MY9700813 STUDY OF AUSTEMPEREVG REACTION IN AUSTEM^WED DUCTILE IRON

Ja'far Farhan Al-sharab. D.G.R.Sharma, and Samsul Bahar Sadli School of Materials and Mineral Resources Engineering USM (kcp), Perak, MALAYSIA

Abstract : Austempered Ductile Iron (ADI) is an important engineering material which is gaining popularity. The conventional belief that austempered ductile iron, when heat treated satisfactorily, contains bainite, is now disproved by recent experiments. Our present work on the study of the reaction products of heat treated ADI by x-ray diffraction confirms the recent view. The results of x-ray diffraction studies on the structural constituents of ADI for various durations of austempering are presented and discussed.

Introduction

Cast iron is one of the important engineering materials which finds extensive applications in automotive, machine tool, textile, chemical and many other industries1. The development of ductile iron has improved the scope of applications of cast iron due to the improved properties available in ductile iron. In recent years, Austempered Ductile Iron (ADI) has attracted the attention of materials engineers because it has superior strength, good impact resistance, light weight etc., compared to low-alloy steel2 '3. ADI has a good wear resistance4, excellent damping capacity, high toughness5, and very good thermal conductivity. ADI is also easier for production. Austempered Ductile Iron is produced by the heat treatment -Austempering- of the cast ductile iron component. The reactions during the Austempering of ductile iron are different compared to the austempering reactions in steel6. Further, the composition of ADI influences the heat treatment and final properties significantly. In ADI, recent studies have shown that the reaction products of the austempering heat treatment does not contain bainite in the initial stage (i.e. the first reaction). Since the material is still in a state of development, many aspects of the behavior of ADI are not fully understood6.

Austempering Reactions

In the case of steels austempering produces a bainitic structure (upper of lower bainite depending on the temperature of austempering ). The reaction in austempering of ductile iron is considerably different compared to that in steels. It is observed that the austempering heat treatment causes an increase of the strength up to a certain value and this is called the first stage reaction. Heat treatment beyond the first reaction will produce high strength and low ductility and this is called the second stage reaction. The microstructure of the ADI appears quite similar to the bainitic structure but recent studies have shown that the structure is a mixture of austenite and acicular ferrite. During austempering, ferrite precipitates out of, and grows into the austenite. At the same time, carbon is rejected from the growing ferrite platelets into the surrounding austenite. The remaining austenite

265 ACXRI '96 continues to absorb carbon as the austempering reaction proceeds. As the austenite becomes enriched with carbon, growth of the ferrite platelets is inhibited and the reaction is arrested This is called the first reaction in the austempering of ADI . The second reaction starts when the high carbon austenite starts to decompose to ferrite and carbide. The high toughness in these irons is attributed to ferrite - austenite structure produced by the first reaction, while the second reaction is undesirable because it produces embrittlment due to the presence of carbides.

Experimental Details :

Ductile iron having a composition suitable for austempering was cast in the form of round bars of size 30 mm diameter x 300 mm length. The composition of the cast alloy is C-3.6%, Si-2.69 %, Mn-0.31 %, P-0.U32 %, S-0.018 %, Mo-0.3 %, and Cu- 0.97 %. From these castings, specimens of 7 mm diameter and 20 mm length were machined for the present study.

Heat treatment :

Heat treatment of the specimens involved austenitization in a tube furnace at 900 °C for 1.5 hour. All the specimens were dipped in a copper sulphate solution so as to minimize decarburization. Austempering was accomplished at a temperature of 316 or 371 °C by 7 quenching into a salt bath ( 55.2 % KN03 and 44.8 % NaNO2 ) and holding at the desired temperature for 0 5, 1, 1.5, 2, and 5 hours as shown in the schematic diagram in figure 1. Alter austempering, the specimens were quenched into water for subsequent analysis by x-rays.

X-ray Analysis:

The samples were subjected to fine grinding and polishing for analysis using x-ray diffraction X-ray analysis was performed using graphite monochromated Cu Ka radiation at 40 KV and 20 mA. with a Philips diffractometer Scanning was carried out over the range of 20 - 145 °C for the values of 29 . This range could detect the austenite and ferrite. The location of the fee austenite and bec ferrrite diffraction peak positions and measurement of their integrated areas allowed the estimation of two important parameters; 1) the angular position of the austenite peaks gives an estimate of the carbon content of the austenite and 2) the integrated area under the austenite and ferrite peaks allowed an estimate of the volume fraction of austenite

Results and Discussion

The values of carbon content and volume fraction of austenite are included in table 1. Volume fraction estimates can be made from measurements of the integrated intensities of

266 ACXRI 96 the bcc ferrite and fee austenite phases assuming that they are the only matrix phases present. The ratios of the intensities of diffraction peaks from these two phases is piven by8

I /? X Y 'a (hkl) lya(hkl) ^ a where ly(hki) = Integrated intensity from a given (hkl) plane from the austenite phas1, Itt(hki) = Integrated intensity from a given (hkl) plane from the ferrite phase, Xy = Volume fraction of austenite. Xa = Volume fraction of ferrite. ly(hki) and Ia(hki) could be obtained by measuring the area under the peaks by using a planimeter. The constants Ra(hkl) and Ry(hkl) are given by the expression for each peak.:

R'actor =7^" l^r XPX

where V - atomic volume of the unit cell, F = structure factor p = multiplicity factor, Lp= Lorentz-polarization factor, and e1M = temperature factor. The temperature factor can either be calculated or read from a curve plotting the temperature as a function of for iron at 20 °C.°. A. The R values have been calculated for the (220) peak from austenite and (211) peak from ferrite and these are listed in table 3. The carbon was calculated from the relation1"

ao = 3.548 + 0.044X,

where X is the percentage of carbon.

The results of x-ray analysis are given in table 1 These results are obtained using the R value for austenite ( given in table 2) and the R values for ferrite (given in table 3). The amount of retained austenite in the specimens for various durations of austempering heat treatment are plotted in figure 2. The graph shows a peak in the volume fraction of retained austenite at 5400 seconds (1.5 hr) for heat treatment at 316 °C and at 3600 seconds (1 hr) for the heat treatment at 371 °C. The SEM micrograph of a specimen corresponding to the peak retained austenite value austempered at 316 °C is given in figures 3. The presence of carbides can be easily seen in a 5 hours heat treated sample given in figure 4 ( as indicated by the arrow). The variation in mechanical properties i.e. tensile strength and impact eneigy with austempering time are given in figures 5 and 6. These results confirm the view that the second reaction starts after about one hour of the austempering heat treatment, which is the optimum time of austempering heat treatment for this composition.

267 ACXRI '96 Conclusions

X-ray diffraction is a powerful tool for the study of austempering reaction in ADI. In the present study, it is found that if the specimen is austempered up to one hour, the reaction products are a mixture of austenite and acicular ferrite. Beyond one hour, the amount of the retained austenite decreases and the carbide starts to form. The optimum duration of austempering heat treatment is up to the attainment of the peak value of retained austenite and this is 1.5 hr at 316 °C and 1 hr at 371°C for the alloy studied.

Acknowledgments

We would like to express our thanks to Associate professor Dr. Azmi Rahmat; Dean of the School of Materials and Mineral Resources Engineering for providing the research facilities, and financial support. Special thanks go to Dr. Sofiane Benhadad for his help in the analysis of x-ray data.

References

1. L. R. Parks. 'Austempered irons and automotive industry', Materials in Action, 1985,1, 53 2. G. J. Cox. ' The heat treatment of S.G. iron', The Metallurgist and Materials Technologist, 1980, Vol. 12, (11), 629 3. Cast metals Development Ltd. 'Austempered ductile - iron castings - advantages production, properties, and specifications', Materials and Design, 1992, Vol. 13, (5), 285 4. Jack R. Laub, 'Cast austempered ductile - iron for high strength and long wear', Advanced materials and processes, 1994, 2, 40 5. Richard Gundlach and Jay F. Janowak, 'Austempered ductile iron combines strength with toughness and ductility', Metal Progress, 1985, Vol. 127, (2), 19 6. B. V. Kovacs, 'Austempered ductile iron : Fact and Fiction', Modern Casting, 1990, 3, 38 7. S. -C. Lee, C. -C. Lee, 'The effects of heat treatment and alloying elements of Fracture toughness of bainitic ductile cast iron', AFS Transactions, 1988, 145 8. K. B. Rundman, R. C. Klug, 'An X-Ray and Metalographic Study of an Austempered Ductile Cast Iron', AFS Transactions, 1982, 115 9. B. D. Cullity, ' Elements of X-Ray Diffraction', 1978, Second Edition, 136 10. Robert E. REED-Hill. 'Physical mettalurgy principles', 1964, First Edition , 497

268 ACXRI '96

40.00 35.00 —«-~316cL 30.00 -Eh-371 C L 25.00 Auatenitizin JOC o —-—„ for 15 hrs 20.00 / 900C • 1 / Temp 15.00 I 10.00 / Austempering at 316 or 371 C 5.00 0.00 / 0 4000 8000 12000 16000 20000

0 05 1 15 2 Time (seconds) Auatempenng lime (hre) Fig. 1. Schematic of the austempering Fig. 2. Retained austenite volume vs patterns used in this study. austempering time

*

Fig. 3. The SEM micrograph for Fig. 4. Carbides in a specimen specimen austempered at 316 C for austempered for 5 hrs.ammonium 2 hours. Nitai etch persulfate etch.(500X)

_. 1600 200 —•— 316 °C J16°C S 160 J71°C 1 A. L , "'• —. • • •••Hi i 120 t — •Q 80 ly | 400 I 40 t n 0 4000 8000 12000 16000 20000 4000 8000 12000 16000 20000 Time (seconds) Time (seconds)

Fig. 6. Tensile strength vs austempering Fig 5. Impact energy vs austempering time time

269 n>

Table 1. Results of x-ray diffraction measurements 2 Austempered at 3 i 6 =C Austempered at 37! 'C Integrated intensity Integrated intensity Time (Relative units) Austenite lattice Parameter Carbon Content (Relative units) Austenite lattice Parameter Carbon Content (hours) 0X211) y(220) Volume (A") (C %) a(211) y(220) Volume (A")

Quench 549 84 18.50 3.5850 1.2; 549 84 18.50 3.5850 1.21 0.5 473 93 22.73 3.6010 1.41 448 110 26.84 3.6100 1.40 1 448 110 26.93 3.6160 1.60 372 10! 29.1! 3.6155 1.54 1.5 355 84 26.54 3.6310 1.93 346 110 32.47 3.6314 1.90 , 431 93 24.75 3.6340 2.00 473 93 22.96 3.6345 1.96 j 5 448 76 20.56 3.6360 1.99 524 84 19.75 3.6359 2.00 I

Table 2. R vaiue calculations for austenite plane (220) ai 316 °C lattice parameter temp, factor i time d-spacing A (a.) sintf LP factor R : (hours) 26 F F (A°v6 (A°) (A0) (A°) ~T (A°"') quench 74.8635 1.2673 1.5406 3.5850 0.395 15.344C 3767.00 12 0.899 0.000471 3.6385 69.68 0.5 74.4545 1.2733 1.5406 3.6010 0.393 15.3937 3791.47 12 0.900 0.000458 3.6752 68.94 l.C 74.1120 1.2783 1.5406 3.6160 0.391 15.4355 3812..07 12 0.900 0.000448 3.67066 68.33 1.5 73.7355 1.2839 1.5406 3.6310 0.390 15.4815 3834.83 12 0.901 0.000436 3.7420 67.67 2.C 73.6635 1.2850 1.5406 3.6340 0.389 15.4903 3839.20 12 0.901 0.000434 3.7489 67.54 5.0 73.6305 1.2855 1.5406 3.6360 0.389 15.4944 3841.21 12 0.901 0.000433 3.7520 67.48

fable 3. Rvalue calculations for ferrite plane (211) ai316°C lattice parameter temp.factor 1 time d-spacing 1 (a.) sinfi' LP factor R 2 (hours) 28 A f F p X (A°y( (A°) (A°) 0 (A ) (A-1) quench 82.4000 1.1694 1.5406 2.8645 0.4277 14.6738 861.28 24 0.882 0.00181 3.1151 102.80 0.5 82.3255 1.1703 1.5406 2.8667 0.4274 14.6798 861.99 24 0.882 0.00180 3.1190 102.58 1.0 82.4250 1.1692 1.5406 2.8638 0.4278 14.6718 861.04 24 0.882 0.00181 3.1138 102.88 1.5 82.3355 1.1702 1.5406 2.8664 0.4274 14.6790 861.89 24 0.882 0.00180 3.1185 102.61 2.0 82.2995 1.1706 1.5406 2.8674 0.4273 14.6819 862.24 24 0.882 0.0018 3.1204 102.50 5.0 82.4505 1.1689 1.5406 2.8631 0.4279 14.6697 860.80 24 0.882 0.00182 3.1125 102.95