Wse2013 Paper Deng 3

8/12/2019 Wse2013 Paper Deng 3

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Proceedings of the WSE2013

Investigation of Influence of Phase Transformation on Welding ResidualStress in P91 Steel

Bi Tao*, Deng Dean*, Tong Yangang*, Liu Xiaozhan*, Zhou Yijun** College of Materials Science and Engineering, Chongqing University, No. 174, Shazhengjie,Shapingba, Chongqing, 400045, China

KEY WORDS: ( Phase transformation), (Residual stress), (Numerical simulation), (TRIP), (Laser Welding)

1. Introduction Ferritic-martensite steels containing 9-12 wt.%

chromium have been widely used in high temperatureservice equipment manufacturing, because of their superior

properties such as high thermal conductivity, low thermalexpansion coefficient, and good resistance to stresscorrosion crack [1] . However, the potential efficiency gainsin using 9-12 wt.% steels are often impeded by prematurefailures in welded joints due to type-IV cracking. Recentresearch has shown that high tensile residual stresses aregenerated in the type-IV cracking [2] region in welded P91

boiler pipe girths, whereas such stresses can hardly berelieved by conventional post-weld heat treatment. In thecurrent study, a computational approach based onSYWELD software was developed to simulate the weldingresidual stresses in P91 welded joint, particularly as suchstresses might be affected by solid-state phasetransformation taking place in the weld and theheat-affected zone. Three factors, namely, the volumetricchange, the variation of mechanical properties, and thetransformation induced plasticity (TRIP), are taken intoaccount as the main factors that may significantly affectwelding residual stresses via the processes of phasetransformation. In this study, a finite element model wasdeveloped to simulate welding temperature field, residualstress field induced by laser beam welding in a butt welded

joint. Meanwhile, the welding residual stresses predicted bythe FEM were compared with the experimental resultscontacted by Residual stresses in laser welded ASTM A387Grade 91 steel plates [3] .

]2. Finite element model

To investigate the influence of solid-state phasetransformation on the formation of welding residual stress,a finite element model was developed. The finite elementmodel is shown in Fig. 1 . The thickness of the model is9mm, and the total number of nodes is 66, 531, and that ofelement is 57,600. In this study, it was assumed that laser

beam welding process was used to perform the welding.The welding parameters are shown in Table 1 . SYSWELDsoftware was used to simulate welding temperature fieldand residual stress field.

In the thermal analysis, Goldak heat source model wasused to simulate the heat input of laser welding, as shownin Fig.2 . The thermal physical and mechanical properties ofthe weld metal were assumed to be the same as those of

base metal because filler wire was not used in the laserwelding process. The temperature-dependent thermal

physical properties are shown in Fig. 3 . In the simulation,the K-M relationship was used to model theaustenite-martensite transformation.

In the mechanical analysis, the elastic strain-stressrelationship was modeled using the isotropic Hookes law,and plastic behavior is considered through Von Missescriterion. The effect of work hardening was neglected inthis study. The thermal strain was considered using thermalexpansion coefficient. During the solid-state phasetransformation, the variation of yield strength wasconsidered. The temperature-dependent mechanical

properties of base metal (tempered at 770 ) are shown inFig. 4 .

Fig.1 Finite element model

Fig. 2 Heal flux distribution in Goldak heat source model

a 1

a 2

x

y

z b

c

Rear half ellipsolid Front half ellipsolid

q= ( x,y,z )

200

Welding Line

100


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Influence of TRIP on calculated results of residual stress in a low temperature transformation steel joint

To examine the influence of TRIP (TransformationInduced Plasticity) on the welding residual stress, two caseswere simulated in this study. These two cases aresummarized in Table 2 .

Fig. 3 Thermal physical properties of P91

Fig.4 Temperature-dependent mechanical properties of P91

Table 1 Welding parameters

weldingcurrent

arcvoltage

weldingspeed

arcefficiency

300A 25V 12.5mm/s 85%

Table 2 Simulation cases

Case Volumechange

Yield strengthchange

TRIP

A Yes Yes NoB Yes Yes Yes

3. Simulation results and comparison withmeasurements3.1 Results of thermal analysis

The fusion zone simulated by the finite element modelwas shown in Fig.5 . The picture of fusion zone [3] is shown

in Fig. 6 . Comparing Fig. 5 and Fig. 6 , we can find thatalthough the reinforcement and sunken of the weld beadwere neglected in the numerical model, however the fusionzone of FEM is roughly similar to that of the experiment.

Fig. 5 Fusion zone predicted by FEM

Fig. 6 Fusion measured by experiment

3.2 Results of mechanical analysisFig. 7 shows the contour of longitudinal residual stress

distribution of the top surface in Case B . Fig.8 shows thelongitudinal residual stress distribution of the middlecross-section of Case B. These two figures tell us that thereare compressive longitudinal stress in the weld zone and theheat affected zone (HAZ). This information indicates thatthe phase transformation especially the volume change hasa significant influence on the final longitudinal stressdistribution. Volume change can largely reduce theaccumulated longitudinal stresses before the start ofaustenite- martensite transformation. In this case, becausethe martensite start temperature is relatively low, the solid state phase transformation not only changed themagnitude of longitudinal stress but also altered its sign(tension /compression).

Carefully observing Fig.8 , we can find that the peak

value of longitudinal residual stress appears at the regionsnear the HAZ, and it is larger than the yield strength of the base metal.


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Proceedings of the WSE2013

Fig. 7 Contour of longitudinal residual stress of top surface(Case B)

Fig.8 Longitudinal residual stress distribution ofMid-section (Case B)

Fig. 9 Contour of transverse residual stress of top surface(Case B)

Fig. 10 Transverse residual stress distribution ofMid-section (Case B)

Fig. 9 shows the contour of transverse residual stress ofthe top surface of Case B. From this figure we can see thatthere are compressive transverse stresses near the two endsof the FE model. Except for the two ends, there arerelatively large tensile transverse stresses around the weldzone. On the whole, the transverse residual stresses aresmaller than the yield strength of base metal. Fig. 10 showsthe transverse residual stress distribution of Mid-section

predicted by Case B. It is interesting that there are tensiletransverse residual stresses near both the top surface and the

bottom surface. In the inside of the plate, there arecompressive transverse stresses near the weld beam.

Fig. 11 compares the simulation results and the measureddata [3] . In this figure, the longitudinal residual stress

Fig. 11 Comparison of the simulation results and themeasured data [3]

distributions on the top surface of the middle cross-section

predicted by these two cases and the measured data are plotted. From Fig.11 , we can know that compared withCase A, the simulated results of Case B are closer to theexperiment at and near the weld zone. Although the

predictions of Case A generally match the measurements atand near the fusion zone, there are some discrepancies atthe weld zone and the HAZ. On the whole, the compressivelongitudinal stresses predicted by Case A are larger thanthose measured by experiment. This information suggeststhat to accurately simulate welding residual stresses in thefusion zone and the HAZ for alloy steels such as P91, theTRIP should also be considered in the numerical model.From the same figure, we can also obverse that there areerrors between the predictions and the measurement in theranges away from the weld zone. Generally, the simulatedresults are larger than the measured data. In the aspect ofnumerical model, perhaps the temperature-data mechanical

properties used in the finite element model did not exactlyreflect the real status of the experimental mock-up. In theaspect of experiment, the measuring method also has someshortcoming and can cause some systematic errors. Inaddition, there are initial residual stresses in the weldmentdue to manufacturing process before welding. In this study,we only considered welding process in the numericalsimulation, but the other manufacturing processes wereneglected completely. Generally when the distance fromthe weld zone is large, the initial residual stress is moredifficult to be affected by welding process [4]. So, thediscrepancies can also be caused by the initial residualstresses.

4. ConclusionsIn this study, we developed a computational approach

based on SYSWELD software to simulate welding residualstress with considering solid-state phase transformation.Using the developed approach, we simulated the weldingresidual stresses induced by laser welding in P91 steel. Inaddition, we also compared the simulated results with theexperimental measurements in the opening literature. Wecan draw the following conclusions:

1) Based on SYSWELD software, we have developed a


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Influence of TRIP on calculated results of residual stress in a low temperature transformation steel joint

computational approach to simulate welding residual stressin P91 steel with considering solid-state phasetransformation.

2) The simulated results show that the solid-state phasetransformation has a significant influence on the welding

residual stress, and it not only can change the magnitude ofresidual stress but also can alter the sign of longitudinalstress in the present study.

3) It seems that TRIP has influence on the weldingresidual stress in P91 steel. When this factor wasconsidered in the numerical model, the predictions matchthe measurements better.

Acknowledgements

This research was supported by National Natural ScienceFoundation of China (Project No.51275544), theOpen-Fund Research of State Key Laboratory of Advanced

Welding and Joining, Harbin Institute of Technology and by the fundamental Research Funds for Central University(Project No. CDJZR12130036).

References[1] J. A. Francis, W. Mazur, H. K. D. H. Bhadeshia,

Review Type IV cracking in ferritic power plant steels,Science and Technology of Welding & Joining, Vol.22,

No. 12, 2006, pp.1387-1395.[2] K. Laha, K. S. Chandravathi, P. Parameswaran, K. B.

S. Rao, S. L. Mannan, Characterization ofmicrostructures across the heat-affected zone of themodified 9Cr-1Mo weld joint to understand its role in

promoting type IV cracking Metallurgical andMaterials Transactions A, 38A , 2007, pp. 58 68.

[3] Santosh Kumar, A.Kundu, K.A.Venkaka, C.E.Truman,J.A.Francis, Residual stresses in laser welded ASTMA387 Grade 91 steel plates, Materials Science &Engineering A. 575, 2013, pp.160-168

[4] D. Deng, S. Kiyohsima, Numerical simulation ofresidual stresses induced by laser beam welding in aSUS316 stainless steel pipe with considering initialresidual stress influences, Nuclear Engineering andDesign, Vol.240, Issue 4, 2010, pp.688 696

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