A model for the strength of the As-deposited regions of steel weld metals
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I.
INTRODUCTION
MUCHfundamental
work has recently been done on the prediction of the microstructure of steel weld deposits, t''2'31 and it is now possible to estimate the as-welded microstructure as a function of chemical composition and thermal history. While the work on microstructure prediction has made good progress, it is the properties of welds which ultimately determine the quality of that weld. This work is part of a project aimed at obtaining quantitative and widely applicable relationships between weld microstructure and properties, so that the latter can then be estimated for calculated microstructures. The aim of this work is to try to predict the strength of the as-deposited weld as a function of alloy concentration and microstructure, and also over a wide temperature range. Many welds are used or tested at nonambient temperatures, and it is then not sufficient only to be able to predict their strength at room temperature. The solidification of low-alloy steel weld deposits starts with the epitaxial growth of delta-ferrite (6) from the parent plate grains at the fusion boundary. The high temperature gradients involved in arc welding cause solidification to proceed in a cellular manner with the grains having their major axes following the direction of maximum heat flow. On further cooling, allotriomorphs of austenite (30 nucleate at the 3 / 6 cell boundaries, and anisotropic 7 growth along these boundaries leads to the formation of columnar austenite grains which closely resemble the original 6-ferrite morphology. On cooling to temperatures below the me3 temperature, the first phase to form is allotriomorphic ferrite (a). The ferrite nucleates at the columnar-austenite grain boundaries, which rapidly become covered with a nearly uniform layer of c~. Following this, Widmanst~itten ferrite (aw) nucleates at the ~/3~ boundaries and grows by a displacive mechanism in the form of thin, wedge-shaped plates at a rate approximately controlled by the diffusion of carbon in the austenite ahead of the interface. At the same time, a third phase, acicular ferrite (C~a), which consists of a series of nonparallel arrays of bainite laths, nucleates intragranularly. I4'51 Finally, very small volume fractions of "microA. A. B. SUGDEN, Research Student, 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 August 3, 1987.
METALLURGICAL TRANSACTIONS A
phases" are found within the acicular ferrite consisting of mixtures of martensite, degenerate pearlite, and retained austenite, all resulting from the austenite remaining untransformed after a, aw, and c~a have formed. However, microphases comprise typically only 1 to 3 pct of the weld microstructure, and it is the three morphologically distinct phases - - allotriomorphic, Widmanst~itten, and acicular ferrite - - which can be said to form the primary micro structure. [~l
II.
METHOD
It is normal practice to express th
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