Measurement and Numerical Prediction of Residual Stresses in Steel Welds subject to Heat Treatment

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)

Frazer-Nash Consultancy Ltd 2014. All rights reserved. SYSTEMS AND ENGINEERING TECHNOLOGY

Introduction Description of Specimens Residual Stress Measurements Residual Stress Modelling Post Weld Heat Treatment Comparison of Results Conclusions

Overview


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


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


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


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


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


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


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

ND

DHD

TT

ST

HD

CM

P

E

N

E

T

R

A

T

I

O

N


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



Strain Scanning Diffractometer at the National Research Universal (NRU) reactor located at Chalk River, Canada


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 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


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


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.


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


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

200.0

300.0

400.0

500.0

600.0

700.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Time [Hours]

T

e

m

p

[

d

e

g

C

]

Post Weld Heat Treatment


SBW As-welded longitudinal residual stress

TBW As-welded longitudinal residual stress

Comparison of ResultsAs-Welded


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 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 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



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



SBW

-0.5

0.0

0.5

1.0

1.5

-10.0 10.0 30.0 50.0 70.0 90.0Distance from the weld centre line (mm)

L

o

n

g

i

t

u

d

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S

Phase Trans. After PWHTPhase Trans. Before PWHTR6 Level 2BS7910ND Measurements before PWHTND Measurements After PWHT


-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

-40.0 -20.0 0.0 20.0 40.0 60.0 80.0 100.0Distance from the weld centre line (mm)

L

o

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g

i

t

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d

i

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a

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N

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S

Phase Trans. After PWHTPhase Trans. Before PWHTR6 Level 2BS7910ND Measurements Before PWHTND Measurements After PWHT


TBW


Comparison of ResultsVickers Hardness

PWHT Measurements Vickers Hardness (VH) Comparison

Peak Hardness


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]


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]


Any Questions?

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Measurement and Numerical Prediction of Residual Stresses in Steel Welds subject to Heat Treatment