Effect of Initial Microstructure on the Activation Energy of Second Stage During Austempering of Ductile Iron

Scripta Materialia, Vol. 38, No. 8, pp. 1281–1287, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/98 $19.00 1 .00

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EFFECT OF INITIAL MICROSTRUCTURE ON THE ACTIVATION ENERGY OF SECOND STAGE DURING AUSTEMPERING OF DUCTILE IRON R.E. Campos-Cambranis*, L. Narva´ez Herna´ndez*, M.M. Cisneros-Guerrero**, and M.J. Pe´rez-Lo´pez*** *

Instituto de Metalurgia, UASLP. 78210 San Luis Potosı´, SLP, Me´xico **Depto. Metal-Meca´nica, Instituto Tecnolo´gico de Saltillo, 25280 Saltillo, Coah., Me´xico ***Instituto Tecnolo´gico de Zacatecas, 98000 Zacatecas, Zac., Me´xico (Received August 25, 1997) (Accepted in revised form January 17, 1998)

Introduction The good balance among mechanical properties of austempered ductile irons (ADI) mainly depends on the matrix microstructure, which basically consists of acicular ferrite and carbon-enriched austenite. This structure is produced by isothermal transformation of the austenite over the temperature range of 523 to 673 K. It is well accepted that during the isothermal holding, the transformation takes place in two stages. In the first stage, stage I, the austenite decomposes into acicular ferrite and carbon-enriched austenite. When the austenite is transformed at temperatures higher than 623 K, the acicular ferrite is free of carbides; at temperatures below 623 K, besides the formation of the acicular ferrite and austenite, precipitation of carbides takes place over the plates of the acicular ferrite. The mixture of ferrite and austenite, obtained during the stage I is known as ausferrite being the responsible for the good mechanical properties of ADI. The term ausferrite became the official name assigned by ASTM (A 644 –92) for this mixture. In this work, the ausferrite obtained above and below 623 K will be termed high and low temperature ausferrite respectively. Although ausferrite does not transform at room temperature, it is not a thermodynamically stable structure. Consequently, if the isothermal holding is extended, or if ADI is heated at high temperatures (523 to 800 K), the second stage of the austempering reaction, stage II, will occur. During this stage, the carbon rich austenite will decompose into ferrite and carbides. In order to establish the maximum working temperature of ADI, it is necessary to characterize the thermal stability of ausferrite microstructure, since once stage II takes place, the mechanical properties, in particular ductility and toughness, are adversely affected. In the available literature the studies focused on the determination of the kinetic parameters for stage II are scarce. One of the few available is that due to Liu et al. (1) who used measurements of magnetic properties to study the kinetics of this transformation making use of the classical methodology of isothermal holding and evaluating the change in properties as a function of temperature and tempering time. These authors reported the activation energy for the transformation (g) 1 (a) 3 a 1 carbides, using high temperature ausferrite as the initial structure. 1281

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TABLE 1 Composition of the Ductile Cast Iron Used, wt-% C

Si

Mn

S

P

Ni

Mo

Mg

3.51

2.59

0.28

0.01

0.01

0.68

0.15

0.06

In the present work the influence of previous ausferrite microstructure (i.e., that obtained during first stage) of an alloyed ductile iron (0.6%Ni, 0.15%Mo) on the empirical activation energy of stage II is studied. This activation energy was determined making use of the non-isothermal dilatometric technique proposed by Mittemeijer et al. (2). Experimental Procedure A Ni-Mo ductile iron, with the chemical composition and as-cast structure given in Tables 1 and 2, was produced in a 100 kg capacity coreless induction furnace using a charge consisting of low manganese steel scrap, ductile iron returns, high purity graphite, ferrosilicon (75%Si), ferromolybdenum (65%Mo) and electrolytic nickel. The melt was treated with magnesium by the tundish-cover-ladle method and after inoculation it was poured into standard 25.4 mm Y-blocks. Two types of specimens were prepared from the bottom half of one Y-block: specimens of rectangular section (25 3 15 3 4 mm) and cylindrical specimens (2 mm diameter and 12 mm length). The heat treatment applied to both types of specimens involved austenitization at 1143 K for 2 hours followed by austempering at 573, 593, 643 and 673 K for periods of time from 15 up to 300 minutes. Austenitization of rectangular specimens was conducted in a muffle by placing the samples inside a container packed with fresh gray iron chips to minimize decarburization. Austempering was carried out in a salt bath. After austempering, specimens were ground, polished and etched with 2% nital; afterwards, the samples were analyzed by scanning electron microscopy and x-ray diffraction. The XRD spectrum of each sample was obtained in the 2u range from 70 to 105° by using the graphite monochromated CuKa radiation at 36 kV and 30 mA of a Rigaku DMAX-2200 diffractometer. The volume fraction of austenite (Xg) was determined from the integrated intensities of five diffraction peaks. The austenitization/austempering cycle for the cylindrical specimens was performed in the vacuum chamber of an Adamel dilatometer Mod. LK-02 and using an argon jet for cooling the samples to the austempering temperature. During the isothermal holding, the linear expansion of the sample was recorded to obtain its DL vs. time curve. The results of the Xg measurements, the scanning electron microscopy analysis and the DL vs. time curves, were used to select the optimum isothermal holding time for each austempering temperature. Such a time corresponding to the time at which a microstructure consisting of high or low temperature ausferrite, without martensite, and with a minimum development of stage II is obtained. This structure is considered the optimal and was the initial structure used for the reheating treatment experiments. As mentioned before, the experiments were designed to study the kinetics of the ausferrite decomposition reaction (stage II). TABLE 2 Phase Composition and Characteristics of as-cast Ductile Iron Nodule count, mm22 106

Graphite, vol. %

Ferrite, vol. %

Pearlite, vol. %

11.1

49.3

39.6

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Figure 1. Expansion and % volume of austenite as a function of isothermal holding time at 643 K.

In order to determine the activation energy of the ausferrite transformation according to stage II, the previously mentioned non-isothermal dilatometric technique was used. This method involves the determination of the absolute temperature (Ti, K) at which an inflection point in the dilatometric curve (expansion vs. temperature) occurs, as a function of heating rate H (K/min). The reheating experiments were performed with the cylindrical samples treated to produce ausferrite by the previously described method. The specimens were heated from room temperature to 973 K in the dilatometer chamber; several heating rates were tested: 0.2, 0.5, 1, 2, 5 and 10 K/s. During the reheating, the expansion was recorded and for each austempering temperature and heating rate, the inflection point temperature (Ti) was determined from the curve. According to Mittemeijer et al., the activation energy Q is related to Ti and H by the equation:

SD S D

ln

T 2i Q Q 5 ln 1 H RK 0 RT i

(1)

The parameters Q and Ko are calculated from the slope and intercept of the straight line obtained when plotting ln(Ti2/H) against 1/Ti]. Results and Discussion Figure 1 shows the volume fraction change of the austenite and the linear expansion of the sample as a function of the holding time at 643 K. It is observed in the figure that between 60 and 100 minutes, the slope of the DL vs. time curve is close to zero. In a preliminary study (3), it was established that such a behavior could be associated to a microstructure presenting an advanced development of stage I and an incipient contribution of stage II. This behavior and the results of XRD analyses that revealed a maximum of austenite volume fraction, together with the SEM observations of the microstructure, indicate that an austempering time of 90 minutes was necessary to produce the optimal ausferrite microstructure at that temperature. Applying the same criterion for austempering temperatures of 673, 593 and 573 K, the austempering times shown on Table 3 were determined. These treatment conditions were used to produce the initial microstructure for the reheating treatments. The microstructures corresponding to the austempering treatments at 673 and 593 K are presented in Figures 2a and 2b. These structures correspond to high and low temperature ausferrite, respectively,

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TABLE 3 Austempering Conditions for Obtaining Initial Ausferritic Microstructures Temperature, K

Time, min.

673 643 593 573

45 90 60 60

both of them consisting of a mixture of acicular ferrite and a high carbon austenite. At 673 K, the structure is coarser compared to that obtained at 593 K. Although in the literature it is established that the low temperature ausferrite contains fine carbides, these precipitates were not observed during the SEM analysis of the samples. The structures obtained at 643 and 573 K were similar to those presented in Figures 2a and 2b, respectively. Figure 3 presents the DL vs. temperature curves of specimens previously austempered at 643 K, and reheated at a heating rate of 0.2 and 5 K/s. Both curves show the ausferrite expansion at temperatures higher than 673 K. Depending on the heating rate, the high carbon austenite starts to decompose into ferrite and a transition carbide different from cementite (4), when the temperature reaches 709 and 763 K for 0.2 and 5 K/s, respectively. This transformation induces a significant contraction that ends at 772 and 857 K, again, depending on the heating rate. In both cases, a second contraction of lower magnitude is observed; this contraction probably being associated to the conversion of the transition carbide to cementite (5). The Ti temperature, previously defined, increases as the heating rate increases as shown in Figure 3. Table 4 presents the Ti] values as a function of the heating rate (H) for the austempering temperatures studied. Figure 4 presents the plot of the natural logarithm of the quotient of square temperature (Ti]) to heating rate (H) against the inverse of Ti]. It is observed in the figure that the data fall into two sets: one set, whose initial austempering temperatures were 573 and 593 K, fit to a straight line with a slope of 33670 K, the other set having an initial austempering temperature of 643 and 673 K, fit to a second straight line with a slope of 23329 K. Applying equation (1) to these slope values,

Figure 2. Microstructures of samples austempered at (a) 673 and (b) 593 K.

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Figure 3. DL vs. T curves at heating rates of 0.2 and 5 K/s for samples austempered 643 K.

the corresponding activation energy for the transformation reaction (g) 1 (a) 3 a 1 carbides was determined. Q5279.9 kJ/mol for austempering at 573 and 593 K. Q5193.9 kJ/mol for austempering at 643 and 673 K. In the available literature, the activation energy for stage II has been reported only for initial structures consisting of high temperture ausferrite. Liu et al. (1) reported values between 251 and 262 kJ/mol for irons austempered at 653 K, while Dorazil, as quoted by Liu (1), reported an activation energy of 230 kJ/mol for a Fe-Si-C alloy austempered at 643 K. Using TEM observations of the microstructure, several authors (4, 6, 7) have reported the presence of free carbide ausferrite at temperatures above 623 TABLE 4 Measured T i Temperatures Corresponding to the Inflection Point on DL vs. T Curves as a Function of Heating Rate, H, and Austempering Temperture. Austempering temperature, K

573

593

H K/min

Ti K

12.00 30.30 60.12 117.96 332.34 589.50 12.48 30.42 61.80 122.10 304.02 609.12

756 770 779 793 810 824 758 767 781 791 814 824

Austempering temperature, K

643

673

H K/min

Ti K

12.12 31.02 60.60 120.30 306.54 617.28 12.18 30.66 60.48 124.08 304.62 611.76

746.0 767.0 782.0 807.0 824.0 842.5 746 770 791 810 831 842

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Figure 4. Plot of ln(Ti2/H) vs. 1/Ti to determine the activation energy of the reaction (g) 1(a) 3 a 1 carbides in an alloyed 0.6Ni-0.15Mo ductile iron.

K. However, the presence of several types of carbides has been reported for very long holding times. At temperatures below 593 K, and similar holding times to those used in the present work (60 min), these authors reported evidences of carbides precipitation both inside the ferrite needles and at the ferrite/austenite interface. These carbides have been identified as e (6, 8), although at austempering temperatures as low as 523 K, the presence of cementite has been claimed (6). The dependence of the activation energy value upon the temperature of ausferrite structure formation, suggests that the energy barrier that must be overcome in order to carry out the transformation, is related to the nature of the initial microstructure. Due to the possible role that carbides might play on the mobility of the a/g interface, when the initial structure is lower ausferrite, the transformation (g) 1 (a) 3 a 1 carbides requires a high activation energy. This hypothesis will be confirmed by future studies using transmission electron microscopy. Conclusions 1. The empirical activation energy for the reaction (g) 1 (a) 3 a 1 carbides on a Ni-Mo alloyed ductile iron was experimentally determined. When the iron was austempered at 573 and 593 K, an activation energy of 279.9 kJ/mol was calculated; for the irons treated at 643 or 673 K a value of 193.9 kJ/mol was determined. 2. The initial ausferrite structure has a significant influence on the activation energy of stage II. Low temperature ausferrite requires activation energies higher than those required by high temperature ausferrite. 3. Non-isothermal dilatometry resulted a successful technique in determining the activation energy of stage II of the austempering treatment of ductile iron. Acknowledgments The authors are grateful to Mr. Armando Rodrı´guez for his help in the experimental work.

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References 1. 2. 3. 4. 5. 6. 7. 8.

Y. C. Liu, J. M. Schissler, and A. Munteanu, Rev. Me´t. 93, 815, (1994). E. J. Mittemeijer, A. Van Gent, and P. J. Van der Schaaf, Met. Trans. 17A, 1441 (1986). R. Campos, M.S. Thesis, Instituto Tecnolo´gico de Saltillo, Saltillo, Coah. (1993). Y. C. Liu, J. M. Schissler, J. P. Chobaut, and H. Vetters, in Fifth International Symposium on the Physical Metallurgy of Cast Iron, Nancy, France, October (1994). Y. C. Liu, J. M. Schissler, J. P. Chobaut, and H. Vetters, Metall. Sci. Technol. 13, No. 12 (1995). L. Sidjanin, R. E. Smallman, and J. M. Young, Acta Metall. Mater. 42, 3149 (1994). J. Aranzabal, I. Gutierrez, and J. J. Urcola, Mater. Sci. Technol. 10, 728 (1994). S. Korichi and R. Priestner, Mater. Sci. Technol. 11, 901 (1995).