Core Hardenability Calculations for Carburizing Steels
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INTRODUCTION
FORany steel used in the quenched or quenched and tempered condition, hardenability is one of its most important properties. It is this factor which determines which microconstituents will form on quenching a piece of given size in a given quenchant, and hence determines the final mechanical properties of the heat treated material. The ability to predict hardenability from chemical composition and to calculate Jominy end-quench curves is of great benefit to the materials engineer who wishes to know if a proposed new steel or a given heat of steel can meet the hardenability requirements of a given application. The engineer is often interested in the level of hardness he can obtain in a specific location on a certain part subjected to a given quench, and he often knows the "Jominy Equivalent Cooling Rate" of the location in question, i.e., the position on the standard Jominy end-quench bar which has the same cooling rate as the part location of interest. A commonly used index of hardenability is the ideal critical diameter (Dr). This is the diameter of a cylindrical steel bar Which will form 50 pct martensite at its center when subjected to an ideal quench. An ideal quench is one in which the temperature of the surface of the piece is instantaneously lowered to the temperature of the quenchant, on immersion therein, so that the cooling rate is controlled solely by the thermal diffusivity of the material. The purpose of this paper is to describe a new method of determining Dr values experimentally using a standard Jominy test and to present regression equations for calculating both Dr and Jominy curves from composition. It should be mentioned at the outset that these regression equations are valid only for calculating the core hardenability of boron-free carburizing steels with 0.15 to 0.25 pct* carbon. *All alloy contents in this paper are in weight percent.
presented by Grossmann. 2 DI is dependent on the composition and grain size of the steel, and various empirical formulae have been proposed to express this interrelationship. Grossmann3 was among the first to publish such empirical formulae; he used the model that Dt is equal to some base value (dependent only on carbon content and grain size) times various multiplying factors for the different alloying elements. Grossmann's work was based on data for steels with 0.6 pct carbon. 3 Using this same model, DeRetana and Doane4 have determined the multiplying factors for lower carbon steels (0.15 to 0.25 pct C). Their revised alloy multiplying factors5 and their extension4'5 of the original base Dr values first published by Kramer et al. 6 are shown in Figures 1 and 2, respectively. A comparison of the various methods for predicting Dr values was published
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