Collins SuppÑ10/03 corrected zaida

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Specifications	Collins SuppÑ10/03 corrected zaida
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WELDING RESEARCH OCTOBER 2003-S288 ABSTRACT. Ductility dip cracking (DDC) is a solid-state, elevated tempera- ture phenomenon

Specifications	Collins SuppÑ10/03 corrected zaida
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Content	WELDING RESEARCH OCTOBER 2003-S288 ABSTRACT. Ductility dip cracking (DDC) is a solid-state, elevated tempera- ture phenomenon that has been observed in thick-section, multipass austenitic stainless steel and nickel-based alloy weld metals where large grain size and high re- straint are characteristic. The mechanism has been postulated to be the result of “ductility exhaustion” along the grain boundary with grain boundary sliding and the relative orientation of a grain bound- ary to an applied strain increasing suscep- tibility to DDC. Although DDC is rela- tively uncommon, in applications where there is low defect tolerance, its occur- rence can be very costly. In Part I of this investigation, the strain-to-fracture (STF) test, a Gleeble- based test developed by Nissley and Lip- pold (Ref. 1), was employed to develop STF DDC susceptibility curves for one heat of Filler Metal 52 and three heats of Filler Metal 82. Filler Metal 52 was found to be more susceptible to DDC than Filler Metal 82, with Filler Metal 82 exhibiting a heat-to-heat variation in susceptibility. Additions of hydrogen and sulfur to Filler Metal 82 were also investigated and found to increase the susceptibility of the weld metal to DDC. Metallurgical analysis of the weld metal microstructures revealed contrast- ing grain boundary characteristics. Whereas Filler Metal 52 microstructures contained both straight and tortuous mi- grated grain boundary paths, Filler Metal 82 contained tortuous boundary paths only. Under low orders of strain and when oriented favorably to the applied load (45–90 deg), straight migrated grain boundary paths were found to be more conducive to DDC than tortuous bound- ary paths. Based on these grain boundary path differences, the STF test results re- vealing increased susceptibility to DDC for Filler Metal 52 are understood. Introduction Ductility dip cracking has been re- ported in a number of alloys. From a mechanistic standpoint, relatively little is known or understood about this particular type of solid-state cracking. Ductility dip cracking forms below the effective solidus temperature, and separation of grain boundaries has been reported to be char- acteristic of materials susceptible to DDC (Refs. 2, 3). Figure 1 schematically illus- trates ductility as a function of tempera- ture and reveals ductility dip behavior for susceptible alloys. Where thermal strains caused by manufacture or welding- induced strains intercept the ductility dip temperature range (DTR), cracking oc- curs. Ductility dip cracking is often associ- ated with the welding of heavy sections commonly encountered in critical high- pressure steam, nuclear, and power gen- eration applications (Refs. 4–6). Although a study by Honeycombe and Gooch(Ref. 3) demonstrated that microcracking in fully austenitic weld metal is unlikely to have a detrimental effect on mechanical or fatigue properties of the weldment, DDC can have serious consequences in critical applications where defect toler- ance is low and repair is both difficult and costly. A number of factors have been re- ported to contribute to the development of DDC, including specific alloy, impurity and interstitial element content; solute, impurity and interstitial element segrega- tion; grain growth; grain boundary sliding; grain boundary precipitation; grain boundary orientation relative to the ap- plied strain; and multipass welding opera- tions. While the DDC mechanism is not well understood, neither are the relative effects of each factor reported to con- tribute to the overall mechanism. Further- more, preventive methods to avoid DDC have proven elusive. This investigation addresses cracking that occurs in dissimilar welds between SA533 pressure vessel steel and Alloy 690, a nickel-based alloy that has excellent stress corrosion cracking resistance prop- erties. Ductility dip cracking has been ob- served in the butter layers applied to SA533 and in the closure weld between SA533 and Alloy 690. Initially, Filler Metal 52 was used for both the butter lay- ers and the closure weld. Cracking in Filler Metal 52 was found to be particularly se- vere and thus a switch to Filler Metal 82 was made. Although cracking was reduced through the application of Filler Metal 82, it was not completely eliminated. The overall goal of this investigation was to quantify DDC susceptibility and develop a better overall understanding of the DDC mechanism in nickel-based Filler Metal 52 and Filler Metal 82. Part I quantifies DDC susceptibility in Filler Metals 52 and 82. Hydrogen and sulfur ad- ditions to the weld metal were also evalu- ated. Part II summarizes metallographic and fractographic studies that provided insight into the factors responsible for causing DDC while Part III uses optical microscopy, high resolution scanning elec- tron microscopy, and electron backscat- tered diffraction (EBSD) techniques to further explore the factors that contribute to DDC in highly restrained Ni-base weld metals and to provide insight into the mechanism of DDC. The STF test has proven to be a suit- able, robust test technique for evaluating DDC susceptibility in weld metals. Appli- cation of this test technique successfully quantified susceptibility differences be- tween Filler Metals 52 and 82 while also providing definitive evidence of the nega- An Investigation of Ductility Dip Cracking in Nickel-Based Filler Materials — Part I The strain-to-fracture test has been used to develop temperature-strain relationships for ductility dip cracking BY M. G. COLLINSANDJ. C. LIPPOLD M. G. COLLINS and J. C. LIPPOLD are with The Ohio State University, Columbus, Ohio. KEYWORDS Ductility Dip Cracking Nickel-Based Filler Metals Gas Tungsten Arc Welding Strain-to-Fracture Test Grain Boundary Characteristics Spot Welds
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