Indian Journal of Engineering & Materials Sciences Vol. 25, August 2018, pp. 330-334

Influence of cooling rate on the amount of graphite nodules and interlamelar space in perlite phase in a ductile iron

J R Benavides-Treviño, F A Perez-Gonzalez, M A L Hernandez, E Garcia & A Juarez-Hernandez* Universidad Autónoma de Nuevo León, Facultad de IngenieriaMecanica y Electrica, Ciudad Universitaria, San Nicolas de los Garza,

Nuevo Leon, C.P. 66450, Mexico

Received 10 October 2016; accepted 11 October 2017

The microstructures of a ductile iron solidified under six different cooling rates have been studied, using optical microscopy (OM), scanning electron microscopy (SEM) and tensile test. Results show that cooling rates increase from 0.58 to 0.95°C/s, so perlite growth temperature (TPG) decreases, nodules size (N) decreases and nodules per volume (Nv) increases. Perlite interlamellar spacing (λ) between cementite and ferrite, shows no significant change. Finally, as cooling rate increases, yield and tensile strengths increase, but the hardness shows a minimum increase.

Keywords: Ductil iron, Solidification, Graphite nodules, Scanning electron microscopy

Nodular iron or spheroidal graphite (SG) irons have excellent mechanical properties and a good castability. The feature that nodular iron distinguishes from other irons is the roughly spherical shape of the graphite nodules1,2. However, a number of variables including chemical composition, cooling rate, type and amount of graphite, method of post inoculation, amount of residual magnesium and pouring temperature; they can control the matrix structure of nodular irons3-5 and process parameters6-8. Also a subsequent heat treatment is another route to produce an iron similar than nodular irons. The importance of controlling the type of microstructure is emphasized by the mechanical properties that will be required in the application. Whereas a ferritic matrix provides low properties in strength, with good ductility, impact resistance, tensile and yield strength; a perlitic matrix provide an iron with high hardness and yield strength and also good wear resistance. In a matrix containing both ferrite and perlite properties obtained are intermediate between ferritic and perlitic grades, with good machinability and low production costs, however cooling rate play an important role on the microstructure formation. The aim of the present study was to determine the influence of different cooling rates on perlite phase (focused on interlaminar space) and nodules (count and size) on its mechanical properties.

Experimental Procedures It was designed a casting system comprising of 6

plates (Fig. 1), where it was observed the changes occurring in the microstructure and precipitation of graphite nodules. The size of each plate is given in Table 1. As it is observed only thickness varies resulting in a change of cooling rate.

The ductile cast iron used in the present work has the following chemical composition (wt%): 3.66 C, 2.33 Si, 0.371 Mn, 0.007 S, 0.024 Cr, 0.004 Sn, 0.495 Cu, 0.047 Mg, balance Fe. The casting process was made in a green sand mold where R type thermocouples were placed at the ends of each plate (Fig. 1a), to monitor the cooling rate since the metal begins to fill the mold until its solidification. Liquid metal was poured into the mould cavity for 40 s at 1400°C.

Plates were mechanized, near the thermocouple region where the readings were made in order to analyze the microstructure. Later samples were polished and etched with nital at 4% of concentration as a reagent, the immersion method was applied for a time between 10 s to 45 s to obtain the microstructure of nodular iron.

——————— *Corresponding author (E-mail: artjua@yahoo.com)

Table 1 — Dimensions of experimental plates

No. plate Length (mm) Width (mm) Thickness (mm)

1 190 50 25 2 190 50 30 3 190 50 35 4 190 50 40 5 190 50 45 6 190 50 50

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Inspection was performed to characterize the microstructure the perlite phase by optical microscopy (OM) and scanning electron microscopy (SEM/EDS) in order to see the interlaminar space of perlite and number and size of nodules. Image J software was used for phase analysis using the grayscale technique. Commercial software ProCast was used to simulate the filling and the solidification behavior of the ductile iron alloy. Results and Discussion

Simulation results (Fig. 1b), show that thinner the plate higher the under cooling, Fig. 2 presents the cooling curves associated to each plate. It is seen a slope change between 600ºC and 700ºC which corresponds to perlite formation (Fig. 2a), but

simulation results present a constant under cooling compared with experimental results, the software is for a thermodynamic stable system, and the real case is a thermodynamic unstable system where back diffusion and segregation phenomena are present, also boundary conditions are unstable such as temperature inlet and the heat transfer coefficient. On the other side, perlite growing temperature (TPG) decreases as cooling rate (V) increases, linear fit to the data shown (Table 2) thus accord with Eq. (1), it is appreciable how the average cooling rate of each plate during the experiment tends to decrease linearly as the thickness is increased, as:

TPG= -360 log V + 605 … (1)

Fig. 1 — Casting (a) experimental model and (b) simulated model by varying the cooling rate in each plate

Fig. 2 — Cooling curves of samples (Table 2) from plates 1 to 6, (a) experimental and (b) predictions

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Table 2 — Growth temperatures of perlite phase (TPG) vs cooling rates

No. plate TPG (°C) Cooling rates (°C/s)

1 616.2 0.95

2 627.2 0.88

3 635.4 0.82

4 639.9 0.76

5 673.5 0.64

6 695.7 0.58

Microstructure Figure 3 shows the microstructure of the present

material. Spherical graphite embedded in a perlitic-ferritic matrix, and of graphite, the difference between images is that it changes the number of graphite nodules and their respective size.

All the micrograph show typical “bull’s eye” structure in which many of the graphite nodules are surrounded by an envelope of ferrite. Both the graphite nodules and their ferrite envelopes are embedded in a perlitic matrix as expected9.

Fig. 3 — Microstructures obtained at a magnification of 100X using nital (4%) (a) plate 1, (b) plate 2, (c) plate 3, (d) plate 4, (e) plate 5 and (f) plate 6

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The precipitation of graphite nodules tends to arise as the cooling rate is increased and observed by other researchers5,10. Table 3 shows the calculation of nodules per unit volume (Nv) using the expression of Richoz11, it was necessary to count the number of nodules in the image and compared to the total image area to obtain the amount of nodules per unit area (Na).

Results of SEM analysis show the morphology and distribution of the laminar cementite in perlite phase. In Fig. 4, it is observed how the cementite is oriented

Table 3 — Count of graphite nodules

No. plate Nodule count Na (1/cm2) Nv (1/cm3) Standard deviation

(%)

1 275.5 23760 9851879 0.17

2 253.4 23020 9395451 0.22

3 252.5 22788 9253410 0.18

4 189.2 17437 6193891 0.21

5 182.5 16780 5846929 0.14

6 134.4 12349 3691318 0.18

Fig. 4 — Micrograph taken with SEM of interlaminar space of perlite phase (a) plate 1, (b) plate 2, (c) plate 3, (d) plate 4, (e) plate 5 and (f) plate 6

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randomly depending as the grain growth, the interlaminar space is between 0.14 and 0.45 μm. For the preset cooling rates perlite interlaminar space does not present a significant dependence12. Mechanical properties

The mechanical properties of nodular iron are closely related to the microstructure13. Table 4 shows the effect of the cooling rate on the mechanical properties, whereas the highest the cooling rate there is a trend to higher tensile strength and yield stress. Due the cooling rate apparently did not affect the percentage of ferrite and perlite in the matrix, the increase in the mechanical properties can be assumed that at higher cooling rates more nodules will precipitate with a better distributed in the matrix (Table 4). The hardness in the samples had no significant change.

Conclusions Perlite growing temperature (TPG) decreases as

cooling rate (V) increases. The amount of nodules per volume (Nv) in the matrix increase from 3691318 to 9851879 1/cm3, as the cooling rate increase from 0.58 to 0.95 C/s due to thickness of the plates decrement from 50 to 25 mm, respectively; thus properties of tensile strength and elongation have a trend to increase as (Nv) are higher in the matrix, but there was not a significant change in the brinell hardness. Distribution of lamellas of cementite in the matrix depends mainly on the orientation and grain growth and did not show changes with the different cooling rates.

Acknowledgements The authors would like to thank CONACYT for its

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Table 4 — Mechanical properties

No. sample Yield strength Tensile strength Standard deviation (%)

% Elongation (2,0") Standard deviation (%)

HB MPa

Psi MPa Psi MPa

1 64738 446 110683 763 0.51 12.61 4.63 255 2 64622 445 110670 758 1.80 11.37 2.08 249 3 64594 445 106623 735 0.49 11.26 4.97 255 4 63520 438 106222 732 0.11 10.23 1.33 255 5 61710 425 105108 725 1.41 10.68 3.28 244 6 60382 416 103655 715 0.12 10.60 7.67 241