7/25/2019 Heat Input Model-root Openings

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Heat Input Model

It is important that heat input from thearc is exactly duplicated in modeling be-cause the heat input model has an im-portant effect on the accuracy of analysisfrom the temperature distribution, cool-ing rate, size of fusion zone and heat-affected zone to the strength of the weld-ment. The heat input model for analysisof weld residual stresses and deforma-tions was distinguished as a ramp heatinput model and a lumped pass model. Aramp heat input model was developed toavoid numerical convergence problemsbecause of an instantaneous increase int e mp e rature near the fusion zone, andenabled the model to include the effectof a moving arc in a 2-D plane. It takesinto account the variation of plane en-ergy flow in a 2-D model as the arc ap-proaches, travels across and departs fromeach plane under investigation.

The lumped pass model for thick mul-tipass welds developed to reduce analy-sis times and costs is useful in predictingresidual stress, but it is troublesome inpredicting weld deformation. In a single-V groove, the center of the total shrink-age force does not coincide with the neu-tral axis because the bead deposits arenot symmetric about the neutral axis.

This results in larger bend stress and pro-duces different final stress or strain rates.

The lumped pass amplifies these effects,so the ramp heat input model is recom-mended for welds that have unsymmetri-cal bead depositions such as a single-V-groove weld. The level is particularlys e vere in cases that have many weldpasses such as the 30-mm root openingweld in this study.

As previously described, the follow-ing heat input model is used to reduce

calculation costsand times throughthe ramp heat inputmodel in this study.Small time incre-ments are applied toa period of instanta-neous tempera t u r efluctuation that gen-erates stiff tempera-ture gradients due tothe localized highheat input and ra p i dcooling near the fu-sion zone; largetime increments areapplied for other pe-riods. Next, 100%ramp ratio is pro-vided to improve thec o nvergence of theanalyses. These methods make a differ-ence between the total time of actualwelding and the analytical one, includ-ing cool ing time. Figure 5 shows the heatinput model of the 100% ramp ra tio.

In other words, the interpass temper-ature (the temperature to which the weldregion cools between passes) exists in theactual weld. The total weld time is thesum of the welding time of each layer andthe cooling time required for each inter-pass temperature. However, if ramp ratiois increased to improve analyses, thetotal time of analysis is greater than theactual welding time. Therefore, this timedifference must be considered in theanalysis. The heat input rates of the 2-Danalysis are given by the surface flux andexpressed as the following:

Q = EI/bL (3)

where represents 0.8 as the arc effi-

ciency, E and I are arc volts and amperes,and b and L represent the width of the weldbead and the length of the weld directionof the heat input region, that is, unit lengthat the 2-D analysis, respective ly.

Results and Discussion

Experimental Results and Considerations

Tensile test results for each specimenare presented in Fig. 6. The figure showsapproximately 534540 MPa for the 0-mm root opening, 528537 MPa for the6-mm root opening and 536547 MPa forthe weld with the 24-mm buildup and 6-mm root opening. In total, the differencesare not large. The tensile strengths foreach specimen were shown to be satis-fied because fractures were generated inthe base metal, not the weld metal. The30-mm root opening weldment had suf-ficient static strength, but dy n a m i c

84-s MARCH 2001

Fig. 5 Shape of ramp heat input function. The area under the rampfunction curve was kept constant to maintain the same net heat input energy and to study the effect on thermal and stress responses.

Fig. 6 Plot of the tensile strength of each weld specimen.

Fig. 7 Plot of the impact energy of each weld specimen.