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  • 1.
    Domeij, Björn
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Compacted graphite iron: On solidification phenomena related to shrinkage defects2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    An improved understanding of solidification of compacted graphite iron (CGI) is necessary to predict shrinkage related casting defects. Spheroidal graphite is not only found in ductile iron (SGI) but usually also in CGI, but its characteristics and role in this material is less understood. Uncertainties also remain regarding the segregation of alloying elements in cast irons. This is important because of its important role in the solidification process. The dendritic austenite structure has received little attention in cast iron research. A good understanding of the development of this structure is important to understand feeding of melt through the solidifying material.

    Nodularity is a measure of the amount of spheroidal versus compacted graphite in the microstructure. At a lower nodularity, the compacted graphite tips were found to grow in contact with the melt for a longer time before being encapsulated in the austenite. Moreover, as the nodularity was reduced, the subpopulation of larger spheroidal graphite gradually disappeared, reducing the bimodal size distribution to unimodal.

    Segregation of Si, Mn and Cu in SGI and CGI with a solidification time of near 10 min was found to be rather predictable under assumptions of no diffusion in austenite and complete mixing in the melt. Gradients of these elements contribute to a decrease in the driving force for diffusion of carbon into the austenite from the liquid, which is important for the growth of graphite which is separated from the liquid by austenite.

    During solidification of a near-eutectic CGI, the carbon concentration of austenite was found to deviate considerably from local equilibrium with graphite during solidification. This is important to consider in growth models for graphite by diffusion of carbon through a barrier of austenite.

    Micropores were shown to have displaced liquid from the solid structure at a late stage of solidification when solidification was slowing down and the temperature of the casting was falling at an increasing rate.

    The development of dendritic austenite in a near eutectic CGI was investigated. The contact area between liquid and the dendritic structure exceeded the contact area between liquid and eutectic cells through the dominant part of solidification. This highlights the importance of good understanding of the development of this structure in order to predict feeding of melt through the solidifying material. The coarsening of the structure was found to proceed at a higher rate compared to studies under isothermal condition. The dendritic structure continued to grow in parallel with the eutectic by a combination of thickening and dendritic growth.

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  • 2.
    Domeij, Björn
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    On the solidification of compacted and spheroidal graphite irons2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    A good understanding of the solidification process of a cast material is essential to understand how the combination of alloy composition and the casting process variables combines into the solid cast component and its performance. The wrong combination may result in poor performance or casting defects. Spheroidal graphite has been well researched in ductile irons (SGI) where it is predominant. Spheroidal graphite is also present in smaller amounts in compacted graphite irons (CGI), but its nature and role in this material is less understood. Recent associations of spheroidal graphite in CGI with shrinkage porosity problems highlights the need for better understanding in this area. The importance of the dendritic austenite structure to the properties and solidification behaviour of cast irons has been highlighted in recent research. However, progress is to a degree limited by lack of practical means to characterize the structure.

    In the present work, the transition of a cast iron from SGI to CGI though remelting was studied. As the fraction of SG dropped, the tips of the compacted graphite tended to lose contact with the melt at a later stage. After this occurred, solidification appeared to be dominated by spheroidal graphite. Compacted and spheroidal graphite was found to solidify mostly segregated, and the increased recallescence induced by a higher fraction of compacted graphite displayed small influence on the size distribution of spheroidal graphite apart from the total number and fraction. The partitioning of Si, Mn and Cu in SGI and CGI was found to agree well with each other, as well as with theoretical predictions under the assumptions of zero diffusion of the elements in the solid. This shows that the proportions of spheroidal and compacted graphite has small or no influence on the evolution of these elements in the melt during solidification. A method for characterization of the dendritic austenite in quenched cast irons was introduced and evaluated. The method includes a technique for producing a visual contrast between the ledeburite matrix and the dendritic austenite, and a scheme for producing binary images from the resulting micrographs which are suitable for automatic image analysis. Measurements of the volume fraction and surface area per unit volume of the dendritic austenite structure using the introduced method was found to agree reasonably with traditional point counting and line intercept techniques. The difficulty in finding the exact boundary was proposed to be the major source of systematic disagreement.

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  • 3.
    Domeij, Björn
    et al.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Diószegi, Attila
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Inferring the development of microsegregation and microstructure in Spheroidal and Compacted Graphite Iron using EPMA‐WDS2017In: Solidification Processing 2017: Proceedings of the 6th Decennial International Conference on Solidification Processing / [ed] Zhongyun Fan, Brunel Centre for Advanced Solidification Technology (BCAST) , 2017, no 1, p. 455-458Conference paper (Refereed)
    Abstract [en]

    Microsegregation is closely related to the solidification characteristics and microstructure development of a material. In this paper, the microsegregation of Si, Cu, Mn, and P was investigated on a spheroidal and a compacted graphite iron using a modified EPMA equipped with WDS spectrometers. On the basis of the approximated local equilibrium eutectic temperature, the solidification sequence of the matrix was estimated. The inferred microstructure development appeared to correspond well to published interrupted solidification experimental results. Solute profiles and effective partition coefficients were constructed using the solidification sequence. While the spatial microsegregation patterns clearly differed between the two materials, solute profiles and effective partition coefficients were very similar. For Si, Cu and Mn, the solute profiles corresponded reasonably with simulation results produced using the Scheil-Gulliver module of the Thermo-Calc software with the TCFE7 databank, indicating back-diffusion of these elements is negligible for SGI and CGI with solidification times up to 10 min. The effective partition coefficients for Si, Cu and Mn were fairly constant until about 90% of the matrix had solidified, after which they appeared to approach unity.

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  • 4.
    Domeij, Björn
    et al.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Diószegi, Attila
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Solidification Chronology of the Metal Matrix and a Study of Conditions for Micropore Formation in Cast Irons Using EPMA and FTA2018In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 925, p. 436-443Article in journal (Refereed)
    Abstract [en]

    Microsegregation is intimately coupled with solidification, the development of microstructure, and involved in the formation of various casting defects. This paper demonstrates how the local composition of the metal matrix of graphitic cast irons, measured using quantitative electron microprobe analysis, can be used to determine its solidification chronology. The method is applied in combination with Fourier thermal analysis to investigate the formation of micropores in cast irons with varying proportions of compacted and spheroidal graphite produced by remelting. The results indicate that micropores formed at mass fractions of solid between 0.77 and 0.91, which corresponded to a stage of solidification when the temperature decline of the castings was large and increasing. In 4 out of the 5 castings, pores appear to have formed soon after the rate of solidification and heat dissipation had reached their maximum and were decreasing. While the freezing point depression due to build-up of microsegregation and the transition from compacted to spheroidal type growth of the eutectic both influencing solidification kinetics and the temperature evolution of the casting, the results did not indicate a clear relation to the observed late deceleration of solidification.

  • 5.
    Domeij, Björn
    et al.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Diószegi, Attila
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    The Distribution of Carbon in Austenite Studied on a Water-Quenched Compacted Graphite Iron Using Electron Probe Microanalysis2020In: International Journal of metalcasting, ISSN 1939-5981, E-ISSN 2163-3193Article in journal (Refereed)
    Abstract [en]

    Spheroidal graphite is the defining microstructural feature of ductile iron and also plays an important role in compacted graphite iron. It is widely accepted that graphite spheroids are engulfed by austenite at an early stage of solidification after which their growth is impeded by the slow diffusion of carbon through a layer of austenite. In this work, a compacted graphite iron-containing spheroidal graphite was studied after interruption of its solidification by water quenching. Selected areas of a cross section of the castings were investigated using quantitative electron probe microanalysis, with emphasis on the distribution of carbon in austenite. The measured carbon concentration near graphite was generally below the theoretical carbon concentration in austenite at equilibrium with graphite at 1140 °C. Numerical simulations of diffusion of carbon in austenite around spheroidal graphite suggest that a zone of austenite around graphite was likely depleted of carbon during quenching, possibly explaining the low measured concentrations. The measured carbon concentration near graphite varied by as much as 0.3 wt%, with the lowest concentrations consistently found in the central region of compacted graphite–austenite eutectic cells. Regardless of whether these differences were present prior to quenching or are consequences thereof, they seem to reflect either departures from, or displacements of, the carbon concentration in austenite at equilibrium with graphite. This indicates that there is something about growth of graphite embedded in austenite which is not well understood. Concentrations of Si, Mn and Cu are near equal in the compared regions and do not explain the observed differences in carbon content near graphite.

  • 6.
    Domeij, Björn
    et al.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Elfsberg, J.
    Diószegi, Attila
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    The distribution of carbon during solidification of a compacted graphite ironManuscript (preprint) (Other academic)
  • 7.
    Domeij, Björn
    et al.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Elfsberg, J.
    Diószegi, Attila
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    The evolution of dendritic austenite in a solidifying near‐eutectic compacted graphite iron under varying cooling conditionsManuscript (preprint) (Other academic)
  • 8.
    Domeij, Björn
    et al.
    Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    Hernando, Juan Carlos
    Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    Diószegi, Attila
    Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    Quantification of dendritic austenite after interrupted solidification in a hypoeutectic lamellar graphite iron2016In: Metallography, Microstructure, and Analysis, ISSN 2192-9270, Vol. 5, no 1, p. 28-42Article in journal (Refereed)
    Abstract [en]

    This paper presents an unconventional etching technique to reveal the microstructure in a hypoeutectic lamellar graphite iron that has been quenched after isothermal heat treatment in the proeutectic semi-solid temperature region. A technique for quantifying the dendrite microstructure using the aforementioned etching technique involving a combination of a raster graphics editor and an image analysis software is outlined. The agreement between this quantification technique with regard to volume fraction and surface area per unit volume of the dendritic austenite and corresponding point counting and line intercept techniques is analyzed. The etching technique was found useful but sporadic tinting of martensite was problematic. Some measurements showed significant systematic disagreement which correlated with the coarseness of the measured dendrites. Most systematic disagreement is attributed to difficulties in defining the dendrite boundary in the analogues and much of the random disagreement to easily identified discrepancies between the analogue and the micrograph.

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  • 9.
    Domeij, Björn
    et al.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Hernando, Juan Carlos
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Diószegi, Attila
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Size distribution of graphite nodules in hypereutectic cast irons of varying nodularity2018In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 49, no 5, p. 2487-2504Article in journal (Refereed)
    Abstract [en]

    An SGI was machined into 400 g cylindrical pieces and remelted in an electrical resistance furnace protected by Ar gas to produce materials ranging from SGI to CGI. The graphite morphology was controlled by varying the holding time at 1723 K (1450 °C) between 10 and 60 minutes. The discrete sectional size distribution of nodules by number density was measured on cross sections of the specimens and translated to volumetric distribution by volume fraction. Subpopulations of nodules were distinguished by fitting Gaussian distribution functions to the measured distribution. Primary and eutectic graphite, were found to account for most of the volume of nodular graphite in all cases. For holding times of 40 minutes and greater, corresponding to nodularity roughly below 40 pct, the primary subpopulation was very small and difficult to distinguish, leaving eutectic nodules as the dominant subpopulation. The mode and standard deviation of the two subpopulations were roughly independent of nodularity. Moreover, the nodular and vermicular graphite were segregated in the microstructure. In conclusion, the results suggest that the parallel development of the vermicular eutectic had small influence on the size distribution of eutectic graphite nodules.

  • 10.
    Hernando, Juan Carlos
    et al.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    Domeij, Björn
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    Diószegi, Attila
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    Influence of Ti and Mo additions on the isothermal coarsening process of primary austenite in Lamellar Graphite Iron2017Conference paper (Refereed)
  • 11.
    Hernando, Juan Carlos
    et al.
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    Domeij, Björn
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    González, Daniel
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Amieva, José Manuel
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing.
    Diószegi, Attila
    Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
    New experimental technique for nodularity and Mg fading control in compacted graphite iron production on laboratory scale2017In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 48, no 11, p. 5432-5441Article in journal (Refereed)
    Abstract [en]

    The narrow production window for compacted graphite iron material (CGI) drastically reduces the possibilities to produce it in small batches outside an industrial environment. This fact hinders laboratory-scale investigations on CGI solidification. This work presents a solution to that issue by introducing an experimental technique to produce graphitic cast iron of the main three families. Samples of a base hypereutectic spheroidal graphite iron (SGI) were re-melted in a resistance furnace under Ar atmosphere. Varying the holding time at 1723 K (1450 °C), graphitic irons ranging from spheroidal to lamellar were produced. Characterization of the graphite morphology evolution, in terms of nodularity as a function of holding time, is presented. The nodularity decay for the SGI region suggests a linear correlation with the holding time. In the CGI region, nodularity deterioration shows a slower rate, concluding with the sudden appearance of lamellar graphite. The fading process of magnesium, showing agreement with previous researchers, is described by means of empirical relations as a function of holding time and nodularity. The results on nodularity fade and number of nodules per unit area fade suggest that both phenomena occur simultaneously during the fading process of magnesium.

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