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A R C H I V E S

O f

F O U N D R Y E N G I N E E R I N G

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310) Volume 11

Issue 2/2011 55–60

11/2

Microstructure of hypereutectic

Fe-C-V alloys

M. Kawalec

AGH University of Science and Technology, ul. Reymonta 23, 30-059 Cracow, Poland e-mail contact: kawalec@uci.agh.edu.pl

Received 11.04.2011; accepted for print 26.04.2011

Abstract The study shows the results of examinations of the microstructure of the in-volume-solidifying Fe-C-V alloys containing carbon in a range of 1.53÷3.09% and vanadium in a range of 14.73÷15.71%. All the examined alloys were hypereutectic alloys as proved by the cooling curves, relevant calculations (Sc) and examinations under the microscope. It has been confirmed that the primary carbides of vanadium are crystallising in the form of non-faceted dendrites, acting as a substrate for the growing fibres of vanadium eutectic. The content of primary carbides in the microstructure depends on the content of carbon and vanadium in the alloy. During the conducted research an attempt was also made to determine the content of primary and eutectic carbides and to establish a relationship between carbon and vanadium content in the alloy and the content of primary carbides. In Fe-C-V alloys, besides primary carbides, also a fibrous eutectic composed of ferrite and vanadium carbides of the VC1-x type crystallises. To conduct the previously planned examinations, a series of melts with a variable carbon and vanadium content was carried out under argon atmosphere in a Balzers vacuum furnace. Specimens were examined under an optical microscope, but to investigate in more detail the geometry of individual phases, they were deep etched with aqua regia and examined by scanning electron microscopy. In the investigated alloys, the chemical composition of the crystallised phases was determined by EDS microanalysis. Keywords: Cast composites; Microstructure; Primary carbides; Fe-C-V alloys; High-vanadium cast iron.

1. Introduction Owing to its favourable properties, both technological and mechanical, cast iron is used in nearly every branch of industry. Yet, due to the increasing demands from users of this material, search is still ongoing for new types of cast iron with better mechanical and functional properties. In most cases, the search relates to:

• changing the physico-chemical state of liquid cast iron through its inoculation [1], spheroidising treatment [2] or vermicularising [2],

• heat treatment [2], • optimising chemical composition and adding to cast

iron certain alloying elements [3]. The last of these trends in research is related with alloying elements introduced to cast alloy to change the type of matrix or

the morphological characteristics of graphite, or to induce crystallisation of carbides. This leads to changes in mechanical and functional properties of the investigated material. One of the elements offering very interesting, though still little explored, properties is vanadium [3]. The research described in [4, 5] provides information on the structure of Fe-12.9% V-2.94% C alloys, in which the occurrence of a fibrous γ + VC1-x eutectic with about 20 vol.% content of vanadium carbide has been stated. Various studies [6-8] present the results of microstructural examinations of Fe-C-V alloys with carbon content of 1.38 ÷ 4.16% and vanadium in the range of 6.7 ÷ 15.5%, disclosing also their mechanical and tribological properties. The main sources of information on the interaction of alloying elements and their distribution between the individual system-forming phases are phase diagrams, whose degree of intricacy and cost of development increase disproportionately to the number of elements included in the system [9]. There are several

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items in reference literature which describe the Fe-C-V phase equilibrium system [10-13]. Unfortunately they are not fully consistent with the metallographic practice. In [14] it was demonstrated that the experimentally determined eutectic line is shifted to the left in respect of the eutectic line calculated from the above mentioned phase equilibrium diagram. A relationship was also derived for the degree of eutectic saturation, which allows predicting if the examined Fe-C-V alloy is of a hypoeutectic, eutectic or hypereutectic type, basing only on the results of the chemical analysis, and without the need to carry out the time-consuming metallographic examinations. The purpose of this study was to examine the microstructure of Fe-C-V alloys of hypereutectic composition, and investigate carbon effect on the content of primary and eutectic carbides.

2. Research methodology To perform the anticipated mechanical tests, a series of melts with the varying content of carbon and vanadium was carried out under argon atmosphere in a Balzers vacuum furnace. To make these alloys, an Fe-V master alloy with 81.7% of vanadium, armco iron, technically pure graphite and commercial 98.5% Si silicon were used. Moulds prepared from molochite flour with CO2-hardened sodium silicate were preheated to a temperature of 550oC, and poured next with molten iron at a temperature of 1600oC. After knocking out of castings, the specimens were cut out for metallographic examinations. Unetched specimens were examined under a LEICA MEF4 M

optical microscope and JEOL 5500LV scanning microscope with secondary electrons. This enabled differentiating between the vanadium carbides and other phases, which was not always possible when optical microscope was the only tool used in investigations. For more detailed examinations of the geometry of individual phases, the specimens were deep etched with aqua regia and examined by scanning microscopy. The percent content of structural constituents was determined on a LEICA QWin automatic image analyser.

3. Discussion of results Table 1 compares the chemical composition of the examined alloys, the degree of eutectic saturation and the content of primary and eutectic vanadium carbides. Figure 1 shows the effect of the C/V ratio (where C, V is the percent content in alloy of carbon and vanadium, respectively) on the content of primary carbides crystallising in the alloy. Besides the vanadium eutectic and primary carbides, depending on the C/V ratio in cast iron, the following other microstructural constituents were detected:

• alloyed ferrite (Fig. 2a), when C/V is 0.10÷0.14 (melts nos. 3 - 6);

• granular pearlite (Fig. 2b), when C/V is 0.17 (melt no. 2); • lamellar pearlite (Fig. 2c) + granular pearlite, when C/V is

0.20 (melt no. 1).

Table 1. Chemical composition and content of primary and eutectic carbides Chemical

composition Carbide content

Melt No. C

[%] V

[%]

C/V Sc fw [%]

fp [%]

fe [%]

fp/ fw

1 3.09 15.24 0.20 2.20 21.47 13.40 8.37 0.61 2 2.60 15.71 0.17 1.89 20.01 11.46 8.55 0.57 3 2.12 15.50 0.14 1.53 17.98 9.32 8.66 0.62 4 1.78 15.22 0.12 1.27 14.85 5.07 9.79 0.34 5 1.55 14.73 0.11 1.08 13.40 4.38 9.02 0.33 6 1.53 14.81 0.10 1.07 12.78 3.15 9.63 0.25

f w – content of all vanadium carbides f p – content of primary carbides f e – content of eutectic carbides

Sc – degree of eutectic saturation calculated from the equation: 617.0618.7 −⋅

=V

CSc

where C, V – content in alloy of carbon and vanadium, respectively [14]

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Fig. 1. Effect of carbon/vanadium ratio in alloy on the content of primary vanadium carbides

a)

b)

c)

Fig. 2. Microstructure of hypereutectic Fe-C-V alloys

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Deep etching with aqua regia and examinations of microstructure by scanning electron microscopy enabled very accurate observations of individual grains of the eutectic microstructure (Fig. 3b), and also of the primary vanadium carbides crystallising in the form of non-faceted dendrites (Fig. 3a, c, d). It is clearly visible that on these carbides the eutectic fibres have just started growing (Fig. 3c, d). The study defines quantitative composition of the crystallised phases using an EDS point microanalysis. An example of the analysed microregions is shown in Figure 4, while below this

drawing, the results of measurements are given. Figure 5 shows an example of the X-ray energy spectrum of primary vanadium carbides. The research has proved that vanadium content in the alloy does not affect the composition of primary carbides or eutectic. Vanadium content in primary carbides is in the range of 72-80%, while in eutectic carbides it reaches 54-73%. On the other hand, an increase of vanadium content dissolved in the matrix from 2.2% in alloy no. 1 to 6.4% in alloy no. 6 has been observed.

a) b)

c) d)

Fig. 3. Microstructure of hypereutectic Fe-C-V alloys after deep etching with aqua regia (SEM)

Fig. 4. Chemical composition of individual phases as determined by EDS microanalysis

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Region 1 – primary carbides

Region 2 – eutectic carbides

Region 3 – alloyed ferrite

Fig. 5. Example of X-ray energy spectrum of primary vanadium carbides

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4. Conclusions 1. Primary vanadium carbides crystallise in the form of non-

faceted dendrites on which vanadium eutectic fibres are growing.

2. In hypereutectic Fe-C-V alloys containing from 1.53 to 3.09 %C and from 14.73 to 15.71 %V, besides primary carbides and vanadium eutectic, depending on C/V ratio, the following other microstructural constituents have also been found to crystallise: • alloyed ferrite, when C/V ratio is 0.10÷0.14; • granular pearlite, when C/V ratio is 0.17; • lamellar pearlite + granular pearlite, when C/V ratio is

0.20. 3. With increasing C/V ratio, the content of primary carbides

crystallising in the alloy increases, too. 4. The content of eutectic carbides in the examined alloys is 8-

9%. 5. Reduced carbon content in the alloy increases the

concentration of vanadium dissolved in matrix.

Literatura [1] E. Fraś, Cz. Podrzucki: Inoculated cast iron, AGH Brochure

No. 675, Cracow, 1978 (in Polish). [2] E. Guzik: Cast iron refining processes. Selected problems,

Archiwum Odlewnictwa, PAN, Katowice, 2001 (in Polish). [3] A. Kosowski, Cz. Podrzucki: Alloyed cast iron, AGH

Brochure No. 825, Cracow, 1981 (in Polish).

[4] E. Fraś, E. Guzik, Primary Microstructure of the Fe-C-V Alloys, Archiwum Hutnictwa, vol. 25, No. 4, (1980) 757-772 (in Polish).

[5] E. Fraś, Crystallization of metals, WNT, Warsaw, 2003 (in Polish).

[6] M. Kawalec, E. Fraś, Structure, Mechanical Properties and Wear Resistance of Highvanadium Cast Iron, ISIJ International, vol. 48, No. 4 (2008) 518-524.

[7] E. Fraś, M. Kawalec, Structure, Mechanical Properties and Wear Resistance of the Fe-C-V Alloys, Inżynieria Materiałowa, vol.29, No. 2 (2008) 78-85 (in Polish).

[8] M. Kawalec, E. Fraś: Abrasive wear resistance of high-vanadium cast iron. Archives of Foundry Engineering, vol. 9, Issue 4/2009, s. 103-108.

[9] J. Kaczyński, S. Prowans: Theoretical fundamentals of metal science. Katowice, 1972 (in Polish).

[10] R.Kesri, M. Durand – Charre: Metallurgical structure and phase diagram of Fe-C-V system: comparison with other systems forming MC carbides. Materials Science and Technology, August 1988, vol.4

[11] M.J. Collins: Fe-V-C phase diagram. Journal of the Iron and Steel Institute, 1972, 210, s. 284

[12] G. P. Dmitrieva, A. K. Shurin, A. D. Vasilev: Mechanicostructural properties of iron alloys with vanadium carbide. Metal Science and Heat treatment of Metals, 4, 1978, s. 325–327

[13] H. Frey, H. Holleck: DTA investigations of high temperature phase equilibria in ternary transition metal-carbon system. Thermal Analysis Proceedings of the International Conference, vol.1, 1975, s. 339-348

[14] M. Kawalec M., E. Fraś: Shaping of the microstructure in near-eutectic Fe-C-V alloys, Archives of Foundry Engineering, Vol. 10, Issue 4/2010, pp. 83-88.

Mikrostruktura nadeutektycznychstopów Fe-C-V Streszczenie W pracy przedstawiono wyniki badań mikrostruktury stopów Fe-C-V, o zawartości węgla w zakresie 1.53÷3.09% i wanadu w zakresie 14.73÷15.71%, krystalizujących w sposób objętościowy. Wszystkie badane stopy są stopami nadeutektycznymi, co potwierdziły krzywe stygnięcia, odpowiednie obliczenia (Sc) oraz obserwacje mikroskopowe. Potwierdzono, że pierwotne węgliki wanadu krystalizują w posta-ci dendrytów nieścianowych, na których wzrastają włókna eutektyki wanadowej. Udział węglików pierwotnych w mikrostrukturze jest zależny od zawartości węgla i wanadu w stopie. W pracy określono także udział węglików pierwotnych i eutektycznych oraz wyznaczono zależność pomiędzy zawartością węgla i wanadu w stopie a udziałem węglików pierwotnych. Oprócz węglików pierwotnych w stopach Fe-C-V krystalizuje eutektyka włóknista składająca się z ferrytu i węglików wanadu typu VC1-x. W celu wykonania założonych badań przeprowadzono serię wytopów o zmiennej zawartości węgla i wanadu w piecu próżniowym typu Balzers w atmosferze argonu. Próbki obserwowano za pomocą mikroskopu optycznego, a w celu bardziej szczegółowego zbadania geo-metrii poszczególnych faz trawiono je głęboko wodą królewską i obserwowano za pomocą mikroskopu skaningowego. W badanych sto-pach przeprowadzono również pomiary składu chemicznego wykrystalizowanych faz metodą punktowej mikroanalizy EDS.