An analytical electron microscopy study of paraequilibrium cementite precipitation in ultra-high-strength steel
- PDF / 1,143,745 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 47 Downloads / 224 Views
I.
INTRODUCTION
THE desired property objectives in a modern ultra-highstrength (UHS) steel are achieved by controlling the kinetics of a series of solid-state phase transformations. These include the martensitic transformation upon quenching from the solution treatment temperature, cementite precipitation prior to secondary hardening, coherent M2C precipitation giving rise to secondary hardening, and austenite precipitation for dispersed-phase transformation toughening. Depending on the alloy composition, there can be a significant overlap in the kinetics of last three precipitation processes. Developing appropriate models and tools that reflect the synergistic interplay between the various kinetic processes governing microstructural evolution is the cornerstone of the systems approach to alloy design.[1] In UHS steels, a fully martensitic microstructure ensures a supersaturated state for the precipitation of extremely fine alloy carbides. Although a comprehensive model to predict the kinetics of martensitic transformation is not available, a fully martensitic microstructure can be obtained if the martensite start temperature (Ms) is sufficiently high. A model for predicting Ms for multicomponent ferrous alloys with good accuracy is available.[2,3] Secondary hardening, using coherent M2C carbides (where M 5 Mo, Cr, Fe, or V), is exploited in the commercial alloys AERMET100*[4] *AERMET100 is a trademark of Carpenter Technology, Redding, PA.
and AF1410[5,6] to obtain UHS, fracture-resistant alloys. During stage IV tempering, coherent M2C carbides are pre-
G. GHOSH, Research Assistant Professor, and G.B. OLSON, Professor, are with the Department of Materials Science and Engineering, Robert R. McCormick School of Engineering and Applied Science, Northwestern University, Evanston, IL 60208. C.E. CAMPBELL, formerly Graduate Student, Department of Materials Science and Engineering, Robert R. McCormick School of Engineering and Applied Science, Northwestern University, is Postdoctoral Fellow, Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. Manuscript submitted February 4, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
cipitated in a ferrite matrix. However, the precipitation of metastable cementite precedes the precipitation of M2C carbides, which, in turn, results in the dissolution of cementite. To achieve the desired high-fracture toughness, tempering must be continued until all the cementite is dissolved,[7] because the undissolved cementite particles serve as sites for microvoid nucleation. To maintain a fine M2C particle size during cementite dissolution, the growth regime of the M2C carbide precipitation reaction should be suppressed. The kinetics of M2C carbide precipitation in high–Co-Ni secondary-hardening steels have been studied using atom probe field-ion microscopy (APFIM), transmission electron microscopy (TEM), and small-angle neutron scattering (SANS) measurements to determine particle size, number density, volume fraction, and composition.[8,9,10] The kine
Data Loading...
