phase within the PFZ is γ ′. Figure 17 shows electron backscattered diffraction (EBSD) patterns acquired across the boundary that contains a PFZ. The EBSD patterns demonstrate that the PFZ is located entirely in Grain 2. Detailed comparisons of creep samples from a wide range of test conditions also showed that there was no relation between the PFZ orientation to that of the applied stress. It is worth noting that similar PFZs are occasionally observed in the base metal of IN740H after creep. However, the base metal requires significantly longer times (or higher temperatures) for PFZ formation. Thus, the reduced creep strength in welds of IN740H can be attributed to enhanced susceptibility to PFZ formation associated with DPC. The enhanced driving force for PFZs in the weld metal can be understood with reference to Figs. 18 and 19. Figure 18 shows results from EDS traces acquired across the dendritic substructure of the weld. The data points reflect the average and standard deviation from multiple measurements, while the lines represent results from Scheil solidification simulations (Ref. 20). Note there is good agreement between the model and experimental results. In this case, the calculations are probably more reflective of the actual compositions within the very edge of the interdendritic regions due to the spatial resolution limitations of the electron beam (which is about 1 μm3) and the large concentration gradient within the interdendritic region. The results also show that, due to microsegregation, the concentrations of Ti and Nb are higher at the grain boundary and interdendritic regions (the Al concentration is also slightly higher). The increased localized concentration of these γ ′ forming elements is important, since it causes an increased supersaturation beyond the solubility limit, thus leading to the enhanced grain boundary precipitation shown in Fig. 19. Classical solutions to the diffusion equations associated with precipitation (Ref. 35) reveal that the growth rate of the precipitate/ matrix interface is directly proportional to the degree of supersaturation in the matrix, ΔCo. Here, ΔCo can be viewed as the driving force for precipitation and is given by the difference between the actual matrix concentration and the solubility limit at a given temperature. Figure 20 shows the ΔCo values (at 800°C, the typical operating temperature) associated with the γ ′ forming elements Nb, Ti, and Al that demonstrate the significantly enhanced driving force for precipitation associated with interdendritic microsegregation. Also note from Fig. 19 that there is significant grain boundary curvature. This could be associated with the original (nonequilibrium) grain boundary curvature typical in the as-solidified condition, WELDING JOURNAL 37-s WELDING RESEARCH Fig. 11 — Corrosion fatigue cracks observed in the GMAW cladding. A— 25 cycles; B — 50 cycles; C — 100 cycles; D — 442 cycles. Fig. 13 — Maximum allowable stress as a function of temperature for a wide range of stainless steels and nickel alloys. Fig. 12 — A — Average crack depths; B — maximum crack depths for the GMAW and laser weld cladding samples. Fig. 14 — Recent results from Oak Ridge National Laboratory that compares the stress rupture properties of wrought IN740H tube material to that of crossweld samples. A B C D
Welding Journal | February 2014
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