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SYSTEMS AND ENGINEERING TECHNOLOGY
FATIGUE 2014, 11TH International Fatigue Congress
Measurement And Numerical Prediction Of Residual Stresses In Steel Welds Subject To Heat
Treatment
Dr Roger Dennis (Frazer-Nash) & Dr Anna Paradowska (ANSTO)
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Introduction Description of Specimens Residual Stress Measurements Residual Stress Modelling Post Weld Heat Treatment Comparison of Results Conclusions
Overview
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Introduction
Fatigue lives of welded structures is an ongoing matter of concern and focus of research in the field of structural integrity
Welded joints are generally treated as defective and that a crack has already initiated, usually at the weld toe, with the failure mechanism being the propagation of that crack
Many factors affect fatigue lives including cyclic stress-state, geometry, surface quality, residual stresses, environment and temperature
Here the Neutron Diffraction (ND) technique is used to investigate the residual stress distributions in carbon steel specimens and compared to current fitness-for-purpose assessments (BS7910, R6) and finite element weld modelling methods
Residual stress mitigation benefits of PWHT investigated using both measurements and modelling
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Description of Specimens
Two full penetration welds were studied Single-V butt weld preparations, angle of 60 and a root gap of 3mm Carbon Steel (AS 1548-7-460) Dimensions of each parent plate were:
Width (x-direction) of 150mm Thickness (z-direction) of 25mm Length (y-direction) of 250mm
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Description of SpecimensStringer Bead Welding (SBW)
Constructed with fourteen beads First bead (root bead) was carried out at a lower heat input than the
remaining filling beads Schematic is what the welder was requested to produce Clear that the welder did not follow the schematic particularly
accurately in this case and consequently the capping passes are offset Final
Pass
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Description of SpecimensTemper Bead Welding (TBW)
Temper bead weld procedure aims to provide better metallurgical structure than conventional welding
Two layers, with 38 passes in total First (tempering) layer contains 18 beads deposited at a low heat input Remaining 20 filling beads were deposited at a higher heat input Method much more expensive and time consuming than SBW method.
Final Pass
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Weld parameters were recorded Pass 1 of SBW and Pass 1-18 of TBW were conducted at a higher
traverse speed to provide a lower input temperature
SBW Pass 1 Pass 2-14
TBW Pass 1-18 Pass 19-38
Electrode Diameter [mm] 1.6 1.6Current Range [A] 260-280 260-280Voltage Range [V] 28-30 28-30Traverse Speed [mm/sec] 8.0 6.0
Description of SpecimensWeld Parameters
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Description of SpecimensMaterial Properties
Parent material used in this study was a carbon steel (AS 1548-7-460). Tensile properties of the parent material were examined in accordance with Australian Standard AS 13911991
Weld metal tensile properties were examined in accordance with Australian Standard AS 2205.2.21997
Materials were tested at ambient temperature Weld specimens were taken from the weld in the direction
longitudinal (y) to the weld. Average mechanical properties shown Yield Stress
(0.2% Proof Stress) [MPa] Tensile Strength
[MPa] Elongation
[%] Parent Metal 430 525 30.0 Weld Metal 465 590 29.5
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Lab. XR: Laboratory X-ray
SXD: Synchrotron X-Ray Diffraction
ND: Neutron Diffraction
HD: Hole Drilling
TT: Trepanning Technique
CM: Contour Method
ST: Slitting Technique
DHD: Deep Hole Drilling Technique
Residual Stress MeasurementsOptions
0 10m
10m
100 m
100 m 1 mm
1 mm
10 mm 100 mm
100 mm
10 mm
SPATIAL RESOLUTION
Lab. XR
SXD
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Residual Stress Measurements
Non-destructive Deep penetration Full triaxial stress state Individual phases Fast Minimal sample preparation In situ studies Typically used for:
Stress and texture characterisation of novel processes Validation of models and other measurement methods Microstructural characterisation Specific residual stress problems
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Residual Stress Measurements
Strain Scanning Diffractometer at the National Research Universal (NRU) reactor located at Chalk River, Canada
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d
Change in lattice spacing detected as a shift in the diffraction peak From the Bragg equation the strain can be determined Assuming an isotropic solid, stresses can be calculated
Braggs Law:
d = / (2 sin)
Residual Stress Measurements
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Residual Stress Modelling
Specimens modelled using a sequentially-coupled FE analysis 2D cross section, static heat source modelling approach adopted. Start/stop effect not captured but ok since measurments at mid-length of
weldSBW FE Model TBW FE Model
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Residual Stress ModellingOverview
Overview of weld modelling approach:1. Calculation of heat inputs2. Construction of models and meshes3. Heat source modelling4. FE thermal analysis5. FE mechanical analysis6. Post weld heat treatment analysis
Many papers which describe in detail the modelling approach: Smith M.C., Bouchard P.J., Turski M., Edwards L. and Dennis R.J., Accurate Prediction of
Residual Stress in Stainless Steel Welds, Computational Materials Science. COMMAT-D-11-00770, 54, pp. 312-328, 2011.
Bray D.P., Dennis R.J. and Bradford R.A.W., Modelling The Complex Manufacturing History Of A Pipework Joint And Assessment Of Its Through Life Creep-Fatigue Damage Using Finite Element Based Methods, Proceedings of ASME PVP 2010-25702, Washington, 2010.
Bray D.P., Dennis R.J. and Smith M.C., Prediction Of Welding Residual Stresses, Crack Initiation And Creep Crack Driving Forces C(T) Within A Continuous Finite Element Solution, Proceedings of ASME PVP 2010-25699, Washington, 2010.
SBW As-Welded RS
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Residual Stress ModellingHardening Models
Isotropic hardening (1st) and linear kinematic hardening (2nd) models were considered in the FE analysis
Isotropic model the most common form in FE modelling and is more conservative than the kinematic hardening model
Courtesy: LM Smith. Metal Forming Lecture Notes, Chapter 4: Hardening. Department of Mechanical Engineering, Oakland University.
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A 3rd hardening model was considered, kinematic hardening with solid state phase transformations
Residual stress state influenced by volumetric changes associated with phase change
Vickers hardness predicted using the numerical routines which can be used as an indicator of the strength of welded joints. Vickers hardness of each phase calculated accounting for composition and cooling rate
Residual Stress ModellingHardening Models
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As-welded residual stresses output from weld modeling process Heat treatment modelled by a transient analysis after welding Material behavior described using a simple secondary creep model
with creep strain rate a function of stress and temperature Temperature history from thermocouple data
0.0
100.0
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0.0 2.0 4.0 6.0 8.0 10.0 12.0
Time [Hours]
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p
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d
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Post Weld Heat Treatment
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SBW As-welded longitudinal residual stress
TBW As-welded longitudinal residual stress
Comparison of ResultsAs-Welded
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Comparison between SBW and TBW respectively, As-Welded, Path 1 Longitudinal Stress results
All results are in terms of Normalised Residual Stress (NRS)
SBW peak stress above fitness-for-purpose assessment levels
Comparison of ResultsAs-Welded
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Comparison between SBW and TBW respectively, As-Welded, Path 1, Transverse Stress results
SBW and TBW transverse stresses below the fitness-for-purpose assessment levels
Comparison of ResultsAs-Welded
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Comparison between SBW and TBW respectively, As-Welded, Path 2, Transverse Stress results
SBW and TBW transverse stresses are below BS7910 recommendation, but in places are underestimated by R6 Level 2
Comparison of ResultsAs-Welded
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PWHT Measurements:
Stress results normalised with respect to the room temperature yield strength of the parent and weld materials. All results are in terms of Normalised Residual Stress (NRS)
Plots compare experimental data with current fitness-for-purpose assessments and modelled results
Modelled results are plotted along the same path as ND results
Comparison of ResultsPWHT
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Comparison of ResultsPWHT
SBW
-0.5
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-10.0 10.0 30.0 50.0 70.0 90.0Distance from the weld centre line (mm)
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Phase Trans. After PWHTPhase Trans. Before PWHTR6 Level 2BS7910ND Measurements before PWHTND Measurements After PWHT
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-1.0
-0.5
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-40.0 -20.0 0.0 20.0 40.0 60.0 80.0 100.0Distance from the weld centre line (mm)
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Phase Trans. After PWHTPhase Trans. Before PWHTR6 Level 2BS7910ND Measurements Before PWHTND Measurements After PWHT
Comparison of ResultsPWHT
TBW
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Comparison of ResultsVickers Hardness
PWHT Measurements Vickers Hardness (VH) Comparison
Peak Hardness
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The following key conclusions can be drawn:
High values of longitudinal residual stress are present in both types of weld. The peak residual stresses in most cases are located around the fusion boundary of the last pass
The predictions of as-welded residual stresses correlated well with the experimental results, especially in the case of the TBW specimen providing confidence in the modelling approach
Accounting for solid state phase transformations provides a notable improvement in the accuracy of the modelling relative to the ND measurements
Conclusions [1/2]
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The TBW and SBW welding procedures induce similar magnitudes of residual stress
The fitness-for-purpose assessments were both found to under-estimate longitudinal residual stresses and over-estimate transverse residual stresses
The influence of PWHT on the as-welded residual stresses was both significant and quantified using both modelling and measurements. ND measurements were limited in number but gave confidence in accuracy of the PWHT modelling
Modern ND measurement and modelling techniques can provide detailed knowledge of residual stresses both in as-welded and after PWHT state, a key factor when calculating the fatigue lives of welded joints
Conclusions [2/2]
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