Prediction of the mechanical properties of gray cast iron (probabilistic approach)

Abstract

The article analyses the literature on the dependence of mechanical properties (tensile strength, hardness, etc.) of grey cast iron castings on the percentage of chemical elements in its composition and on the carbon equivalent, degree of eutecticity, and the ratio of carbon to silicon content. On the example of assessing the mechanical properties of cast iron with lamellar graphite, these dependencies are considered in the probabilistic aspect. It is emphasised that even within the same ranges of percentage of chemical elements in cast iron, as defined in standards or specifications, the values of its carbon equivalent, eutecticity, and the ratio of carbon to silicon content may be different. On the other hand, with different percentage ratios of chemical elements in the cast iron composition, the values of its carbon equivalent may be the same. It is shown that the conclusion about the presence or absence of correlation, in particular, between the hardness of castings and the carbon equivalent of the grey cast iron from which they are made, depends on the interval of variation of this argument. If the variation in the hardness of castings is significant and the interval for changing its carbon equivalent, calculated according to the chemical composition of cast iron specified in the standard for a particular casting product, is relatively short, the dependence of cast iron hardness on its carbon equivalent may not be evident. The disadvantage of the known empirical formulas intended for the approximate calculation of the tensile strength and hardness of cast iron based on its carbon element and eutecticity is the uncertainty of the choice of these indicators. It is proposed to use in such formulas the mathematical expectations (average values) of the carbon equivalent, eutecticity degree and the ratio of carbon to silicon content determined in the probabilistic approach using the Monte Carlo method. It is recommended that the distributions of chemical elements in grey cast iron within their intervals provided for in the standards be considered in accordance with the normal Gaussian law. Based on the information on the chemical composition and properties of grey cast iron with lamellar graphite smelted in accordance with the requirements of DSTU 8833-2019, the dependence of the carbon equivalent and the degree of eutecticity on the ratio of carbon to silicon content, as well as the strength and hardness of castings on these characteristics of the chemical composition of cast iron, have been identified. The formulas for determining the mechanical properties of grey cast iron with lamellar graphite have been calculated. Keywords: cast iron, casting, chemical composition, carbon equivalent, carbon to silicon ratio, degree of eutecticity, hardness, strength.