Rapid Solidification of Steels
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RAPIDLY SOLIDIFIED AMORPHOUS AND CRYSTALLINE and M. Cohen, editors B.H. Kear, B.C. Giessen,
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ALLOYS
317
RAPID SOLIDIFICATION OF STEELS
BRIAN CANTOR Department of Metallurgy & Science of Materials, University of Oxford, Parks Road, Oxford OXi 3PH
ABSTRACT This paper reviews some fundamental aspects of the rapid solidification of iron-based alloys and commercial steels, concentrating on heat flow during cooling, the thermodynamics and kinetics of solidification, post-solidification solid state phase transformations, and the effect of these processes on the resulting steel microstructures.
INTRODUCTION Compared with conventional casting methods, rapid solidification produces small grain sizes, enhanced solubility of alloying elements and impurities, low or zero levels of macrosegregation and microsegregation, and removal or finescale dispersion of second-phase particles formed during secondary solidification or precipitation reactions. These effects can lead to desirable improvements in alloy properties [1-4]. Compared with Al or Ni based alloys, there have been relatively few investigations of the microstructure and properties of rapidly solidified steels, and Wood and Honeycombe [5,6] have recently reviewed the available information. It is pointless to reproduce Wood and Honeycombe's comprehensive compilation of data. Instead, this paper concentrates on fundamental aspects of the rapid solidification of steels, in particular the relationship between heat flow during rapid solidification and the resulting steel microstructure. COOLING RATES At Consider a thin, parallel-sided specimen at an initial temperature To. time t = 0, let one face of the specimen be brought into contact with a substrate at a lower initial temperature Ts. For unidirectional heat flow normal to the specimen/substrate interface, the temperature distributions in specimen and substrate, T(x,t) and T'(x,t) can be found by solving one-dimensional diffusion equations: 2
dT/dt = ad T/dx dT'/dt = a'd
2
2
T'/dx
(1) 2
(2)
where a, a' are thermal diffusivities of specimen and substrate. The rate of heat removal from the specimen per unit area of interface is given by: q = k(dT/dx)i = h(TI-T{)
= k'(dT'/dx)1
(3)
where k, k' are specimen and substrate thermal conductivities, h is the interface heat transfer coefficient, TI, TI' are specimen and substrate temperatures at the interface, and (dT/dx)i, (dT'/dx)I are specimen and substrate thermal gradients at the interface. when no solidification or other phase transformation takes place, the specimen has an average cooling rate t given by:
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q = dpCT
(4)
where d, p, C are specimen thickness, density and specific heat respectively. Ruhl [7] has solved eqs (1-3) by finitedifference calculations to obtain T(x,t) and T'(x,t) for an Fe specimen on a Cu substrate, and determined specimen cooling rates for different assumed values of heat transfer coefficient h. The primary resistance to heat flow is determined by specimen and substrate For small h, with N > 1 and N' >> 1, t
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