In this second of two papers, the microstructural and microchemical evolution of Alloy 600 (A600), Alloy 690 (A690), Filler Metal 82H (FM82H), and Filler Metal 52 (FM52) during the weld thermal cycle was investigated and compared to the hot ductility data presented in the first paper (Ref. 1). The Gleeble® hot ductility test was used to subject these four alloys to a simulated weld thermal cycle. Water quenching was conducted at select temperatures so that the elevated temperature microstructure could be subsequently characterized. Microstructural and microchemical characterization was carried out using scanning electron microscopy, transmission electron microscopy, and analytical electron microscopy techniques. Complete dissolution of intergranular carbides was observed in A690 and FM52 at ∼2400°F (1316°C), both of which exhibit an on-cooling ductility minimum at 1600°F (871°C). Of all four alloys, the greatest resistance to ductility dip cracking (DDC) was observed in A600 and A690 during on-heating, which had coarse, homogenously distributed intergranular carbides. FM82H, which formed NbC intergranular carbides, had the most stable intergranular microstructure and serrated grain boundaries, which corresponded to the best overall DDC resistance. Modifications to the thermal cycle that resulted in increased intergranular carbide coverage in FM82H and FM52 also reduced DDC susceptibility. AEM analysis did not reveal any sulfur or phosphorous segregation in FM52 at 1600°F (871°C) on-heating, on-cooling, or after a 60s hold. Samples with microstructures that consisted of coarsened carbides and/or serrated grain boundaries, which are expected to decrease grain boundary sliding, were found to be resistant to DDC. Based on the results of this work and the results previously presented in Paper I (Ref. 1), grain boundary sliding contributes to DDC. Conversely, sulfur and phosphorous embrittlement do not play a role in DDC of FM52 at the concentrations investigated. The dynamic precipitation of partially coherent intergranular M 23 C 6 carbides at intermediate temperatures may exacerbate DDC in A690 and FM52, but requires further investigation.

Metallurgical investigation into ductility dip cracking in Ni based alloys

The strain-to-fracture (STF) test has been developed to determine susceptibility to ductility dip cracking (DDC) and other elevated-temperatures cracking phenomena. Samples are tested over a range of temperature and strains, producing temperature-strain cracking envelopes. Threshold strain for fracture (e min ) and the ductility dip temperature range (DTR) can then be extracted from these envelopes and used to compare susceptibility among materials. The test is robust and allows for changes in the testing parameters and material condition to develop a fundamental understanding of factors that affect cracking. Three austenitic base metals were studied: Type 310 stainless steel, Ni-based Alloy 690, and super-austenitic Alloy AL-6XN. Autogenous spot and transverse welds in the three austenitic alloys were tested onheating using the strain-to-fracture test. Alloy 690 was found to be the most susceptible to DDC with a low threshold strain to fracture and wide DTR. Type 310 stainless steel exhibited a similar DTR to Alloy 690 but had a higher threshold strain. Alloy AL-6XN was found to have the greatest resistance to DDC with a narrow DTR, although the threshold strain over a narrow temperature range was comparable to Alloy 690. Metallographic examination revealed that intergranular cracking occurred preferentially along migrated grain boundaries (MGBs) in the fusion zone. Cracking was most severe in materials that were free of second phases or precipitates and accordingly had large grain size and straight MGBσ. Recrystallization was observed at temperatures near the upper end of the DTR and was accompanied by a recovery of ductility. loys, Cu-based alloys, and titanium alloys (Ref. 1). These normally ductile materials can exhibit a loss of ductility over a temperature range below the solidus temperature. Under the proper conditions, solid-state intergranular cracks form within this elevated temperature range. Such behavior has been called ductility dip cracking (DDC) (Ref. 2). The mechanism for DDC is unclear and tests to determine susceptibility to DDC are often inconclusive. The purpose of this investigation was to develop a test technique that could determine susceptibility to weld metal DDC using a thermomechanical simulator. As a result, the STF test (Refs. 3, 4) has been developed to determine susceptibility to DDC and other elevated temperature cracking phenomenon.

Development of the strain-to-fracture test: A new test has been established for evaluating ductility dip cracking susceptibility in austenitic alloys

........................................................................................................ii Acknowledgments ..........................................................................................vi Vita............................................................................................................... vii List of Tables............................................................................................... xiii List of Figures...............................................................................................xv List of Equations ......................................................................................... xxii List of Abbreviations......... .............................................................................xxiii

Intermediate temperature grain boundary embrittlement in nickel-base weld metals

The ductility-dip cracking (DDC) behavior of Ni-Cr-Fe alloys has been evaluated by the use of in situ scanning electron microscopy (SEM) involving high temperature deformation This unique approach provides information regarding the DDC phenomenon and its dependence on microstructural features such as grain boundaries and second phase precipitates Experimental materials were prepared based on the ERNiCrFe-7 alloy (Filler Metal 52) with additions of Nb, Mo, and Hf These additions provided a range of precipitates and gamma matrix chemical composition, which had an important effect on high temperature ductility Evidence of grain boundary sliding (GBS), the role of precipitates, grain boundary morphology, and gamma phase composition were evaluated and related with fundamental properties governing the material deformation The in situ observations revealed that DDC is a high temperature intergranular fracture phenomenon dominated by Rachinger-like grain boundary sliding Finally, this new approach provided valuable information to understand material ductility at high temperature

In Situ Observations of Ductility-Dip Cracking Mechanism in Ni-Cr-Fe Alloys

In this second of two papers, the microstructural and microchemical evolution of Alloy 600 (A600), Alloy 690 (A690), Filler Metal 82H (FM82H), and Filler Metal 52 (FM52) during the weld thermal cycle was investigated and compared to the hot ductility data presented in the first paper (Ref. 1). The Gleeble® hot ductility test was used to subject these four alloys to a simulated weld thermal cycle. Water quenching was conducted at select temperatures so that the elevated temperature microstructure could be

Metallurgical Investigation into Ductility Dip Cracking in Ni-Based Alloys: Part II Microstructural and microchemical development is characterized during simulated weld reheat thermal cycle and correlated to ductility dip cracking susceptibility

/pdf/weldability-of-fe-36-ni-alloy-report-ii-effect-of-chemical-5fs4g3qob7.pdf

Weldability of Fe-36% Ni Alloy (Report II) : Effect of Chemical Composition on Reheated Hot Cracking in Weld Metal(Materials, Metallurgy & Weldability)

https://ir.library.osaka-u.ac.jp/repo/ouka/all/8666/jwri09_02_197.pdf

Nobuharu Sakabata

Papers

Weldability of Fe-36% Ni Alloy (Report II) : Effect of Chemical Composition on Reheated Hot Cracking in Weld Metal(Materials, Metallurgy & Weldability)

Weldability of Fe-36%Ni Alloy (Report I) : Hot Cracking with Cross-Bead Test(Materials, Metallurgy, Weldability)

Cold cracking in electron beam welding of carbon and low alloy steels - A study on electron beam welding - (I)

Ductility-dip cracking in weld metal of Fe-36%Ni alloy (Report I) - Behaviors of hot cracking and its dependency on chemical composition.