Catalyst effects in heterogeneous nucleation of acicular ferrite
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I.
INTRODUCTION
ASPECTSof acicular ferrite (AF) formation in lowalloy steel weld metals have been extensively reported and investigated in the literature. I1-71 It is well established that this microconstituent nucleates at indigenous nonmetallic inclusions in the transformation temperature range between Widmannst/itten ferrite and lower bainite. fS,9j Based on the electron backscattering pattern (EBSP) technique, Kluken et al. tl~ have shown that the AF plates exhibit an orientation relationship with both the austenite and the prior delta ferrite columnar grains in which they grow. The observed orientation relationship lies within the Bain orientation region and can be described by three texture components, i.e., a (100) component and two complementary (111) components. Each of these texture components is orientated approximately parallel with the original cell/dendrite growth direction. Moreover, measurements of the spatial misorientation between neighboring plates confirm that the morphology of AF in low-alloy steel weld metals bears a close resemblance to upper bainite, t~~ This suggests that growth of the AF plates occurs either by a ledge mechanism or by a pure shear transformation, similar to that documented for bainite in wrought steel. However, from the literature reviewed, it is obvious that conflicting views are held about the major controlling nucleation mechanism. I5-121 In particular, it is essential to clarify whether the AF plates also adopt rational orientation relationships with the substrate in order to minimize the energy barrier to nucleation. This, in turn, could explain why certain types of inclusions (particularly those containing A1 and Ti) appear to be much more favorable nucleation sites for AF than others. 15,7J At present, a complete understanding of the O. GRONG and J. HJELEN, Professors, are with the Department of Metallurgy, University of Trondheim, The Norwegian Institute of Technology, N-7034 Trondheim, Norway. A.O. KLUKEN, Senior Research Metallurgist, is with Hydro Aluminium, Research CentreKarm6y, N-4265 H~vik, Norway. H.K. NYLUND, Ph.D. Student, is with SINTEF-Materials Technology, N-0314 Oslo, Norway. A.L. DONS, Senior Research Metallurgist, is with SINTEF-Materials Technology, N-7034 Trondheim, Norway. Manuscript submitted October 18, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
development of the AF microstructure in low-alloy steel weld metals is lacking. II.
EXPERIMENTAL DETAILS
A. Materials For this investigation, a 1.6-mm-diameter tubular electrode wire was selected. Bead deposition was carried out with a mechanized unit for submerged arc (SA) welding under a commercial basic flux, using the same welding procedure as described in Reference 13. Details of the weld metal inclusion characteristics and solidification microstructure have been reported elsewhere.tl3'14] Table I contains a summary of weld chemical composition and operational conditions.
B. Preparation of Thin Foils The thin foils for the transmission electron microscope (TEM) analysis were taken from
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