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Investigation of mechanical properties of boro-tempered ductile iron
Materials and Design 31 (2010) 1799–1803
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Investigation of mechanical properties of boro-tempered ductile iron Yusuf Kayali, Sukru Taktak, Sinan Ulu, Yilmaz Yalcin * Afyonkarahisar Kocatepe University, Technical Education Faculty, Department of Metal Education, Afyonkarahisar 03200, Turkey
a r t i c l e
i n f o
Article history: Received 10 September 2009 Accepted 7 November 2009 Available online 12 November 2009 Keywords: Ductile iron Boro-tempering Micro-hardness Mechanical properties
a b s t r a c t In this study, the effects of boro-tempering heat treatment on mechanical properties of ductile iron were investigated. Standard tensile test samples and unnotched Charpy specimens were boronized at 900 °C for 1, 3 and 5 h and then tempered at four different temperatures (250, 300, 350 and 450 °C) for 1 h. Micro-hardness measurements were performed on cross-section of the metallographically prepared samples, where cut from fractured impact test samples. The hardness of boride layers was measured in the range of 1654–1867 HV0.05. It was observed that tempering temperature was more effective on the mechanical properties of the material than boronizing time. Optimum mechanical properties were obtained for the samples boronized for 1–3 h and then tempered between 250 and 350 °C for 1 h. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Austempered ductile iron (ADI) has attracted attention as a substitute for forged steel components in several structural applications such as, automotive, agricultural, railroad, construction, defense, etc. [1–5]. The combination of high levels of strength with good ductility, good fracture toughness, high fatigue strength and wear resistance make ADI attractive for these applications [6–8]. Furthermore, it has the advantage of design and manufacturing flexibility, lower material cost, lower production cost, lower density, better machinability and higher damping capacity than steels [2,9]. Up to now, numerous studies have been performed on microstructural and mechanical properties of the material [3,10–14]. New studies have been focused to improve its present properties, especially surface properties. It has been well known that the mechanical properties of engineering materials largely depend on their surface conditions. Surface conditions can be modified by means of case hardening or coating [15]. Unfortunately, case hardening techniques such as, carburizing, nitriding, carbonitriding, cyaniding, chromizing, boronizing and chemical vapor deposition (CVD) are not available to apply austempered ductile irons without damaging of original ausferritic matrix. On the other hand, modern surface engineering techniques which are applied at low-temperatures can be used to protect original ausferritic matrix after austempering heat treatment [16–19]. However these techniques necessitate at least two-stepped operations which are austempering and surface modification or coating.
* Corresponding author. Tel.: +90 272 228 13 11; fax: +90 272 228 13 19. E-mail address: [email protected] (Y. Yalcin). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.11.017
Austempering heat treatment caused ductile iron to gain superior properties such as high strength, fracture toughness and fatigue resistance. Also high surface hardness (between 1450 and 2500 HV), excellent wear resistance, and corrosion resistance were achieved by boronizing heat treatment [20,21]. Boro-tempering combines these two heat treatments, i.e. austempering together with boronizing. Then it may be possible to have both matrix with high strength and fracture toughness and surface with superior hardness and wear resistance in ductile iron. This heat treatment can contribute to improved protection of ductile iron surfaces. In the literature there are many works focusing on austempering and boronizing of ductile cast iron, separately. In most of these studies, a hard boride layer was developed over the alloy having ferritic and/or pearlitic and/or austenitic microstructure. However, there is little literature about combined austempering and boronizing, i.e. boro-tempering process. In a previous study [22] boro-tempering heat treatment of ductile iron, which combined boronizing with austempering, has been applied to cast ductile iron. This study showed that boro-tempering heat treatment could be successfully applied to ductile iron and it provided a high surface hardness of 1600 HV0.1. Baydogan and Akray [23] have carried out boronizing with austempering heat treatment for GGG-40 grade ductile iron to combine the advantages of both processes in a single treatment. They reported that combined austempering and boronizing procedure exhibited considerably higher wear resistance than conventional boronizing having a subsurface matrix structure consisting of ferrite and pearlite. Unfortunately, there is not any knowledge about mechanical properties of boro-tempered ductile iron (BDI). Therefore, the purpose of this study is to investigate mechanical properties of ductile iron which was boro-tempered under several heat treatment conditions.
1800
Y. Kayali et al. / Materials and Design 31 (2010) 1799–1803
Table 1 The chemical composition (wt.%) of ductile iron. C
Si
S
Mo
Cr
Cu
Sn
Mn
Ti
Mg
P
3.840
2.273
0.010
0.001
0.021
0.044
0.005
0.141
0.020
0.039
0.048
Fig. 1. Schematic diagram of applied boro-tempered heat treatment processes.
2. Experimental procedures 2.1. Ductile iron materials and test samples The experimental ductile iron materials were poured into standard Y-block casting. Microstructure of the as-cast ductile iron consists of ferritic matrix, few pearlite and graphite nodules which had a good nodularity. The bulk composition of this ductile cast iron is provided in Table 1. Unnotched Charpy test specimens with dimensions of 10 10 55 mm and tensile test samples with 8 mm gauge diameter and 40 mm gauge length were machined from the leg of the Y-blocks. 2.2. Boro-tempering heat treatment Tensile test and Unnotched Charpy specimens were subjected to boro-tempering heat treatment which combines boronizing with tempering. Boronizing was performed in a solid medium containing commercial EkaborÒ2 powder under atmospheric pressure at a temperature of 900 °C for 1, 3 and 5 h. This was followed by quenching in a molten salt bath at one of temperatures 250 °C, 300 °C 350 °C or 450 °C for 1 h and finally, air-cooled to room temperature. All of the heat treatments are summarized in Fig. 1. 2.3. Mechanical tests The hardness of the boride layers and substrates was measured on the cross-sections using a Shimadzu HMV-2 Vickers indenter with a 50 g load. The tensile tests were performed on a 100 kN hydraulic Instron 8801 universal testing machine with a constant cross-head speed of 1.0 mm min 1. The unnotched Charpy impact tests were carried out by a PSd 300/150-1 impact test device. For each heat treatment condition, at least three tensile and three impact specimens were tested to ensure the reproducibility of the test data. The fracture surface characteristics of the tensile and impact test specimens were investigated by using a Leo 1430VP scanning electron microscope. 3. Results 3.1. Micro-hardness of boro-tempered ductile iron Fig. 2 shows the optical micrographs of the cross-sectioned ductile cast iron samples which were boronized at 900 °C for 3 h and
Fig. 2. The optical micrographs of the cross-sectioned ductile cast iron samples boronized at 900 °C for 3 h and then tempered for 1 h: (a) at 250 °C, (b) 300 °C and (c) 450 °C.
then tempered at 250, 300 and 450 °C for 1 h. Fig. 2 a and b clearly showed a boride layer having a tooth-shaped morphology and the layer/matrix interface was seen to be rather smooth. As seen in Fig. 2a-c, ductile iron samples had different matrix structure with lower ausferrite at 250 °C, upper ausferrite at 300 °C and fully pearlitic matrix structure at 450 °C underneath the boride layer. Fig. 3 shows the micro-hardness variation of the boro-tempered ductile iron from the surface to the interior. Surface hardness
Y. Kayali et al. / Materials and Design 31 (2010) 1799–1803
2000
a
Miicrohardness (HV0.05)
1800 1600 1400 1200
250 300 350 450
1000 800
°C °C °C °C
600 400 200 0
0
20
40
60
80
100
120
140
Distance from the surface (µm) 2000
b
Microhardness (HV0.05)
1800 1600 1400 1200
250 300 350 450
1000 800
°C °C °C °C
400 200 0
20
40
60
80
100
120
140
Distance from the surface (µm) 2000
c
Microhardness (HV0.05)
1800 1600 1400 1200 1000 800
250 300 350 450
600 400 200
°C °C °C °C
0 0
20
40
60
80
100
120
process, various phases having different hardness formed from surface to interior. It was observed that boronized metals have three distinct regions which are: (i) layers having borides (i.e. FeB and Fe2B), (ii) the region below boride layers, where boron makes solid solution, which has hardness less than that of borides, and (iii) matrix, which is not affected by boron [22]. The hardness of the matrix was affected by tempering temperature rather than boronizing time. Fig. 3 showed that the highest matrix hardness was obtained for tempering at 250 °C due to lower ausferrite matrix structure and measured hardness values ranged from 370 to 415 HV. At temperatures above 250 °C, matrix hardness was lower than that of 250 °C on account of the fact that upper ausferritic matrix formed. In fact, at traditional austempering treatment, the lower ausferrite with high hardness, forms at temperature of 300 °C. However, previous study [22] indicated that upper ausferritic matrix structure formed instead of lower ausferritic matrix structure because of low austemperability at 300 °C in the boro-tempering treatment. On the other hand, at 450 °C, fully pearlitic matrix structure formed. Consequently, in the boro-tempering treatment, the temperatures of lower and upper ausferrite were lower than that of traditional austempering treatment. The results presented in this work suggest that tempering temperature would be selected in the range of 250–350 °C. 3.2. Tensile test results
600
0
1801
140
Distance from the surface (µm) Fig. 3. The micro-hardness variation of the boro-tempered ductile iron from the surface to interior at different tempering temperatures after boronizing at 900 °C for: (a) 1 h, (b) 3 h and (c) 5 h.
values were taken at 15 lm depth underneath the surface and the hardness values of boride layers vary between 1654 and 1867 HV0.05. It has been reported that hardness values of Fe2B and FeB are 1800–2000 HV and 1900–2100 HV, respectively [20]. Therefore, the results of hardness measurement in the present work suggest that the major constituent of the surface layer should be Fe2B. Fig. 4 shows the X-ray diffraction patterns of the ductile cast irons boronized at 900 °C for 1–5 h and then tempered at 300 °C for 1 h. As can be seen in Fig. 4a and b, the major constituent of the compound layers is the Fe2B phase. As seen in Fig. 3, the micro-hardness decreased from the surface to interior and remained almost unchanged at definite distance. Owing to the fact that boronizing is thermo-chemical diffusion
The effects of boro-tempering conditions on tensile strength, 0.2% yield strength, elongation and impact energy are given in Fig. 5. As seen in Fig. 5a and b, the tensile strength and 0.2% yield strength had same trend. They sharply decreased with increasing tempering temperature and then nearly remained unchanged. It is well known that matrix structure directly affects the strength of materials. The lower ausferrite that forms at lower temperatures has high strength, high hardness, low elongation and low impact resistance. However, the upper ausferrite structure that forms at higher temperatures has low strength, low hardness, high ductility and high toughness compared with the lower ausferrite. As seen in Fig. 5a, high strength was obtained at 250 °C and then the strength sharply decreased at 300 °C. This could be attributed to formation of upper ausferrite matrix structure at 300 °C. Although the matrix had upper ausferrite structure at the temperatures of 300–350 °C, the strength at 350 °C was lower than that of 300 °C because of coarser ausferrite structure. Pearlitic matrix structure forms instead of ausferritic matrix structure at tempering temperature of 450 °C (Fig. 2c). However, aim of the boro-tempering treatment is to obtain ausferritic matrix structure. At the boro-tempering conditions, which were achieved ausferritic structure, for borotempering samples, maximum and minimum tensile strengths were obtained as 1547 MPa and 795 MPa, respectively. Similarly, for 0.2% yield strength, maximum and minimum values were 1402 and 590 MPa, respectively. Fig. 5a and b showed that boronizing time also affected tensile and 0.2% yield strengths, but it is not as effective as tempering temperature. As the boronizing time increased, both tensile strength and 0.2% yield strength decreased at 250 °C. As shown in Fig. 5c, elongation increased as the tempering temperature increased to 300 °C, thereafter decreased. Maximum elongation was obtained as 11.3% for boronizing of 1 h followed by tempering at 300 °C. However, boronizing for 3 h caused to a lower elongation (8%). Increasing boronizing time decreased elongation at temperatures of 250–450 °C. 3.3. Impact test results As seen in Fig. 5d, impact energy and elongation (Fig. 5c) graphs had similar trend. Like elongation, maximum impact energy values
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Fig. 4. The XRD patterns of ductile cast iron samples tempered at 300 °C for 1 h followed by boronizing at 900 °C for: (a) 1 h and (b) 5 h.
12
a
1400
10
1200
8
Elongation (%)
Tensile strength (MPa)
1600
1000 800
1h 3h 5h
600 400 200
250
300
c
6 4
1h 3h 5h
2
350
400
450
0 200
500
250
1600
400
450
500
d
1400
100
Impact energy (J)
0.2% yield strength (MPa)
350
120
b
1200
1h 3h 5h
1000 800 600 400 200
300
Tempering temperature (ºC)
Tempering temperature (ºC)
80 60 40
1h 3h 5h
20
250
300
350
400
450
500
0 200
250
300
350
400
450
500
Tempering temperature (ºC)
Tempering temperature (ºC)
Fig. 5. The effects of tempering temperature for different boronizing time on: (a) tensile strength, (b) 0.2% yield strength, (c) elongation and (d) impact energy of ductile iron.
for all boronizing times were obtained from tempering at 300 °C. The highest impact energy was achieved as 111 J at 300 °C for 1 h boronizing. It was noted that for all tempering temperatures, 1 h boronizing time ensured better impact resistance, whereas impact resistance decreased to about 10 J at tempering temperatures of 350–450 °C for 5 h boronizing time.
4. Discussion It is evident that there are considerable differences between boro-tempering and austempering, when the results of this study are compared with the results of conventional austempering. While the mechanical properties obtained by austempering can
Y. Kayali et al. / Materials and Design 31 (2010) 1799–1803
also be achieved by boro-tempering treatment, it additionally provides much higher surface hardness values up to 1800 HV [24]. There is no negative effect of the boride layer on the tensile and yield strengths. Furthermore, it can be said that at 250 °C the mechanical properties of boro-tempered ductile iron is better than austempered ductile iron. It is reported that generally classical austempering process gives maximum ductility and toughness at 350– 375 °C temperatures [8,25]. However, low strength and high toughness and ductility obtained at 300 °C in this study may be due to decreasing formation temperatures of lower and upper ausferrite. This lower formation temperatures can be attributed to the fact that boride layers of lower thermal conductivity compared to ductile iron reduces austemperability [22]. There seems to be an optimum boride layer thickness or matrix hardness for the conservation of good mechanical properties of ductile iron. Otherwise much thicker boride layer or lower matrix hardness would result in a dramatic drop in impact resistance. Based on the experimental results in this study it can be said that a boro-tempering treatment up to 3 h at temperatures in the range of 250–350 °C would be useful to obtain mechanical properties comparable to austempering with additional surface hardness about four or five times better.
5. Conclusions Mechanical properties of boro-tempered ductile iron were investigated in this study. The following conclusions can be drawn from the results. The boride layer is formed by boro-tempering heat treatment on the ductile iron and its micro-hardness is in the range of 1654– 1867 HV. Depending on the boronizing time, micro-hardness decreases from surface through interior and reaches the matrix hardness beneath 50–80 lm of surface. The hardness of the matrix was affected by tempering temperature rather than boronizing time. Also the hardness of the matrix is in the range of 295–416 HV depending on tempering temperature. The strength sharply decreases with increasing tempering temperature and then remains unchanged. At 250 °C tempering temperature, maximum tensile and 0.2% yield strengths are obtained as 1547 and 1432 MPa, respectively. However, it is ascertained that increasing boronizing time decreases the strength. The graphs of the impact energy and elongation show similar trend depending on boro-tempering conditions. Generally, the impact energy and elongation increase up to tempering temperature of 300 °C, thereafter decrease. At temperatures above 300 °C, boronizing for 5 h extremely lowered the impact energy. The boride layers, which are formed by boro-tempering on the ductile iron, decrease the austemperability. Optimum mechanical properties are obtained for the samples boronized for 1–3 h and then tempered between 250 and 350 °C for 1 h.
1803
Acknowledgements The authors gratefully acknowledge the Scientific and Technical Research Council of Turkey (TUBITAK) for financial support with the Grant Number of 104M398. The authors also thank to Scientific Research Project Council of Afyon Kocatepe University.
References [1] Fuller AG. Austempered ductile irons – present applications. Mater Des 1985;6:127–30. [2] Harding RA. The production, properties and automotive applications of austempered ductile iron. Kovove Mater 2007;45:1. [3] Putatunda SK. Development of austempered ductile cast iron (ADI) with simultaneous high yield strength and fracture toughness by a novel two-step austempering process. Mater Sci Eng 2001;A315:70–80. [4] Krohn BR. Austempered ductile iron emerging as a structural material. Modern Casting; 1984. p. 26. [5] Hayrynen KL. ADI: another avenue for ductile iron foundries. Modern Casting; 1995. p. 35–7. [6] Kovacs BV. Development of austempered ductile iron (ADI) for automobsßle crankshafts. J Heat Treat 1987;5(1):55. [7] Rao PP, Putatunda SK. Investigations on the fracture toughness of austempered ductile iron alloyed with chromium. Mater Sci Eng 2003;A346:254–65. [8] Blackmore PA, Harding RA. The effect of metallurgical process variables on the properties of austempered ductile iron. J Heat Treat 1984;3:310–25. [9] Harding RA. Opening up the market for ADI. The Foundryman; 1993. p. 197– 208. [10] Gudlanch RB, Janowak JF. Development of a ductile iron for commercial austempering. AFS Trans 1983;94:377. [11] Harding RA, Gilbert GNJ. Why the properties of ductile irons should interest engineers. Brit Foundryman 1986;79:489. [12] Yang J, Putatunda SK. Effect of microstructure on abrasion wear behavior of austempered ductile cast iron (ADI) processed by a novel two-step austempering process. Mater Sci Eng A 2005;406:217–28. [13] Putatunda SK, Bartosiewicz L, Krause AR, Singh I, Alberts FA. Influence of microstructure on high-cycle fatigue behavior of austempered ductile cast iron. Mater Char 1993;30:221–34. [14] Hughes ICH. Austempered ductile irons their properties and significance. Mater Des 1985;6:124–6. [15] El-Bahloul AMM. Surface capacity of gears of circular – arc tooth-profile. Wear 1996;93:146–54. [16] Roy A, Mana I. Laser surface engineering to improve wear resistance of austempered ductile iron. Mater Sci Eng 2001;A297:85–93. [17] Pepe A, Galliano P, Cere S, Aparicio M, Duran A. Hybrid silica sol–gel coatings on austempered ductile iron (ADI). Mater Lett 2005;59:2219. [18] Hsu CH, Lu JK, Tsai RJ. Effects of low-temperature coating process on mechanical behaviors of ADI. Mater Sci Eng 2005;A398:282–90. [19] Hsu CH, Lu JK, Tsai RJ. Characteristics of duplex surface coatings on austempered ductile iron substrates. Surf Coat Tech 2006;200:5725–32. [20] Sinha AK. Boriding (boronizing). ASM international handbook, vol. 4. Materials Park (OH, USA): The Materials International Society; 1991. [21] Bindal C, Erdemir A. Ultralow friction behavior of borided steel surfaces after flash annealing. Appl Phys Lett 1996;68(7):923–5. [22] Yalcin Y, Yazici AM. The effect of boro-tempering heat treatment on the properties of ductile cast iron. Kovove Mater 2007;45:51. [23] Baydogan M, Akray SI. Successive boronizing and austempering for GGG-40 grade ductile iron. J Iron Steel Res Int 2009;16(2):50–4. [24] A897M-90, Standard specification for austempered ductile iron castings [Metric];1990. p. 506 [25] Darwish N, Elliott R. Austempering of low manganese ductile irons. Part 3 Variation of mechanical properties with heat treatment conditions. Mater Sci Tech 1993;9:882–9.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Investigation of mechanical properties of boro-tempered ductile iron Yusuf Kayali, Sukru Taktak, Sinan Ulu, Yilmaz Yalcin * Afyonkarahisar Kocatepe University, Technical Education Faculty, Department of Metal Education, Afyonkarahisar 03200, Turkey
a r t i c l e
i n f o
Article history: Received 10 September 2009 Accepted 7 November 2009 Available online 12 November 2009 Keywords: Ductile iron Boro-tempering Micro-hardness Mechanical properties
a b s t r a c t In this study, the effects of boro-tempering heat treatment on mechanical properties of ductile iron were investigated. Standard tensile test samples and unnotched Charpy specimens were boronized at 900 °C for 1, 3 and 5 h and then tempered at four different temperatures (250, 300, 350 and 450 °C) for 1 h. Micro-hardness measurements were performed on cross-section of the metallographically prepared samples, where cut from fractured impact test samples. The hardness of boride layers was measured in the range of 1654–1867 HV0.05. It was observed that tempering temperature was more effective on the mechanical properties of the material than boronizing time. Optimum mechanical properties were obtained for the samples boronized for 1–3 h and then tempered between 250 and 350 °C for 1 h. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Austempered ductile iron (ADI) has attracted attention as a substitute for forged steel components in several structural applications such as, automotive, agricultural, railroad, construction, defense, etc. [1–5]. The combination of high levels of strength with good ductility, good fracture toughness, high fatigue strength and wear resistance make ADI attractive for these applications [6–8]. Furthermore, it has the advantage of design and manufacturing flexibility, lower material cost, lower production cost, lower density, better machinability and higher damping capacity than steels [2,9]. Up to now, numerous studies have been performed on microstructural and mechanical properties of the material [3,10–14]. New studies have been focused to improve its present properties, especially surface properties. It has been well known that the mechanical properties of engineering materials largely depend on their surface conditions. Surface conditions can be modified by means of case hardening or coating [15]. Unfortunately, case hardening techniques such as, carburizing, nitriding, carbonitriding, cyaniding, chromizing, boronizing and chemical vapor deposition (CVD) are not available to apply austempered ductile irons without damaging of original ausferritic matrix. On the other hand, modern surface engineering techniques which are applied at low-temperatures can be used to protect original ausferritic matrix after austempering heat treatment [16–19]. However these techniques necessitate at least two-stepped operations which are austempering and surface modification or coating.
* Corresponding author. Tel.: +90 272 228 13 11; fax: +90 272 228 13 19. E-mail address: [email protected] (Y. Yalcin). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.11.017
Austempering heat treatment caused ductile iron to gain superior properties such as high strength, fracture toughness and fatigue resistance. Also high surface hardness (between 1450 and 2500 HV), excellent wear resistance, and corrosion resistance were achieved by boronizing heat treatment [20,21]. Boro-tempering combines these two heat treatments, i.e. austempering together with boronizing. Then it may be possible to have both matrix with high strength and fracture toughness and surface with superior hardness and wear resistance in ductile iron. This heat treatment can contribute to improved protection of ductile iron surfaces. In the literature there are many works focusing on austempering and boronizing of ductile cast iron, separately. In most of these studies, a hard boride layer was developed over the alloy having ferritic and/or pearlitic and/or austenitic microstructure. However, there is little literature about combined austempering and boronizing, i.e. boro-tempering process. In a previous study [22] boro-tempering heat treatment of ductile iron, which combined boronizing with austempering, has been applied to cast ductile iron. This study showed that boro-tempering heat treatment could be successfully applied to ductile iron and it provided a high surface hardness of 1600 HV0.1. Baydogan and Akray [23] have carried out boronizing with austempering heat treatment for GGG-40 grade ductile iron to combine the advantages of both processes in a single treatment. They reported that combined austempering and boronizing procedure exhibited considerably higher wear resistance than conventional boronizing having a subsurface matrix structure consisting of ferrite and pearlite. Unfortunately, there is not any knowledge about mechanical properties of boro-tempered ductile iron (BDI). Therefore, the purpose of this study is to investigate mechanical properties of ductile iron which was boro-tempered under several heat treatment conditions.
1800
Y. Kayali et al. / Materials and Design 31 (2010) 1799–1803
Table 1 The chemical composition (wt.%) of ductile iron. C
Si
S
Mo
Cr
Cu
Sn
Mn
Ti
Mg
P
3.840
2.273
0.010
0.001
0.021
0.044
0.005
0.141
0.020
0.039
0.048
Fig. 1. Schematic diagram of applied boro-tempered heat treatment processes.
2. Experimental procedures 2.1. Ductile iron materials and test samples The experimental ductile iron materials were poured into standard Y-block casting. Microstructure of the as-cast ductile iron consists of ferritic matrix, few pearlite and graphite nodules which had a good nodularity. The bulk composition of this ductile cast iron is provided in Table 1. Unnotched Charpy test specimens with dimensions of 10 10 55 mm and tensile test samples with 8 mm gauge diameter and 40 mm gauge length were machined from the leg of the Y-blocks. 2.2. Boro-tempering heat treatment Tensile test and Unnotched Charpy specimens were subjected to boro-tempering heat treatment which combines boronizing with tempering. Boronizing was performed in a solid medium containing commercial EkaborÒ2 powder under atmospheric pressure at a temperature of 900 °C for 1, 3 and 5 h. This was followed by quenching in a molten salt bath at one of temperatures 250 °C, 300 °C 350 °C or 450 °C for 1 h and finally, air-cooled to room temperature. All of the heat treatments are summarized in Fig. 1. 2.3. Mechanical tests The hardness of the boride layers and substrates was measured on the cross-sections using a Shimadzu HMV-2 Vickers indenter with a 50 g load. The tensile tests were performed on a 100 kN hydraulic Instron 8801 universal testing machine with a constant cross-head speed of 1.0 mm min 1. The unnotched Charpy impact tests were carried out by a PSd 300/150-1 impact test device. For each heat treatment condition, at least three tensile and three impact specimens were tested to ensure the reproducibility of the test data. The fracture surface characteristics of the tensile and impact test specimens were investigated by using a Leo 1430VP scanning electron microscope. 3. Results 3.1. Micro-hardness of boro-tempered ductile iron Fig. 2 shows the optical micrographs of the cross-sectioned ductile cast iron samples which were boronized at 900 °C for 3 h and
Fig. 2. The optical micrographs of the cross-sectioned ductile cast iron samples boronized at 900 °C for 3 h and then tempered for 1 h: (a) at 250 °C, (b) 300 °C and (c) 450 °C.
then tempered at 250, 300 and 450 °C for 1 h. Fig. 2 a and b clearly showed a boride layer having a tooth-shaped morphology and the layer/matrix interface was seen to be rather smooth. As seen in Fig. 2a-c, ductile iron samples had different matrix structure with lower ausferrite at 250 °C, upper ausferrite at 300 °C and fully pearlitic matrix structure at 450 °C underneath the boride layer. Fig. 3 shows the micro-hardness variation of the boro-tempered ductile iron from the surface to the interior. Surface hardness
Y. Kayali et al. / Materials and Design 31 (2010) 1799–1803
2000
a
Miicrohardness (HV0.05)
1800 1600 1400 1200
250 300 350 450
1000 800
°C °C °C °C
600 400 200 0
0
20
40
60
80
100
120
140
Distance from the surface (µm) 2000
b
Microhardness (HV0.05)
1800 1600 1400 1200
250 300 350 450
1000 800
°C °C °C °C
400 200 0
20
40
60
80
100
120
140
Distance from the surface (µm) 2000
c
Microhardness (HV0.05)
1800 1600 1400 1200 1000 800
250 300 350 450
600 400 200
°C °C °C °C
0 0
20
40
60
80
100
120
process, various phases having different hardness formed from surface to interior. It was observed that boronized metals have three distinct regions which are: (i) layers having borides (i.e. FeB and Fe2B), (ii) the region below boride layers, where boron makes solid solution, which has hardness less than that of borides, and (iii) matrix, which is not affected by boron [22]. The hardness of the matrix was affected by tempering temperature rather than boronizing time. Fig. 3 showed that the highest matrix hardness was obtained for tempering at 250 °C due to lower ausferrite matrix structure and measured hardness values ranged from 370 to 415 HV. At temperatures above 250 °C, matrix hardness was lower than that of 250 °C on account of the fact that upper ausferritic matrix formed. In fact, at traditional austempering treatment, the lower ausferrite with high hardness, forms at temperature of 300 °C. However, previous study [22] indicated that upper ausferritic matrix structure formed instead of lower ausferritic matrix structure because of low austemperability at 300 °C in the boro-tempering treatment. On the other hand, at 450 °C, fully pearlitic matrix structure formed. Consequently, in the boro-tempering treatment, the temperatures of lower and upper ausferrite were lower than that of traditional austempering treatment. The results presented in this work suggest that tempering temperature would be selected in the range of 250–350 °C. 3.2. Tensile test results
600
0
1801
140
Distance from the surface (µm) Fig. 3. The micro-hardness variation of the boro-tempered ductile iron from the surface to interior at different tempering temperatures after boronizing at 900 °C for: (a) 1 h, (b) 3 h and (c) 5 h.
values were taken at 15 lm depth underneath the surface and the hardness values of boride layers vary between 1654 and 1867 HV0.05. It has been reported that hardness values of Fe2B and FeB are 1800–2000 HV and 1900–2100 HV, respectively [20]. Therefore, the results of hardness measurement in the present work suggest that the major constituent of the surface layer should be Fe2B. Fig. 4 shows the X-ray diffraction patterns of the ductile cast irons boronized at 900 °C for 1–5 h and then tempered at 300 °C for 1 h. As can be seen in Fig. 4a and b, the major constituent of the compound layers is the Fe2B phase. As seen in Fig. 3, the micro-hardness decreased from the surface to interior and remained almost unchanged at definite distance. Owing to the fact that boronizing is thermo-chemical diffusion
The effects of boro-tempering conditions on tensile strength, 0.2% yield strength, elongation and impact energy are given in Fig. 5. As seen in Fig. 5a and b, the tensile strength and 0.2% yield strength had same trend. They sharply decreased with increasing tempering temperature and then nearly remained unchanged. It is well known that matrix structure directly affects the strength of materials. The lower ausferrite that forms at lower temperatures has high strength, high hardness, low elongation and low impact resistance. However, the upper ausferrite structure that forms at higher temperatures has low strength, low hardness, high ductility and high toughness compared with the lower ausferrite. As seen in Fig. 5a, high strength was obtained at 250 °C and then the strength sharply decreased at 300 °C. This could be attributed to formation of upper ausferrite matrix structure at 300 °C. Although the matrix had upper ausferrite structure at the temperatures of 300–350 °C, the strength at 350 °C was lower than that of 300 °C because of coarser ausferrite structure. Pearlitic matrix structure forms instead of ausferritic matrix structure at tempering temperature of 450 °C (Fig. 2c). However, aim of the boro-tempering treatment is to obtain ausferritic matrix structure. At the boro-tempering conditions, which were achieved ausferritic structure, for borotempering samples, maximum and minimum tensile strengths were obtained as 1547 MPa and 795 MPa, respectively. Similarly, for 0.2% yield strength, maximum and minimum values were 1402 and 590 MPa, respectively. Fig. 5a and b showed that boronizing time also affected tensile and 0.2% yield strengths, but it is not as effective as tempering temperature. As the boronizing time increased, both tensile strength and 0.2% yield strength decreased at 250 °C. As shown in Fig. 5c, elongation increased as the tempering temperature increased to 300 °C, thereafter decreased. Maximum elongation was obtained as 11.3% for boronizing of 1 h followed by tempering at 300 °C. However, boronizing for 3 h caused to a lower elongation (8%). Increasing boronizing time decreased elongation at temperatures of 250–450 °C. 3.3. Impact test results As seen in Fig. 5d, impact energy and elongation (Fig. 5c) graphs had similar trend. Like elongation, maximum impact energy values
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Fig. 4. The XRD patterns of ductile cast iron samples tempered at 300 °C for 1 h followed by boronizing at 900 °C for: (a) 1 h and (b) 5 h.
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Fig. 5. The effects of tempering temperature for different boronizing time on: (a) tensile strength, (b) 0.2% yield strength, (c) elongation and (d) impact energy of ductile iron.
for all boronizing times were obtained from tempering at 300 °C. The highest impact energy was achieved as 111 J at 300 °C for 1 h boronizing. It was noted that for all tempering temperatures, 1 h boronizing time ensured better impact resistance, whereas impact resistance decreased to about 10 J at tempering temperatures of 350–450 °C for 5 h boronizing time.
4. Discussion It is evident that there are considerable differences between boro-tempering and austempering, when the results of this study are compared with the results of conventional austempering. While the mechanical properties obtained by austempering can
Y. Kayali et al. / Materials and Design 31 (2010) 1799–1803
also be achieved by boro-tempering treatment, it additionally provides much higher surface hardness values up to 1800 HV [24]. There is no negative effect of the boride layer on the tensile and yield strengths. Furthermore, it can be said that at 250 °C the mechanical properties of boro-tempered ductile iron is better than austempered ductile iron. It is reported that generally classical austempering process gives maximum ductility and toughness at 350– 375 °C temperatures [8,25]. However, low strength and high toughness and ductility obtained at 300 °C in this study may be due to decreasing formation temperatures of lower and upper ausferrite. This lower formation temperatures can be attributed to the fact that boride layers of lower thermal conductivity compared to ductile iron reduces austemperability [22]. There seems to be an optimum boride layer thickness or matrix hardness for the conservation of good mechanical properties of ductile iron. Otherwise much thicker boride layer or lower matrix hardness would result in a dramatic drop in impact resistance. Based on the experimental results in this study it can be said that a boro-tempering treatment up to 3 h at temperatures in the range of 250–350 °C would be useful to obtain mechanical properties comparable to austempering with additional surface hardness about four or five times better.
5. Conclusions Mechanical properties of boro-tempered ductile iron were investigated in this study. The following conclusions can be drawn from the results. The boride layer is formed by boro-tempering heat treatment on the ductile iron and its micro-hardness is in the range of 1654– 1867 HV. Depending on the boronizing time, micro-hardness decreases from surface through interior and reaches the matrix hardness beneath 50–80 lm of surface. The hardness of the matrix was affected by tempering temperature rather than boronizing time. Also the hardness of the matrix is in the range of 295–416 HV depending on tempering temperature. The strength sharply decreases with increasing tempering temperature and then remains unchanged. At 250 °C tempering temperature, maximum tensile and 0.2% yield strengths are obtained as 1547 and 1432 MPa, respectively. However, it is ascertained that increasing boronizing time decreases the strength. The graphs of the impact energy and elongation show similar trend depending on boro-tempering conditions. Generally, the impact energy and elongation increase up to tempering temperature of 300 °C, thereafter decrease. At temperatures above 300 °C, boronizing for 5 h extremely lowered the impact energy. The boride layers, which are formed by boro-tempering on the ductile iron, decrease the austemperability. Optimum mechanical properties are obtained for the samples boronized for 1–3 h and then tempered between 250 and 350 °C for 1 h.
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Acknowledgements The authors gratefully acknowledge the Scientific and Technical Research Council of Turkey (TUBITAK) for financial support with the Grant Number of 104M398. The authors also thank to Scientific Research Project Council of Afyon Kocatepe University.
References [1] Fuller AG. Austempered ductile irons – present applications. Mater Des 1985;6:127–30. [2] Harding RA. The production, properties and automotive applications of austempered ductile iron. Kovove Mater 2007;45:1. [3] Putatunda SK. Development of austempered ductile cast iron (ADI) with simultaneous high yield strength and fracture toughness by a novel two-step austempering process. Mater Sci Eng 2001;A315:70–80. [4] Krohn BR. Austempered ductile iron emerging as a structural material. Modern Casting; 1984. p. 26. [5] Hayrynen KL. ADI: another avenue for ductile iron foundries. Modern Casting; 1995. p. 35–7. [6] Kovacs BV. Development of austempered ductile iron (ADI) for automobsßle crankshafts. J Heat Treat 1987;5(1):55. [7] Rao PP, Putatunda SK. Investigations on the fracture toughness of austempered ductile iron alloyed with chromium. Mater Sci Eng 2003;A346:254–65. [8] Blackmore PA, Harding RA. The effect of metallurgical process variables on the properties of austempered ductile iron. J Heat Treat 1984;3:310–25. [9] Harding RA. Opening up the market for ADI. The Foundryman; 1993. p. 197– 208. [10] Gudlanch RB, Janowak JF. Development of a ductile iron for commercial austempering. AFS Trans 1983;94:377. [11] Harding RA, Gilbert GNJ. Why the properties of ductile irons should interest engineers. Brit Foundryman 1986;79:489. [12] Yang J, Putatunda SK. Effect of microstructure on abrasion wear behavior of austempered ductile cast iron (ADI) processed by a novel two-step austempering process. Mater Sci Eng A 2005;406:217–28. [13] Putatunda SK, Bartosiewicz L, Krause AR, Singh I, Alberts FA. Influence of microstructure on high-cycle fatigue behavior of austempered ductile cast iron. Mater Char 1993;30:221–34. [14] Hughes ICH. Austempered ductile irons their properties and significance. Mater Des 1985;6:124–6. [15] El-Bahloul AMM. Surface capacity of gears of circular – arc tooth-profile. Wear 1996;93:146–54. [16] Roy A, Mana I. Laser surface engineering to improve wear resistance of austempered ductile iron. Mater Sci Eng 2001;A297:85–93. [17] Pepe A, Galliano P, Cere S, Aparicio M, Duran A. Hybrid silica sol–gel coatings on austempered ductile iron (ADI). Mater Lett 2005;59:2219. [18] Hsu CH, Lu JK, Tsai RJ. Effects of low-temperature coating process on mechanical behaviors of ADI. Mater Sci Eng 2005;A398:282–90. [19] Hsu CH, Lu JK, Tsai RJ. Characteristics of duplex surface coatings on austempered ductile iron substrates. Surf Coat Tech 2006;200:5725–32. [20] Sinha AK. Boriding (boronizing). ASM international handbook, vol. 4. Materials Park (OH, USA): The Materials International Society; 1991. [21] Bindal C, Erdemir A. Ultralow friction behavior of borided steel surfaces after flash annealing. Appl Phys Lett 1996;68(7):923–5. [22] Yalcin Y, Yazici AM. The effect of boro-tempering heat treatment on the properties of ductile cast iron. Kovove Mater 2007;45:51. [23] Baydogan M, Akray SI. Successive boronizing and austempering for GGG-40 grade ductile iron. J Iron Steel Res Int 2009;16(2):50–4. [24] A897M-90, Standard specification for austempered ductile iron castings [Metric];1990. p. 506 [25] Darwish N, Elliott R. Austempering of low manganese ductile irons. Part 3 Variation of mechanical properties with heat treatment conditions. Mater Sci Tech 1993;9:882–9.