Effect of homogenization heat treatment on the microstructure and heat- affected zone microfissuring in welded cast allo
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I.
INTRODUCTION
INCONEL* 718 is a T"-precipitation-hardened Ni-Fe-Cr *INCONEL is a trademark of Inco Alloys International, Inc., Huntington, WV.
base alloy that was developed for aerospace applications in the 1960sYJ The presence of Nb in this alloy makes it free from strain-age cracking; however, it promotes intergranular and interdendritic cracking in the fusion and heat-affected zones (HAZs) in welded regions. The alloy was originally utilized in wrought form, and most of the welding research reported in the literature was conducted on wrought alloy. It was found that HAZ microfissuring in this condition can be controlled through reducing the grain size.t21 Recently, an increasing number of large components, such as compressors and turbine frames, are being made of cast alloy 718, they require a reduced amount of machining and welding.[31Compared to wrought alloy 718, more severe microfissuring has been observed in cast alloy 718, which could be attributed to the coarse grain size and greater presence of a low-melting-point Laves phase as a result of heavy dendritic segregation. The effect of heat treatment prior to welding has been examined by several investigators;t~s] however, no systematic study has been conducted in the past. This article describes the results of an investigation on the influence of
heat treatment on the microstructure and HAZ microfissuring of cast alloy 718. Heat-affected-zone microfissuring has been discussed in terms of the volume fraction of second phases, grain boundary precipitates, hardness, and grain boundary elemental segregation. II.
EXPERIMENTAL
TECHNIQUES
The as-received material was alloy 718 plates (1.6 • 5 • 15 cm) produced by investment casting at P.C.C. (Portland, OR). The chemical composition of the as-received material is given in Table I. The plates were sectioned into 1.6 • 5 • 5-cm test specimens and were given various preweld heat treatments in an air furnace within the temperature range of 1037 ~ to 1163 ~ (summarized in Table II). The heat-treated specimens were gas tungsten arc spot welded onto a large plate to establish restraint and then were welded by the bead-on-plate technique by a Sciaky Mark VII electron-beam welder using a sharp focus (without beam oscillation) at 44 kV, 79 mA, and 152 cm/min. A partial penetration weld was achieved. The microstructure of the base material was examined by scanning electron microscopy (SEM) using the secondary and backscattered electron image modes. Phases were identified by their morphologies and chemical compositions. A JEOL* 840 scanning electron microscope equipped with a *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo, Japan.
XIAO HUANG, Material & Processes Engineer, is with Bristol Aerospace Ltd., and M.C. CHATURVEDI, Professor, is with the Department of Mechanical Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 2N2. N.L. RICHARDS, Manager, Material & Processes/Facilities Engineering, is with Bristol Aerospace Ltd., Winnipeg, MB, Canada. Manuscript submitted September 3, 1993. METALLUR
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