FE simulation of hydrogen induced cracking in pipeline X70 steel using cohesive zone elements.

1. Cohesive simulation of hydrogen
assisted crack initiation in X70
steel and welded joints
Presentation at EuroCorr2010, Moscow, 13.-17.
September 2010
V. Olden(SINTEF), O. M. Akselsen (SINTEF/NTNU), H. Fjær
(IFE)
Materials and Chemistry 1

2. Content
 Background
 Description of the FE model
 Experimental and FE simulation results
 Main conclusions
Materials and Chemistry 2

3. Deep water repair welding and hot tapping
(DEEPIT 2009-2013)
The overall objective is to establish basic
understanding of deep water pipeline repair Budget: 26 mill NOK
welding and hot tapping.
Subgoals:
 Develop fundamental knowledge on Participants:
hyperbaric welding technology for full SINTEF, IFE,
remote control in welding of normal pipes
and clad pipes. NTNU, StatoilHydro,
 Study the behaviour of different Gassco, Technip and EFD
consumables and shielding gas
 Develop mathematical models for
electromagnetic induction heating. The project is open for new
 Develop relevant models for simulation of participants
heat flow in hyperbaric MIG with different
bevel configurations.
 To develop models tailored for cold
cracking susceptibility predictions
capturing hydrogen pickup, diffusion,
microstructure evolution, restraint
intensity, and integrity assessment.
Materials and Chemistry

4. PRS
Recent large projects:
Hyperbaric Welding Langeled (2004-2007)
North stream (2009-2012)
SINTEF is responsible for all
welding procedure Strategy:
development and Qualify MIG and TIG welding
qualifications for pipeline
tie-ins and repair welding
for the operators on the
Norwegian shelf.
Materials and Chemistry 4

5. Nord Stream: Sub-sea gas pipeline from Russia to Germany
under construction
 1200 km
 Two lines in parallel
 48”, ID 1153 mm, X70
 WT 26.8 – 41.0 mm
 Max. sea depth: 210 m
 Design pressure:
Up to 220 bar
 Two hyperbaric weld
tie-ins:146 and 76msw.
 Line I: June 2011
 Line II: April 2012
 Installation of the first
line started in April
2010
Materials and Chemistry 5

6. The remote PRS welded sleeve concept
Sleeve
Materials and Chemistry 6

7. Nord Stream Hyperbaric Welding Procedures -
Development/qualification
Materials and Chemistry 7

8. Hydrogen pick up
 If moisture is present in the hyperbaric weld chamber it
can cause hydrogen pick up during welding, which gives a
risk of cold cracking in the weld and heat affected zone.
 A possible hydrogen source during service is hydrogen
evolution and absorbtion due to cathodic protection.
”Cold cracking in hyperbaric welds: A critical combination of residual
stress after welding, applied load, a sensitive microstructure and
hydrogen from welding and/or CP”
Materials and Chemistry 8

9. The model
 The cohesive element is
 Described by an energy criterion, the traction separation law, TSL
A traction separation law (TSL) is a
function described by the cohesive
stress, σ, and separation, δ.
The area below the curve
represents the critical separation
energy, Гc.
 Embedded in a continuum element model
Materials and Chemistry 9

10. Energy reduction due to hydrogen
Estimation of the cohesive energy (Γc) without hydrogen influence
σθ
= 1 − 1.0467θ + 0.1687θ 2
σc
Yiang & Carter, 2004
C
θ=
C + exp(−∆g 0 / RT )
b
Materials and Chemistry 10

11. Estimation of initial cohesive energy
J-r SENT testing in air at 4°C 9
Γc = σ c δ c
16
Measured streching
zone width
prior to ductile
fracture (~0.1mm)
Materials and Chemistry 11

12. Stress and diffusion
 Elastic plastic
 Mises material model (ABAQUS 6.9)
 Material specific stress strain curves
 E=205000 MPa, Poisson’s rate= 0.3
 Fick’s law
 With influence of hydrostatic stress (ABAQUS 6.9):
∂C V
H
V
H
= D∇ C + D ⋅
2
∇C ⋅∇p + D ⋅ C∇ 2 p
∂t (
R⋅ T −T Z ) R ⋅ T −T Z ( )
 Trapping
 Hydrogen concentration is corrected with respect to plastic strain
within the cohesive element formulation.
Materials and Chemistry 12

13. Experiments
C Mn Si P S Cu Mo Ni Cr Ti Pcm
X70 0.047 1.74 0.10 0.01 7 0.32 0.04 0.25 0.05 0.01 0.16
(CGHAZ) ppm
X70 0.09 1.71 0.30 0.01 10 0.02 0.05 0.07 0.02 0.20
(BM) ppm
 BM  CGHAZ
Upper
bainite
Martensite
Pearlite UB M
Ferrite
10µm
10µm
Rp 0.2=485 MPa Rp 0.2=810 MPa
Materials and Chemistry 13

14. Testing
•SENT samples, pre charged
at 80°C and -1050mVSCE (1.5
ppm)
•Constant load testing at 4°C
and -1050mVSCE
Fracture
Materials and Chemistry 14

15. FE model geometry
Crack
tip
Element size: 15-20 μm close
to the crack tip to ensure sufficient
resolution of the local stress field.
Materials and Chemistry 15

16. Net section stress and cohesive
parameters
BM CGHAZ
625 Mpa~1.3σRp0.2 550 Mpa~0.7σRp0.2
δc = 0.3 mm, σc = 1700 MPa δc = 0.3 mm and σc = 3900 MPa
Materials and Chemistry 16

17. Application of the model for simulation of cold
cracking in pipe after welding Distribution of hydrogen
Residual stress distribution after welding concentration
WeldSimS
Materials and Chemistry 17

18. Cohesive simulation of crack
susceptability in weld toe
Material:
CGHAZ material and
diffusion properties
(Rp0.2=809 MPa, E=208000 MPa,
ν=0.3, D=3.4 ×10-6 mm2/s)
Load:
10 ppm H in weld, cooling down
to RT, storage for 24 hrs, tensile
loading at 500 and 600 MPa for 1 year
at subsea conditions (1.5 ppm H
at surface)
Materials and Chemistry 18

19. With 30 μm surface ”crack”
No cracking
when loaded at 600 MPa for
1 year
Materials and Chemistry 19

20. Conclusions
 The tested base metal X70 steel revealed low sensitivity to hydrogen
embrittlement. Did not fail at net section stresses lower than 1.29
times the 0.2% yield strength.
 Weld simulated coarse grained heat affected zone is prone to fracture
at stresses above 70% of the yield strength, which indicates hydrogen
embrittlement susceptibility.
 A polynomial traction separation law with intrinsic hydrogen
dependant energy works well for the cohesive FE simulations.
 Cohesive parameters best fitting the experiments for base metal:
δc=0.3 mm and σc=1700 MPa (3.5∙σy)
 Cohesive parameters applied for CGHAZ: δc=0.3mm and σc=3900
MPa (4.8∙σy).
 An example is given proving that the model, given the correct input,
can be applied in evaluating the fracture integrity of a welded joint.
Materials and Chemistry 20

21. Acknowledgements
The present work was financed by the Research Council of Norway
(Petromaks project 192967/S60), Statoil, Technip and EFD Induction.
Materials and Chemistry 21

22. WeldsimS
 WeldsimS is a finite element code
developed by IFE and SINTEF for simulating
welding of steels.
 Features include:
 Phase transformations
 Thermal expansion
 Transformation plasticity
 Flow stress as a function of temperature,
strain rate, and deformation.
 Input:
 Geometry, material data, welding parameters,
H concentration in weld pool
 Output:
 Temperature, residual stress/strain,
hardness, phase composition, H distribution
Materials and Chemistry 22