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Science and Technology of Welding and Joining

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Influence of alloy type, peak temperature andconstraint on residual stress evolution in Satohtest

J. P. Galler, J. N. DuPont & J. A. Siefert

To cite this article: J. P. Galler, J. N. DuPont & J. A. Siefert (2016) Influence of alloy type, peaktemperature and constraint on residual stress evolution in Satoh test, Science and Technologyof Welding and Joining, 21:2, 106-113

To link to this article: http://dx.doi.org/10.1179/1362171815Y.0000000071

Published online: 25 Feb 2016.

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RESEARCH PAPER

Influence of alloy type, peak temperature andconstraint on residual stress evolutionin Satoh test

J. P. Galler*1, J. N. DuPont1 and J. A. Siefert2

The evolution of residual stress in welds is affected by a variety of factors such as joint design,

welding parameters, material properties and the possible presence of phase transformations. The

Satoh test can be useful as a method to understand differences in welding residual stresses

among various materials. This study compares the evolution of residual stress measured in the

Satoh test for ferritic and stainless steels. For weld thermal cycles with a peak temperature above

Ac3, the ferritic alloys exhibit lower residual stresses than the austenitic alloys due to the well

known reduction in stress from the martensitic or bainitic transformation. However, the residual

stress levels among these alloys are similar for peak temperatures below Ac3. The application of

constraint during both the heating and cooling portions of the thermal cycle affects the final

magnitude of residual stress in a way that is not often considered.

Keywords: Welding residual stress, Satoh test, Phase transformation

IntroductionResidual stresses in welds can be quite significant andhave an appreciable influence on weldability and serviceperformance.1 The evolution of welding residual stress isaffected by a number of factors, including weld geo-metry, joint design, welding parameters and the tem-perature dependent thermophysical and mechanicalproperties.2–4 The large difference in temperaturedependent material properties between unique alloys oralloy systems is particularly important and expected toproduce wide variations in behaviour. Further compli-cations arise in alloys that experience a solid state phasetransformation during cooling, as this transformationalso affects the magnitude of accumulated weld residualstress.6–16 The wide range of complex interactionsbetween welding parameters, weld design and materialproperties makes it difficult to make straight forwardcomparisons between expected differences of residualstress among engineering alloys. It is clear that a fun-damental assessment of welding residual stress for agiven alloy or alloy system is important in assessing itspotential performance in a given application or forproviding critical feedback such as in a root causeanalysis of a component failure. Welding residualstresses can have significant effects in service behavioursuch as reheat cracking in ferritic alloys, stress relaxationcracking in stainless steels and stress corrosion crackingin many alloy systems.

The Satoh test has recently been applied as a means tostudy residual stress evolution in welds in a controlledmanner for the purpose of making direct comparisonsbetween engineering alloys.5–7,10,12–17 As shown sche-matically in Fig. 1, a sample is locally heated and cooledunder conditions that simulate the heating and coolingrates during fusion welding. The peak temperaturerepresentative of the coarse grain heat affected zone(CGHAZ) is often used in the weld thermal cyclesimulation. In a conventional Satoh test, the sample ispermitted to expand freely during the heating stage.When the peak temperature is reached, the sample iscooled while contraction is restricted, and the residualstress that accumulates due to the restricted contractionis then measured with a load cell in line with the sample.This procedure is thought to simulate the restrainedcontraction that accounts for residual stress accumu-lation in real welds.5

Work carried out by Dai et al.6 and Frances et al.7

showed that the transformation temperature in steelsaffects the final residual stress when using the Satoh test.Jones andAlberry8 conclude that residual stresses are bestavoided by suppressing the transformation temperaturelow enough so that the phase change compensates for thethermal contraction. Not only can this reduce residualstresses at ambient temperature, but this can lead to thepresence of favourable compressive residual stresses nearthe weld surface. A lower transformation temperaturemerits a smaller temperature range after the transfor-mation in which the residual stress can begin toreaccumulate. This effect is well known and is being usedto develop welding consumables with low transformationtemperatures to help minimise residual tensile stresses.9

Shirzadi and Bhadeshia10 used a modified Satoh testdesign to help ensure uniform temperature and stress

1Department of Materials Science and Engineering, Lehigh University,Bethlehem, PA 18015, USA2Electrical Power Research Institute, 1300 West W. T. Harris Blvd,Charlotte, NC 28262, USA

*Corresponding author, email jpg211@lehigh.edu

� 2016 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 06 May 2015; accepted 24 June 2015DOI 10.1179/1362171815Y.0000000071 Science and Technology of Welding and Joining 2016 VOL NO21 2 106

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conditions within the test material. The concept involvedthe bonding of two pieces of non-transforming metal(alloy IN617) to the test material. This was comparedwith a sample made entirely of the test material. Bothsamples behaved identically until the start of the phasetransformation, with the monolithic sample transform-ing at a higher temperature than the bimetallic sample.This was attributed to the regions outside of the samplecentre that transform before the centre of the sample.The bimetallic sample showed a greater level of residualstress at room temperature. This was attributed to theaddition of alloy IN617, which has a higher coefficient ofthermal expansion than the test material. Thus, thermalcontraction stresses were able to accumulate moreduring cooling due to this added material. Also descri-bed in that work is a design to completely eliminateresidual stresses outside of the HAZ by the use of Invar,a material that has zero thermal expansion. With thisapproach, the material of interest would be the onlycontributor of residual stress accumulation in the HAZ.

Satoh test results have been provided to date that showthe accumulation of stress for different alloys. However,these tests generally only used one (or two) peak tem-perature(s). In the case of two peak temperatures,13,15 peaktemperature values above and below Ac3 were used tocompare the effect of the phase transformation. However,the HAZ experiences peak temperatures from the solidustemperature to ambient temperature. Thus, use of a single(or two) peak temperature(s) cannot provide informationon the range of residual stresses expected in the HAZ.In addition, most tests conducted to date allow freeexpansion during the heating stage of the thermal cycle.However, this does not represent an actual weld thermalcycle where there is constraint during both heating andcooling. Another style of Satoh tests was conducted wherethe sample is under constraint during the entire thermalcycle.16,17 In this case, stress accumulates in compressionduring heating due to the restricted thermal expansion.Depending on peak temperature, compressive stressesmaystill be present during cooling and will eventually turn totension due to the applied constraint. The tensile stresseswill start to form at lower temperatures since it takes timefor the compressive stresses to reduce and turn to tension,thus resulting in a lower final level of residual tensile stress.These studies demonstrate the rather wide range of resultsthat can be observed depending on the test procedureemployed. The Satoh tests appear to become a common

method for understanding residual stress formation inwelds. Thus, it is useful to consider how various proceduralparameters affect the results of the tests.

The main objectives of this research are to compare theevolution of residual stress measured in the Satoh testbetweenvariousalloys and investigate the influenceofpeaktemperature on residual stress evolution. Preliminary teststhat apply constraint during both the heating and coolingstages of the thermal cycle are also conducted, and theresults are compared to those obtained with the standardSatoh test. The results of this work should be useful formaking direct comparisons of residual stress levels in var-ious alloys and should also shed light on procedural issuesthat should be considered with the Satoh test.

ExperimentalIn this study, a Gleeble 3500 thermomechanical simu-lator was used to conduct Satoh tests on the alloyssummarised in Table 1. Peak temperatures Tp of 1350,1100, 1000, 900 and 750uC were used, along with heatingand cooling rates of 10 and 30uC s21 respectively. Theseheating and cooling rates were chosen as they represent areasonable range of heat inputs for traditional arcwelding processes. Cylindrical samples (110 mm longand 10 mm in diameter) with threaded ends were usedfor all of the tests. The samples had a reduced centregage section 15 mm long and 6 mm in diameter.

Most tests were conducted in which the samples wereheated without constraint to the Tp, locked in place andallowed to cool to room temperature Tr, while stress wasmeasured as a function of temperature. Selected testswere also conducted with constraint applied during boththe heating and cooling portions of the thermal cycle.The Gleeble has the added advantage that phase trans-formations can be monitored in situ during a Satoh testwith a dilatometer placed on the centre of the sample(i.e. the same location in which the temperature ismonitored and controlled). The dilatometer was used tomeasure the radial expansion and contraction of thesample during the thermal cycle to detect the Ac1 andAc3 transformation during heating and the martensiticor bainitic transformation during cooling. Figure 2shows an image of the specimen set up in the Gleeblebefore the installation of the dilatometer. Copper grips

1 Schematic illustration of Satoh test comparing evolution

of temperature and stress as function of time

Table 1 Compositions of alloys used in this study/wt-%

Element T24 P91 304H 347H

C 0.08 0.10 0.04–0.10 0.08Si 0.28 0.28 0.75 0.75Mn 0.51 0.56 2.0 2.0P 0.007 0.017 0.045 0.045S 0.005 ,0.001 0.03 0.03Cu 0.04Ni 0.12 0.17 8–10.5 9–13Cr 2.43 8.63 18–20 17–19Mo 0.95 0.89Al 0.010 0.003Ti 0.070 ,0.003V 0.24 0.21Nb ,0.01 0.080 0.54B 0.0042N 0.007 0.0386Zr ,0.001W ,0.01

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were used for the tests, as well as conical jacks thatrestrict axial expansion of the specimen in the grips.

Results from some of the preliminary tests are shownin Fig. 3 and demonstrate abnormal behaviours in thestress at low temperatures, shown by the red circle.At temperatures close to Tr, the stress deviated fromlinear accumulation and in some cases began todecrease, suggesting a source of slippage in the testapparatus. As shown in Fig. 2, the specimen has threa-ded ends with nuts that apply pressure on the outersurface of the copper grips. During thermal contraction,the grips were observed to slip inwards during the testdue to the force from the nuts, thus causing a reductionin the stress. This problem was avoided by pulling thesample to 90% of the yield strength before the test andtightening the outermost nuts that constricted expansionof the sample in the grips. This induced a compressivestress on the sample, thus securing the copper grips intothe pocket jaw assembly. The sample was again pulled intension to 90% yield strength and relieved by tighteningthe conical nuts. This process was repeated a coupletimes to secure the copper grips in the Gleeble jaws.Any stress imposed during the securing of the samplewas zeroed out by controlling the stroke of the Gleeble.Any presence of stress (tensile or compressive) is alsoeliminated once the test starts since, during the heatingcycle of the test, the sample is not constricted and

allowed to achieve a stress free state. This procedure wasconducted before each test to eliminate sample slippageand provide accurate residual stress measurements.Although the exact cause of the sample slippage has notbeen considered in detail, evidence for similar types ofstress reduction at the lower temperatures has beenobserved on other Satoh tests results.15

The temperature distribution was also measured alongthe length of the sample during the thermal cycle forvarious peak temperatures. Thermocouples were spotwelded to different locations of the sample in order tomeasure thermal cycles for locations further from thesample centre. Figure 4a and b shows the temperaturedistribution for alloy P91 and 304H respectively. Thesemeasurements show the temperature gradient throughthe sample at the moment the peak temperature ofinterest is reached. Figure 4c demonstrates the differencein temperature gradients between materials. This can beattributed to the lower thermal conductivity in 304H(14.5 W m21 K21) than P91 (26.7 W m21 K21).

To summarise, the procedure for conducting a Satohtest in the Gleeble involves the following critical aspects:

(i) placing the sample in the copper grips and intothe Gleeble jaws

(ii) pulling the sample in tension to 90% of the yieldstrength

(iii) tightening both conical nuts (this will relieve thetensile force and create a compressive force)

(iv) pulling the sample in tension again and securingthe copper grips in the jaws

(v) tightening both conical nuts to relieve the tensileforce

(vi) repeating steps 2–4 as necessary.

As described above, any force on the sample from thisprocedure will be relieved when test starts.

Results and discussion

Ferritic alloysFigure 5a shows the accumulation of residual stress as afunction of temperature for alloy P91 exposed to variouspeak temperatures. These results were obtained with norestraint applied during heating. The dilatometry results

2 Image of Satoh test specimen in Gleeble before addition of dilatometer

3 Preliminary Satoh test results demonstrating influence of

slippage during test that causes reduction in final stress,

shown by red circle

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acquired for a Tp of 1350uC are shown in Fig. 5b, wherethe dilation is shown as a function of temperature. Thereis a rather large deviation between the dilation onheating and cooling that is associated with additionaldilation from restricted contraction of the sample. TheAc1 and Ac3 represent the start and finish of the auste-nite phase transformation during heating respectively,while the Ms and Mf represent the martensite start andfinish temperatures of the alloy during cooling. Thereare several interesting observations from the results inFig. 5. The alloy exhibits a reversal of residual stressat temperatures near Ms temperature. However, for aTp of 1350uC, the stress reversal in the Satoh testoccurs *500uC, whereas the dilatometry results showthe martensite transformation does not begin until375uC. The results also show that decreasing Tp pro-duces higher final residual stress values for peak tem-peratures that are above the Ac3. In contrast, theresidual stress is among the highest when the Tp is belowthe Ac3 (peak temperature of 750uC). This observationmatches the reported behaviour of single pass laserwelds, such as in Ref. 18 where the highest weldingresidual stresses are in a location in the HAZ, removedfrom the fusion line and consistent with a TpvAc1.

The reversal of stress associated with the martensitestart temperature is well recognised and is associatedwith the volume expansion from the austenite to mar-tensite transformation that counteracts the tensile strainduring cooling.6–16 However, the difference between theactual Ms temperature and the temperature at which thestress begins to reverse in a Satoh test is not alwaysrecognised. The cause for this difference becomesapparent when examining the Satoh test results in whichthe stress reversal begins at *500uC (Fig. 5a), thedilatometry results in which the martensite start tem-perature is 375uC (Fig. 5b), and the temperature gradientthrough the sample (Fig. 4a). The temperature ismeasured at the centre of the sample, and this is thetemperature that is conventionally plotted in the Satohtest results. The dilatometer also detects the phasechange at the centre of the sample. However, there areparts of the sample away from the centre that are heatedabove the Ac3 temperature but are at a lower tempera-ture than the sample centre. This region of the samplewill undergo the phase transformation before the centreof the sample. The phase transformation in this regioncounteracts the thermal contraction, so there is areduction in stress before the phase transformationmeasured with the dilatometer at the sample centre. Thiseffect accounts for the apparent differences between thedilation reading and the Satoh test stress reversal.Thus, care must be taken not to use the temperature atthe start of the stress reversal on the Satoh test as anindication of the start of the phase transformation whena temperature gradient exists through the sample.

The difference between transformation temperatureand temperature at which the stress reversal begins hasbeen noted in other work on SA508 steel.15 In this case,the stress reversal occurred at *600uC, whereas thereported bainite start temperature was 550uC. In pre-vious work, the apparent increase in the bainite starttemperature was attributed to the presence of stress,

4 Qualitative temperature distribution in Satoh test

specimen showing temperatures at different locations for

a P91 and b 304H and c difference between two materials;

location of thermocouples in a and b are indicated by

symbols at axial locations 27, 33, 41, 49 and 55 mm, with

41 mm being specimen centre

5 a Satoh test results and b dilatometry test results for alloy

P91 with heating rate of 108C s21 and cooling rate of

308C s21

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but results presented in this work demonstrate that thetemperature gradient in the sample is responsible for thisdifference. Figure 6 shows dilatometry results on alloyP91 comparing dilation results from a Satoh test anddilation results for athermal cycle under zero stress. Theresults show that both tests produce the same martensitestart temperature, indicating no significant influence ofstress on the transformation start temperature.

The influence of Tp on the magnitude of the residualstress in the steels used here can also be understood withreference to Fig. 5a. For peak temperatures above Ac3,a lower Tp generally accumulates greater stress beforethe transformation (at a temperature just before thestress reversal), as well as when cooled to Tr. This is alsoattributed to the temperature distribution throughoutthe sample. Regions away from the centre of the sampleexhibit a lower temperature, but are still above Ac1(shown in Fig. 4a) and therefore exhibit the phasetransformation during cooling before the sample centre.At higher peak temperatures, larger portions of thesample reach a temperature above Ac1. Thus, for ahigher Tp, a larger portion of the sample undergoes thevolume expansion associated with the phase transform-ation during cooling, leading to a larger reduction instress accumulation. Furthermore, since there is lessstress accumulated before the transformation, there isless final residual stress at Tr. Note that the peak tem-perature of 750uC is below Ac1 and therefore will notexperience a phase transformation during cooling thatcan counteract the residual tensile stress. Also note thisresults in the highest residual stress value at Tr. The finalresidual stress for the test conducted with a peak tem-perature of 900uC lies between the results for the samplesheated above and below the Ac3, since this test rep-resents an intercritical peak temperature that is betweenAc1 and Ac3. Thus, this intermediate residual stress levelis attributed to partial transformation that occurredduring heating.

These results carry important implications for futuretests. Satoh test results reported to date typically utilise asingle Tp (typically associated with the CGHAZ). This isoccasionally carried out to evaluate the effectiveness ofan alloy to counteract residual stresses via the martensitictransformation.5–7,10,12,14,16,17 However, the HAZ willexperience the full range of peak temperatures duringwelding (from ambient to the solidus temperature),

and the residual stress behaviour can thus not becaptured with results from a single Tp. In fact, Onsoienet al. suggested that it is most relevant to compareresidual stress levels in the Satoh test with the maximumaxial stress in girth welds.16 They measured residualstress levels in X70 pipeline steel using Satoh test andcompared that to values measured in real welds.19

The good correlation between maximum residual stressesfound in the Satoh test and the maximum valuesmeasured in real welds led to this conclusion. In this case,the maximum residual stress acquired in the Satoh testshould then also be used for a proper comparison. Use ofa single Tp well above Ac3 in the Satoh test wouldproduce a relatively low residual stress level that may notbe representative of the maximum residual stressmeasured in the HAZ. The results in Fig. 5a suggest thatSatoh tests should be conducted over a range of peaktemperatures to adequately capture this behaviour.

The results shown for P91 in Fig. 5a were duplicatedfor alloy T24, and the results are summarised in Fig. 7 inwhich the final residual stress is shown as a function ofpeak temperature. The martensite or bainite start tem-perature was measured for each alloy and is also notedin Fig. 7. Note that the residual stress level is reduced asthe transformation temperature is reduced. With a lowertransformation temperature, there is a smaller tem-perature range after the transformation in which theresidual stress can begin to reaccumulate. However,as shown in Fig. 5a, it is important to note that thedifference in residual stress level depends on Tp. Alsonote that the highest residual stress levels are expected tooccur for regions of the HAZ in which the peak tem-perature is below the Ac1. These effects may notalways be recognised and should be considered in futureSatoh tests.

Austenitic alloysFigure 8a and b shows typical Satoh test results for twoaustenitic alloys 304H and 347H respectively. The aus-tenitic alloys do not experience a solid state phasetransformation (other than possible carbide formationat longer times/lower cooling rates) and thus accumulate

7 Effect of peak temperature on residual stress measured at

room temperature during Satoh testing of T24 and P91

6 Dilatometry test, conducted on alloy P91, shows applied

stress had no significant effect on transformation

temperatures during heating and cooling stages

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greater residual stress levels than the ferritic alloys whencooled to Tr. In contrast to the ferritic alloys, use of alower Tp leads to the accumulation of less residual stressdue to the smaller temperature range associated with thethermal contraction.

A summary of the Satoh test conducted on all alloysof interest can be seen in Fig. 9. Use of peak tempera-tures above Ac3 produces significantly less stress inferritic alloys, especially at temperatures in the CGHAZ.This can be attributed to the transformation thatcounteracts the thermal contraction. However, for a Tp

of 750uC, where the ferritic alloys do not transform, theresidual stress can be comparable or even higher thanthe austenitic alloys, depending on the alloy.

Effect of applied constraintThe Satoh test is typically conducted by heating thesample without constraint. However, in actual welds,there is significant restraint during both the heating andcooling portions of the weld thermal cycle. Thus, tests

were conducted to investigate the possible effect ofrestraint applied during both the heating and coolingstages. Ferritic alloy P91 and alloy 347H stainless steelwere heated to three peak temperatures, 1350, 1100 and750uC to compare the difference in residual stress of theconventional Satoh test with a test conducted under fullconstraint. The results can be seen in Figs. 10 and 11.A Tp of 1350uC shows a compression stress accumu-lation upon heating for both alloys due to the restrictedexpansion from the constraint. The stress decreases atelevated temperatures due to the lack of strength andassociated plastic deformation. At 1350uC, the stress isalmost zero, so the accumulation during cooling followsthe same path as the conventional Satoh test. Somewhatsimilar results are observed for both alloys when a peaktemperature of 1100uC is used, regardless of the presenceof a phase transformation. These results suggest that theuse of constraint does not have a significant effect duringheating when peak temperatures associated with theCGHAZ are used.

However, at 750uC, there is a significant differencebetween the two methods of measuring stress. Duringheating, the sample reaches a state of compressiveresidual stress. This stress is still present when coolingbegins because the peak temperature was too low to

8 Satoh test results for a alloy 304H and b 347H at various

peak temperatures with heating rate of 108C s21 and

cooling rate of 308C s21

9 Satoh test results for alloys P91, T24, 347H and 304H at

peak temperatures of 750, 900,1000, 1100 and 13508C

10 Satoh test results comparing general Satoh test with

fully constrained specimen for alloy P91 at peak tem-

peratures of a 13508C, b 11008C and c 7508C with heating

rate of 108C s21 and cooling rate of 308C s21; red and

blue arrows show heating and cooling stages

respectively

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permit stress relaxation. During cooling, the compres-sive stress needs to be relieved before tensile stressesaccumulate. The constraint during heating causing thecompressive stress has a major effect on the magnitudeof residual stress when cooled to Tr. Tensile stresses donot start to form until *500uC, so there is a smallertemperature range for the stress to accumulate. This is incontrast to the conventional Satoh test, where the tensileresidual stresses start to form at their respected Tp.

ConclusionsSatoh tests were conducted on ferritic and austeniticalloys. The influence of peak temperature and theapplication of constraint applied during heating wereinvestigated. The following conclusions can be drawnfrom this work.

1. For the ferritic alloys investigated in this study, themagnitude of residual stress decreases withdecreasing martensite/bainite start temperature fortests conducted with peak temperatures above theAc3. This is attributed to the volume expansionassociated with the austenitic–martensitic trans-formation. Lower transformation temperatures

permit less of a temperature range for the residualstress to reaccumulate with continued cooling.

2. For ferritic steels tested without constraint duringheating, lower peak temperatures above the Ac3accumulate greater residual stress when cooled toroom temperature because less of the sample ex-periences the austenite–martensite/bainite trans-formation that counteracts the thermal contraction.

3. Ferritic alloys tested above the Ac3 generallyshowed lower residual stresses relative to austeniticalloys tested at comparable peak temperaturesbecause of the stress reduction associated with theaustenite–martensite/bainite transformation duringcooling. However, this large difference among theresidual stress levels in the ferritic alloys does notoccur when the peak temperature is below Ac1.

4. The application of constraint during both theheating and cooling portions of the thermal cycleeffects the final magnitude of residual stress relativeto the cycle in which constraint is only appliedduring cooling. The difference is negligible at highpeak temperatures due to stress relaxation. How-ever, at lower peak temperatures, the compressivestresses cannot completely relax during the heatingcycle, and the corresponding final tensile stressesare therefore reduced when the sample is con-strained during the heating stage.

AcknowledgementsThe authors gratefully acknowledge the financial supportfrom the Electric Power Research Institute and the NSFIndustry/University Collaborative Research Center forIntegrative Materials Joining Science for Energy Appli-cations (CIMJSEA) under contract no. IIP-1034703.

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