Phase chemistry and precipitation reactions in maraging steels: Part I. Introduction and study of Co-containing C-300 st
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I.
INTRODUCTION
M A R A G I N G steels are a special class o f ultrahighstrength martensitic steels which are age-hardened by the precipitation of intermetallic compounds,tl,z,3J Since their initial development in the late 1950s, maraging steels have found a wide range o f applications. A large amount o f research work was carried out on the physical metallurgy o f these alloys in the early years after their invention. This interest has been renewed in the last decade, prompted by the sharp drop in the availability o f cobalt in the late 1970s. Although much work has focused on what happens in hardening reactions during aging, the exact nature o f the precipitation process is still, to a large extent, unknown. This is due to the high density o f extremely fine precipitates embedded in the martensite matrix ( o f the order o f 10 n m in size and 1023 m-3 in density4). Generally, it is believed that Ni3X (X = Ti, Mo, V, W) type phases form at the earlier aging stage, ]1-3,5,6~ while the more stable Fe-Y (Y = Mo, W) type phases t~-3,7,81 appear after longer aging times. However, different resuits have been reported even on the same material and after the same heat treatment. The similarity o f diffraction patterns from the different possible precipitate phases makes their identification difficult using transmission electron microscopy (TEM) techniques. Vasudevan e t al. studied the crystallography, structure, and composition o f the maraging precipitates in two W . SHA, formerly Graduate Student with the Department of Materials, Oxford University, is with the Department of Materials Science and Metallurgy, Cambridge University, Cambridge CB2 3QZ, United Kingdom. A. CEREZO, Royal Society Research Fellow, and G.D.W. SMITH, University Lecturer, are with the Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom. Manuscript submitted July 3 1 , t 9 9 2 . METALLURGICAL TRANSACTIONS A
commercial steels, V A S C O M A X C-250 and T-250,* *VASCOMAX C-250 and T - 2 5 0 are trademarks of Teledyne Vasco (formerly Vanadium Alloy Steel Company), Latrobe, PA.
using analytical electron microscopy and computersimulated electron diffraction patterns, t9,1°j The kinetics of precipitation were studied by electrical resistivity and microhardness measurements. The results indicate that precipitate nucleation occurs on dislocations and that growth proceeds by a mechanism in which the dislocations serve as collector lines f o r solute from the matrix. The strengthening o f the Co-free, higher Ti T-250 steel is caused by a refined distribution o f Ni3Ti precipitates. High strength is maintained at longer aging times due to the combined effect o f the high resistance o f these precipitates to coarsening and the small volume fraction o f reverted austenite. In the case o f Co-containing, l o w e r Ti, C-250 steel, these authors report the combined presence o f Ni3Ti precipitates (initially) and Fe2Mo (at longer aging). Loss o f strength at longer times is partly associated with overaging but is mainly due to the l a r
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