FCC and BCC Solidification Products in a Rapidly Solidified Austenitic Steel.
- PDF / 2,331,663 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 94 Downloads / 167 Views
349
FCC AND BCC SOLIDIFICATION PRODUCTS IN A RAPIDLY SOLIDIFIED AUSTENITIC STEEL. THOMAS F. KELLY, JOHN B. VANDER SANDE AND MORRIS COHEN Department of Materials Science and Engineering, M.I.T., Cambridge, Massachusetts 02139 ABSTRACT The microstructures and local composition variations in centrifugally atomized high-sulfur stainless steel powder are investigated. Both fcc and bcc are found to be primary solidification phases in the as-solidified powder of this nominally austenitic steel where the smaller powder particles (ý 70 micron diameter) tend to be bcc. Cellular solidification structures, with sulfide precipitates (100 to 200 nm diameter in size) at the cell walls, are observed in both fcc and bcc particles. The bcc structure, however, has many small sulfide precipitates (10 to 20 nm diameter) in the cell interior with few larger sulfide precipitates at the cell walls. The small precipitates, observed only in the bcc structures, form on cooling from a supersaturated solid solution that results from reduced solute partitioning during solidification. Partitioning of chromium and nickel is minimal in these cellular structures. A non-cellular bcc structure is also observed with small sulfide precipitates throughout the entire structure. This non-cellular bcc structure results from smooth-front massive solidification. Analysis of the nucleation process for solidification indicates that a transition from fcc nucleation to bcc nucleation occurs with increasing wetting angle in heterogeneous nucleation. Thus bcc should nucleate in the smaller droplets of a liquid dispersion where catalytic surfaces of low potentcy (large wetting angle) tend to be the only heterogeneous nucleants available. EXPERIMENTAL PROCEDURE An austenitic 303 stainless steel was rapidly solidified'by centrifugal atomization5 and forced convective cooling in helium at cooling rates on the order of 10 K/s and greater. Electron-transparent specimens of the powder were prepared from composite foils of powder in electrodeposited nickel by a combination of conventional jet electropolishing and ion beam milling. Specimens were observed in scanning transmission electron microscopy where compositions are determined by analysis of X-ray fluorescence spectra. For more details see Reference 1. *Nominal composition wt% Fe Cr Ni Mn Cu Si Mo S Co C N P bal 17.3 8.7 1.60 0.78 0.66 0.37 0.34 0.17 0.034 0.032 0.028 PThe powders were kindly supplied by Pratt and Whitney Aircraft Group, Government Products Division, United Technologies Corporation, West Palm Beach, Florida 33402.
350
RESULTS Both fcc and bcc structures are found in the rapidly solidified powder of the 303 stainless steel. In the as-solidified powder which is sieved to -140 mesh (n,120 micron diameter), fcc is the primary solidification phase in the larger particles (%u 70 micron diameter) and bcc is the primary solidification phase in the smaller particles (f 70 jimdiameter). Secondary electron micrographs (SEM) of the magnetically separated particles, Figure 1, clearly show the relative sizes o
Data Loading...
