The concept of an effective quench temperature and its use in studying elevated-temperature microstructures
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J.M. VITEK and S. A. DAVID Elevated-temperature structures are routinely studied at room temperature by assuming such structures can be quenched-in by appropriate means. Undoubtedly this assumption is suitable in many circumstances, but its validity is difficult to check. In particular, when the phase equilibrium at elevated temperatures is not drastically different from that found at room temperature, small changes may be hard to detect. The possibility is often ignored that microstructures change during quenching, and that the final product is not truly representative of the structure at the temperature from which one has quenched. A water quench, for example, is all too often assumed to be sufficiently fast to avoid any alterations during cooling. It is the objective of this paper to propose a method of determining an effective quench temperature. An effective quench temperature, corresponding to a given solid-state cooling rate, represents the temperature below which the structure can be quenched-in without any substantial structural alterations during cooling. The effective quench temperature is determined by means of compositional analyses of the constituent phases in a structure, but in theory it can be determined by any property that is uniquely defined as a function of temperature. The idea of an effective quench temperature will be applied to welded or rapidly quenched type 308 austenitic stainless steels. Its application leads to several interesting conclusions regarding the relative cooling rates of these steels produced by various means. The proposed method of obtaining an effective quench temperature, Tq, is analogous to the method used in the study of glasses for defining a glass transition temperature, Tg. 1 For glasses, the liquid structure and properties are continuously changing during cooling in an attempt to remain in metastable equilibrium during the cooling process. At any given cooling rate, metastable equilibrium can be maintained only to a certain extent. Below a critical temperature, defined as the glass transition temperature, Tg, sufficient time is not available during cooling for the structure to maintain metastable equilibrium. Hence, when cooling a glass from the liquid state (T > Tg) the structure at Tx is effectively quenched-in. Heat treating this quenched-in structure may result in relaxation as the structure strives to achieve metastable equilibrium at the aging temperature. A similar approach may be applied to the case of a system in multiphase equilibrium. The composition of each phase is defined by the equilibrium tie lines and this composition is likely to vary as a function of temperature. Therefore, as a J.M. VITEK, Member of Research Staff, and S.A. DAVID, Group Leader, are with Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Manuscript submitted July 20, 1984. METALLURGICAL TRANSACTIONS A
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COOLING AT T2
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COOLING AT Tt
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f,>t2 lt2) TEMPERATURE Fig. 1 --Schematic diagram illustrating the departure from equilibrium
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