M. Górny

Effect of Copper and Nickel on the Transformation Kinetics of Austempered Ductile Iron

The kinetics of reaction occurring during the austempering treatment of ductile iron (DI) containing different additions of Cu and Ni was investigated in this work. DI bars were heat treated in an instrumented dilatometer in order to follow the exhibited transformation kinetics. The dilatometric results indicated that the addition of Cu alone did not have a significant effect on the incubation times for the austempering transformation. Also, the addition of both, Cu and Ni resulted in a significant effect on reducing the transformation rates. It was found that the austempering process is characterized by two clearly distinguished transformation stages. In the initial stage, the addition of Cu, and to a greater extent, additions of both Cu and Ni led to reductions in the transformation rates shifting the maximum transformation rate values toward longer times. The outcome of this work indicates that during the first stage of austempering, nucleation of the ferrite plates occurs via a diffusionless mechanism while their growth is diffusion controlled. Moreover, after the maximum in the transformation rate has been reached, the growth of ferrite plates becomes dominant with the rate-limiting step becoming the diffusion of C into the surrounding austenite. A qualitative model for the austempering transformation is proposed in this work to account for the experimental observations.

Effect of nodule count on austenitising and austempering kinetics of ductile iron castings and mechanical properties of thin walled iron castings

Abstract In the present work, the potential for producing thin walled ductile iron castings with an ausferritic matrix is presented. Experimentally, thin walled iron castings of 2 mm in thickness were obtained and characterised by a nodule count of 1992 mm−2. In addition, a reference casting was produced with a 25 mm thick wall and a nodule count of 330 mm−2. Austenitising was carried out at 920°C, whereas austempering was implemented in the 300–400°C temperature range. The austenitising and austempering transformation rates were determined by dilatometry, and the results were confirmed by microstructural analyses. It was found that in thin walled castings, the austenitising and austempering times were reduced by either one-half or one-third of the ones corresponding to the reference casting. The exhibited mechanical properties of the thin walled castings were also determined as a function of austempering time and temperature. It was found that austempering at 300°C for 1200 s leads to thin walled castings with a tensile strength of 1500 MPa. Accordingly, from this work, it is plausible to produce high strength thin walled castings that satisfy all the ASTM 897M grades of ausferritic ductile iron through proper heat treating.

Nodule count in ductile iron: theoretical model based on Weibull statistics

Abstract In this work, a model is proposed for heterogeneous nucleation on substrates (sites) whose size distribution can be described by Weibull statistics. It is found that the nuclei density N can be given in terms of the maximum undercooling ΔTm by N = N s exp[(−b/ΔT m)Γ(1 + 1/n)n]; where n is a positive integer, N s is the overall nucleation site density in the melt and b is the nucleation coefficient (b>0)(Γ denotes the gamma function). In the case when nucleation occurs on all the possible sites, the graphite nodule density N V after solidification equals N. Consequently, measurements of N V values were carried out on experimental nodular iron castings processed under various inoculation conditions. In addition, maximum undercooling ΔT m was measured by thermal analysis and volumetric nodule count (nodule density).N V was estimated from area densities N A on cast iron surface sections using stereology. The experimental results indicate that nucleation can be described by the proposed expression for n = 1 (i.e.N = N s exp[−b/ΔT m]). Moreover, N s and b values were experimentally determined. Finally, the proposed model suggests that the site size distribution is exponential in nature.

Theoretical model for heterogeneous nucleation of grains during solidification

Abstract Grain nucleation during solidification is generally a heterogeneous process. In this work, a model for heterogeneous grain nucleation on substrates is proposed where the size distribution is given by a Weibull type expression. In this model, the nuclei density λz is described by a function of the maximum undercooling ΔT m through an expression of the type λ z =λ exp[(-b/Δ T m) Γ (1 + 1/n), where n is a positive integer, λ is the substrate density in the melt, b is the nucleation coefficient (b > 0), and Γ is the second order Euler function. When nucleation occurs on all possible substrates, the grain density N V after solidification equals λ z. Hence, measurements of N V can be employed to disclose the active nucleation process. The proposed model was experimentally corroborated for cast iron solidification after inoculation. In this case, precise determinations of grain densities versus cast iron undercooling were made experimentally. A comparison of the experimental results with the proposed model indicates that the nucleation process is in good agreement with this model for n = 1, with the λ and b terms being time dependent functions. In particular, the outcome suggests that the substrate size distribution is of an exponential nature.

Microstructure and Properties of Ductile Iron and Compacted Graphite Iron Castings

This book provides an overview of the surface effects at the interface boundary of metal/sand moulds, and their influence on the surface quality, microstructure and mechanical and anticorrosive properties of high-quality cast iron. It explores utilitarian aspects of the production of high-quality cast iron castings, including thin-walled castings of high-quality cast iron alloys, and examines problems related to the determination of moulding sands and reclaim quality, and their influence on castings. Presenting new material, this book takes into account the influence of metal quality, pouring temperature, solidification time, the quality of moulding sand with the reclaim application, as well the binders of moulding sands, on the formation of the degenerated graphite near surface layers. It also employs the latest research methods, such as a wavelength-dispersive spectrometer (WDS) analysis and thermodynamic calculations, which were carried out on the reactions occurring in the study area. Providing a valuable resource to academics and researchers interested in materials science, metal casting and metallurgy, this book is also intended for metal industry professionals

Equiaxed Grain Count in Aluminum Alloy Castings: Theoretical Background and Experimental Verification

In this work, a theoretical model is proposed for heterogeneous nucleation on substrates the size distributions of which can be described by the Weibull statistics. In particular, the proposed model suggests that the size distribution of the various nucleation sites is exponential in nature. It is found that the nuclei density can be given in terms of the maximum undercooling. Measurements of grain count were carried out on single-phase Al-1.3 Si and Al-5.0 Cu (wt pct) alloys inoculated using an Al-5Ti-1B (wt pct) master alloy. In the single-phase alloys, the area of equiaxed dendritic grains was estimated using EBSD analysis and by stereological means on polished and etched surface sections. In addition, maximum undercoolings were determined by thermal analysis. The experimental outcome indicates that the volumetric grain density can be properly described by an exponential expression. Finally, the magnitudes of the nucleation parameters were experimentally determined in this work.

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.

Numerical Simulation of Austempering Heat Treatment of a Ductile Cast Iron

This paper presents a coupled thermo-mechanical-metallurgical formulation to predict the dimensional changes and microstructure of a ductile cast iron part as a consequence of an austempering heat process. To take into account the different complex phenomena which are present in the process, the stress-strain law and plastic evolution equations are defined within the context of the associate rate-independent thermo-plasticity theory. The metallurgical model considers the reverse eutectoid, ausferritic, and martensitic transformations using macro- and micro-models. The resulting model is solved using the finite element method. The performance of this model is evaluated by comparison with experimental results of a dilatometric test. The results indicate that both the experimental evolution of deformation and temperature are well represented by the numerical model.

Effect of Reclamation on the Skin Layer of Ductile Iron Cast in Furan Molds

The paper presents the results of investigations of the influence of the quality of molding sand with furan resin hardened by paratoluenesulfonic acid, on the formation of microstructure and surface quality of ductile iron castings. Within the studies different molding sands were used: molding sand prepared with fresh sand and molding sands prepared with reclaimed sands of a different purification degree, determined by the ignition loss value. Various concentrations of sulfur and nitrogen in the sand molds as a function of the ignition loss were shown in the paper. A series of experimental melts of ductile iron in molds made of molding sand characterized by different levels of surface-active elements (e.g., sulfur) and different gas evolution rates were performed. It was shown that there exists a significant effect of the quality of the sand on the formation of the graphite degeneration layer.

Eutectic cell and nodule count in grey and nodular cast irons

Abstract In the present work the predictions of a proposed analytical theory were compared with the experimental outcome on metallographic determinations of eutectic cell count N and nodule count N n. Accordingly, processing of flake graphite and ductile cast irons was carried out under various inoculation conditions in order to achieve a range of physicochemical states for the experimental melts. In addition, plates of various wall thicknesses s were cast and the resultant eutectic cell and nodule counts were established. Moreover, thermal analysis was employed for determinations of the maximum degree of undercooling for the graphite eutectic ΔT m. A relationship was found between the eutectic cell N or the nodule count and ΔT m. In addition, it was found that N and N n can be related to the casting modulus M and hence to s in plate shaped castings. Finally, the present work provides insights into the effect of technological factors such as the melt chemistry, the inoculation practice and the holding temperature and time on the resultant N and N n values. In general, it was found that the predictions of the present analysis are in good agreement with the experimental outcome.