Robin lead by the US nuclear authority for
validation of weld residual stress simulation
2015-04-16
International Weld Residual Stress Validation Project - lead by US NRC
Project aims and problem description:
Project aims:
‒
Determine the relevance of weld residual
stress FE modelling, for nickel base
Alloy 82/182 welds between ferritic
nozzle and stainless steel pipe
‒
a)
b)
Validate predicted stress profiles through
comparison with experimental
measurements (iDHD)
‒
d)
Identify uncertainties associated with the
modeling process
•
Predictions were made blind, with limited
access to underlying data.
•
Predict the residual stresses for a
nozzle mockup containing the
following welds:
a) Buttering
b) Dissimilar metal weld (DMW)
c) Fill-in / Back weld (RBW)
d) Safe end weld (SSW)
14
2015-04-16
c) Fill-in Weld
Weld residual stress analysis
Incremental Deep Hole Drilling (DHD/iDHD)
International Weld Residual Stress Validation Project - lead by US NRC
2015-04-16
International project for validation of weld residual stresses
Validation project by U.S. Nuclear Regulatory Commission, 2010
Participants in the project for comparison of blindly
measured and calculated residual stresses:
Hoop stress profile after welding the stainless steel pipe
– comparison of experiments and detailed modelling by Inspecta
17
2015-04-16
Conclusions from the NRC international validation project
1
8
•
Weld residual stresses in dissimilar metal welds has been validated in a
blind round robin managed by U.S. NRC, with 16 leading organizations.
•
Measurements were performed by incremental deep hole drilling (iDHD).
•
Inspecta is one of the modeling groups with best agreement with the
experimentally measured residual stress profiles, during all phases of the blind
round robin.
•
It was large scatter between the predicted residual stress profiles from
different analysis groups.
•
Our results show that reliable predictions can be achieved if detailed and
careful modeling is used, especially with respect to heat source modeling
and high temperature properties and material hardening behavior.
•
The material model should consider the constraints for the welding geometries
of interest and the relevant deformation cycles. Biaxial material testing is
recommended if applying mixed hardening models.
Residual Stress Design
19
2015-04-16
Residual Stress Design
Produce beneficial residual stress by design of welding and/or surface treatments.
(A) Design of welding and welds
§ Design of new welds
Optimized weld joint geometry, sequence of beads and
joints, heat input, pre-heating, active cooling, etc
§ Design of repairs and weld overlays
Optimization and qualification weld overlay repairs for
the actual loading, geometry and welding.
Weld overlay
Original weld
20
Surface treatment methods
(B) Surface treatment methods:
‒ Shoot peening, hammer penning,
ultrasonic impact peening
‒ Cold hole expansion
‒ Roller burnishing, cold rolling
‒ Water jet cavitation peening, laser shock peening, …
• Produce local plastic deformation
• Optimize controlling parameters to the actual process,
material and operational loads in order to assure the effect
o Simulations are very useful
• Monitoring of controlling process parameters during
manufacturing.
21
Reducing Residual Stress
• Stress relaxation methods include:
− PWHT (annealing)
− Local PWHT
− Pre-straining, over pressurizing
− Machining and grinding
− …
• Accurate selection and control of procedure
to avoid e.g. micro structural changes
− Detailed specification of procedure and limits
− Validate procedure effectiveness
22
Examples
23
2015-04-16
Beneficial sequence of welds – Example for nozzle weld
Before:
After:
24
2013-11-27
Effect of bending on weld residual stresses
Roller Plate Bending Machine
Assess the difference by bending before or after welding:
As welded
Bending
25
2014-09-23
Result: welding after bending
26
2014-09-23
Result: bending after welding
27
2014-09-23
Optimization of burnishing process – Example
Validation - Predicted and measured
compressive stress from well chosen
burnishing of bearing steel:
28
Optimization – identification governing process
parameters and optimized limits for the process
Weld overlay repair
Original weld – as welded
Overlay weld
Original weld
Validation of the effect of PWHT to Weld Residual Stress in
nickel base weld (Alloy 182) to CS – full scale mockup
Path1 - Final state mock-up (Block 1)
30
Recommendations for WRS profiles in nickel base welds Alloy 182
Updated Appendix in the ProSACC fracture mechanical handbook
(current handbook version is SSM 2008:01)
Example:
Weld Type III.1
t =35 mm,
R= 300mm
Effect of operational loads – example nozzle in cladded vessel
Insticksstuts i plåterat kärl
32
2013-11-27
Analysis considering 3D effects for 5 bead weld in stainless steel
33
2013-11-27
Conclusions
• Improved residual stresses increase design life and reliability
• Welding Design and Adapted Surface Treatments have the
potential to produce beneficial residual stress
• Optimize to the actual process, material and operational loads is
needed in order to assure the effect
• Monitoring of parameters is necessary during production for the
quality assurance of critical equipment
34
Thank you!
Questions and ideas?
For more information contact:
Jens Gunnars,
+46 8 5011 3087,
jens.gunnars@inspecta.com
35
Fogningsdagarna 2015
Svetskommissionen
Svetssimulering – möjligheter och utmaningar
Jens Gunnars, Inspecta Technology AB
Svetsning ger deformationer som ibland kan påverka funktion och utseende. För att innan
tillverkningen säkerställa symmetri och deformation inom toleranser, så kan svetssimulering
användas för att ta fram lämpliga stöd, initiala vinklar i riggar samt svetssekvens. Detta är
särskilt användbart inför tillverkning av dyrbara lågserieobjekt, eller vid användning av mer
höghållfasta material som ger slankare konstruktioner.
Svetsning ger även restspänningar vilka ofta har stor inverkan på komponenters livslängd
och driftsäkerhet. Efter svetsning finns i materialet restspänningar i nivå med materialets
sträckgräns. Restspänningarna är självbalanserande i komponenten och visar inga yttre
tecken. Detta kan vara en bidragande orsak till att restspänningar är mindre kända och sällan
beaktas vid produktutveckling.
Restspänningar har stor inverkan på bland annat utmattning, spänningskorrosion och
slagskador. Den ökade förståelsen för hur restspänningarna uppstår vid svetsning har gett
möjligheten att gå ifrån att enbart acceptera de restspänningar som uppkommer, till att
istället designa för att skapa mer gynnsamma restspänningar. Olika strategier kan användas
för att inkludera restspänningar, i samverkan mellan konstruktion och tillverkning.
Förbättringspotentialen kan vara stor genom att begränsa dragspänningar i högt påkända
ytor eller uppnå kompressiva spänningar.
Analys av restspänningar innebär modelleringstekniska utmaningar. Validering är viktigt för
beräkning och även för de mätmetoder som finns. Det finns också utmaningen att
medvetandegöra tillverkare och brukare om möjligheterna, samt nyutveckling i för hållande
till användning av standarder.