| Specifications | Collins SuppÑ10/03 corrected zaida |
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| Specifications | Collins SuppÑ10/03 corrected zaida |
| Business section |
| Specifications | Collins SuppÑ10/03 corrected zaida |
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| Content | WELDING RESEARCH -S293WELDING JOURNAL ditions, with cracks occurring along mi- grated grain boundaries. Additions of hy- drogen simply increase the number of sus- ceptible boundaries. A higher prevalence for cracking near triple-point grain boundary intersections was observed in these samples. Discussion Alloy Element Effects Nickel-based alloys containing Nb and Ti can form both NbC and Laves phase [Ni2(Nb,Ti)] eutectic constituents during solidification (Ref. 8). Because it contains Nb, Filler Metal 82 is capable of forming both constituents while Filler Metal 52 can only form Laves phase (Ni2Ti). Addition- ally, Filler Metal 52 has a lower carbon content than all heats of Filler Metal 82, while Filler Metal 82 has lower carbon content than carbon contents reported in literature to be beneficial to ductility (Refs. 2, 9–13). It is postulated that the dif- ference in STF DDC susceptibility behav- ior between Filler Metals 52 and 82 is ex- plained by the addition of Nb and its ability to form NbC and, possibly, Laves phase eutectic constituents that inhibit grain boundary motion and contribute to grain boundary tortuosity. These con- stituents along the boundary also inhibit grain boundary sliding, which has been re- ported to be most prevalent along grain boundaries absent of such impediments (Refs. 7, 14–18). The tortuosity of the boundary can provide a mechanical lock- ing effect that resists grain boundary slid- ing and subsequent cracking. Hydrogen Effects As shown in Figs. 8 and 9, hydrogen has a pronounced negative effect on the STF behavior of Filler Metal 82. Hydrogen cracking typically is not a concern in fully austenitic structures based on the high sol- ubility of hydrogen and its low diffusivity in austenitic microstructures. Regardless, atomic hydrogen is an extremely mobile interstitial addition and hydrogen crack- ing may occur in austenitic materials if suf- ficient hydrogen is present. The Decohe- sion Theory for hydrogen cracking may help explain the increase in DDC suscep- tibility upon adding hydrogen to the weld pool (Refs. 19, 20). Based on this theory, hydrogen diffuses to regions of the crystal lattice where tensile stress concentrations occur. Hydrogen diffusing through a metal lattice accumulates most easily at metallurgical inhomogeneities or “traps.” Perhaps at a critical temperature (~950°C) within the DTR, diffusion to the grain boundary is maximized, allowing atomic hydrogen to recombine and form molecular hydrogen (H2) that “de-traps,” subsequently decreasing grain boundary cohesion leading to intergranular frac- ture. Additionally, triple-point grain boundary intersections may be classified as shallow, reversible traps. These partic- ular traps permit rapid hydrogen transport to the crack tip resulting in a critical con- centration of hydrogen necessary to initi- ate cracking, further enhancing fracture under low orders of strain (Ref. 21). Optical microscopy was performed to determine if, in fact, the frequency of DDC at triple-point grain boundary inter- sections increased when hydrogen was in- tentionally added to the weld metal. Based on metallographic observations, the fre- quency of DDC near triple-point grain boundary intersections increases when adding hydrogen to the weld metal mi- crostructure. At relatively low levels of strain (<3%), cracking is predominant at these intersections (Fig. 15), supporting the hypothesis that hydrogen diffuses to regions of the crystal lattice where tensile stress concentrations are present, decreas- ing grain boundary cohesion and ulti- mately increasing DDC susceptibility. Effect of Triple-Point Junctions The presence of triple-point grain boundary intersections in polycrystalline materials has been shown to influence sev- eral material properties, including ductil- ity, grain boundary migration, sliding, and recrystallization (Refs. 22, 23). Watanabe (Ref. 22) reported that a computer simu- lation study of intrinsic stress distributions at triple-point grain boundary intersec- tions revealed that uncompensated Fig. 10 — Filler Metal 82 (Heat YN6830) STF DDC susceptibility curve — effect of sulfur additions. Fig. 11 — DDC along straight, migrated grain boundaries in Filler Metal 52 at 802°C and 4.4% strain (arrows indicate DDC along grain boundary triple-point intersections). Fig. 12 — DDC along a triple-point intersection in Filler Metal 52 at 986°C and 2.6% strain (ar- rows indicate loading direction of the sample). |
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