Applications of Solidification Fundamentals: Solidification of Iron and Steel
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Solidification Committee
Program Organizers: Andre Phillion, McMaster University; Amber Genau, University of Alabama at Birmingham; Lifeng Zhang, University of Science and Technology Beijing
Wednesday 8:30 AM
March 1, 2017
Location: San Diego Convention Ctr
Session Chair: Andrew Kao, University of Greenwich; Mahdi Torabi Rad, University of Iowa
Spheroidal Graphite Growth Studied by Synchrotron X-ray Tomography: Mathias Bjerre1; Mohammed Azeem2; Niels Tiedje1; Jesper Hattel1; Peter Lee2; 1Technical University of Denmark; 2University of Manchester
Spheroidal graphite iron is a widely used cast material which combines strength with low price and good castability. The material properties are closely linked to the presence of spherical graphite particles which form during the solidification process. Thus, a precise description of nucleation and growth of the graphite particles is important in order to properly understand and predict the final mechanical properties of the cast component.We present the first in-situ observations from solidification of cast iron resolved in three spatial dimensions and time by synchrotron x-ray tomography. The cylindrical sample of near-eutectic SGI solidified at a constant cooling rate of 0.03 C/s. The slow cooling rate allows us to resolve the growth of spheroidal graphite particles which provides basis for quantification of nucleation and direct comparison and validation of models of graphite growth. Sectioning of the sample confirmed the nature of the observed particles.
Effect of Solidification Parameters and Alloying Elements on Graphite Morphology in Ni-C Alloys: Amir Ardalan Rezaie1; Haamun Kalaantari2; Reza Abbaschian1; 1University of California, Riverside; 2California State Polytechnic University, Pomona
Graphite morphologies in castings have a pivotal role in mechanical and physical properties of cast irons and other alloys. These morphologies, which often consist of lamellar graphite (LG), compacted graphite (CG) and spheroidal graphite (SG), are achieved by adding alloying elements and controlling cooling rates. However, the mechanisms for the formation of these morphologies, are not clearly understood yet. In this study, graphite morphologies in arc-melted hypo and hyper-eutectic Ni-C alloys were investigated using electromagnetic levitation (EML) technique and rapid solidification. Additionally, spheroidizing and anti-spheroidizing elements such as Mg, Ce and S were added to the alloys in order to investigate their influence on the resultant graphite morphologies. The microstructures were analyzed via optical and scanning electron microscopy to elucidate the mechanisms of graphite growth and the influence of solidification conditions on graphite morphologies mentioned above.
Effects of Rare Earth Oxides on the Precipitation of Graphite in Fe-C-Si Alloy: Kok Long Ng1; Hideaki Sasaki2; Hisao Kimura3; Takeshi Yoshikawa3; Masafumi Maeda3; 1University of Tokyo ; 2Ehime University; 3University of Tokyo
Despite decades of research in finding the controlling formation mechanism of spheroidal graphite, a general explaination has yet to emerge. It is undeniable that an elucidation of the formation mechanism of spheroidal graphite is both scientifically and industrially important. Heterogeneous nucleation theory states that, non-metallic inclusions such as Mg and rare earth element oxides, sulfides and oxy-sulfides act as sites for spheroidal graphite nucleation during solidification. However, the actual spheroidization process involves a high level of complexities caused by interactions between additive elements with impurities in cast iron melt. Hence, a better understanding on the properties of inclusions and other impurities during the solidification process is necessary. In current study, the contribution of rare earth oxides to the formation of nodular graphite is investigated by melting Fe-C-Si alloy on sintered rare earth oxide pellets and the effect of sulfur on the heterogeneous nucleation of graphite is discussed.
Evolution of Microstructure in Directionally Solidified Compacted Graphite Iron: Subhojit Chaktaborty1; Amber Genau1; Charles Monroe1; 1University of Alabama at Birmingham
Compacted graphite iron (CGI) has advantages of both gray and ductile iron, however its sensitivity to cooling rate and composition makes manufacturing difficult. In order to investigate the graphite morphology transition conditions, various amounts of either only Ce or both Ce and Mg were added as spherodizing elements to a near-eutectic base iron composition. Samples of these alloys were directionally solidified at different velocities, and some were also quenched to examine the solid/liquid interface. The microstructures of the samples were analyzed using optical microscopy and scanning electron microscopy. The microstructures are quantified using surface factor, roundness, nodularity, etc. in order to understand the transition of graphite in the solidified structure from flake through vermicular to nodular as a function of composition, velocity and distance from original melt interface.
An Electron Microscopy Study of Graphite Growth in Nodular Cast Irons: Rawen Jday1; Lydia Laffont1; Jacques Lacaze1; 1CIRIMAT
Growth of graphite during solidification and high temperature solid-state transformation has been investigated in samples cut out from a thin wall casting solidified partly in the stable (iron-graphite) and partly in the metastable (iron-cementite) systems. Transmission electron microscopy has been used to characterize graphite nodules in as-cast state and in samples having been fully graphitized at various temperatures in the austenite field. Nodules in the as-cast material show a multi-fold structure characterized by an inner zone where graphite is misoriented and an outer zone where it is well crystallized. In heat-treated samples, graphite nodules consist of well crystallized sectors radiating from the nucleus. These observations suggest that the misoriented zone appears because of mechanical deformation when the liquid contracts during its solidification. During heat-treatment, this zone disappears by recrystallization. In turn, it can be concluded that nodular graphite growth mechanism is the same during solidification and solid-state transformation.
10:10 AM Break
Discovery of New Grain Refiners Utilizing Crystallographic Data: Hunter Martin1; Brennan Yahata2; Tresa Pollock1; 1University of California, Santa Barbara; 2HRL Laboratories
Grain refinement is a crucial part of processing commercial alloys. Fine equiaxed grains result in improved properties for both cast and wrought alloys. While grain refinement can be accomplished several ways introducing particles or phases which promote nucleation is one of the most effective methods. Discovering new grain refining systems has been a difficult and expensive task. We will demonstrate a new method of searching massive crystallographic data to determine possible grain refining structures using classical nucleation theory. Once candidates are found thermodynamic and kinetic analysis can be used to determine the system viability. The approach is alloy agnostic and enables the discovery of previously unknown or underutilized grain refiners.
Mechanisms of Surface Stability in Al-Zn Coated Steel: Matthew Gear1; Kazuhiro Nogita1; Stuart McDonald1; Dongdong Qu1; David StJohn1; 1University of Queensland
Thin coatings of 55%Al-Zn based alloys are frequently applied to mild steel to provide protection against corrosion. . There are economic and environmental advantages associated with minimising the thickness of these surface coatings. However, achieving a uniform thin stable coating in continuous production processes requires a deep understanding of the relationship between process variables and alloy microstructure. This research has used a variety of advanced techniques to characterise the solidification process and microstructure of the Al-Zn system. These techniques include synchrotron work for in situ observation of nucleation, X-ray fluorescence microscopy and synchrotron X-Ray diffraction for crystallographic studies.