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ANÁLISE TÉRMICA

Solidificação dos Ferros Fundidos

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!O potencial de nucleação de um banho de ferro fundido é um indicativo da facilidade de formação da fase grafita.

Este potencial de nucleação pode ser medido basicamente de duas maneiras:

!1- teste de cunha: consiste na medição da altura de coquilhamento de um corpo-de-prova em formato de

cunha

é o método mais simples e tradicional.

a sua grande desvantagem é que a altura de coquilhamento da amostra é muito influenciada pela

temperatura de vazamento do metal, o que pode gerar diferentes resultados para um mesmo banho.

!2- análise térmica: consiste no levantamento da curva de solidificação de uma amostra do metal.

exige o uso de recursos computacionais para a gravação e análise dos dados. Isto permitiria levantar

parâmetros a partir da curva, que descrevem o estado de nucleação do banho, de maneira rápida o suficiente

para adequar o processo de pós-tratamento ao estado de nucleação do banho.

!


Avaliação do Potencial de Nucleação dos Banhos de Ferros Fundidos

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ASTM A367-60


Medidas da cunha Cunha A (mm) B (mm) 1 25 5 2 35 10 3 40 20 4 50 30

Região coquilhada

Altura de coquilhamento

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Análise Térmica

31. J.P. Sadocha and J.E. Gruzleski, The Mechanism of Graphite Spheroid Formation in Pure Fe-C-Si Alloys,in The Metallurgy of Cast Iron, B. Lux et al., Ed., Georgi Publishing, 1975, p 443

32. I. Minkoff and B. Lux, Graphite Growth From Metallic Solution, in The Metallurgy of Cast Iron, B. Lux etal., Ed., Georgi Publishing, 1975, p 473

33. K. Herfurth, Investigations Into the Influence of Various Additions on the Surface Tension of Liquid CastIron With the Aim of Finding Relationships Between the Surface Tension and the Occurrence of VariousForms of Graphite, Freiberg Forschungs, Vol 105, 1965, p 267

34. E.N. Pan, K. Ogi, and C.R. Loper, Jr., Analysis of the Solidification Process of Compacted/VermicularGraphite Cast Iron, Trans. AFS, Vol 90, 1982, p 509

35. P. Zhu, R. Sha, and Y. Li, Effect of Twin/Tilt on the Growth of Graphite, in The Physical Metallurgy ofCast Iron, Vol 34, H. Fredriksson and M. Hillert, Ed., Proceedings of the Materials Research Society,North Holland, 1985, p 3

36. X. Den, P. Zhu, and Q. Liu, Structure and Formation of Vermicular Graphite, in The Physical Metallurgy ofCast Iron, Vol 34, H. Fredriksson and M. Hillert, Ed., Proceedings of the Materials Research Society,North Holland, 1985, p 141

37. M. Hillert and H. Steinhauser, The Structure of White Iron Eutectic, Jernkontorets Ann., Vol 144, 1960, p520

38. M. Hillert and V.V. Subba Rao, "The Solidification of Metals," Publication 110, The Iron and SteelInstitute, 1968

Interpretation and Use of Cooling Curves (Thermal Analysis)

H. Fredriksson, The Royal Institute of Technology, Sweden

Introduction

THERMAL ANALYSIS is a classical method of determining phase diagrams. By melting and cooling an alloy of knowncomposition and registering the temperature-time curves, the liquidus temperature for the respective alloy can bedetermined. Figure 1 shows an idealized view of the relationship between the cooling curves and the phase diagram for analloy A-B. The starting point for solidification is given by a change in the slope of the cooling curve due to the evolutionof the heat of solidification. This starting point is normally used to define the liquidus temperature. The method isdescribed more fully in Ref 1.

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Análise Térmica

with with Teno

CEL = 14.45 – 0.0089·TLA

%C = 0.0178·TES – 0.0084·TLA – 6.51

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Análise Térmica

Fig. 3(a) The fraction of solid phases as a function of time for the alloy with a cooling curve shown in Fig. 2.

The Fe-3.2C alloy discussed earlier in this article can be used as an example to illustrate this analysis. According to theiron-carbon phase diagram, the liquidus temperature for this alloy is 1268 °C (2314 °F). At this temperature, dendriticcrystals of austenite are formed. Because carbon has a very high diffusivity in austenite, the lever rule* can be used todetermine the fraction of solid as a function of temperature. The rule is derived with respect to temperature, and thederivate df /dT is obtained. This derivate is inserted into Eq 2. The temperature-time curve is then easily calculated. Theresult is shown in Fig. 3(b) as a dashed line. The deviation from the real curve and that calculated under equilibriumconditions is greatest at the beginning of the precipitation of austenite. After the initial stage during the cooling down tothe eutectic temperature, the lever rule will describe the fraction of solid formed. The eutectic reaction for the equilibriumcase occurs without any undercooling. This derivation from equilibrium will be interpreted below.

Fig. 3(b) Deviation of the cooling shown in Fig. 2 resulting from equilibrium solidification condition (see dashedline).

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Análise Térmica

accepted for publication - International Journal of Metalcasting

5

Fig. 10. Theoretical (equilibrium) and experimental (non-equilibrium) cooling curves for a hypoeutectic alloy

A correlation between composition and the liquidus arrest temperature (in oC) was also established by Heine14,15

for hypoeutectic gray irons:

TLA = 1569 – 97.3(%C+ 0.25∙%Si) Eq. 2

Further work16 at BCIRA in England using two cups, a standard one and one that had some tellurium addi-tion, resulted in the ability to determine both C and Si from CCA. The tellurium addition has the effect of pro-moting metastable (white) solidification which, because of the high growth rate of the white eutectic (ledeburite) all but eliminates the eutectic undercooling. The eutectic arrest, TEwhite, becomes flat. The equations proposed by Heraeus Electro-Nite17 are:

CEL = 14.45 – 0.0089∙TL Eq. 3

%C = – 6.51 – 0.0084∙TL + 0.0178∙TEwhite

%Si = 78.411 – 4.28087∙Si-adj. – 0.06831∙TEwhite

where Si-adj. is a correction factor, mainly depending on the phosphorus content of the iron. For SG iron the same equation as for LG iron can be used to calculate CEL by substituting TL with (TL + 5).

A three-cup system is also used by to determine the magnesium content of Mg treated irons. The system in-cludes a standard cup, one with tellurium, and one with tellurium and sulfur (Fig. 8).

Prediction of the eutectic type and the degree of inoculation in cast iron Another application of single thermocouple TA in cast iron is for the prediction of the general eutectic struc-

ture, i.e. gray (stable) or white (metastable). If both the start and end of the eutectic solidification are above the metastable temperature, Tmet, the iron solidifies gray, i.e. without carbides (Fig. 11a). If the end of solidification is under Tmet the iron is mottled (mixed gray and white Fig. 11b). If both the beginning and end of solidification are under Tmet, the iron is white (Fig. 11c). Note that Tst and Tmet are not straight line, because they reflect the effect of solute build-up or depletion during solidification. Carbide promoting elements are rejected in the liq-uid, their content increases, and Tmet goes up. The opposite is true for graphite promoting elements.

a) gray iron b) mottled iron c) white iron

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TL: temperatura liquidus TE: temperatura de equilíbrio do eutético TLA: temp. de arraste liquidus ΔT: recalescência TEmin: temp. de super-resfriamento do eutético TEmax: temp. de recalescência do eutético ΔTmax: super-resfriamento máximo ΔTmin: super-resfriamento mínimo


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Análise Térmica

accepted for publication - International Journal of Metalcasting

6

Fig. 11. Interpretation of cooling curves to predict stable or metastable solidification in cast iron

Various inoculation additions to the iron greatly affect the eutectic temperature. As shown in Fig. 12, the addi-tion of 0.002% Bi lowers the TEmin by 9F, and raises the TEmax by 3F. A combined addition of 0.002% Bi and a commercial inoculant in the sprue raised the TEmin by 25F (Strong 1983).

Fig. 12. Effect of Bi inoculation on SG iron18 ()

The degree of inoculation of LG iron can be estimated by using two cups: one before inoculation, and one after inoculation. The ratio (∆𝑇 ) (∆𝑇 )⁄ steadily increases with better inoculation (higher number of eutectic grains), as shown schematically in Fig. 5.

Kanno et al.19 used three cups with one thermocouple each to predict the eutectic graphitization ability of LG iron. They defined the eutectic graphitization ability, EGA, through the equation (see Fig. 13 for definitions):

𝐸𝐺𝐴 = = ( ) ( )( . ) ( ) Eq. 4

Here, DTE symbolizes the difference between the stable and metastable eutectic temperatures. DT1 is the differ-ence between the maximum eutectic temperature of the inoculated iron and the metastable temperature. As shown in Fig. 14, there is a clear correlation between EGA, the type of graphite and the chill depth. A good correlation was also found between EGA and the tensile strength, as follows:

TS = (180∙EGA + 170)∙(4.4 - CE) + 160 Eq. 5

where TS is the tensile strength.

Fig. 13. Three-thermocouple thermal analysis of gray iron19

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Figure 3: Graphite structures as a function of eutectic undercooling in grey iron.

Table 1: Experimental results showing effects of Mn and S contents and Mn:Sratio on eutectic cell count and chill level in grey iron.

% Mn % S Mn:S Cell Count[mm]

Chill [mm]

0.8 0.012 67 15 13

1.0 0.022 46 15 10

0.8 0.065 12 21 7

0.3 0.20 1.5 69 23

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