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Surface residual stresses in multipass weldsproduced using low
transformationtemperature filler alloys
T. I. Ramjaun*1, H. J. Stone1, L. Karlsson2, M. A. Gharghouri3,
K. Dalaei4, R. J.Moat5 and H. K. D. H. Bhadeshia1
Tensile residual stresses at the surface of welded components
are known to compromise fatigue
resistance through the accelerated initiation of microcracks,
especially at the weld toe.
Inducement of compression in these regions is a common technique
employed to enhance
fatigue performance. Transformation plasticity has been
established as a viable method to
generate such compressive residual stresses in steel welds and
exploits the phase transformation
in welding filler alloys that transform at low temperature to
compensate for accumulated thermal
contraction strains. Neutron and X-ray diffraction have been
used to determine the stress profiles
that exist across the surface of plates welded with low
transformation temperature welding alloys,
with a particular focus on the stress at the weld toe. For the
first time, near surface neutron
diffraction data have shown the extent of local stress variation
at the critical, fusion boundary
location. Compression was evident for the three measurement
orientations at the fusion
boundaries. Compressive longitudinal residual stresses and
tensile transverse stresses were
measured in the weld metal.
Keywords: Transformation induced plasticity, Martensite,
Residual stress, Welding, Neutron, X-ray diffraction
IntroductionThe welded joints of engineering structures are
often thefeatures that limit both the service loads that can
betolerated and their fatigue life. Stress at the boundarybetween
the weld metal and base material caused bygeometrical changes,
coupled with local microstructuralchanges, often leads to
preferential crack initiation andfatigue failure at this site.1
Susceptibility to microcrackinitiation/propagation can be further
exacerbated bytensile residual stresses in this region, which
accumulateas a result of thermal contraction strains during
coolingof the weld to ambient temperature.In order to increase the
longevity of welded compo-
nents, post-weld treatments may be applied that areeither
mechanical or thermal in nature. Heat treatmentsare able to relax
internal stresses, while mechanicalprocesses tend to impart
compression into the compo-nent surface or modify the weld toe
geometry to reduce
stress concentration. As such treatments are typicallycostly or
impractical, it may be preferable to modify thewelding process to
minimise the occurrence of suchstresses. One method, proposed
originally by Jones andAlberry,2 is to counter the thermally
induced tensilestresses through exploitation of the strains
associatedwith solid state phase transformation of the weld
filler.Application of this mechanism has led to a number
ofmartensitic welding alloys being developed.311
The transformation of austenite to martensite (cRa9)is
displacive and results in a shape deformation that is aninvariant
plane strain with a large shear component.12
The magnitude of the shear, in conjunction with adilatational
strain, is sufficient to not only cancel thetensile stresses, but
even create compression in the weldmetal. Detailed reviews of this
mechanism and itsinfluence on residual stresses in welds are
availableelsewhere.13,14 However, the beneficial effects of
stressalleviation through transformation plasticity are depen-dant
on the temperature at which the cRa9 transforma-tion occurs. If the
transformation takes place above anoptimal temperature, continued
thermal contraction ofthe transformed product to ambient
temperature leadsto further accumulation of tensile stress. Thus,
the initialbenefits associated with the transformation are
elimi-nated. To avoid this problem, the weld metal should
bedesigned with a low martensite start temperature (MS),typically
200uC.
1Materials Science and Metallurgy, University of Cambridge,
CambridgeCB3 0FS, UK2Department of Engineering Science, University
West, SE-461 86Trollhattan, Sweden3Canadian Neutron Beam Centre,
Chalk River Laboratories, Chalk River,Ont. K0J 1J0, Canada4ESAB AB,
Lindholmsallen 9, 40277 Gothenburg, Sweden5Materials Engineering,
The Open University, Milton Keynes MK7 6AA,UK
*Corresponding author, email [email protected]
2014 Institute of Materials, Minerals and MiningPublished by
Maney on behalf of the InstituteReceived 20 May 2014; accepted 11
July 2014DOI 10.1179/1362171814Y.0000000234 Science and Technology
of Welding and Joining 2014 VOL 19 NO 7 623
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These low transformation temperature (LTT) weldingalloys have
proved effective in enhancing fatigueperformance.1520 Although,
judicious alloy design21,22
has been required in order to ensure that the
carbonconcentration is sufficiently low to avoid the embrittle-ment
associated with hard martensite. The ability ofLTT welding alloys
to provide stress relief in single andmultipass welds has also been
established.23 However,surface stress measurements obtained from
synchrotronX-ray data are seemingly in contradiction with
measure-ments made in the bulk using neutron diffraction, asthey
reveal tensile stress in the weld metal surface.24,25
In this work, near surface neutron diffraction hasbeen used to
probe the residual stress state in the vicinityof the free surface
and these results are criticallycompared with residual stresses
determined using X-ray diffraction. This study highlights the
differencesbetween the residual stresses at the surface and those
inthe bulk immediately below it, and serves to rationalisethe
apparent disparity between the residual stress dataobtained with
X-ray and neutron diffraction in previousstudies.
Experimental methodThree multipass welds were prepared from
differentwelding consumables (LTT-1, LTT-2, HTT) depositedon a high
strength ferritic steel plate (BP700) in order tomeasure the
surface residual stresses. The specificwelding procedures are given
in Table 1, where LTT-1and LTT-2 are martensitic stainless steel
filler alloyswith a low MS, while HTT is a commercially
availablefiller with a much higher MS.
The equation by Steven and Haynes26 was used tocalculate the MS
of the welding fillers. Dilatometry wasthen used to verify the MS
of LTT-1 in its undilutedstate. The measured value (16412uC) was in
agreementwith the calculation. The welding alloy compositionsand
predicted martensite start temperatures are shownin Table 2. LTT-2
is highly alloyed and was designed tocompensate for dilution that
occurs when the filler mixeswith the baseplate.
The baseplate was prepared from 5006150615 mmsections, machined
with a 60u, 8 mm deep V groovealong the long direction and a root
radius of 4 mm. Theplates were clamped to the bench before
mechanised gasshielded metal arc welding, which was
performedhorizontally in the down hand position (Fig. 1). Allthree
welding alloys were deposited using metal coredelectrode wire, with
an initial preheat of 50uC and aninterpass temperature of 100125uC.
The heat inputs forthe LTT and HTT welding alloys were y1?0 andy1?5
kJ mm21 respectively. Details of the weldingparameters are in Table
3. The mechanical propertiesof the welding fillers and baseplate
are shown in Table 4;the LTT data are measured values.
Welds L1 and H1 were produced to compare thesurface stress
distributions that develop during thecooling of a weld fabricated
with an LTT filler withthose that develop using a conventional
filler, with the
Table 1 Combinations of baseplate and weld ller alloys
Weld Baseplate Pass 1 Pass 2 Pass 3
L1 BP700 LTT-1 LTT-1 LTT-1L2 BP700 HTT HTT LTT-2H1 BP700 HTT HTT
HTT
Table 2 Compositions (wt-%) and calculated MS of undiluted
welding alloys and baseplate
Material C Si Mn Cr Ni Mo MS/uC
LTT-1 0?010?03 0?60?8 1?21?7 12?513?0 5?56?5 ,0?1 169LTT-2 ,0?02
,1 ,2 1518 68 ,0?1 87HTT 0?12 0?65 1?50 0?70 2?80 0?85 372BP700
0?15 0?29 0?98 0?25 0?043 0?15 447
1 Photograph of weld being produced by automated gas
shielded metal arc welding
Table 3 Welding parameters
Voltage/V Current/A Shielding gas Gas flow/L min21
Welding speed/cm min21
Pass 1 Pass 2 Pass 3
24?7 y250 Arz2%CO2 18 36 30 23
Ramjaun et al. LTT surface stresses
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aim of correlating with fatigue data. Weld L2 wasassessed to
determine whether a single capping passwould be sufficient to
provide the same surface stressdistribution as a full LTT weld and
hence, the desiredfatigue improvement. Surface residual stresses
weremeasured with both neutrons and X-rays.
Neutron diffractionThe residual stresses measured using neutron
diffractionwere performed on the L3 beamline at the CanadianNeutron
Beam Centre.27 Strain scanning was performedacross a plane
perpendicular to the weld at the positionalong the sample shown in
Fig. 2. Measurements weremade at 0?15 and 2?5 mm below the top
surface in theweld, heat affected zone (HAZ) and baseplate
toascertain how the stresses vary with depth and acrossboth sides
of the weld.
A 0?360?363 mm gauge volume was defined using0?3 mm wide slits
on the incident and scattered sides,with a height limiter on the
incident side for thelongitudinal direction. The transverse and
normaldirections were measured using a gauge volume of0?360?3620
mm, with the long dimension of thisvolume parallel to the weld
direction. The use of anelongated gauge volume in the welding
direction wasdeemed appropriate as the residual stress is not
expectedto vary significantly along the central portion of
thewelded plates. The gauge volume selected was deemed tobe the
smallest possible that was capable of producingdefined diffraction
peaks. The centroid of the gaugevolume could therefore be
positioned at 0?15 mm belowthe surface, while simultaneously
avoiding partialimmersion errors. It was assumed that the
principalstresses were parallel to the plate edges.
A monochromatic beam with a wavelength ofy1?66 A was provided
from a squeezed Ge (004)monochromator set to a take off angle
2hM571?88u.The diffracted intensities were recorded on a
positionsensitive detector. Measurements of the {211}
ferritediffraction peak were performed around a scatteringangle of
2hS
-
The elastic strain ehkl at each measurement locationand
direction was then found as follows
ehkl~dhkl{d0,hkl
d0,hkl(1)
where dhkl is the interplanar spacing in the weld for
areflection (hkl).The stress sii in each of the three
orthogonal
directions was found from the strain measurementsaccording
to:
sii~E
1zneiiz
n
1{2ne11ze22ze33
h i(2)
where the Youngs modulus E5220 GPa and Poissonsratio n50?28, are
the diffraction elastic constants for the{211}.30 i51, 2, 3 denotes
the direction of the latticespacing measurement and hence, strain
and stressdirection with respect to the welded plate geometry.
X-ray diffractionThe residual stresses measured using X-rays
wereperformed on an Xstress 3000 instrument using Cr Karadiation on
the {211} for the ferrite phase. A 2 mmcollimator was used and
penetration depths were,10 mm from the sample surface, along the
planeidentified in Fig. 2. Stresses in the longitudinal
andtransverse orientations of the welded plate were inferredthrough
the sin2 y technique.31 The y angle was variedbetween 245 and z45u
(eight angles in total).Measurements were performed on the as
receivedwelded plates, with no prior grinding or polishing.
Results and discussionFor each of the welded plates, three sets
of stress profilesare presented. These include surface stresses
measuredby neutron diffraction and X-rays. Further measure-ments
were made at 2?5 mm below the surface, to allowcomparison with data
collected from greater depthsbelow the free surface at other
neutron sources usinglarger gauge volumes. It should be noted that
the surfacestress results measured by neutron diffraction in
this
work were volume averaged over 00?3 mm below thesurface.
Macrographs of the three welds, as shown inFig. 4, reveal the weld
layer structure. The boundarybetween passes is particularly
pronounced when LTT-2is deposited on the HTT filler.
Surface stresses: neutronsThe surface residual stresses measured
in three orienta-tions by neutron diffraction are presented in Fig.
5.Larger experimental uncertainties are apparent in theweld metal
than the baseplate for all three specimens,which may be attributed
to solidification texture.However, trends are evident and symmetry
along thecentreline would suggest that the data are reliable.The
surface stress profile for Weld L1 (Fig. 5a) shows
the characteristic high tensile longitudinal stresses in theHAZ
with compression in the weld metal, which ischaracteristic of these
types of LTT alloys and arises as aresult of the shape deformation
and net expansionfollowing transformation.13 Conversely, the
transversestresses are tensile in nature in the weld metal and
mildlycompressive in the HAZ. The normal stresses arecompressive in
the weld metal. Given their proximityto a free surface they may be
expected to be closer tozero, but it is possible for stress to be
retained within afew hundred micrometres of material.The filler for
weld H1 (Fig. 5b) has a sufficiently high
MS that the benefits of transformation plasticity areeradicated
on further cooling to ambient temperatureand this is reflected by
the residual tensile longitudinalstresses in the weld metal. As
with weld L1, high tensilelongitudinal stresses exist within the
HAZ. The trans-verse stress is tensile in the weld metal and does
notdisplay the sharp change to a compressive stress at thefusion
boundary, which is evident for weld L1. Thenormal stress fluctuates
about the zero stress mark inboth the weld metal and baseplate.Weld
L2 (Fig. 5c), which has a capping pass of a
highly alloyed LTT filler only, displays a stress profilemore
akin to weld L1. The longitudinal stress iscompressive in the weld
metal with high tensile stressesin the HAZ. Continuing the trend of
the previous two
4 Macrostructures of a weld L1, b weld H1 and c weld L2
Ramjaun et al. LTT surface stresses
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specimens, tensile transverse stresses are found in theweld
metal. The normal stress is generally mildly tensile,but also
fluctuates about zero.
The transverse stress measured at the critical locationof the
weld toe differs significantly between weld L1(about 2600 MPa) and
weld H1 (about 2200 MPa).Also, the compressive region for weld L1
extends furtherinto the baseplate and weld metal. The filler MS
clearlyinfluences the transverse residual stress distribution,
buttension remains in the central portion of the weld metal.This is
not the case for the stresses measured in thelongitudinal
orientation. A possible explanation for thisbehaviour is that the
longitudinally generated stressesare the greatest because this is
the direction of maximumthermal constraint during cooling. External
stresses areknown to initiate variant selection during the
earlystages of the cRa9 transformation.32 It is, therefore,presumed
that the martensitic variants initially orientthemselves in order
to cancel the dominant longitudinalstresses. However, as the
transformation continues,subsequent variants are less free to align
themselves insuch a manner as to minimise the tensile stresses in
thetransverse orientation. This hypothesis may be con-firmed by
detailed microstructural analysis; however,this is beyond the scope
of this paper.
The fusion boundary at the sample surface is the siteof greatest
interest due to this being the predominantlocation of failure
during fatigue experiments.33 Theresidual stresses at the fusion
boundary for weld L1show compression in all orientations. In
contrast, thelongitudinal stresses in weld H1 are tensile in
nature.The maximum tensile stresses at the fusion boundary for
weld L2 are less tensile compared with weld H1, whichwould
suggest that there are benefits to applying an LTTcapping pass in
multipass welds, but not to the extent ofa full LTT weld (weld
L1).
Surface stresses: X-raysLongitudinal and transverse residual
stresses measuredwith X-rays using the sin2 y method are presented
withthe surface measurements made with neutrons overlaidin Fig. 6.
The error bars from the neutron data have notbeen included for
clarity, but can be referred to in Fig. 5.Some of the X-ray results
had excessive errors and havebeen omitted, which may be attributed
to solidificationtexture. While it is useful to compare both the
X-ray andneutron data and anticipate similarities, it should
benoted that measurements were made with differentsampling volumes
and at slightly different depths belowthe surface.It is broadly
apparent that the stresses measured by X-
rays and neutrons are in agreement. The X-ray results alldisplay
tensile transverse stresses in the weld metal andthe effects of
using an LTT filler show a reversal oftensile stress (Fig. 6b) to
compression (Fig. 6a) in theweld in the longitudinal orientation.
The X-ray mea-surements do not appear to fully replicate the
peaklongitudinal tensile stresses in the HAZ, which wereidentified
by neutron diffraction. Perhaps the profilesgenerated by the two
techniques may show greatercorrelation if the number of X-ray
measurementlocations were to be increased. However,
completeagreement between the X-ray and neutron diffractionresults
cannot be expected due to the difference inexperimental
conditions.
a weld L1; b weld H1; c weld L25 Near surface residual stresses
measured at depth of 0?15 mm by neutron diffraction
Ramjaun et al. LTT surface stresses
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Bulk stresses: neutronsThe small gauge volume necessary to
measure surfacestresses deceases the volume of material sampled
andtherefore, the number of grains. In order to verify theeffects
of the reduced sampling volume, measurementswere made at 2?5 mm
below the surface and comparedwith previously collected data (Fig.
7).23,34 The reduc-tion in sampling volume is significant, 1006 for
weldsL1/L2 and 306 for weld H1. The trends identified forboth sets
of data for the three welded plates arecomparable, the only
exception being the extent ofcompressive stresses measured in the
weld metal for weldL2. This is not necessarily a discrepancy
because thelarger gauge volume will average the strains measured
ata depth of 2?51?5 mm, while the small gauge volumerange is only
2?50?15 mm. This could have asignificant effect, depending on the
stress gradients inthis region. Error bars have been included for
thelongitudinal orientation and are representative of
allorientations for the small gauge volume. Error bars
areencompassed within the marker for the large gaugevolume
measurements. Critically, the stress distributionsmeasured in the
vicinity of the surface by both neutronsand X-rays are
significantly different to those measuredat 2?5 mm below the
surface, with high tensile transversestresses at the surface in the
weld metal being the majordifferential.
ConclusionsMeasurement of the surface residual stresses
producedfollowing the welding of a series of ferritic steel
plateswith low transformation temperature filler alloys has
been performed using X-ray and neutron diffraction.From this
study, the following conclusions and recom-mendations have been
drawn:
1. The adoption of a small gauge volume haspermitted the
measurement of near surface residualstresses by neutron
diffraction. For the first time, thistechnique has been employed to
measure the residualstresses in LTT welds and reveal the local
stressvariations at the critical fusion boundary location.
2. The surface stress distributions measured byneutron
diffraction are comparable with those obtainedfrom laboratory
X-rays using the sin2 y technique.3. Two LTT welding alloys have
been shown capable
of inducing compressive longitudinal residual stressesinto the
surface layers of the weld metal for multipasswelds. Both neutron
and X-ray diffraction confirm thesefindings.
4. Tensile transverse stresses were measured in theweld metal
for all three welded plates. The stressesbecame compressive at the
fusion boundaries, with weldL1 displaying the greatest levels of
compression.
5. Weld L1 appears to display the most desirableresidual
stresses as they are compressive in nature at thefusion boundary,
which is the expected site of crackinitiation and subsequent
propagation during service. Incontrast, weld H1, which was produced
with a conven-tional filler alloy, produces tensile longitudinal
stressesat the fusion boundary.
6. Weld L2, which has a singular capping pass madeby an LTT
filler, is capable of inducing compressivelongitudinal stress in
the weld metal near the surface andthe stress profile across the
fusion boundary appearspreferable to weld H1.
a weld L1; b weld H1; c weld L26 Surface residual stresses
measured using X-rays (solid lines) and neutrons (dashed lines)
Ramjaun et al. LTT surface stresses
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Acknowledgements
We are grateful to ESAB AB for sponsoring thisresearch. This
work is based upon experiments per-formed on the L3 beam line at
the Canadian NeutronBeam Centre, Chalk River Laboratories,
Ontario,Canada.
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