Variation of mechanical properties of metals due to nonisothermal tempering
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LATION UDC 669.017:621.78
VARIATION OF MECHANICAL PROPERTIES OF METALS DUE TO NONISOTHERMAL TEMPERING B. M. Shlyakman1 Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 3 – 8, December, 2012. A computational-experimental method for determining the mechanical properties of metals after nonisothermal tempering with the use of results of isothermal tests is suggested. Formulas based on the principle of additivity are derived. Computed and experimental values of mechanical properties obtained under laboratory and industrial conditions are compared.
Key words: heat treatment, tempering, mechanical properties, isothermal and nonisothermal processes, additivity principle, simulation. Methods for determining the properties of materials in kinetic processes from data obtained at constant (but the time) values of parameters can be found in studies of various inherent processes, to which tempering belongs too. For example, creep curves plotted at a constant load are used to plot the curves of stress relaxation for which these stresses are variables [1]. Another known method is determining the phase composition due to nonisothermal decomposition of supercooled austenite from isothermal curves [2, 3]. All these methods involve substitution of smooth curves by broken stepped lines, computation of the change in the parameters in every time step, and summation of the computed results in accordance with the additivity rule [4]. The aim of the present work was to develop a method for determining mechanical characteristics after nonisothermal tempering from experimental data obtained under isothermal conditions.
INTRODUCTION Tempering as a kind of heat treatment is often a finishing operation responsible for the main mechanical properties of articles. Therefore, the choice of correct parameters of tempering is an important and critical procedure. As a rule, the parameters of tempering (the temperature and the duration) are determined from experimental dependences of mechanical properties on the hold time obtained at a constant treatment temperature. Figure 1 presents typical dependences of conventional yield strength, ultimate rupture strength and hardness on duration of isothermal tempering (known as curves of tempering resistance). After specifying the desired level of properties these curves are used to choose the temperature and duration of the tempering. This approach does not allow for the nonisothermal nature of the tempering process conducted under actual conditions. Structural transformations occur in the metal continuously with different activity at different temperatures, which can affect substantially the final result. In the case of nonisothermal tempering the mechanical properties can be determined by reproducing the actual temperature regime during an experiment. However, this is a laborious and expensive procedure, which explains the interest in the development of a method for determining the mechanical properties formed by nonisothermal tempering from isothermal curves of tempering resis
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