Microstructure and mechanical properties of a 2000 MPa Co-free maraging steel after aging at 753 K
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6/30/04
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Microstructure and Mechanical Properties of a 2000 MPa Co-Free Maraging Steel after Aging at 753 K Y. HE, K. YANG, W. SHA, and D. J. CLELAND The precipitation kinetics at the aging temperature of 753 K in a 2000 MPa grade Co-free maraging steel (Fe-18.9Ni-4.1Mo-1.9Ti, wt pct) has been studied. Under the peak-aged condition at 753 K, Ni3Ti precipitates of moderate size were uniformly distributed in the martensite matrix, leading to optimal combination of strength (2000 MPa of yield strength) and fracture toughness (70 MPa 1m). The ultra-high strength of the maraging steel subjected to long time aging at 753 K is attributed to the high resistance to coarsening of precipitates. The orientation relationship between martensite and Ni3Ti was observed as (011)M//(0001)-Ni3Ti and [111]M//[1120]-Ni3Ti. The Orowan mechanism is the dominant strengthening mechanism.
I. INTRODUCTION AND EXPERIMENTAL PROCEDURE
THIS article reports systematic work on the microstructure and its relation to mechanical properties in a 2000 MPa maraging steel after aging at 753 K. Strengthening and toughening mechanisms and other controversial issues are also discussed. This article follows previous research[1] and concentrates on aging at an intermediate, or normal, temperature, 753 K, of the 2000 MPa grade Co-free maraging steel. The steel composition is Fe-18.9Ni-4.1Mo-1.9Ti (wt pct). The subject of study was introduced in Reference 1. The main conclusions from that work can be summarized in the following points. (1) The phase transformation temperatures of the 2000 MPa grade maraging steel studied in the present work are Ms 408 K, Mf 321 K, As 783 K, and Af 993 K. Cooling to room temperature can achieve a single-phase martensitic structure. (2) There is rapid age hardening across the aging temperature range of 713 to 813 K in the Co-free maraging steel. (3) In the early stages of aging at 713 K, the Co-free maraging steel has extremely fine precipitates with average diameter 2 to 3 nm. Ni3Ti precipitates are identified, but the crystallography of another family of spherical precipitates has not been identified. Ni3Ti is the main precipitate, growing and coalescing into its normal needle or rod shape along the 111 direction with increasing aging time. This type of precipitate has strong resistance to coarsening at this aging temperature. (4) The spherical precipitates exist in a stable manner after low-temperature aging, with strong resistance to coarsening. (5) When aged at 713 K, there is no reverted austenite.
Y. HE, formerly Ph.D. Student, with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China, is now a Research Associate with the Laboratory for Materials Science, Delft University of Technology, 2628 AL Delft, The Netherlands. K. YANG, Professor, is with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China. W. SHA, University Reader, and D. J. CLELAND, Professor, are with the Metals Research Group, School of Civil Engineering, The Qu
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