Charpy V-notch properties and microstructures of narrow gap ferritic welds of a quenched and tempered steel plate
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I.
INTRODUCTION
WHEN quenched and tempered low carbon low alloy steels[1] are welded, it is necessary to control the microstructures of both the heat-affected zone[2] and the weld deposit.[3] One method of ensuring adequate toughness in a ferritic weld deposit is to minimize the percentage of Widmansta¨tten sideplate ferrite and maximize the acicular ferrite content.[4] Widmansta¨tten sideplate ferrite nucleates from the prior austenite grain boundaries, while acicular ferrite is nucleated intragranularly on oxide inclusions.[5,6] Recent research has focused on the heterogeneous nature of the oxide inclusions and especially on the crystalline compounds present in the oxide inclusions. Two methods have been adopted to identify these crystalline compounds. One is to electrolytically extract the oxide inclusions and determine the presence of crystalline compounds by X-ray diffraction.[7] The other is to identify the crystalline compounds in individual inclusions by electron diffraction.[8] Widmansta¨tten sideplate ferrite grows as parallel laths of similar crystallographic orientation and hence offers little resistance to crack propagation.[9] Acicular ferrite grows in many directions and a crack is required to change directions repeatedly, thus providing better toughness.[4] The International Institute of Welding has issued guidelines for the quantitative determination of the constituents of ferritic steel weld metals.[10] In these guidelines, acicular ferrite is designated AF while Widmansta¨tten sideplate ferrite is approximated by FS (ferrite with second microconstituent phase). G.L.F. POWELL, Metallurgist, and G. HERFURTH, Metallographer, are with Manufacturing Science and Technology, Commonwealth Scientific and Industrial Research Organization (CSIRO), Woodville, South Australia 5011. Manuscript submitted December 27, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
This article reports the relationship of Charpy V-notch (CVN) results to microstructure, including oxide inclusion size and ferrite morphologies for submerged arc welding (SAW) and gas metal arc welding (GMAW) narrow gap weld deposits in 50-mm-thick-plates of a quenched and tempered steel. A correlation of acicular ferrite percent to the crystalline compound content of oxide inclusions is presented and related to the Al-to-Ti ratio of both the oxide inclusions and the weld deposits. II.
EXPERIMENTAL PROCEDURE
Fifty-millimeter-thick quenched and tempered plate to Australian Standard 3597[11] (similar to American Society for Testing and Materials (ASTM) A514[1]) was welded by a single wire narrow gap process using both GMAW and SAW. Details of the welding conditions are given in Table I. The chemical compositions of the plate and the filler wires are presented in Table II. Charpy V-notch specimens were taken from the top and bottom of the weld with the notch aligned vertically 2 mm from the plate surface (Figure 1) and tested at 220 7C per Australian Standard 1544.2 (similar to ASTM E-23 with a slight difference in the curvature of the impacting s
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