Inclusion phases and the nucleation of acicular ferrite in submerged arc welds in high strength low alloy steels
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I.
INTRODUCTION
FOR good toughness
over a range of temperatures, submerged arc welds (SAW) in high strength low alloy (HSLA) steels should have a high proportion of acicular ferrite. However, the factors which promote acicular ferrite formation are not fully understood. During the decomposition of austenite, 1'2 the first a-ferrite often forms as coarse proeutectoid ferrite at the austenite boundaries, from which side plates develop. As transformation proceeds, fine grained acicular ferrite may be nucleated at suitable intragranular sites. At still lower temperatures, any remaining austenite transforms to bainite or martensite. The relative proportions of these transformation products are governed by their nucleation and growth rates, but are now thought to be influenced by the inclusions in the weld deposit, 3'4'5 which provide nucleation sites for acicular ferrite within the prior austenite grains. However, it should be emphasized that transformation of austenite to acicular ferrite will occur only if conditions in the weld pool are favorable. The effects of weld composition and cooling rate must be such that the acicular ferrite transformation is promoted. If these conditions are not met, the austenite will transform to bainite or coarse ferrite depending on the cooling rate. 3 The fraction of acicular ferrite depends to some extent upon the weld metal oxygen content. It has been shown that too few oxide inclusions allow a bainitic structure while too many promote early proeutectoid ferrite nucleation at the expense of the acicular ferrite. 2 More recently, the uncombined weld pool oxygen content has been proposed as the critical factor:3 if this oxygen level lies within a certain range prior to and during solidification, the oxide inclusions formed will promote acicular ferrite formation. Alternatively, the oxide inclusions may determine the trans-
formation product by controlling the grain size of the prior austenite, large grain sizes being associated with bainitic structures. ~'6 Other reports have proposed that acicular ferrite is nucleated by certain inclusion compositions and phases. Compounds such as titanium oxide, TiO, 7 boron nitride BN precipitated on rare earth metal oxysulfides,s aluminum-rich inclusions, 9 and titanium nitride, TiN, ~~ have all been suggested as nucleant phases. Inclusions rich in manganese and inclusions covered with a skin of sulfide are reported to be ineffective nucleants. 9A2 Some authors consider that inclusion phase or composition is less important than low mismatch between inclusion phases and a-ferdte. ~ Others have proposed that high surface energy of inclusions is the main requirement for ferrite nucleation. The inclusions then act as inert substrates and reduce the energy barrier for nucleation. 4 The nucleation barrier may also be reduced by strain and dislocation fields which arise from differences in contraction around the inclusions during weld cooling. 6 The present paper is part of a larger investigation on microstructure-toughness relationships in submerged
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