Lower acicular ferrite
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INTRODUCTION
T H E R E is general agreement t~-4j that a weld microstructure containing mainly acicular ferrite will exhibit high strength and optimum toughness due to its small ~grain size" and the way in which the plates of ferrite are dispersed in the microstructure, causing the plane of a cleavage crack to be frequently deflected. In contrast, the presence of allotriomorphic ferrite, parallel formations of Widmanst/itten ferrite plates, or grain-boundary nucleated sheaves of bainite is considered to be detrimental to the toughness of the weld, because these constituents allow cracks to propagate without much deflection. The problem is, in fact, more complicated, since it appears that there are instances when large amounts of acicular ferrite do not lead to the best toughness, tS] Whatever the optimum microstructure, it is clear that a better understanding of the phases involved would permit more detailed investigation of the relationship with mechanical properties. In this context, acicular ferrite is the least understood of all the main phases that occur in steel welds. The nature of the acicular ferrite phase has been the cause of much research. In fact, the term "acicular ferrite" is a misnomer. In two dimensions, acicular ferrite appears as "randomly" oriented, needle-shaped particles, but this belies its true morphology, which is that of a thin, lenticular plate. For a typical low-alloy C-Mn steel weldment, acicular ferrite will begin to appear during cooling in the range of 500 ~ to 440 ~ ]6] a temperature range which is consistent with the observation of plate morphologies of ferrite in wrought steels. It is well established that acicular ferrite nucleates at nonmetallic inclusions which occur frequently in arc-weld deposits, but its mechanism of growth has not been clear. Its appearance alone has sometimes led to proposals that it is Widmanst~itten ferrite, t7,8,91 However, few investigations have focused on the details of the transformation mechanism. There is some evidence that the formation of acicular ferrite causes an invariant plane strain shape
A.A.B. SUGDEN, Research Scientist, and H.K.D.H. BHADESHIA, University Lecturer, are with the Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom. Manuscript submitted November 28, 1988. METALLURGICAL TRANSACTIONS A
deformation of the transformed region, t~~ The acicular ferrite always has an orientation relationship with the austenite grain in which it grows, such that one of its closest packed {110} planes is nearly parallel to a closepacked {111} plane of the parent austenite; within these planes, a close-packed ( 11 i) direction of the acicular ferrite is found to be near a close-packed (101) direction of the austenite, t~~ These considerations indicate that the growth of acicular ferrite occurs by displacive transformation, and the strain energy due to the accompanying shape deformation is a major contribution to the - 4 0 0 Jmo1-1 of stored energy associated with the acicul
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