Phase chemistry and precipitation reactions in maraging steels: Part II. Co-free T-300 steel
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I.
INTRODUCTION
T H E development o f Co-free maraging steels was prompted by a sharp drop o f Co availability and a rise in Co price in the late 1970s. In fact, Co is present only in small quantity in precipitates in maraging steels, and so its contribution to overall strength must be due to an indirect, rather than a direct, mechanism, t1'2~ The strength o f the steels normally drops as Co content decreases but can be regained by increasing the Ti content. As in the case o f Co-containing steels, the exact nature o f precipitates in Co-free grades is not well understood. There have been reports suggesting that they are hardened by Ni3Ti only,t3~ but other reports claim the formation o f Mo-containing precipitates as well. t4,5] The VASCOMAX* Co-free T-series steels have been *VASCOMAX T - 2 5 0 , C-250, T - 3 0 0 and C-300 are trademarks of Teledyne Vasco (formerly Vanadium Alloy Steel Company), Latrobe, PA.
relatively recently designed, with the same strength level as the Co-containing alloys (C-series). [6,7~ Vasudevan e t al. compared the maraging precipitates in the commercial steels T-250 and C-250.14'5] Their main conclusions were outlined in Part I.[~l W e have examined a n u m b e r o f maraging steels using the high mass resolution energy-compensated Oxford FIM100 system. Preliminary results have been reported.[8-~ 11 The current article is intended to present our overall microstructural and microchemical investigation o f the Co-free V A S C O M A X T-300 steel (for composition, see Table 1). This steel was chosen partly because it has been the subject o f investigations by Vanderwalker. ]3,~z~ More importantly, comparing the aging behavior o f the Co-containing and the Co-free steels, the role o f Co can b e understood,t~31 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. C E R E Z O , R o y a l 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 , 1992. METALLURGICAL TRANSACTIONS A
Hickey and Thomas characterized mechanical properties o f the V A S C O M A X T-250 maraging steel.J7] F r a c ture toughness increased with increasing aging temperature in the range o f 450 °C to 510 °C, and Charpy energy was the highest in the 510 °C aged condition. The 510 °C aging temperature resulted in the maximum tensile strength and toughness properties. The main aging temperature in the present work was therefore chosen to be 510 °C. O t h e r temperatures ranging from 450 °C to 610 °C have also been studied. II.
EXPERIMENTAL
The T-300 steel were originally made by Vanadium Alloy Steel Company, Latrobe, PA. Small plates o f steels in the as-received condition (about 10-mm thick) were supplied courtesy o f Dr. D . M . Vanderwalker o f the United States Army Materials Technology Laboratory, Watertown, MA. These were cut into
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