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  • The experimental validation of temperature distribution was

    2018-10-25

    The experimental validation of temperature distribution was carried out by physically measuring temperature on plate surface using thermocouples placed at 10 mm, 15 mm and 20 mm away from the weld bead. Figs. 7 and 8 shows the validation of predicted thermal cycles in A-GTAW and SMAW process respectively. Figs. 7 and 8 indicate higher temperature gradients at areas closer to the welding heat input and lower temperatures away from the weld center. The highest heating and cooling rate is observed at the weld center point. The thermal orexin has a steep rise in the curve gradient and the slope of curve is lesser for cooling after the heat source passes, validating higher heating rate than the cooling rate. The heating rate for both experimental and simulated results was observed to be similar. Figs. 7 and 8 exhibit that the experimental cooling rate was lesser than the simulated cooling rate. It was also observed that thermocouple readings are lower near to weld centerline as compared to FEM but higher as distance from centerline increases, showing higher cooling rate in FEM. The simulated thermal profile showed steeper slope and higher cooling rate as the heat transfer mode was assumed to be primarily conduction with modified higher thermal conductivity for molten weld pool. The comparison of peak temperature values for each pass measured at the predetermined locations of thermocouple and as deduced from the FEM of A-GTAW and SMAW processes is given in Table 4. The maximum peak temperature measured at molten weld cenetr in FEM for A-GTAW and SMAW depict the melting temperature and was found to be similar as mentioned in Table 4. The difference between maximum temperatures observed at different distances from weld center for A-GTAW and SMAW is attributed to the difference in heat input for the two welding techniques. The higher heat input of 5 kJ/mm for A-GTAW compared with 2 kJ/mm for SMAW process resulted in higher temperatures in the A-GTAW welded plate. Also, the cooling rate decreases at large heat inputs so that larger quantities of Widmanstatten ferrite are obtained with corresponding reduction in the amount of acicular ferrites. Widmanstatten structure is characterized by its low impact values so the A-GTAW weld joint is expected to have lower impact toughness value at sub zero temperatures as compared to SMAW weld joint. The residual stress profile in the A-GTAW and SMAW process are given in Fig. 9(a)–(b). The residual stresses as measured experimentally using XRD, UT (Lcr) techniques and FEM simulation for plates using A-GTAW and SMAW processes are shown in Fig. 10. The various studies on RS measurements reported existence of maximum tensile RS generally up to yield stress of the base material or even up to values matching the tensile strength. RS of 250 MPa for 2219 Al, 300 MPa in 316LN steel, 320 MPa in 316L and 750 MPA in Z8CD12 steels have been reported [24–27]. In Fig. 10(a)–(b), the tensile RS are shown to be present up to 20 mm to 40 mm on either side of the weld line. Fig. 10 and Table 5 show that experimental stress measurements are comparable with the numerical prediction. The profiles of the residual stresses measured experimentally and using FEM were observed to be similar. The minor variations in absolute values of stresses is attributable to the difference in residual stress gradients for different welding techniques and inherent volume of inspection of surface and bulk residualstresses of respective technique [5–14,19,20]. The XRD measurements are sensitive to surface conditions with assumed depth of penetration in the order of 5 to 30 microns. For the LCR measurements, residual stress is the average value over effective penetration of 3 mm. The FEM simulated values are average stress values interpreted from the SYSWELD model. The microstructure studies of base metal DMR-249A and arc welded joints were undertaken using optical microscope. The microstructure of base metal and weld metal of different arc welded joints at 500× magnification are given in Fig. 11. The microstructure of base metal show predominantly fine grained equiaxed ferrite and some percentage of banded structured pearlite. For weld metals, the optical images showed arc welded joints with grain boundary ferrite, Widmanstatten ferrite with aligned second phase along with veins of ferrite, acicular ferrite, polygonal ferrite and microphases. The grain boundary ferrite has equiaxed form or thin veins delineating prior austenite grain boundaries. The sideplate Widmansttten ferrite is seen as the parallel ferrite laths emanating from prior austenite grain boundaries.