Grain structures in gas tungsten-arc welds of austenitic stainless steels with ferrite primary phase
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I.
INTRODUCTION
THE fact that
weld pool solidification can readily start from the parent metal partially molten zone with a minimum degree of undercooling leads to epitaxial growth, resulting in a coarse columnar grain structure with a preferred orientation. The growth of columnar grains to the center of the weld, particularly in full penetration welds of thin sheet, can lead to a weak centerline, as evidenced by either hot cracking during welding or poor bending ductility of the solid weld. Moreover, in austenitic stainless steels, ultrasonic inspection is more difficult for columnar than for equiaxed grains because the velocity anisotropy of ultrasonic waves can give spurious results. The formation of a central equiaxed zone in the weld helps to avoid these problems. Different refinement mechanisms have been proposed to assist the columnar-to-equiaxed transition (CET) in gas tungsten-arc (GTA) welds including dendrite fragmentation, grain detachment, and heterogeneous nucleation. Early work by Matsuda et al. I~'2] proposed dendrite fragmentation to explain the occurrence of equiaxed grains in aluminum GTA welds. However, Kerr and coworkers I3,41 and Kou and Le I5,6] have suggested that observed Ti-rich particles act as heterogeneous nucleation sites. Pearce and Kerr [41 also have proposed partially molten zone grain detachment as a second refining mechanism for some alloys. The role of titanium as grain refiner in cast steels has been known for some time and is generally related to heterogeneous nucleation on TiN or Ti(C, N) particles due to low misfit between these particles and ferrite. [71 Consequently, Fe-Ti and/or Ti compound additions have been made to low alloy and stainless ferritic alloy welds to promote the CET. I8'9,1~ In submerged arc welds, in which the cooling rate is slower than in GTA welds, J.C. VILLAFUERTE, Graduate Student, and H.W. KERR, Professor, are with the Department of Mechanical Engineering, University of Waterloo, Waterloo, ON N2L 3GI, Canada. Manuscript submitted July 5, 1989. METALLURGICAL TRANSACTIONS A
Heintze and McPherson till reported the two-stage nucleation of TiN on A120~, followed by the nucleation of ferrite on the TiN. This observation is consistent with earlier studies of cast steels, t~21 This sequence recently has been observed for GTA welds in type 409 ferritic stainless steels containing titanium and aluminum, t~31 so that the CET is promoted by these additions. In austenitic stainless steels, however, the situation is less clear. Heintze and McPherson t~lj reported that for submerged arc welds, the austenite phase is not refined by titanium additions. On the other hand, Matsuda et al. fl4j have reported some grain refinement in type 321 stainless steel when aided by magnetic stirring but attributed the effect to dendrite fragmentation. In many commercial austenitic stainless steels, the primary metallic phase is ferrite, followed by the formation of austenite which begins solidification at the ferriteliquid interface and increases its volume fraction by a solid-s
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