Heat transfer during Nd: Yag pulsed laser welding and its effect on solidification structure of austenitic stainless ste
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INTRODUCTION
O N E of the major problems encountered during fusion joining of austenitic stainless steels is the propensity for hot-cracking, tl,2] One way of controlling the crack sensitivity of austenitic stainless steel welds is by adjusting the composition so as to produce approximately 5 pct delta (6) ferrite in the weld metals, t31 It is well recognized that the amount of ferrite is related to the composition of the weld metal. To predict the amount of delta ferrite in the weld metal microstructure, Schaeffler t41 developed a constitution diagram in terms of the important austenite stabilizers and ferrite stabilizers in the weld deposit. This diagram was later modified by Delong ~5] to introduce the austenite stabilizing effect of nitrogen. However, the ferrite morphology (size, shape, and distribution) in the weld metal microstructure often exhibits considerable variation and is not addressed in these constitution diagrams. The rate at which the weldment cools significantly influences the ferrite morphology and distribution, t61 It is well documented that, for GTA welds, the ferrite morphology is strongly dependent on the process parameters.IT.8] For high heat inputs with relatively low cooling rates, the solidification substructure tends to be coarse and results in a widely spaced ferrite network. Lower heat inputs, on the other hand, produce a finer ferrite network. In the case of high energy beam processes such as laser welding, other types of microstructural modification have been found. For example, changes in the mode of solidification were observed after laser weldT. Z A C H A R I A , S.A. D A V I D , and J.M. VITEK are with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. T. D E B R O Y is with the Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802. Manuscript submitted April 1, 1988. METALLURGICAL TRANSACTIONS A
ing. t9-~2] As a result, the rapid solidification of the weld metal during laser welding makes predictions of ferrite morphology from conventional constitution diagrams impossible. Recognizing the need for including the effect of cooling rate, David et a l . , t12~ in a recent publication, proposed modifications to the Schaeffler constitution diagram to incorporate the effect of rapid solidification on weld metal microstructure. The authors showed that, during laser welding, both weld pool cooling rates and postsolidification solid-state cooling rates can significantly influence the weld metal microstructure by changing the primary mode of solidification and/or suppressing subsequent solid-state transformation. Since cooling rates are sensitive to power density and scanning speed of the laser, it is of considerable interest to describe quantitatively the effect of process parameters on the solidification behavior and the resultant structure. Accurate measurement of puddle temperature and cooling rates during laser welding are extremely difficult due to the small size of the weld pool and large t
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