Dilatometric technique for evaluation of the kinetics of solid-state transformation of maraging steel
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I.
INTRODUCTION
M A R A G I N G steel is a class of low-carbon highalloy steel that can provide a combination of ultrahigh strength and moderate toughness. Its mechanical properties are governed by a number of solid-state reactions that take place in the alloy during heat treatment. The basic microstructure of 18Ni maraging steel consists of martensite with a lath morphology. At an intermediate aging temperature, uniform precipitation of fine intermetallic phases occurs within the martensite laths, thereby imparting excellent mechanical properties. However, prolonged aging of the material at higher temperatures leads to the formation of substantial amounts of austenite. This reverted austenite significantly influences the mechanical properties of maraging steel, limiting its usefulness as a high-strength material. In order to optimize the heat treatment parameters that lead to desirable properties, the kinetics of solid-state reactions in maraging steel have been evaluated. The transformation kinetics of the solid-state reaction in a material can be analyzed by measuring the changes in a physical property such as hardness, dilation, electrical resistivity, or enthalpy as a function of reaction time and temperature. Experimental methods are generally restricted to those based on isothermal annealing. Recently, much effort has been directed toward the development of nonisothermal techniques for the evaluation of transformation kinetics, because such techniques are simple and precise. These experiments generally have been confined to thermal analysis using a differential scanning calorimeter, where the temperature corresponding to the m a x i m u m reaction rate is measured from the enthalpy peak signal and the data are then analyzed in terms of the Kissinger method tm~ in order to obtain the activation energy associated with the transformation. Dilatometry is a sensitive and convenient method for studying solid-state transformation, but its
use for this purpose has been largely ignored until recently, t3m For example, tracing the temperatures corresponding to the inflection points in the dilatometric curve at different heating rates allowed determination of the activation energy of tempering in iron-carbon and iron-nitrogen martensites, t3~ This article presents the results of nonisothermal dilatometric experiments performed on a 350 grade commercial maraging steel. The activation energy estimated for the precipitation is compared with that obtained from isothermal experiments based on hardness measurements. The kinetics of the reversion of martensite to austenite at higher temperatures is examined in terms of activation energy, and the results are supplemented with microstructural evidence.
II. METHOD, MATERIAL, AND EXPERIMENTAL PROCEDURE Reaction kinetics can be analyzed using simplified procedures based on the m a x i m u m transformation rate of the Kissinger equationJ 1,21 The total rate of reaction during a transformation can be expressed m as
dx dt
- - = A(1
- x) e - e / R r
[1]
where T is absolute te
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