The nonunjform distribution of inclusions in low-alloy steel weld deposits |
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Authors: | A A B Sugden H K D H Bhadeshia |
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Affiliation: | (1) Department of Materials Science and Metallurgy, University of Cambridge, CB23QZ Cambridge, UK |
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Abstract: | Nonmetallic inclusions in low-alloy steel welds have an important effect on the microstructure and properties of weld deposits.
This work is an attempt at understanding the factors controlling the spatial distribution of such inclusions, with particular
emphasis on the uniformity of the distribution and the effect of solidification mode during manual-metal-arc welding. The
solidification mode has been controlled by using unusual combinations of base plates and experimental electrodes. It is found
that the first phase to solidify (in the form of columnar grains) is delta-ferrite (δ) when a medium carbon electrode is deposited onto a low carbon substrate, but that it is austenite (γ) when a low carbon electrode is deposited onto a high carbon substrate. Relatively large inclusions have been found to position
themselves preferentially, during solidification, to the columnar grain boundaries of the first phase to solidify, whether
this is 8-ferrite or austenite. The results can be understood qualitatively in terms of a surface tension driven Marangoni
effect, or in terms of the pushing of solid inclusions by the solidification front. Both mechanisms drive the larger inclusions
into cusps in the interface while smaller ones are passively trapped. The implications of the observed nonuniform distribution
of inclusions are more severe for solidification with austenite as the primary phase, since the larger inclusions are in that
case located in the weakest region of the weld where they also do not contribute to the intragranular nucleation of acicular
ferrite. |
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