Edward Fras

Structure and mechanical properties of unidirectionally solidified Fe-Cr-C and Fe-Cr-X-C alloys

In this work four different microstructures were obtained by unidirectional solidification of Fe-Cr-C eutectic alloys. Conditions for zone coupled growth were determined in alloys containing approximately 30 wt pct chromium. Furthermore, mechanical testing indicated that the maximum strength was exhibited by Fe-30Cr-C alloys with cerium or titanium additions. These alloys had the largest volume fraction of eutectic fibers and their ultimate tensile strength was of the order of 3250 MPa. Correlations between the rate of crystal growth(u) and fiber spacing (λ) or tensile strength(Rm) were found and an expression of the typeRm = Aλ-b2 was obtained whereb2 varied between 0.283 and 0.685. Finally, manganese or chromium (35 wt pct Cr) additions did not lead to appreciable improvements in composite strength for this alloy system.

Nucleation and grains density in grey cast iron—A theoretical model and experimental verification

A model for heterogeneous nucleation on substrates (places of the nucleation) which size distribution is of the Weibull type is proposed. The nuclei density λn is following function of the maximum undercooling ΔTm: λn=λexpl[(-b/ΔTm)—Γ(1+1/n)]n; where: n is a positive integer, λ—is the substrate density in melt and b is the nucleation coefficient (b > 0) (Γ- denotes the gamma function). In the case when nucleation occurs on all possible substrates, the grain density Nv after solidification is equal to λn. Consequently, from Nv measurements it is possible to conclude about the nucleation process. The experimental verification of the proposed model is performed for the solidification of cast iron. The grain density Nv was estimated from the density NA of grain sections by a stereological method. The experiment shows that nucleation follows the model for n = 1, ie.: λn=λexp[-b/ΔTm]. For inoculated cast iron, the empirical λ and b values are time dependent functions. The performed model analysis also indicates that the substrates size distribution is of the exponential type.

Eutectic cell and nodule count in cast irons

Abstract In the present work the predictions based on a theoretical analysis aimed at elucidating of eutectic cell count or nodule count N were experimentally verified. The experimental work was focused on processing flake graphite and ductile cast irons under various inoculation conditions in order to achieve various physicochemical states of the experimental melts. In addition, plates of various wall thicknesses s, were cast and the resultant eutectic cell or nodule counts were established. Moreover, thermal analysis was used to find out the degree of maximum undercooling for the graphite eutectic ΔT m. A relationship was found between the eutectic cell or nodule count and the maximum undercooling ΔT m. In addition, it was also found that N can be related to the wall thickness of plate shaped castings. Finally, the present work provides a rationale for the effect of technological factors such as the melt chemistry, inoculation practice, and holding temperature and time on the resultant cell count or nodule count of cast iron. In particular, good agreement was found between the predictions of the theoretical analysis and the experimental data.

Processing and properties of high aluminium Fe-Al-C alloys

A method was investigated to minimise or totally prevent the formation of the undesirable Al4C3 phase in high-aluminium cast iron composites (>19% Al). When aluminium carbide is absent in these castings, both mechanical strength and corrosion resistance are significantly improved. Accordingly, Ti additions were introduced to high-aluminium cast iron to develop Fe-Al-Ti-C alloys, which resemble “in-situ” composites where the inter-metallic FeAl phase is the matrix, and TiC acts as the reinforcement. It was found that the minimal amount of Ti that needs to be added has to be equal or above 4.5 wt% in order to totally remove the Al4C3 phase. It was found that Fe-Al-Ti-C alloys exhibit relatively high oxidation resistance, which easily exceeds that of high-chromium cast iron and cast chromium steels. Finally, in order to improve alloy ductility, minor alloy additions of elements such as B and Cr were added to the melt. It was found that additions of 0.03% B and 0.03%B-5% Cr combined with heat treatment lead to significant improvements in the ductility of the alloy with elongations of up to 15%.

Multiscale Modeling of Ductile Iron Solidification With Continuous Nucleation by a Cellular Automaton

The solidification of metals and alloys is a typical example of multiphysics and multiscale engineering systems. The phenomenon of different time and spatial scales should be taken into consideration in the modeling of a microstructure formation: heat diffusion, the components diffusion in the liquid and solid phases, the thermodynamics of phase transformation under a condition of inhomogeneous chemical composition of growing and vanishing phases, phase interface kinetics, and grains nucleation. The results of a two-dimensional modeling of the microstructure formation in a ductile cast iron are presented. The cellular automaton model (CA) was used for the simulation. The model takes into account the nucleation of two kinds of grains that appear inside of the liquid during solidification: austenite and graphite. The six states of CA cells correspond to the above-mentioned three phases (liquid, austenite and graphite) and to the three two-phase interfaces. A numerical solution was used for the modeling of concentration and temperature fields. The parabolic nonlinear differential equations with a source function were solved by using the finite element method and explicit scheme. In the mono-phase cells the source function is equal to zero. In the interface cells the value of the source function varies depending on the local undercooling. The undercooling value depends on the front curvature, the local temperature and the local chemical composition of the phases. Overlapping lattices with the same spatial step were used for concentration field modeling and for the CA. The time scale of the temperature field for this lattice is about 104 times shorter. Due to the above reasons, another lattice was used with a multiple spatial step and the same time step. The new grain nucleation of solid phases from a liquid is a phenomenon which must be taken into account for correct simulation of a polycrystalline structure formation. The cumulative distribution curve approach was used to calculate the number of substrates on which nucleation takes place as a function of under-cooling below the equilibrium temperature. An algorithm of continuous nucleation modeling during solidification is presented. The undercooling of solid phase grain nucleation was calculated on the basis of the inverse function of the above-mentioned cumulative distribution curve (fractile) with the argument equal to the random number generated in the interval 0[[ellipsis]]1 with uniform density. The domain of correct usage of this algorithm was analyzed.Copyright