Simon N. Lekakh

Detection of Non-metallic Inclusions in Steel Continuous Casting Billets

This work applied automated particle analysis to study non-metallic inclusions in steel. Compared with traditional methods, the approach has the advantage of capturing the morphology, measuring the size, recording the original positions, and identifying the composition of inclusions on a selected area in a short time. The morphology and composition of typical inclusions were analyzed using partial acid extraction and discussed through thermodynamic calculation. Steel samples were collected from the entire cross section of billets cast during times of steady state and ladle change. The spatial distribution of inclusions agreed well with the measurement of the total oxygen. The spatial distribution of inclusions was plotted to represent the entrapment positions of inclusions on the casting strand and their concentration on the cross section of the billet. Also, regarding the different size and type of inclusions, the spatial distribution of classified inclusions was explored such as the distribution of sulfide, oxide, and high sodium and potassium content inclusions. The sufficient information could be used to identify the source of inclusions and guide the steel refining process.

Optimization of Melt Treatment for Austenitic Steel Grain Refinement

Refinement of the as-cast grain structure of austenitic steels requires the presence of active solid nuclei during solidification. These nuclei can be formed in situ in the liquid alloy by promoting reactions between transition metals (Ti, Zr, Nb, and Hf) and metalloid elements (C, S, O, and N) dissolved in the melt. Using thermodynamic simulations, experiments were designed to evaluate the effectiveness of a predicted sequence of reactions targeted to form precipitates that could act as active nuclei for grain refinement in austenitic steel castings. Melt additions performed to promote the sequential precipitation of titanium nitride (TiN) onto previously formed spinel (Al2MgO4) inclusions in the melt resulted in a significant refinement of the as-cast grain structure in heavy section Cr-Ni-Mo stainless steel castings. A refined as-cast structure consisting of an inner fine-equiaxed grain structure and outer columnar dendrite zone structure of limited length was achieved in experimental castings. The sequential of precipitation of TiN onto Al2MgO4 was confirmed using automated SEM/EDX and TEM analyses.

Solidification Kinetics of Graphite Nodules in Cast Iron and Shrinkage Porosity

The shrinkage porosity of castings made from cast iron with spherical graphite (SGI) depends on a combination of intrinsic (density and volume of phases, solidification kinetics) and extrinsic conditions related to casting-mold thermo-mechanical interactions. Precipitation of graphite nodules increases the specific SGI volume, and control of the nucleation rate in solidified castings can be used for improving casting soundness. In this article, the method of structural reconstruction of solidification kinetics was used to link the nucleation rate of graphite nodules to experimentally observed shrinkage porosity in a specially designed test casting. An automated SEM/EDX system was used to determine the “true” two-dimensional graphite nodule distributions in the casting sections. These two-dimensional distributions were converted into the volume particle distribution functions (PDF), and the solidification kinetics were reconstructed by applying inverse simulations. The experiments were performed with variations in inoculation and pouring temperature. The shrinkage porosity was compared to the restored sequence of graphite nodule nucleation in the specific casting volumes. It is shown that the second nucleation wave in low-temperature poured and inoculated SGI eliminated interdendritic microporosity. The suggested method could be used in industry to improve the soundness of SGI castings.

Novel Approaches to Analyze Structure of Ductile Iron

The suggested approaches, including an automated SEM/EDX (Scanning Electron Microscopy/Energy Dispersive X-ray) analysis of graphite nodule nuclei and a special algorithm to convert two-dimensional to three-dimensional graphite nodule size distribution, were tested. The “soft” quenching technique was applied to develop small graphite nodules and increase probability to reveal non-metallic heterogeneous nuclei using automated SEM/EDX analysis. Ternary diagrams present experimental statistics of the graphite nuclei chemistry. A special algorithm for the conversion of diameters of two-dimensional sections of graphite nodules to the real three-dimensional distribution of spherical particles was developed. This algorithm is based on inverse simulation. The developed program calculates a three-dimensional nodule diameter distribution curve, the real average diameter, and volumetric nodule number. The examples of practical applications of these methods for spherical graphite characterization in different ductile iron castings are provided. Heterogeneous nucleation of graphite nodules is discussed based on the novel experimental data.

Thermal Property Database for Investment Casting Shells

Reliable and realistic thermal properties data for investment casting shell molds are required to correctly simulate the solidification and predict the shrinkage. Investment casting shells exhibit several phase transformations during firing and pouring which affect their transient thermal properties. These properties are dependent upon time, temperature and process history. This study presents the thermal properties (thermal conductivity and specific heat capacity) of seven industrially produced ceramic molds using an inverse method in which pure Ni was poured into ceramic molds equipped with two thermocouples (inside the mold cavity and in the shell). MAGMASOFT® software (hereafter known as Software A) was used to simulate virtual cooling curves which were fitted to experimental curves by adjusting the temperature dependent thermal properties of the ceramic mold. The thermal properties data obtained from the inverse method were compared with measurement results from laser flash and the differences were discussed. The dataset thus developed will serve to improve the accuracy of investment casting simulation.

High Strength Ductile Iron Produced by Engineered Cooling: Process Concept

Traditionally, high strength ductile irons are produced by a combination of alloying and heat treatment, both operations substantially increase the cost and carbon footprint of casting production. In this study, the concept of a process for the production of high strength ductile iron using engineered cooling is discussed. The process includes early shakeout of the casting from the mold and application of a specially designed cooling schedule (engineered cooling) to develop the desired structure. The high extraction rate of internal heat is achieved by controlling the thermal gradient in the casting wall and the surface temperature. Experimental “Thermal Simulator” techniques and Computational Fluid Dynamic (CFD) simulations were used to design the cooling parameters. The concept was experimentally verified by pouring plate castings with 1” wall thickness and applying the engineered cooling techniques. The tensile strengths of ductile iron increased from 550–600 MPa for castings solidified in the mold to 1000–1050 MPa after engineered cooling.

Two Inoculation Methods for Refining As-Cast Grain Structure in Austenitic 316L Steel

Two inoculation methods were utilized to introduce titanium nitride (TiN) particles into an AISI 316L steel melt to refine the as-cast grain structure during solidification. The design of the experimental melt treatments and grain refining additions was performed using thermodynamic simulations. The first inoculation method is based on in situ formation of heterogeneous nuclei by TiN co-precipitation on preexisting Mg–Al spinel inclusions. This method included a two-stage melt treatment using spinel forming additions followed by an addition of titanium in the ladle just prior to pouring. The second inoculation method used a newly developed master alloy that contains TiN precipitates which was added in the ladle during furnace tapping. In this method, protective conditions to prevent full dissolution of the TiN nuclei before the onset of solidification were determined by thermodynamic simulations. Grain refinement of the cast macrostructure was observed with both methods. The in situ method provided finer equiaxed grains than the master alloy method, while a thicker zone with columnar grains next to the chill was observed. A scanning electron microscope (SEM) with automated feature analysis was used to quantify the resulting inclusions. The master alloy method eliminated the need for spinel, gave better control of the amount and size of heterogeneous nuclei, and reduced clustering tendency in comparison with the in situ method. However, the in situ formed nuclei method is more effective to refine grain size. The effects of contact angle and nuclei surface geometry on the activity of heterogeneous nucleation were discussed. It is proposed that clustering TiN particles provides numerous sharp, concave corners which favors the heterogeneous nucleation of austenite grains. This is illustrated by SEM images of extracted TiN particles and electron backscatter diffraction analysis of grain orientation.

Effect of Nonmetallic Inclusions on Solidification of Inoculated Spheroidal Graphite Iron

Inoculation treatment of spheroidal graphite cast iron (SGI) controls graphite nodule heterogeneous nucleation and is used for elimination of solidification microporosity and improvement in casting performance. In this study, thermodynamic simulations were performed to predict precipitates formed in the inoculated melt above a liquidus temperature (primary precipitates) and during solidification (secondary precipitates). The experimental inoculation treatments were designed targeting formation of primary precipitates (Ti and Zr additions) and secondary precipitates (S and N additions to inoculant). An automated SEM/EDX analysis was applied to analyze the graphite nodule distribution statistics and a family of nonmetallic inclusions in the experimental castings. In inoculated SGI, the observed bimodal distributions of graphite nodules were related to continuous nucleation with the second nucleation wave that occurred toward the solidification end. The measured microporosity in the castings was linked to graphite nucleation. The origin of the continuous graphite nodule nucleation and the possibility of engineering nonmetallic inclusions to control casting soundness are discussed.

Communication: Characterization of Spatial Distribution of Graphite Nodules in Cast Iron

Important properties of cast iron, such as fatigue strength, wear resistance, and low-temperature toughness, relate to spatial distribution of graphite nodules. Characterization of spatial distribution can also provide insight into the solidification sequence in casting. An automated SEM/EDX analysis was utilized to distinguish graphite nodules from other structural features (pores and inclusions). The two-dimensional near-neighbor distance (NND) between nodule centers was calculated for three equal sets of nodule diameters (small, medium, and large) in each cast iron. Comparison of measured spatial distributions and ideal random distribution was executed by plotting the mean and variance ratios of NND on a spatial distribution quadrant. This method was used to clarify clustering or ordering tendencies of graphite nodules in studied cast irons. The suggested procedure was used to verify the effects of inoculation and the cooling rate on spatial distribution of graphite nodules. Inoculation of sand casting increased nodule counts, decreased mean NND, and eliminated clustering of small graphite nodules precipitated at the solidification end. Intensive surface cooling of a continuously cast bar significantly increased nodule count near the external surface and decreased NND without changing spatial distribution. The suggested analysis can be used as a tool for cast iron quality control and process development.

Engineered Cooling Process for High Strength Ductile Iron Castings

Professor Stefanescu contributed fundamentally to the science of solidification and microstructural evolutions in ductile irons. In this article, the possibility of development of high strength ductile iron by applying an engineered cooling process after casting early shake out from the sand mold was explored. The structures in industrial ductile iron were experimentally simulated using a computer controlled heating/cooling device. CFD modeling was used for process simulation and an experimental bench scale system was developed. The process concept was experimentally verified by producing cast plates with 25 mm wall thickness. The tensile strength was increased from 550 MPa to 1000 MPa in as-cast condition without the need for alloying and heat treatment. The possible practical applications were discussed.