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Effects of successive austempering on the tribological behavior of ductile cast iron
Wear 231 Ž1999. 293–300 www.elsevier.comrlocaterwear
Effects of successive austempering on the tribological behavior of ductile cast iron M. Nili Ahmadabadi b
a,)
, H.M. Ghasemi a , M. Osia
b
a Faculty of Engineering, UniÕersity of Tehran, Tehran 11365-4563, Iran Graduate Student, Faculty of Engineering, UniÕersity of Tehran, Tehran, Iran
Received 21 April 1999; received in revised form 1 May 1999; accepted 11 May 1999
Abstract A 0.75 wt.% Mn ductile iron with different nodule counts was austempered by conventional and successive austempering processes at 315 and 3758C for different periods. Specimens with optimum mechanical properties were used to study the effect of austempering process on the wear behavior of austempered ductile iron ŽADI.. Sliding wear tests were performed in a pin-on-ring wear tester with the test materials rubbing under dry atmospheric condition against a surface hardening tool steel ring at speeds of 0.6 and 1.28 m sy1 and normal loads of 100, 200 and 300 N. To study worn surfaces of test specimens, optical and scanning electron microscopy ŽSEM. were used. It was found that specimens austempered at 3158C ŽLAT, lower austempering temperature. show the highest wear resistance while specimens austempered at 3758C ŽHAT, higher austempering temperature. show the lowest. Specimens austempered by successive austempering process ŽHLAT, high–low austempering temperature. with hardness about equivalent to HAT specimens, in addition to improvement of mechanical properties, show higher wear resistance than that of HAT specimens. Duplex structure, upper and lower bainite, along with higher carbon content of retained austenite and reduction of UAV Žuntransformed austenite volume. can be the main factors which improve mechanical properties and wear resistance of HLAT specimens. The results showed that specimens with longer solidification time have lower wear rate than specimens with shorter solidification time, supposedly due to effect of longer solidification time on the reduction of nodule count and heavier segregation of alloying elements. SEM study of worn surfaces suggests that delamination could be the probable wear mechanism in this investigation. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Austempering; Tribological behavior; Ductile iron; Bainite
1. Introduction Austempered ductile iron ŽADI. has received much publicity as more applications have been reported w1,2x. ADI offers a combination of strength and wear resistance with low cost and good toughness. To provide sufficient hardenability during austempering, ductile iron should be alloyed with alloying elements like, Mo, Mn, Cu or Ni. Mn an inexpensive hardenability promoter may be added to iron for this purpose. However, Mn segregates in the intercellular regions which has adverse effects on the mechanical properties of ADI. As a consequence of negative contribution of Mn, it was recommended to keep Mn content around 0.35% w3,4x. To overcome the negative impact of Mn, one of the authors for the first time, introduced the successive austempering process which improves mechanical properties of 1 wt.% Mn, well compa)
Corresponding author. E-mail: [email protected]
rable with low alloy conventional austempering process and much higher than ASTM values w5,6x. Subsequently, Bayati et al. w7x also applied this process on the same material, and their results confirmed the beneficial effects of successive austempering. The wear behavior of ADI has been studied by some researchers. Nili Ahmadabadi et al. studied wear characteristic of ADI, unalloyed and 3 wt.% P gray cast iron w8x. They observed a higher wear resistance in ADI than the others even if all samples had almost the same hardness. They concluded that higher wear resistance of ADI should be due to the presence of ferritic and retained austenitic Ža carbon enriched austenite which is stable at room temperature. component of bainite. Prado et al. w9x examined the effects of austempering temperature on the dry sliding wear of ADI. They reported that when high loads Ž500 and 800 N. are applied, the best wear resistance is obtained for thermal treatments at 3408C. However, their conclusion is rather in question, since they austempered all of specimens
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 9 . 0 0 1 6 3 - 5
M. Nili Ahmadabadi et al.r Wear 231 (1999) 293–300
294 Table 1 Chemical composition of alloy Žwt.%. C
Si
Mn
Cr
Cu
Mg
3.5
2.73
0.75
0.05
0.18
0.04
for 60 min. It has been well-established that the kinetics of bainitic transformation is a function of austempering temperature w10,11x. Therefore, if one assumes that 60 min is sufficient for completion of austempering at higher austempering temperatures but in other specimens, in particular, specimens austempered at lower temperatures, this austempering period seems to be insufficient. As a consequence of short austempering process at lower austempering temperature some regions may remain untransformed. These regions transform to martensite following cooling to room temperature. The formation of martensite in the matrix will have an important impact on the mechanical properties and wear behavior of ADI. The tribological behavior of austempered spherical graphite cast iron containing aluminum was studied by Boutorabi et al. w12x. They showed that the wear resistance of ADI is a complex function of austempering temperature and time. Inasmuch as successive austempering process extensively improves mechanical properties of ADI as a consequence of formation of duplex structure and higher retained austenite in the matrix w5,6x, it is reasonable to assume that the process should also have some important impacts on the wear behavior of ADI. This work is an attempt to investigate the effect of austempering process and solidification time on the wear behavior of 0.75 wt.% Mn ADI. Special attention is paid to study the effect of successive austempering process on the wear behavior of this alloy.
2. Experimental procedure
Fig. 2. Ultimate tensile strength vs. elongation of higher and lower nodule count specimens with optimum mechanical properties. HNs higher nodule count, LNs lower nodule count.
spheroidization treatment and 1-in. thick Y block castings were poured in green sand molds. Base irons were treated by 5.5% MgFeSi alloy for spheroidization followed by post-inoculation with 75% FeSi. Fig. 1 shows a schematic diagram of the austempering treatment carried out on different specimens. The specimens were austenitized for 90 min in an atmosphere-controlled furnace at 9008C, austempered for various times in a salt bath at 3758C to provide a high austempering temperature ŽHAT process., or at 3158C for a low austempering temperature ŽLAT process., followed by quenching in iced water. In the case of successive austempering ŽHLAT., specimens were first austempered at 3758C, and then austempered at 3158C for different periods, followed by quenching in iced water. To study the effect of solidification time Žnodule count. and austempering variables on the mechanical properties,
High Mn ductile iron of the composition given in Table 1 was prepared. The alloy was produced by a sandwich
Fig. 1. Schematic diagram of different heat treatment processes.
Fig. 3. Optical micrograph of specimen austempered at 3758C for 120 min.
M. Nili Ahmadabadi et al.r Wear 231 (1999) 293–300
295
Fig. 4. The same as Fig. 3 but at higher magnification. F s feather-like bainite, L s lath bainite, Ns nodular bainite.
Fig. 6. Optical micrograph of specimen austempered by successive austempering. LBs lower bainite, M s martensite.
tensile test specimens were prepared from the bottom Žhigher graphite nodule count, 102 nodule graphitermm2 . and the vicinity of riser Žlower graphite nodule count, 65 nodule graphitermm2 .. Tensile tests were carried out on a 10 ton Instron tensile test. The specimens with optimum mechanical properties were used for wear tests. The wafer specimens with optimum mechanical properties obtained from different austempering processes, were ground, polished and etched prior to hardness testing, and optical or electron microscopy. Five hardness measurements were made on each specimen to obtain the average value. Sliding wear tests were performed in a pin-on-ring wear tester with the test materials rubbing under dry atmospheric condition against a surface hardened tool steel ring at speeds of 0.6 and 1.28 m sy1 and loaded with normal loads of 100, 200 and 300 N. Inasmuch as in many ADI application, the combination of optimum properties are required, therefore, the pin specimens with diameter of 5 mm were used with higher and lower nodule count and
optimum mechanical properties obtained from different austempering processes.
Fig. 5. Optical micrograph of specimen austempered at 3158C for 480 min. The morphology of bainite is acicular with a lower density in the intercellular.
3. Results 3.1. Metallography Fig. 2 shows optimum mechanical properties of LAT, HAT and HLAT specimens. This figure shows that mechanical properties of HAT specimens is lower than ASTM standard while mechanical properties of HLAT specimens is higher. Comparison between mechanical properties of HAT and HLAT specimens clearly expresses the drastic impact of successive austempering on the mechanical properties of high Mn ADI. It was shown that this improvement is due to the reduction of UAV Žuntransformed austenite volume, i.e., a low carbon austenite which upon cooling to room temperature transforms to martensite., increase of retained austenite volume fraction and also carbon content of austenite in HLAT specimens w5,6x.
Fig. 7. Retained austenite volume percentage vs. Mn content of specimens austempered by different austempering processes.
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M. Nili Ahmadabadi et al.r Wear 231 (1999) 293–300
Fig. 8. SEM micrograph of worn surface of HLAT specimen at an applied load of 200 N and a running speed of 0.6 m sy1 .
Figs. 3–6 show the microstructure of three specimens: HAT, LAT and HLAT. Figs. 3 and 4 show the microstructure of a specimen austempered at 3758C for 120 min. In Fig. 4, a large volume of untransformed austenite remains in the intercellular region Žhigh Mn region. and three different bainitic morphology as feather-like bainite, bainitic lath and nodular bainite can be distinguished. The feather-like bainite forms around graphite while nodular bainite mostly forms in the intercellular region, near microporosity. This figure shows that the bainitic lath forms in the regions between nodular graphite and intercellular regions. Formation of bainite with different morphologies is likely due to variation of composition in the matrix as examined by Spanos et al. w13x in steel with different carbon contents. Fig. 5 shows microstructure of a specimen austempered at 3158C for 480 min ŽLAT.. In this figure, bainite has an acicular morphology with a lower density in the intercellular region. On contrast to Figs. 3 and 4, all of the bainitic ferrite show the same morphology except for a decrease in the length of acicular ferrite in the intercellular region.
Fig. 9. SEM micrograph of worn surface of HLAT specimen at an applied load of 300 N and a running speed of 0.6 m sy1 .
Fig. 10. SEM micrograph of worn surface of HLAT specimen at an applied load of 100 N and a running speed of 1.28 m sy1 .
Fig. 6 is micrograph of a specimen austempered at 3758C for 30 min, then quenched to 3158C and kept for 240 min ŽHLAT.. The formation of lower bainite in the intercellular region along with a few plate of martensite can be seen in the figure. A rough examination of Figs. 4–6 shows that HLAT and HAT specimens have higher retained austenite than LAT specimens which is consistent with the results of previous work of one of the authors w14x. As shown in Fig. 7 for 0.75 wt.% Mn, HLAT specimens contain the highest retained austenite followed by HAT and LAT specimens. Figs. 8 and 9 show the worn surfaces of HLAT specimens at various loads and running speeds. Fig. 8 shows a SEM micrograph of the worn surface at an applied load of 200 N and a running speed of 0.6 m sy1 . The figure indicates a moderate rough surface which forms as a consequence of adhesion of some debris on the surface and rubbing track. Fig. 9 shows that an increase of load to 300 N enhances the roughness of surface while some surface
Fig. 11. SEM micrograph of cross-section of worn surface of HLAT specimen at an applied load of 100 N and a running speed of 0.6 m sy1 . The porosities, along with the delaminated layer, clearly show the delamination mechanism in this test.
M. Nili Ahmadabadi et al.r Wear 231 (1999) 293–300
Fig. 12. SEM micrograph of cross-section of worn surface of HAT specimen at an applied load of 200 N and a running speed of 0.6 m sy1 . The deformed blocky austenite can be seen. Bs blocky austenite.
regions seem to be cracked. This could supposedly be a delaminated layer initiated from the graphite underneath. An increase of running speed to 1.28 m sy1 at a load of 100 N enhances surface roughness due to severe adhesive mechanism as shown in Fig. 10. Figs. 11 and 12 are SEM micrographs of cross-section of worn surfaces. Fig. 11 shows the surface structure of HLAT under a load of 100 N and Õ s 0.6 m sy1 . In this figure, three different regions can be distinguished: the heavily plastic deformation region in the direction of sliding parallel to the specimen surface where neither ferrite and austenite can be seen Žzone 1., a distorted region where deformed austenite can be seen Žzone 2., and undeformed bulk region Žzone 3.. Since ADI consists of metastable austenite, it is expected that under load during sliding, strain-induced martensite would form in the deformed region, in particular, in zone 1. In the lower deformation region, it is supposed that austenite has higher twin density than strain-induced martensite w15x. In fact, transformation of austenite to martensite depends on carbon content of retained austenite. It is ex-
Fig. 13. SEM micrograph of cross-section of worn surface of HLAT specimen which shows deformation of a graphite nodule.
297
Fig. 14. The same as in Fig. 12 but at higher magnification.
pected that austenite with lower carbon content transformed to martensite under lower loads and strain rates than high carbon content austenite. This matter is more probable in HAT specimens which have lower carbon content austenite ŽFig. 12.. Fig. 12 shows blocky austenite with low carbon content, therefore, it is expected that this austenite easily deformed to martensite. Fig. 13 shows deformation of a graphite nodule due to applied load during wear test. A curved broken surface is formed over the graphite which would provide a channel for the feeding of graphite to the surface. It could be expected that the smearing of the graphite on the surface may prevent the metallic contact and reduce the wear rate of material. Fig. 14 shows the same concept as Fig. 13 but at higher magnification. The figure shows that while the
Fig. 15. Weight loss vs. sliding time of different specimens with higher nodule count when F s 200 N and Õ s 0.6 m sy1 . Specific gravity of ductile iron, r ( 7.1 grcm3. A duration of 50 min corresponds to a sliding distance of 1800 m.
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M. Nili Ahmadabadi et al.r Wear 231 (1999) 293–300
Fig. 16. Weight loss vs. sliding time of different specimens with lower nodule count when F s 200 N and Õ s 0.6 m sy1 . Specific gravity of ductile iron, r ( 7.1 grcm3.
Fig. 18. Weight loss vs. sliding time of different specimens with lower nodule count when F s 300 N and Õ s 0.6 m sy1 . Specific gravity of ductile iron, r ( 7.1 grcm3.
matrix over the graphite channel is heavily deformed and fractured, the region under the channel shows less deformation. In fact, formation of this channel prevents formation of a heavy deformed area in the regions below the graphite channel.
LAT specimens show higher wear resistance. This figure shows that application of successive austempering improves wear resistance of HLAT specimens higher than HAT specimens and close to LAT specimens. Figs. 17 and 18 show the effect of increased applied load from 200 to 300 N on the wear resistance of different specimens. In this case, weight loss also increases in the order of LAT, HAT and HLAT specimens. Figs. 19 and 20 show the effect of running speed, applied load and austempering processes on the weight loss of specimens running for 1000 m. The figures show that increase of applied load when running speed is 0.6 m sy1 increases wear of all specimens. In addition, it is further noticed that increasing the sliding speed for about two-fold would have almost the same effect on the wear as increasing the applied load by the same amount. These
3.2. Wear tests results Figs. 15 and 16 shows weight loss as a function of sliding time for lower and higher nodule count specimens when F s 200 N and Õ s 0.6 m sy1 . In the both cases,
Fig. 17. Weight loss vs. sliding time of different specimens with higher nodule count when F s 300 N and Õ s 0.6 m sy1 . Specific gravity of ductile iron, r ( 7.1 grcm3.
Fig. 19. Weight loss vs. applied load and running speed for different specimens with higher nodule count, run for 1000 m.
M. Nili Ahmadabadi et al.r Wear 231 (1999) 293–300
Fig. 20. Weight loss vs. applied load and running speed for different specimens with lower nodule count, run for 1000 m.
results which would follow Archard’s Wear Law could be clearly seen by comparing wear tests at sliding speed of 0.6 m sy1 and a load of 200 N with that of 1.28 m sy1 and 100 N. However, increasing the load from 100 to 200 N at a sliding speed of 0.6 m sy1 would increase the wear about 15-fold suggesting a different wear mechanism operating at an applied load of 100 N.
4. Discussion The mechanical and wear test results show that the successive austempering process improves both mechanical properties and wear resistance of ADI in comparison with conventional austempering process. It has been concluded by some researchers that in conventional ADI, an increase of hardness and decrease of retained austenite increase wear resistance w8,16x. However, by considering results of wear resistance of HLAT and HAT specimens achieved in this work, a more precise investigation is necessary. It was shown by one of the authors w5,6x that the successive austempering process increases retained austenite and its carbon content in comparison with conventional austempering process ŽHAT and LAT processes.. In addition, it is more informative if one compares hardness of LAT, HLAT and HAT specimens. Fig. 21 shows variation of hardness of different specimens as a function of austempering time. It can be seen that LAT specimens have the highest hardness while HLAT and HAT specimens have an almost the same hardness. The highest hardness of LAT specimens is due to its lower retained austenite volume percentage, acicular ferrite with higher carbon content and dislocation density compared to HAT specimens containing feathery ferrite w17,18x, and precipitation of carbide in the ferrite. Hardness in HAT specimens is a combination of hardness of upper bainite and UAV while hardness in HLAT, is controlled by hardness of upper bainite and
299
lower bainite. Therefore, replacement of UAV by lower bainite keeps hardness of these samples in the same level. Fig. 21 also shows that hardness of LAT specimens with the best process combination is about 33% higher than HLAT and HAT specimens. However, Figs. 19 and 20 indicate that average wear resistance of HLAT specimens in all experimental running speed and applied load except for lowest applied load Ž F s 100 N., is about 80% of wear loss of HAT specimens and about 110% of LAT specimens. Keeping in mind HLAT hardness and its highest retained austenite volume fraction, it can be concluded that in ADI hardness and retained austenite volume fraction are not the only controlling factors accounting for wear resistance but also other factors like morphology of ferrite, UAV and carbon content of retained austenite should be taken into account. In fact, austenite with lower carbon contents transforms to martensite with lower hardness under loads than high carbon content austenite. This matter is more probable in HAT specimens which have lower carbon content austenite than HLAT specimens. It is assumed that these low carbon blocky austenite in HAT specimens as shown in Fig. 12, should be one of the reasons for the lower wear resistance of HAT specimens in comparison with HLAT specimens. By examination of Fig. 11, wear mechanism in this work can be investigated. This figure shows some porosities and voids near the worn surface and in some regions below it. These porosities which form during sliding are the main cause of the observed delaminated layer, suggesting delamination mechanism as a dominant wear mechanism in these tests. As mentioned before, the matrix over the graphite channel in Figs. 13 and 14 is heavily deformed. It could be expected that depletion of graphite out of the channel and increasing the load would accelerate
Fig. 21. Hardness of different specimens vs. austempering time.
300
M. Nili Ahmadabadi et al.r Wear 231 (1999) 293–300
fracture of the material over the channel, and therefore, increasing the wear of material. It seems that the toughness of the matrix and nodule count play an important role on the fracture of the material over the channel. However, the specimens for Figs. 13 and 14 have about the same nodule count, but different microstructure which gives higher toughness for HLAT than other specimens ŽFig. 2.. This higher toughness could be one of the reasons resulting in improvement of wear of HLAT specimen than HAT specimens and closer to LAT specimens. Higher wear resistance of LAT specimens could be due to their higher hardness. It is speculated that at the lower load and speed of 100 N and 0.6 m sy1 , the graphite layer acts as a solid lubricant with sufficient load carrying capacity. This would explain the much lower wear Ž15 times lower. of specimens at these load and speed compared to higher loads andror speeds shown in Figs. 19 and 20. Figs. 15–20 show that the specimens with lower nodule count have higher wear resistance than specimens with higher nodule count. As shown in Figs. 9 and 13, the delaminated regions over graphites are one of the source of wear debris formation. Therefore, it is likely that reduction of nodule count decreases the probable regions for initiation of delamination layer which leads to a reduction in wear debris. The other point that should also be taken into account is an increase of segregation of Mn in the intercellular region as nodule count decreases. The heavier segregation of Mn in the intercellular regions increases the UAV which upon cooling to room temperature transforms to martensite w19x. The martensite with higher hardness can increase wear resistance of ADI.
3. The specimens with lower nodule count Žlonger solidification time. have lower wear rate than specimens with higher nodule count Žshorter solidification time.. 4. High carbon content retained austenite along with good mechanical properties are supposedly the main reason for improvement of wear resistance of HLAT specimens.
Acknowledgements The authors would like to express their sincere thanks to the research council of University of Tehran for financial support of this work.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x
5. Conclusions w13x
Sliding wear tests on specimens with optimum mechanical properties austempered by different processes lead to the following conclusions: 1. The results of this study suggesting delamination mechanism as a dominant wear mechanism. 2. The mechanical and wear test results show that successive austempering process improves both mechanical properties and wear resistance of ADI in comparison with conventional austempering process.
w14x w15x w16x w17x w18x w19x
R. Harding, 2nd Int. Conf. on ADI, Ann Arbor, USA, 1987, 39. B.V. Kovacs, Modern Casting, March 1990, 38. M. Gagne, AFS Trans. 95 Ž1987. 523. D.J. Moore, T.N. Rouns, K.B. Rundman, AFS Trans. 93 Ž1985. 705. M. Nili Ahmadabadi, T. Ohide, E. Niyama, Trans. JFS 11 Ž1992. 40. M. Nili Ahmadabadi, T. Ohide, E. Niyama, Cast Metals 5 Ž2. Ž1992. 72. H. Bayati, R. Elliot, G.W. Lorimer, MST 11 Ž1995. 1007. M. Nili Ahmadabadi, S. Nategh, P. Davami, Cast Metals 4 Ž4. Ž1992. 188. J.M. Prado, A. Pujol, J. Cullell, J. Tartera, MST 11 Ž1995. 294. K.B. Roundman, D.J. Moore, H.L. Hayrynen, W.J. Dubensky, T.N. Rouns, J. Heat Treating, J. Heat Treating 5 Ž2. Ž1998. 79. M. Nili Ahmadabadi, Metall. Mater. Trans. A 28A Ž1997. 2159. S.M.A. Boutorabi, J.M. Young, V. Kondic, M. Salehi, Wear 175 Ž1993. 19. G. Spanos, H.S. Fang, D.S. Sarma, H.I. Aaranson, Metall. Mater. Trans. 21A Ž1990. 1391. M. Nili Ahmadabadi, T. Ohide, Proc. of IFHT 95, Isfahan, Iran, 239, 1995. R. Boschen, H. Vetters, P. Mayer, P.L. Pyder, Prakt. Metallogr. 25 Ž1988. 524. R. Gundlach, J. Janowak, Proceeding of 2nd Int. Conf. on Austempered Ductile Iron, Ann Arbor, MI, 1986, p. 23. S. Dymski, T. Szykowny, MST 12 Ž1997. 385. M. Nili Ahamadabadi, Metall. Mater. Trans. A 29A Ž1998. 2297. M. Nili Ahmadabadi, E. Niyama, T. Ohide, AFS Trans. 99 Ž1994. 279.
Effects of successive austempering on the tribological behavior of ductile cast iron M. Nili Ahmadabadi b
a,)
, H.M. Ghasemi a , M. Osia
b
a Faculty of Engineering, UniÕersity of Tehran, Tehran 11365-4563, Iran Graduate Student, Faculty of Engineering, UniÕersity of Tehran, Tehran, Iran
Received 21 April 1999; received in revised form 1 May 1999; accepted 11 May 1999
Abstract A 0.75 wt.% Mn ductile iron with different nodule counts was austempered by conventional and successive austempering processes at 315 and 3758C for different periods. Specimens with optimum mechanical properties were used to study the effect of austempering process on the wear behavior of austempered ductile iron ŽADI.. Sliding wear tests were performed in a pin-on-ring wear tester with the test materials rubbing under dry atmospheric condition against a surface hardening tool steel ring at speeds of 0.6 and 1.28 m sy1 and normal loads of 100, 200 and 300 N. To study worn surfaces of test specimens, optical and scanning electron microscopy ŽSEM. were used. It was found that specimens austempered at 3158C ŽLAT, lower austempering temperature. show the highest wear resistance while specimens austempered at 3758C ŽHAT, higher austempering temperature. show the lowest. Specimens austempered by successive austempering process ŽHLAT, high–low austempering temperature. with hardness about equivalent to HAT specimens, in addition to improvement of mechanical properties, show higher wear resistance than that of HAT specimens. Duplex structure, upper and lower bainite, along with higher carbon content of retained austenite and reduction of UAV Žuntransformed austenite volume. can be the main factors which improve mechanical properties and wear resistance of HLAT specimens. The results showed that specimens with longer solidification time have lower wear rate than specimens with shorter solidification time, supposedly due to effect of longer solidification time on the reduction of nodule count and heavier segregation of alloying elements. SEM study of worn surfaces suggests that delamination could be the probable wear mechanism in this investigation. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Austempering; Tribological behavior; Ductile iron; Bainite
1. Introduction Austempered ductile iron ŽADI. has received much publicity as more applications have been reported w1,2x. ADI offers a combination of strength and wear resistance with low cost and good toughness. To provide sufficient hardenability during austempering, ductile iron should be alloyed with alloying elements like, Mo, Mn, Cu or Ni. Mn an inexpensive hardenability promoter may be added to iron for this purpose. However, Mn segregates in the intercellular regions which has adverse effects on the mechanical properties of ADI. As a consequence of negative contribution of Mn, it was recommended to keep Mn content around 0.35% w3,4x. To overcome the negative impact of Mn, one of the authors for the first time, introduced the successive austempering process which improves mechanical properties of 1 wt.% Mn, well compa)
Corresponding author. E-mail: [email protected]
rable with low alloy conventional austempering process and much higher than ASTM values w5,6x. Subsequently, Bayati et al. w7x also applied this process on the same material, and their results confirmed the beneficial effects of successive austempering. The wear behavior of ADI has been studied by some researchers. Nili Ahmadabadi et al. studied wear characteristic of ADI, unalloyed and 3 wt.% P gray cast iron w8x. They observed a higher wear resistance in ADI than the others even if all samples had almost the same hardness. They concluded that higher wear resistance of ADI should be due to the presence of ferritic and retained austenitic Ža carbon enriched austenite which is stable at room temperature. component of bainite. Prado et al. w9x examined the effects of austempering temperature on the dry sliding wear of ADI. They reported that when high loads Ž500 and 800 N. are applied, the best wear resistance is obtained for thermal treatments at 3408C. However, their conclusion is rather in question, since they austempered all of specimens
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 9 . 0 0 1 6 3 - 5
M. Nili Ahmadabadi et al.r Wear 231 (1999) 293–300
294 Table 1 Chemical composition of alloy Žwt.%. C
Si
Mn
Cr
Cu
Mg
3.5
2.73
0.75
0.05
0.18
0.04
for 60 min. It has been well-established that the kinetics of bainitic transformation is a function of austempering temperature w10,11x. Therefore, if one assumes that 60 min is sufficient for completion of austempering at higher austempering temperatures but in other specimens, in particular, specimens austempered at lower temperatures, this austempering period seems to be insufficient. As a consequence of short austempering process at lower austempering temperature some regions may remain untransformed. These regions transform to martensite following cooling to room temperature. The formation of martensite in the matrix will have an important impact on the mechanical properties and wear behavior of ADI. The tribological behavior of austempered spherical graphite cast iron containing aluminum was studied by Boutorabi et al. w12x. They showed that the wear resistance of ADI is a complex function of austempering temperature and time. Inasmuch as successive austempering process extensively improves mechanical properties of ADI as a consequence of formation of duplex structure and higher retained austenite in the matrix w5,6x, it is reasonable to assume that the process should also have some important impacts on the wear behavior of ADI. This work is an attempt to investigate the effect of austempering process and solidification time on the wear behavior of 0.75 wt.% Mn ADI. Special attention is paid to study the effect of successive austempering process on the wear behavior of this alloy.
2. Experimental procedure
Fig. 2. Ultimate tensile strength vs. elongation of higher and lower nodule count specimens with optimum mechanical properties. HNs higher nodule count, LNs lower nodule count.
spheroidization treatment and 1-in. thick Y block castings were poured in green sand molds. Base irons were treated by 5.5% MgFeSi alloy for spheroidization followed by post-inoculation with 75% FeSi. Fig. 1 shows a schematic diagram of the austempering treatment carried out on different specimens. The specimens were austenitized for 90 min in an atmosphere-controlled furnace at 9008C, austempered for various times in a salt bath at 3758C to provide a high austempering temperature ŽHAT process., or at 3158C for a low austempering temperature ŽLAT process., followed by quenching in iced water. In the case of successive austempering ŽHLAT., specimens were first austempered at 3758C, and then austempered at 3158C for different periods, followed by quenching in iced water. To study the effect of solidification time Žnodule count. and austempering variables on the mechanical properties,
High Mn ductile iron of the composition given in Table 1 was prepared. The alloy was produced by a sandwich
Fig. 1. Schematic diagram of different heat treatment processes.
Fig. 3. Optical micrograph of specimen austempered at 3758C for 120 min.
M. Nili Ahmadabadi et al.r Wear 231 (1999) 293–300
295
Fig. 4. The same as Fig. 3 but at higher magnification. F s feather-like bainite, L s lath bainite, Ns nodular bainite.
Fig. 6. Optical micrograph of specimen austempered by successive austempering. LBs lower bainite, M s martensite.
tensile test specimens were prepared from the bottom Žhigher graphite nodule count, 102 nodule graphitermm2 . and the vicinity of riser Žlower graphite nodule count, 65 nodule graphitermm2 .. Tensile tests were carried out on a 10 ton Instron tensile test. The specimens with optimum mechanical properties were used for wear tests. The wafer specimens with optimum mechanical properties obtained from different austempering processes, were ground, polished and etched prior to hardness testing, and optical or electron microscopy. Five hardness measurements were made on each specimen to obtain the average value. Sliding wear tests were performed in a pin-on-ring wear tester with the test materials rubbing under dry atmospheric condition against a surface hardened tool steel ring at speeds of 0.6 and 1.28 m sy1 and loaded with normal loads of 100, 200 and 300 N. Inasmuch as in many ADI application, the combination of optimum properties are required, therefore, the pin specimens with diameter of 5 mm were used with higher and lower nodule count and
optimum mechanical properties obtained from different austempering processes.
Fig. 5. Optical micrograph of specimen austempered at 3158C for 480 min. The morphology of bainite is acicular with a lower density in the intercellular.
3. Results 3.1. Metallography Fig. 2 shows optimum mechanical properties of LAT, HAT and HLAT specimens. This figure shows that mechanical properties of HAT specimens is lower than ASTM standard while mechanical properties of HLAT specimens is higher. Comparison between mechanical properties of HAT and HLAT specimens clearly expresses the drastic impact of successive austempering on the mechanical properties of high Mn ADI. It was shown that this improvement is due to the reduction of UAV Žuntransformed austenite volume, i.e., a low carbon austenite which upon cooling to room temperature transforms to martensite., increase of retained austenite volume fraction and also carbon content of austenite in HLAT specimens w5,6x.
Fig. 7. Retained austenite volume percentage vs. Mn content of specimens austempered by different austempering processes.
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Fig. 8. SEM micrograph of worn surface of HLAT specimen at an applied load of 200 N and a running speed of 0.6 m sy1 .
Figs. 3–6 show the microstructure of three specimens: HAT, LAT and HLAT. Figs. 3 and 4 show the microstructure of a specimen austempered at 3758C for 120 min. In Fig. 4, a large volume of untransformed austenite remains in the intercellular region Žhigh Mn region. and three different bainitic morphology as feather-like bainite, bainitic lath and nodular bainite can be distinguished. The feather-like bainite forms around graphite while nodular bainite mostly forms in the intercellular region, near microporosity. This figure shows that the bainitic lath forms in the regions between nodular graphite and intercellular regions. Formation of bainite with different morphologies is likely due to variation of composition in the matrix as examined by Spanos et al. w13x in steel with different carbon contents. Fig. 5 shows microstructure of a specimen austempered at 3158C for 480 min ŽLAT.. In this figure, bainite has an acicular morphology with a lower density in the intercellular region. On contrast to Figs. 3 and 4, all of the bainitic ferrite show the same morphology except for a decrease in the length of acicular ferrite in the intercellular region.
Fig. 9. SEM micrograph of worn surface of HLAT specimen at an applied load of 300 N and a running speed of 0.6 m sy1 .
Fig. 10. SEM micrograph of worn surface of HLAT specimen at an applied load of 100 N and a running speed of 1.28 m sy1 .
Fig. 6 is micrograph of a specimen austempered at 3758C for 30 min, then quenched to 3158C and kept for 240 min ŽHLAT.. The formation of lower bainite in the intercellular region along with a few plate of martensite can be seen in the figure. A rough examination of Figs. 4–6 shows that HLAT and HAT specimens have higher retained austenite than LAT specimens which is consistent with the results of previous work of one of the authors w14x. As shown in Fig. 7 for 0.75 wt.% Mn, HLAT specimens contain the highest retained austenite followed by HAT and LAT specimens. Figs. 8 and 9 show the worn surfaces of HLAT specimens at various loads and running speeds. Fig. 8 shows a SEM micrograph of the worn surface at an applied load of 200 N and a running speed of 0.6 m sy1 . The figure indicates a moderate rough surface which forms as a consequence of adhesion of some debris on the surface and rubbing track. Fig. 9 shows that an increase of load to 300 N enhances the roughness of surface while some surface
Fig. 11. SEM micrograph of cross-section of worn surface of HLAT specimen at an applied load of 100 N and a running speed of 0.6 m sy1 . The porosities, along with the delaminated layer, clearly show the delamination mechanism in this test.
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Fig. 12. SEM micrograph of cross-section of worn surface of HAT specimen at an applied load of 200 N and a running speed of 0.6 m sy1 . The deformed blocky austenite can be seen. Bs blocky austenite.
regions seem to be cracked. This could supposedly be a delaminated layer initiated from the graphite underneath. An increase of running speed to 1.28 m sy1 at a load of 100 N enhances surface roughness due to severe adhesive mechanism as shown in Fig. 10. Figs. 11 and 12 are SEM micrographs of cross-section of worn surfaces. Fig. 11 shows the surface structure of HLAT under a load of 100 N and Õ s 0.6 m sy1 . In this figure, three different regions can be distinguished: the heavily plastic deformation region in the direction of sliding parallel to the specimen surface where neither ferrite and austenite can be seen Žzone 1., a distorted region where deformed austenite can be seen Žzone 2., and undeformed bulk region Žzone 3.. Since ADI consists of metastable austenite, it is expected that under load during sliding, strain-induced martensite would form in the deformed region, in particular, in zone 1. In the lower deformation region, it is supposed that austenite has higher twin density than strain-induced martensite w15x. In fact, transformation of austenite to martensite depends on carbon content of retained austenite. It is ex-
Fig. 13. SEM micrograph of cross-section of worn surface of HLAT specimen which shows deformation of a graphite nodule.
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Fig. 14. The same as in Fig. 12 but at higher magnification.
pected that austenite with lower carbon content transformed to martensite under lower loads and strain rates than high carbon content austenite. This matter is more probable in HAT specimens which have lower carbon content austenite ŽFig. 12.. Fig. 12 shows blocky austenite with low carbon content, therefore, it is expected that this austenite easily deformed to martensite. Fig. 13 shows deformation of a graphite nodule due to applied load during wear test. A curved broken surface is formed over the graphite which would provide a channel for the feeding of graphite to the surface. It could be expected that the smearing of the graphite on the surface may prevent the metallic contact and reduce the wear rate of material. Fig. 14 shows the same concept as Fig. 13 but at higher magnification. The figure shows that while the
Fig. 15. Weight loss vs. sliding time of different specimens with higher nodule count when F s 200 N and Õ s 0.6 m sy1 . Specific gravity of ductile iron, r ( 7.1 grcm3. A duration of 50 min corresponds to a sliding distance of 1800 m.
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Fig. 16. Weight loss vs. sliding time of different specimens with lower nodule count when F s 200 N and Õ s 0.6 m sy1 . Specific gravity of ductile iron, r ( 7.1 grcm3.
Fig. 18. Weight loss vs. sliding time of different specimens with lower nodule count when F s 300 N and Õ s 0.6 m sy1 . Specific gravity of ductile iron, r ( 7.1 grcm3.
matrix over the graphite channel is heavily deformed and fractured, the region under the channel shows less deformation. In fact, formation of this channel prevents formation of a heavy deformed area in the regions below the graphite channel.
LAT specimens show higher wear resistance. This figure shows that application of successive austempering improves wear resistance of HLAT specimens higher than HAT specimens and close to LAT specimens. Figs. 17 and 18 show the effect of increased applied load from 200 to 300 N on the wear resistance of different specimens. In this case, weight loss also increases in the order of LAT, HAT and HLAT specimens. Figs. 19 and 20 show the effect of running speed, applied load and austempering processes on the weight loss of specimens running for 1000 m. The figures show that increase of applied load when running speed is 0.6 m sy1 increases wear of all specimens. In addition, it is further noticed that increasing the sliding speed for about two-fold would have almost the same effect on the wear as increasing the applied load by the same amount. These
3.2. Wear tests results Figs. 15 and 16 shows weight loss as a function of sliding time for lower and higher nodule count specimens when F s 200 N and Õ s 0.6 m sy1 . In the both cases,
Fig. 17. Weight loss vs. sliding time of different specimens with higher nodule count when F s 300 N and Õ s 0.6 m sy1 . Specific gravity of ductile iron, r ( 7.1 grcm3.
Fig. 19. Weight loss vs. applied load and running speed for different specimens with higher nodule count, run for 1000 m.
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Fig. 20. Weight loss vs. applied load and running speed for different specimens with lower nodule count, run for 1000 m.
results which would follow Archard’s Wear Law could be clearly seen by comparing wear tests at sliding speed of 0.6 m sy1 and a load of 200 N with that of 1.28 m sy1 and 100 N. However, increasing the load from 100 to 200 N at a sliding speed of 0.6 m sy1 would increase the wear about 15-fold suggesting a different wear mechanism operating at an applied load of 100 N.
4. Discussion The mechanical and wear test results show that the successive austempering process improves both mechanical properties and wear resistance of ADI in comparison with conventional austempering process. It has been concluded by some researchers that in conventional ADI, an increase of hardness and decrease of retained austenite increase wear resistance w8,16x. However, by considering results of wear resistance of HLAT and HAT specimens achieved in this work, a more precise investigation is necessary. It was shown by one of the authors w5,6x that the successive austempering process increases retained austenite and its carbon content in comparison with conventional austempering process ŽHAT and LAT processes.. In addition, it is more informative if one compares hardness of LAT, HLAT and HAT specimens. Fig. 21 shows variation of hardness of different specimens as a function of austempering time. It can be seen that LAT specimens have the highest hardness while HLAT and HAT specimens have an almost the same hardness. The highest hardness of LAT specimens is due to its lower retained austenite volume percentage, acicular ferrite with higher carbon content and dislocation density compared to HAT specimens containing feathery ferrite w17,18x, and precipitation of carbide in the ferrite. Hardness in HAT specimens is a combination of hardness of upper bainite and UAV while hardness in HLAT, is controlled by hardness of upper bainite and
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lower bainite. Therefore, replacement of UAV by lower bainite keeps hardness of these samples in the same level. Fig. 21 also shows that hardness of LAT specimens with the best process combination is about 33% higher than HLAT and HAT specimens. However, Figs. 19 and 20 indicate that average wear resistance of HLAT specimens in all experimental running speed and applied load except for lowest applied load Ž F s 100 N., is about 80% of wear loss of HAT specimens and about 110% of LAT specimens. Keeping in mind HLAT hardness and its highest retained austenite volume fraction, it can be concluded that in ADI hardness and retained austenite volume fraction are not the only controlling factors accounting for wear resistance but also other factors like morphology of ferrite, UAV and carbon content of retained austenite should be taken into account. In fact, austenite with lower carbon contents transforms to martensite with lower hardness under loads than high carbon content austenite. This matter is more probable in HAT specimens which have lower carbon content austenite than HLAT specimens. It is assumed that these low carbon blocky austenite in HAT specimens as shown in Fig. 12, should be one of the reasons for the lower wear resistance of HAT specimens in comparison with HLAT specimens. By examination of Fig. 11, wear mechanism in this work can be investigated. This figure shows some porosities and voids near the worn surface and in some regions below it. These porosities which form during sliding are the main cause of the observed delaminated layer, suggesting delamination mechanism as a dominant wear mechanism in these tests. As mentioned before, the matrix over the graphite channel in Figs. 13 and 14 is heavily deformed. It could be expected that depletion of graphite out of the channel and increasing the load would accelerate
Fig. 21. Hardness of different specimens vs. austempering time.
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fracture of the material over the channel, and therefore, increasing the wear of material. It seems that the toughness of the matrix and nodule count play an important role on the fracture of the material over the channel. However, the specimens for Figs. 13 and 14 have about the same nodule count, but different microstructure which gives higher toughness for HLAT than other specimens ŽFig. 2.. This higher toughness could be one of the reasons resulting in improvement of wear of HLAT specimen than HAT specimens and closer to LAT specimens. Higher wear resistance of LAT specimens could be due to their higher hardness. It is speculated that at the lower load and speed of 100 N and 0.6 m sy1 , the graphite layer acts as a solid lubricant with sufficient load carrying capacity. This would explain the much lower wear Ž15 times lower. of specimens at these load and speed compared to higher loads andror speeds shown in Figs. 19 and 20. Figs. 15–20 show that the specimens with lower nodule count have higher wear resistance than specimens with higher nodule count. As shown in Figs. 9 and 13, the delaminated regions over graphites are one of the source of wear debris formation. Therefore, it is likely that reduction of nodule count decreases the probable regions for initiation of delamination layer which leads to a reduction in wear debris. The other point that should also be taken into account is an increase of segregation of Mn in the intercellular region as nodule count decreases. The heavier segregation of Mn in the intercellular regions increases the UAV which upon cooling to room temperature transforms to martensite w19x. The martensite with higher hardness can increase wear resistance of ADI.
3. The specimens with lower nodule count Žlonger solidification time. have lower wear rate than specimens with higher nodule count Žshorter solidification time.. 4. High carbon content retained austenite along with good mechanical properties are supposedly the main reason for improvement of wear resistance of HLAT specimens.
Acknowledgements The authors would like to express their sincere thanks to the research council of University of Tehran for financial support of this work.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x
5. Conclusions w13x
Sliding wear tests on specimens with optimum mechanical properties austempered by different processes lead to the following conclusions: 1. The results of this study suggesting delamination mechanism as a dominant wear mechanism. 2. The mechanical and wear test results show that successive austempering process improves both mechanical properties and wear resistance of ADI in comparison with conventional austempering process.
w14x w15x w16x w17x w18x w19x
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