Precipitation hardening in 350 grade maraging steel
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I.
INTRODUCTION
g number of studies have been carried out in the past on the microstructural aspects of different grades of maraging steels, f~,2l A majority of these studies had been in the late sixties and the early seventies. After the lapse of nearly a decade, interest in this type of steels has recently been revived, as reflected in the series of publications on different maraging alloys of Fe-Ni-Co-Mo and Fe-Ni-Mn types. 13"4'51 The manganese-free maraging steel, which is the subject of study in this work, is different from other manganese-free maraging steels of the types T-250 and C-250 on which a considerable amount of work has been done recently, t6,7j The important difference between the steel used in this work and the T-250 and C-250 grade steels can be noticed from Table I, in which the chemical compositions of these steels are given. T-250 is cobalt free, and C-250 contains relatively little titanium. The steel used in the present investigation, on the other hand, contains both cobalt and titanium in a substantial amount. A high-titanium concentration leads to a larger volume fraction of the Ni3Ti type of phase, and the presence of cobalt makes the formation of the Fe2Mo type of phase easier, tvl The initial strength of these steels is achieved by the precipitation of a Ni 3 (Ti, Mo) type of phase. 171This is then followed by precipitation of the Fe2Mo phase which is responsible for the peak strength and also for maintaining high strength on prolonged aging. 13] With this steel being a precipitation-hardenable alloy, the dislocation-precipitate interaction assumes considerable significance. Though this aspect has been discussed by some workers, tT1 experimental studies to examine it are rather few in other category of maraging steels and nonexistent in the category of steel used in the present study. One of the objectives of this work is to ascertain the nature of the dislocation-precipitate interaction at different stages of hardening. This task is made U.K. VISWANATHAN, Scientific Officer, Radiometallurgy Division, and G.K. DEY, Scientific Officer, Metallurgy Division, are with Bhabha Atomic Research Centre, Bombay-400085, India. M.K. ASUNDI, 44, Vibha, Government Colony, Bombay-400051, India, is a Private Metallurgical Consultant. Manuscript submitted August 7, 1992. METALLURGICAL TRANSACTIONS A
difficult by the fact that the volume fraction of precipitates is not very small, and these precipitates are associated with large strain fields. Other objectives of the microstructural studies are the following: identifying the precipitate phases responsible for strengthening, examining the morphology of precipitates, delineating the sequence of precipitation, and investigating austenite reversion.
II.
EXPERIMENTAL
The material used was vacuum arc-melted and vacuum arc-remelted quality 350 grade commercial maraging steel produced by Mishra Dhatu Nigam Limited. The material was in the form of forged and machined bars of 70-mm diameter in double solution annealed condition, a process which consisted of
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