Computation of effective coefficient of thermal expansion of nodular cast iron

FRANCISCO RODRIGUEZ; ADRIAN BOCCARDO; PATRICIA M. DARDATI; DIEGO J. CELENTANO; LUIS A. GODOY

Buenos Aires

Congreso; PANACM 2015 - Buenos Aires; 2015

AMCA

The nodular cast iron is formed by graphite nodules and metallic matrix. The as-cast matrix may be ferritic, ferritic-pearlitic and pearlitic. The nodular cast iron is employed in industry because of its good mechanical properties and facility for casting. It is used in several important parts as brake systems, crankshafts and gears. In some cases, the parts are subjected to temperature changes, therefore in service conditions, such parts experiment thermal expansions. Thermal expansion plays a key role when there are tight tolerances between parts with different coefficients of thermal expansion (CTE). In this paper, the effective CTE (secant) of a nodular cast iron is evaluated using a finite element method. The computational micromechanical model takes into account a three dimensional representative volume element (RVE), which is formed by graphite nodules embedded in a ferritic-pearlitic matrix [1]. The distribution of graphite nodules inside the matrix is random. Ferrite is considered a halo around nodules. The RVE has periodic boundary condition. Mechanical behavior of phases has been modelled as linear elastic. This model takes into account volume fraction, CTE, Young's modulus and Poisson's ratio of each phase. In the present work pearlite is considered as a homogeneous microconstituent and its properties have been calculated using mixture rule. Coefficients of thermal expansion of ferritic, ferritic-pearlitic and pearlitic nodular cast iron have been evaluated using the above mentioned model over a range of temperatures going from ambient temperature up to 600ºC. The results obtained with the computational model were compared with mixture rule model, n-layered sphere model [2] and experimental measurements [3] . In all cases the results of computational model and n-layered sphere model are very close. CTE of ferritic and pearlitic nodular cast iron: from ambient temperature up to 300ºC the models have same trend than the experimental results. The error, respect to the experiments, is around 5% using both computational model and n-layered sphere model, and around 7% using mixture rule. From 300ºC up to 600ºC the models do not have the same trend than the experimental results. The slope of experimental CTE is the smallest. The error, according experiments, is around 7.5% using both n-layered model and computational model, and around 3% using mixture rule. CTE of ferritic-pearlitic nodular cast iron: from 200ºC up to 600ºC the models have same trend than the experimental results. The error, respect to the experiments, is around 5.5% using both computational model and n-layered sphere model. Experimental results and CTE using mixture rule are very close in above mentioned temperaturerange.REFERENCES[1] F.J. Rodríguez, P.M. Dardati, L.A. Godoy and D.J. Celentano, ?Derivation of nodular cast iron elastic properties via computational micromechanics?, Revista Internacional de Métodos Numéricos para Cálculo y Diseño en Ingeniería, in press (2014).[2] A.A. Gusev, ?Effective coefficient of thermal expansion of n-layered composite sphere model: Exact solution and its finite elements validation?, International Journal of Engineering Science, 84, 54-61(2014).[3] Properties and selection: irons, steels, and high performance alloys, ASM Handbook, Vol. I, Ed. 10th (1997).