The effect of alloy composition and welding conditions on columnar-equiaxed transitions in ferritic stainless steel gas-
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I.
INTRODUCTION
W E L D pool solidification can readily start from the parent metal partially molten zone with a minimum degree of undercooling. This leads to the well-known epitaxial growth, rather than requiring any nucleation event, resulting in a coarse columnar grain structure with a preferred orientation. The deleterious influence of this normal weld pool solidification upon certain mechanical properties of the weld deposit has stimulated research into weld pool solidification. In particular, the formation of a central equiaxed zone helps to prevent both centerline hot crackingt~l and a weak centerline in subsequent plastic deformation. Several mechanisms have been proposed to assist the columnar-equiaxed transition (CET) during weld pool solidification, including dendrite fragmentation, [2.3,41 grain detachment,t51 and heterogeneous nucleation. |5,6,71 In aluminum gas-tungsten arc (GTA) welds, Kerr and coworkers t5"61 and later Kou and LetT,sl observed that Tirich particles act as heterogeneous nucleation sites. Pearce and KerrtS] have also proposed partially molten zone grain detachment as a second refining mechanism for some alloys. On the other hand, Matsuda e t a/. [2'3,91 proposed dendrite fragmentation as the mechanism to explain the occurrence of equiaxed grains in aluminum and austenitic stainless GTA welds containing titanium. J.C. VILLAFUERTE, Graduate Student, and H.W. KERR, Professor, are with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada. E. PARDO, formerly Postdoctoral Fellow, Department of Mechanical Engineering, University of Waterloo, is Adjoint Professor of Mechanics, Faculty of Engineering, Institute of Materials Science and Technology (INTEMA), University of Mar del Plata, 7600 Mar del Plata, Argentina. Manuscript submitted August 16, 1989. METALLURGICALTRANSACTIONSA
The role of titanium as a nucleant in cast steels, which cool more slowly than GTA welds, has beeen known for some time. Brarnfittt~~ reported that titanium nitrides and carbides were effective in promoting equiaxed grains in iron due to heterogeneous nucleation of ~-Fe on these phases. In mild steel submerged arc (SA) welds, Tttrnbull e t al. tnl used a hollow electrode containing Fe-Ti and TiC mixtures to refine the grain size. GarlandI~2Jreported grain refinement in mild steel SA welds by inoculating with Fe-Ti mixtures. The effect was ascribed to heterogeneous nucleation on TiC. Willingham and Baileyt~31 used additions of Fe-Ti, TiB2, and mixtures of TiB2 + CaF2 in grain-refining SA welds. Ostrowski and Langer~14~ reported grain refinement in cast ferritic Fe-17 pct Cr stainless steel due to heterogeneous nucleation of 8-ferrite on Ti-rich inclusions. For this to occur, the Ti-rich inclusions first must be nucleated. Ostrowski and Langer proposed a complex heterogeneous nucleation process of TiN or Ti(C, N) on high-temperature oxides, including A1203, SiO2, Ti~Or, Y203, or ZrO2, with the nucleation of sulfide layers third. The heterogeneous nucleation of 8-ferrite on the
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