Svetssimulering – utmaningar och möjligheter

Jens Gunnars, Inspecta Technology AB

2015-04-161

Fogningsdagarna 2015, Svetskommissionen, Sundsvall 16-17 april 2015

Welding simulation – challenges and possibilities

Jens Gunnars, Inspecta Technology AB

2015-04-162

Fogningsdagarna 2015, Svetskommissionen, Sundsvall 16-17 april 2015

2015-04-163

Outline

• Effects from Welding and Residual Stresses

• Residual Stress Modeling

• Measurement Methods

• Validations

• Weld Residual Stress Design

• Examples

• Conclusions

• 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

Effects from welding and residual stresses

What is residual stress?

• Residual stress are caused by non-uniform plastic deformationdue 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

• 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

Why predict weld residual stresses by simulation?

-50

0

50

100

150

200

250

300

0 0.2 0.4 0.6 0.8 1

Pf=0 Pf=0.84 Pf=1.23

u/t

as-welded

after pressure test

In operation

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 accurateheat 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 methodanalyzed

− 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

−=

10

11DD

EtσSlitting; Circumferential mean stress estimate

Validation

– International Blind Round Robin lead by the US nuclear authority for validation of weld residual stress simulation

2015-04-16

2015-04-1614

International Weld Residual Stress Validation Project - lead by US NRCProject 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

‒ Validate predicted stress profiles through comparison with experimental measurements (iDHD)

‒ 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) Butteringb) Dissimilar metal weld (DMW)c) Fill-in / Back weld (RBW)d) Safe end weld (SSW) Fill-in Weld

a)

b)

c)

d)

2015-04-16

Weld residual stress analysis

International Weld Residual Stress Validation Project - lead by US NRCIncremental Deep Hole Drilling (DHD/iDHD)

International project for validation of weld residual stressesValidation project by U.S. Nuclear Regulatory Commission, 2010

Participants in the project for comparison of blindly measured and calculated residual stresses:

2015-04-1617

Hoop stress profile after welding the stainless steel pipe– comparison of experiments and detailed modelling by Inspecta

18

• 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.

Conclusions from the NRC international validation project

2015-04-1619

Residual Stress Design

Residual Stress DesignProduce beneficial residual stress by design of welding and/or surface treatments.

20

(A) Design of welding and welds

§ Design of new weldsOptimized 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

Surface treatment methods

21

(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 effecto Simulations are very useful

• Monitoring of controlling process parameters during manufacturing.

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

2015-04-1623

Examples

Beneficial sequence of welds – Example for nozzle weld

2013-11-2724

Before:

After:

Effect of bending on weld residual stresses

2014-09-2325

Bending

As welded

Roller Plate Bending Machine

Assess the difference by bending before or after welding:

Result: welding after bending

2014-09-2326

Result: bending after welding

2014-09-2327

Optimization of burnishing process – Example

28

Validation - Predicted and measured compressive stress from well chosen burnishing of bearing steel:

Optimization – identification governing process parameters and optimized limits for the process

Weld overlay repair

Overlay weld

Original weld

Original weld – as welded

30

Validation of the effect of PWHT to Weld Residual Stress innickel base weld (Alloy 182) to CS – full scale mockup

Path1 - Final state mock-up (Block 1)

Recommendations for WRS profiles in nickel base welds Alloy 182Updated Appendix in the ProSACC fracture mechanical handbook

Example: Weld Type III.1t =35 mm, R= 300mm

(current handbook version is SSM 2008:01)

Effect of operational loads – example nozzle in cladded vessel

2013-11-2732

Insticksstuts i plåterat kärl

Analysis considering 3D effects for 5 bead weld in stainless steel

2013-11-2733

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.