Metastable precipitate in a duplex martensite +
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RAGHAVAN AYER, L.P. BENDEL, and V.F. ZACKAY The crystal structure and chemical composition of the strengthening precipitates in an Fe11.8 pct Cr-8.5 pct Ni-l.8 pct Ti-3.8 pct Mo martensite + ferrite duplex steel were determined by transmission electron microscopy (TEM). The study revealed that a metastable precipitate phase formed in the ferrite and martensite phases during the initial stages of aging. The metastable phase was proposed to have a hexagonal crystal structure similar to that reported for the omega phase in Ti alloys. Prolonged aging formed an equilibrium orthorhombic precipitate phase with a Ni3(Mo, Ti) stoichiometry. The higher dislocation density in martensite accelerated the precipitation reaction, and the transition to the equilibrium precipitate occurred with shorter aging times compared to ferrite. The details of the electron diffraction and analytical electron microscopy studies of the precipitates are described.
I.
INTRODUCTION
SINCE their development during the early sixties, high nickel maraging steels have found extensive use as structural materials in many applications requiring a combination of high strength and toughness. ~j,21 The basic principle of maraging consists of strengthening Fe-Ni martensitic matrices by the precipitation of fine intermetallic phases such as Ni3Mo, Ni3Ti, etc. A recent research effort at Ethicon, Inc., Somerville, NJ, successfully incorporated the concept of maraging to develop steel wires with superior strength and ductility for use as surgical needles, t31 These wires had tensile strengths in the range of 2760 to 3000 MPa and also exhibited a high degree of bend ductility. As part of this study, a series of alloys with microstructures ranging from single-phase martensite to duplex martensite/ferrite structures were investigated. Transmission electron microscopy (TEM) studies conducted to characterize their microstructure revealed interesting differences in the nature of the strengthening precipitates in the ferrite and martensite phases. In addition, cold work prior to aging influenced the type of precipitate that formed. The present article describes some of the TEM work relating to the strengthening precipitates in a duplex martensite + ferrite steel. II.
EXPERIMENTAL PROCEDURE
The analysis reported in this article was conducted on an Fe-Cr-Ni-Ti-Mo alloy. A 50-kg ingot of the alloy prepared by vacuum induction melt/vacuum arc remelt (VIM/ VAR) was analyzed to have the following composition (wt pct): 11.76 Cr, 8.52 Ni, 1.84 Ti, 3.75 Mo, 70 ppm C, 17 ppm N, 9 ppm S, 13 ppm O, 500 ppm Si, and 100 ppm Mn. The ingot was hot rolled to 7-mmdiameter rod and the rod was shaved and drawn to 6.33-mm-diameter wires. Samples from this wire material were annealed at 1366 K for 1 hour and water RAGHAVAN AYER is with STEM, Inc., Milford, CT 06460. L.P. BENDEL, Manager, Materials Engineering, is with Ethicon, Inc., Somerville, NJ 08876. V.F. ZACKAY, Consultant, resides in New Canaan, CT 06840. Manuscript submitted October 2, 1991. METALLURGICAL TRANSACTIONS A
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