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Residual stresses measurement by neutron diffraction and theoretical

estimation in a single weld bead

John W.H. Price a,*, Anna Paradowska a, Suraj Joshi a, Trevor Finlayson b

aDepartment of Mechanical Engineering, Monash University, Wellington Road, Clayton, Vic. 3800, Australia

bDepartment of Physics, Monash University, Wellington Road, Clayton, Vic. 3800, Australia

Abstract

Welding residual stresses are important in pressure vessel and structural applications. However, residual stress remains the single largest

unknown in industrial damage situations. They are difficult to measure or theoretically estimate and are often significant when compared with the

in-service stresses on which they superimpose. High residual stresses lead to loss of performance in corrosion, fatigue and fracture.

In this research, a measurement of residual stress by the neutron diffraction technique is compared to an analysis of the same geometry by

theoretical finite element procedures. The results indicate good agreement but scope for further understanding of the details of modelling the

welding heat source, heat transfer and variation of material properties with temperature.

q 2006 Elsevier Ltd. All rights reserved.

Keywords: Residual stress; Neutron diffraction; Hole drilling; Welding

1. Introduction

Residual stresses are formed in weld structures primarily as

the result of differential contractions, which occur as the weld

metal solidifies and cools to ambient temperature. These

stresses can have important consequences on the performance

of engineering components[1].Weld residual stresses have a

significant effect on corrosion, fracture resistance and

corrosion/fatigue performance [2] and a reduction of these

stresses is desirable.

There are several ways of directly measuring residual

stresses in small volumes. The most common ones involve

mechanical invasive methods (e.g. hole drilling or cutting

[3,4]) and non-destructive methods using radiation such as

X-ray (laboratory or synchrotron) or neutron diffraction[57].

Of these, only neutron beams can establish stresses in theinterior of components of a metallic material and have a small

volume of measurement (1 mm3).

In this paper, experimental measurements of weld stresses

generated by a single bead-on-plate of low-carbon steel using

MIG welding are presented. In this work, we have concentrated

on the influence of restraint on the residual stresses behaviour

with the intention of providing key data for the validation of

design and fitness for purpose methodologies and finite element

tools.

2. Experimental work

2.1. Material and welding procedure

The material used in this study was a low-carbon steel [8].

The chemical composition of the parent material and weld

metal are shown inTable 1. The dimensions of the plates were

200!100!12 mm. Typical mechanical properties of parent

and weld metal are shown in Table 2.

Sample I was unrestrained and Sample II was fully

restrained. Restraint was achieved by welding Sample II to a

very thick steel plate, which was cut off after cool down.

Distortion of Sample I was overall approximately 18 in

transverse and 0.58 for longitudinal directions, Sample II had

no visible distortion.

The bead-on-plate welds were produced down the centre of

the plates, using a constant current UNI-MIG 375K AC/DC

power source. The specimens were mounted under an

automatic-speed-controlled, welding torch. The electrode was

Super-COR 5, 1.6 mm in diameter (conforming to AWS

A5.20) with a 20 mm contact tip to work distance. The welds

were made with an ARGOSHIELD 52 gas (with gas flow rate

18 l/min), with start and stop positions approximately 25 mm

International Journal of Pressure Vessels and Piping 83 (2006) 381387

www.elsevier.com/locate/ijpvp

0308-0161/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijpvp.2006.02.015

* Corresponding author. Address: Department of Mechanical Engineering,

Monash University, P.O. Box 197, Caulfield East, Vic. 3800, Australia. Tel.:

C61 3 9903 2868; fax: C61 3 9903 2766.

E-mail address: [email protected](J.W.H. Price).
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from each end of the plate. The welding parameters were:

current 260280 A, voltage of 2830 V and a speed of the

electrode 360 mm/min. The width of the welds bead (Fig. 1)

was 14 mm. There was no pre- or post-weld heat treatment.

The characteristic of the weld hardness and evaluation of

microstructure was described in details in previous work[9].

2.2. Determination of residual stress

The neutron diffraction technique is capable of determining

residual stresses non-destructively within the interior of

components[1]. Diffraction techniques exploit the crystalline

lattice of the material as an atomic strain gauge. When a beam

is passed through a polycrystalline material, diffraction occurs

according to Braggs law which is given by the equation

nlZ 2disin qi (1)

wherenis an integer,qiis the Bragg angle for crystallographic

planes, i having interplanar spacing di.

Under an applied tensile (or compressive) stress, the lattice

spacing (di) in individual crystallite grains expands (or

contracts). This change in the lattice spacing can be detected,at constant wavelength, as a shift (Dqi) in diffraction peaks,

which is schematically shown in Fig. 1. From Braggs

equation, the strain (3i) is given by

3iZdiKd0

d0ZKcotqiDqi (2)

where d0 is the strain-free interplanar spacing for the lattice

planes, i.

The orientation of the principal strains in any specimen is

determined from the geometry. The strains (exx, eyy, ezz) in a

solid can be converted to the three-dimensional stress (sxx,syy,

szz) state. For an elastically isotropic solid, Eq. (3) give thestresses in three directions using the xxdirection as an example.

sxxZE

1Cn1C2n 1Kn

3xxCn

3yyC3zz

(3)

whereEis Youngs modulus, and n is Poissons ratio.

2.3. Experiment details and results

Neutron diffraction measurements were undertaken on the

neutron beam stress scanner TASS (The Australian Strain

Scanner) at ANSTO, Sydney, Australia. The same parameters

of measurements were specified for both specimens. The

neutron wavelength used was 1.40 A. Measurements were

made using the (112) reflection, at the detector angle, 2qI, of

approximately 73.58. Measurements were made with thescattering vector (seeFig. 2) parallel to the three axes marked

transverse, longitudinal and normal as shown inFig. 2.

The incident and diffracted beam slits for transverse normal

measurements were 1 mm wide and 20 mm high because the

stresses do not change much in the longitudinal direction.

Because the stresses are changing rapidly in the transverse

direction; for the longitudinal direction the slits can only be

1.5 mm width, 2 mm high. The slits are formed by a mask of a

neutron absorbing material (Cadmium) in the incident and

diffraction beams.

Table 1

Chemical composition of the consumable materials (in wt%)

Compo-

sition

material

C Mn Si S P Ni Cr Mo Cu V

Parent

metal

0.12 0.63 0.13 0.01 0.02 0.02 0.01 0.01 0.01 !0.01

Weld

metal

0.10 1.7 0.68 0.02 0.02 0.05 0.03 0.04 0.04

Table 2

Typical mechanical properties

Mechanical properties Yield

stress

(MPa)

Tensile

strength

(MPa)

Elongation

(%)

Parent metal (experimental

measurements according to

AS 1391:1991)

285 429 38

Weld metal (as manufac-

tured using Argoshield 52

shielding gas)

445 550 29

Fig. 1. Principles of the neutron diffraction technique showing Braggs

reflection from the crystal plane d(grain size greatly exaggerated for clarity).

Fig. 2. The direction of the measurements (transversex, normaly, longitudinal

would be z) using neutron diffraction on the single bead-on-plate.

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The stress-free parameters for the steel were measured on

eight 2!2!2 mm cuboids which had been cut and glued

together (the d0 specimen). The slits for the stress-free

parameters were the same as for longitudinal measurements

(1.5!1.5!2 mm). Scans were made along the transverse line

(Fig. 3) from xZ0 (the centre of the weld) to xZ32 mm. The

centre of the gauge volume was 1.5 mm below the top surface.

Thed0specimen required three measurements for accuracy and

yielded a stress free diffraction angle, 2q, was 73.36.

Counting times are determined by the accuracy required of

the measurements. Counts are collected until the experimenters

are confident that the peak has been clearly defined. The time

required for this varies with each geometry and direction of

measurement. The number of counts of neutrons at the

collectors used in our experiments was approximately 10,000

for each point. For each data point this required times ofapproximately 25 min for transverse and normal strain

measurements and 60 min for longitudinal and d0 parameter

measurements.

The residual stresses were derived from the elastic strain

measurements (Figs. 3a and 4a) using Youngs modulus of

207 GPa, and Poissons ratio of 0.3. The maximum longitudi-

nal tensile residual stress was found near the middle of the weld

and is shown in Figs. 3b and 4b, reaching the approximate

value of 350360 MPa for Sample I and 470490 MPa for

Sample II (Fig. 5).

3. Finite element modelling

The objective here was to perform three-dimensional finite

element modelling of the bead-on-plate experiment to calibrate

the welding procedure. A relatively uncommon but powerful

commercial FEA package called SysweldC was used in this

attempt. The parent and the weld material were assumed to

have the same mechanical and thermal properties, as was

provided in the software database for the material S355J2G3

with chemical composition as follows: C%0.20%,

Mn%1.60%, Si%0.55%, S%0.035%, and P%0.035%. The

solidus temperature was 1440 8C, the liquidus temperature was

1505 8C, and the latent heat of fusion was 270,000 J/kg. The

temperature dependent properties were measured and tabulated

by extensive experimentation and supplied with the software.

Three-dimensional meshes of the substrate plate and the bead

were constructed as illustrated in Fig. 6.

For the sake of geometric convenience, the volume of the

bead was modelled as an elliptical pyramid (i.e. the front and

the back faces of the bead were half-ellipses). Differential

element sizes were used in meshing, with the density being

Fig. 3. Unrestrained Sample I. The longitudinal, transverse and normal components of strain (a) and stress (b) measured by neutron diffraction against distance from

the weld centre line. Error bars based on uncertainty in the value of the peak diffraction angle are shown.

Fig. 4. Fully restrained Sample II. Data for comparison to Sample I.

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ellipsoidal model as provided in the documentation of

SysweldC (Fig. 7b).

3.2. Results of the thermal analysis

The simulation was run for a time period of 7500 s, i.e. more

than 2 h, allowing for the complete thermal cycle, including

heating and subsequent air cooling to occur. The temperature

profiles obtained at about 10 and 20 s are shown in Fig. 9.

Automatic time stepping adjusted itself in such a manner that the

initial time intervals between successive recordings were quite

small during the period of deposition of the bead. As soon as the

weld torch finished depositing the bead, the time steps between

recordings became increasingly large. In either case, the time

steps were non-uniform during the entire 7500 s period.

It was observed that the maximum temperature reached

during the entire process was about 1600 8C with the above

mentioned heat source parameters. The thermal cycle for the

node at the midpoint of the weld trajectory is shown in

Fig. 10.

3.3. Results of the mechanical analysis

The mechanical analysis followed from the results of the

thermal analysis and took much longer to complete. The output

data included stresses in elements, integration points and

element nodes.Fig. 11shows the plots of the residual stresses

at element nodes at 7500 s.The finite element analysis results show that the transverse

stresses are tensile, though small in magnitude and nearing

zero, in the region close to the centreline, and become

compressive as we move towards the outer edges. This is in

good comparison with the measured values, which show the

same pattern. The longitudinal residual stresses, as predicted

by the finite element analysis results, are tensile in the region

near the weld line and turn more and more compressive when

moving towards the edges on either side. This is also in good

qualitative agreement with the experimentally measured values

as illustrated inFig. 3.

4. Discussion

Transverse and normal stresses are low in the unrestrained

Sample I, because the sample deformed during welding.

However, for Sample II transverse and normal stresses are

raised at all points of measurements as shown in Fig. 5.

Longitudinal stress also generally increases but there are some

reductions around the toe of the weld.

The highest increase between the unrestrained and

restrained specimen was observed in middle of the weld

where the normal and transverse stresses change from

compressive for unrestrained sample (Fig. 3b) to tensile forthe restrained sample (Fig. 4b).

The peak stress in the weld (which is in the longitudinal

direction) is significantly higher in the weld area in both

samples, than the specified yield stress of the steel in question

(250 MPa). Hardness tests (on other samples) indicate

Fig. 10. Temperature history of the midpoint of the trajectory.

Fig. 11. (a) Transverse residual stress, (b) longitudinal residual stress, at 7500 s.

J.W.H. Price et al. / International Journal of Pressure Vessels and Piping 83 (2006) 381387386


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significant increases in hardness in the welded region reflecting

higher yield stresses in the weld. The peak stress in this case

does not occur at the toe of the weld but in the middle of weld.

However, the maximum longitudinal stress at the toe is around

150 MPa which will be approximately 60% of the yield

strength.

For comparison purposes, the fitness-for-purpose codeBS7910 [12] states in clause 7.2.4.1 that the following

assumption should be made for welds not subjected to post-

weld heat treatment:

For a flaw lying in a plane transverse to the welding

direction (i.e. the stresses to be considered are parallel to the

weld in the terminology used in this paper longitudinal

stresses) the tensile stress should be assumed to be equal to

the room temperature yield strength of the material in which

the flaw is located.

If this procedure described in BS7910 were used, a uniform

stress of at least 250 MPa would have to be used in the

calculations. The result of this difference will be a very

conservative assessment of the failure point of the weld and the

use of different growth curves for fatigue assessment.

Our work is also currently developing methods of cross

checking these results with finite element modelling of the

welding process. Initial finite element analysis results using

SysweldC were quite promising. While the longitudinal and

normal stresses were found to be in good agreement with the

experimental values, the longitudinal stresses were a bit off

from the measured values. However, qualitatively, the nature

of the residual stresses predicted by the program is in good

agreement with the experimental observation. Continuingeffort is being made to get an accurate calibration of the heat

source and refining the mesh to locate specific nodes at exactly

those points where experimental values were obtained, so that a

perfect tally can be made.

5. Conclusions

The use of a neutron beam as a non-destructive method of

measuring residual stress due to welding has been explored.

Experimental investigation showed that restraint of the plate

during welding has a significant effect.

The analysis of the same weld geometry following the

procedures of SYSWELDC were used. The handbook

suggested method of analysing the weld pool, heat transfer

and material properties were used. The results of the

comparison were promising, but further improvement in the

modelling could be achieved.

Acknowledgements

This work was conducted with the assistance of an

Australian Research Council grant supported by the WeldingTechnology Institute of Australia (WTIA). Other assistance has

been received from the Monash University Research Fund, the

Australian Nuclear Science and Technology Organisation

(ANSTO) and an Australian Institute of Nuclear Science and

Engineering (AINSE) grant.

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