Microstructural development during solidification of stainless steel alloys
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I.
INTRODUCTION
T H E influence of cooling rate on the microstructure of stainless steel (SST) alloys has been an area of active research because rapid solidification processing can dramatically alter the microstructure of these alloys. 1~'2'31 Table I summarizes the typical cooling rates of different solidification processes, indicating that there is a relatively large range of cooling rates for each process and an even wider range of cooling rates for different processing techniques. The slowest cooling rates are produced by directional solidification, resulting in dendritic microstructures that are close to equilibrium. The highest cooling rates are produced by high-power density welding and rapid solidification processing techniques, resulting in microstructures that are far from equilibrium. The cooling-rate range between these two extremes can be as large as eight orders of magnitude and is responsible for the complex microstructural features that develop during the solidification of SST alloys. J.W. ELMER, formerly Graduate Student at the Massachusetts Institute of Technology, is now Metallurgist with Lawrence Livermore National Laboratory, Livermore, C A 94550. S.M. ALLEN, Associate Professor of Physical Metallurgy, and T . W . EAGAR, Professor of Materials Engineering, are with the Department of Materials Science, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted December 16, 1988. METALLURGICAL TRANSACTIONS A
The chemical composition and the solidification conditions of a given alloy have been shown to be equally important variables in the microstructural development. Katayama and Matsunawa f~l and David et a l J 21 have compared the microstructures that form in slow-coolingrate arc welds with those that form in high-cooling-rate laser-beam (LB) welds. Their observations show that (1) the ferrite content of low C r / N i ratio alloys decreases with increasing cooling rate, whereas (2) the ferrite content of high C r / N i ratio alloys increases with increasing cooling rate. These two different behaviors of residual ferrite with cooling rate have also been confirmed in electron-beam (EB) surface-melted SST alloys, t31 From these investigations, it is apparent that the two different behaviors of residual ferrite with cooling rate are related to the primary solidification mode (PSM). The solidification mode of duplex SST alloys can be either primary ferrite or primary austenite, depending on the alloy composition and the cooling rate. In general, the different behaviors of ferrite content with cooling rate can be described as follows. First, low C r / N i ratio alloys solidify with austenite as the primary phase. The ferrite content of these alloys d e c r e a s e s with increasing cooling rate because of a reduction in the amount of solute redistribution at high rates) 31 Second, high Cr/Ni ratio alloys solidify with ferrite as the primary phase. The ferrite content of these alloys i n c r e a s e s with increasing VOLUME 20A, OCTOBER 1989--2117
Table 1. Estimated Cooling-R
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