Additions of nitrogen were made to ERNiCr-3 (FM 82)
weld metal to investigate the effect on weld metal microstructure and solidification cracking susceptibility.
Weld samples were prepared with different argon-nitrogen
shielding gas mixtures to produce nitrogen variations in
the weld metal. The cast pin tear test (CPTT) was then
used to evaluate solidification cracking susceptibility as a
function of weld metal nitrogen content. Phase fraction
and composition of constituents in the final solidification
microstructure were characterized via optical and electron
microscopy. Thermodynamic (Scheil) calculations were
performed to determine the phase formation during solidification, associated solidification path, and solidification
temperature range. Solidification cracking susceptibility
was found to increase significantly with increasing nitrogen content. The overall amount of second phase in the
solidified microstructure increased when nitrogen was
added to the weld metal. Small skeletal constituents in
the interdendritic regions, primarily Nb-rich carbides (NbC),
were more frequently observed with increasing weld metal
nitrogen content. Larger cuboidal Ti-rich nitrides (TiN) and
carbonitrides (Ti, Nb)(N, C) were found only when nitrogen
was added to the weld metal. Their location in dendrite
core regions indicates formation during an earlier stage of
the solidification process. Scheil calculations confirmed
the strong effect of nitrogen additions on the amount and
sequence of phase formation during solidification of ERNiCr-3 weld metal. High nitrogen levels ( 100 ppm) facilitate
primary nitride formation prior to the solidification of the
gamma () dendrites, and increase the amount of phase
constituents in the solidification microstructure. The presence of nitrogen also shifts the start of the eutectic
/NbC formation at the end of solidification to lower temperatures, which results in an increase in the solidification
temperature range. This occurs at much lower nitrogen levels ( 25 ppm) and correlates with the observed increase
in solidification cracking susceptibility.
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In situ synchrotron diffraction and modeling of non-equilibrium solidification of a MnFeCoNiCu alloy
The solidification mechanism and segregation behavior of laser-melted Mn35Fe5Co20Ni20Cu20was firstly investigated via in situ synchrotron x-ray diffraction at millisecond temporal resolution. The transient composition evolution of the random solid solution during sequential solidification of dendritic and interdendritic regions complicates the analysis of synchrotron diffraction data via any single conventional tool, such as Rietveld refinement. Therefore, a novel approach combining a hard-sphere approximation model, thermodynamic simulation, thermal expansion measurement and microstructural characterization was developed to assist in a fundamental understanding of the evolution of local composition, lattice parameter, and dendrite volume fraction corresponding to the diffraction data. This methodology yields self-consistent results across different methods. Via this approach, four distinct stages were identified, including: (I) FCC dendrite solidification, (II) solidification of FCC interdendritic region, (III) solid-state interdiffusion and (IV) final cooling with marginal diffusion. It was found out that in Stage I, Cu and Mn were rejected into liquid as Mn35Fe5Co20Ni20Cu20solidified dendritically. During Stage II, the lattice parameter disparity between dendrite and interdendritic region escalated as Cu and Mn continued segregating into the interdendritic region. After complete solidification, during Stage III, the lattice parameter disparity gradually decreases, demonstrating a degree of composition homogenization. The volume fraction of dendrites slightly grew from 58.3 to 65.5%, based on the evolving composition profile across a dendrite/interdendritic interface in diffusion calculations. Postmortem metallography further confirmed that dendrites have a volume fraction of 64.7% ± 5.3% in the final microstructure.
more » « less- Award ID(s):
- 1847630
- NSF-PAR ID:
- 10369834
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Scientific Reports
- Volume:
- 11
- Issue:
- 1
- ISSN:
- 2045-2322
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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