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The effect of niobium and homogenization on the wear resistance and some mechanical properties of ferritic stainless steel containing 17–18 wt.% chromium
Journal of Materials Processing Technology 91 (1999) 172 – 177
The effect of niobium and homogenization on the wear resistance and some mechanical properties of ferritic stainless steel containing 17–18 wt.% chromium M. Aksoy a,*, V. Kuzucu b, M.H. Korkut c, M.M. Yildirim c a
Uni6ersity of Fırat, Faculty of Engineering, Department of Metallurgical Engineering, Elazig˘, Turkey b Uni6ersity of Firat, Faculty of Sciences and Arts, Department of Physics, Elazig˘, Turkey c Uni6ersity of Firat, Faculty of Technical Education, Department of Metallurgy, Elazig˘, Turkey Received 14 January 1998
Abstract The effect of niobium (Nb) content and homogenization on the abrasive wear resistance, surface hardness, microhardness and toughness of ferritic stainless steels containing 17–18 wt.% Cr was investigated. Abrasive wear resistance was related to surface hardness, microhardness and toughness. It was seen that there is approximately a linear relationship between the wear resistance and microhardness of both homogenized and unhomogenized samples: the harder the sample, the more resistant it is to abrasive wear. A similar linear relationship between the wear resistance and surface hardness of the homogenized samples was also seen, but a deviation from this linear relationship occurred between the wear resistance and surface hardness of the unhomogenized samples. In addition, it was also found that the resistance to abrasive wear decreases with increasing toughness of the sample. It was concluded that Nb has no a significant effect on abrasion resistance and toughness for the unhomogenized samples. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Ferritic stainless steel; Wear resistance; Surface hardness; Microhardness; Toughness
1. Introduction Ferritic stainless steels remain ferritic at all temperatures, they are not allotropic and they cannot be hardened by heating and quenching [1]. Their microstructure consists of ferrite (a-Fe), sigma phase and M23C6 carbide, where M indicates a mixture of metal atoms, but other phases may form after exposure to high temperatures [2]. The sigma phase, which is an intermetallic phase, increases hardness, which is sometimes useful, but it decreases ductility, notch toughness, and corrosion resistance[1]. It is dissolved at temperatures above 900°C [3]. To avoid M23C6 particles, the alloy is cooled rapidly after having been annealed at temperatures of between 1050 and 1100°C for a time sufficient to dissolve these carbides [2]. In their an-
* Corresponding author. Fax: +90-424-233-0062.
nealed condition, i.e. after rapid cooling from elevated temperatures, ferritic stainless steels gain maximum softness, ductility, and corrosion resistance, since the sigma phase and M23C6 particles are absent [1]. It is well known that wear resistant materials can be obtained by reinforcing soft phases with harder phases [4,5]. Therefore, it is the purpose of this work to obtain a hard phase in the soft ferrite phase by adding various amounts of Nb as a strong carbide maker into the ferritic stainless steels, and to combine the advantage of energy absorption of the softer phase and greater wear resistance of the harder phase. With this aim, the authors have investigated the microstructures, the wear properties and some mechanical properties such as surface hardness, microhardness and toughness, of ferritic stainless steels containing different amounts of niobium (Nb), using scanning electron microscopy (SEM) and applying surface hardness, microhardness, Charpy V-notch impact and abrasive wear tests.
0924-0136/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 8 ) 0 0 4 4 6 - 4
M. Aksoy et al. / Journal of Materials Processing Technology 91 (1999) 172–177
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2. Experimental The alloys (chemical compositions are shown in Table 1) were cast and forged into the form of bars of 20 mm diameter. Some samples cut from these bars were rapidly cooled after homogenizing at 1100°C for 1/2 h. After being polished, all samples were etched electrolitically by holding them in 50 ml HNO3 and 50 ml pure water solution at 1.5 volt for 1.5 min. Micrographs of the etched surfaces were taken by SEM. The surface hardness of the samples were measured in the range of 93 error band with HB hardness scale under 612.5 N load. The microhardness of the samples were measured in the range of 95 error band with HV hardness scale under 0.5 N load. In addition, the toughness of the samples was evaluated in the range of 90.6 error band using Charpy V-notch specimens. A pin-ondisk apparatus was used for evaluating the abrasive wear resistance. The wear samples for the pin-on-disk tests were machined to cylinders of 10 mm diameter and 30 mm length. These samples were polished metallographicaly. All of the wear tests were performed under a load of 10 N in a dry air atmosphere at room temperature. SiC abrasive paper of 80 mesh size was used. The mass loss of these samples was measured with a precision balance and the averages of these results were taken. The wear surface was observed using SEM. The wear resistance was determined in the range of 9 0.2 error band for each sample using the equation [6,7]: Wr =d·F·S/DG where DG, d, S and F are mass loss (mg), density (g cm − 3), length of wear path (cm) and load (N), respectively.
3. Results and discussion The microstructure of the samples considered here was examined in previous paper [8]. Therefore, only the results are summarised here. The microstructure of the Nb-free sample consists of ferrite, sigma phase and M23C6 carbide. Additionally, NbC forms in the samples Table 1 The chemical compositions of the samples Element (wt.%) Sample no.
C
Mn
Si
Cr
Nb
P
S
1 2 3 4 5
0.048 0.054 0.052 0.055 0.053
0.25 0.26 0.30 0.30 0.35
0.28 0.26 0.30 0.33 0.47
18.21 17.95 17.75 17.59 17.07
— 0.5 1.0 1.5 3.0
0.020 0.020 0.020 0.020 0.020
0.010 0.010 0.010 0.010 0.010
Fig. 1. Scanning electron micrographs ( × 500) of the homogenized samples 2 (a) and 5 (b).
containing Nb: it also forms Nb2C in sample 5, being different from the other samples. However, Nb2C is dissolved during heating at 1100 °C. After homogenization, the microstructure of samples 2,3,4 and 5 consist of only two phases, ferrite and NbC. Both the amount and size of the NbC particles increase with increasing Nb content in the samples, as seen in Fig. 1. The white phases seen in SEM micrographs are NbC particles. The surface hardness of the unhomogenized samples decreases with increasing Nb content (Fig. 2(a)). However, the increase in the surface hardness of sample 5 is a result of the existence of Nb2C particles in this sample [8]. Homogenization has decreased the surface hardness of samples 1 and 5, although it has increased the surface hardness of samples 2 and 3 (Fig. 2(b)). However, the surface hardness of sample 4 has not been influenced by homogenization. The reason for the increase in the surface hardness of sample 2 is the formation of new NbC in the form of very small particles both in the matrix and at the grain boundaries, during homogenization [9]. At the same time, the decrease in the surface hardness of samples 1 and 5 is the partial
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Fig. 2. Surface hardness and wear resistance versus Nb ratio for: (a) the unhomogenized; and (b) the homogenized samples.
dissolution of M23C6 and the completely dissolution of Nb2C, respectively. In addition, dissolution of the sigma phase is also an important factor in decreasing of the surface hardness of these samples. In sample 1, a very small part of the M23C6 particles was absent,
Fig. 3. Scanning electron micrograph ( × 500) of the homogenized sample 1.
Fig. 4. Microhardness and wear resistance versus Nb ratio for: (a) the unhomogenized; and (b) the homogenized samples.
because the annealing time was insufficient (see Fig. 3). If M23C6 particles were absent completely, then it could be expected that the surface hardness of sample 1 would be less. The microhardness of the samples decreased with increase in the Nb content, and the microhardnesses of the samples that contain 1.5–3.0 wt.% Nb became equal to that of the Nb-free sample (Fig. 4(a)). The reason for the decreasing in the microhardness with increasing Nb content may be NbC particles becoming longer, as seen in SEM micrographs. Homogenization has increased the microhardness of all samples (Fig. 4(b)). This increase in the microhardness can have originated from both the diffusion of the elements that are components of M23C6 particles and of the sigma phase dissolved due to the homogenization into the matrix, and the formation of new NbC particles in the matrix during annealing. Since M23C6 particles have been formed to a lesser extent in samples 4 and 5 [8], the dissolution of these carbides has contributed very little to the microhardness of these samples.
M. Aksoy et al. / Journal of Materials Processing Technology 91 (1999) 172–177
Nb has not affected significantly the toughness of the unhomogenized samples (Fig. 5(a)). However, the toughness of samples 4 and 5 has been affected somewhat by the homogenization process (Fig. 5(b)). This situation leads to the result that a Nb content of greater than 1.5% incites the formation of the sigma phase, which is a hard, brittle intermetallic phase [10] in unhomogenized samples. Since the sigma phase that is heavily formed in samples 4 and 5 has been rendered absent by homogenization [8], the toughness of these samples has increased considerably. Nb has also not affected the wear resistance considerably for the unhomogenized samples, as seen in Figs. 2, 4 and 5. After being homogenized, the wear resistance of all samples has increased. This increase has originated from the dissolution of the sigma phase during homogenization. With regard to the other samples, the much greater increase in the wear resistance of sample 2 can also be associated with high hardness values besides the dissolution of the sigma phase. In addition, both the amount and the size of the NbC particles has increased with increase in the Nb ratio, as can be seen
Fig. 5. Toughness and wear resistance versus Nb ratio for: (a) the unhomogenized; and (b) homogenized samples.
175
Fig. 6. Scanning electron micrograph ( × 500) of wear scars of the homogenized sample 2 subjected to the pin-on-disk wear test.
in Fig. 1. SEM micrograph of wear scars of the homogenized sample 2 having the greatest wear resistance amongst all of the samples is shown in Fig. 6. The relationships of the surface hardness, microhardness, toughness and wear resistance to the Nb ratio are given in Figs. 2, 4 and 5. There is a correlation between wear resistance and surface hardness for the unhomogenized samples, except for1 and 5, as seen in Fig. 2(a): as the surface hardness decreases, the wear resistance decreases. Although the sample 1 has the greatest surface hardness, it has the lowest wear resistance. The reason for this contradiction may be a result of the formation of the sigma phase in band form around the M23C6 particles as seen in Fig. 7. The brittle sigma phase is easily broken or cracked by abrasive particles during the wear test and it partially empties around M23C6 particles, thus the support of M23C6 particles becomes weak. Hence, this phenomenon causes a decrease in the
Fig. 7. Optic microscope micrograph ( × 1000) of the unhomogenized sample 1 [8]. The sigma phase has formed in a band form around M23C6 particles.
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Fig. 8. Optic microscope micrograph ( × 1000) of the unhomogenized sample 3. The sigma phase has formed in individual and large block form, not in band form, around the M23C6 particles.
Fig. 9. The variation of the wear resistance versus the surface hardness.
contribution of the M23C6 particles to the wear resistance. This situation is not the case for the other samples. Because the sigma phase in these samples, as seen in Fig. 8, is in individual and large block form, it is not in band form around M23C6 particles as in sample 1. Sample 5 has greater surface hardness than that of the other samples containing Nb. The reason for this is the formation of Nb2C particles in this sample besides the other phases [8]. However, these carbides haven’t contributed to the wear resistance. This situation can be explained by the hypothesis that Nb2C particles with hexagonal close packed structure (hcp) creates a weak bond with the matrix with body centered cubic (bcc) structure, and these carbides are easily pulled from their sites during the wear test. The contradiction between surface hardness and wear resistance of samples 1 and 5 has disappeared after homogenization because the sigma phase formed around the M23C6 particles in sample 1 and the Nb2C particles formed in sample 5 have dissolved during homogenization (Fig. 2(b)). As seen in Fig. 9, there is an approximately linear relationship between the wear resistance and surface hardness of both homogenized and unhomogenized samples containing Nb, except for the unhomogenized sample 5. As seen in Fig. 10, there is also a linear relationship between the wear resistance and microhardness of the unhomogenized samples containing Nb: the harder the sample, the more resistant it is to abrasive wear. However, this linear relationship for the homogenized samples is not perfect. If Fig. 2(a) is compared with Fig. 4(a), it is apparent that the main factor that affects the wear resistance is the microhardness of the matrix. Other investigators have also done detailed studies on the effect of hardness and its
relationship to wear resistance [11], during these studies, it being found that the wear resistance increases with increase in the microhardness. As seen in Fig. 5, there is also a correlation between wear resistance and toughness: the wear resistance decreases with increase in toughness.
4. Conclusions 1. Nb does not have a significant effect on wear resistance and toughness of the unhomogenized samples. 2. Homogenization has increased the wear resistance of all samples. The greatest ratio of increase in the wear resistance due to homogenization has occurred in the sample containing 0.5 wt.% Nb. The homoge-
Fig. 10. The variation of the wear resistance versus the microhardness.
M. Aksoy et al. / Journal of Materials Processing Technology 91 (1999) 172–177
nization has also increased considerably the toughness of the samples containing 1.5 – 3.0 wt.% Nb. 3. There is an approximately linear relationship between the wear resistance and the microhardness of both homogenized and unhomogenized samples containing Nb, as was seen between the wear resistance and the surface hardness of the homogenized samples: however a deviation from this relationship between the wear resistance and the surface hardness occurred for the unhomogenized samples. 4. It was established that there is a correlation between the abrasive wear resistance and the toughness: the greater the toughness of the sample, the less resistant it is to abrasive wear. 5. For wear applications, homogenization should be done at such temperature and for such time as to enable the dissolution of the sigma phase.
.
177
References [1] P.M. Unterweiser, H.E. Boyer, J.J. Kubbs (Eds.), Heat Treater’s Guide: Standard Practices and Procedures for Steel, ASM, USA, 1993 p. 418. [2] G.F.V. Voort, H.M. James, K. Mills, J.R. Davis, J.D. Destefani, D.A. Dieterich, G.M. Crankovic, M.J. Frissell (Eds.), ASM Handbook: Metallography and Microstructures, Vol. 9, USA: ASM International, 1992, p. 284. [3] R.A. Lula, Stainless Steel, 5th ed., American Society for Metals, Metals Park, Ohio, USA, 1993, pp. 10 – 57. [4] M. Aksoy, B. Karamis¸, E. Evin, Wear 193 (1996) 248 –252. [5] A. Wang, H.J. Rach, Wear 147 (1991) 355 – 424. [6] S. Seomantri, A.C. Mc. Gee, I. Finnie, Wear 104 (1985) 77–91. [7] DIN 50320, Verschleiss, December, 1979, pp. 38 – 51. [8] V. Kuzucu, M. Aksoy, M.H. Korkut, M.M. Yildirim, Mater. Sci. Eng. A 230 (1997) 75 – 80. [9] G.D.A. Soares, L.H. Almeida, T.L. Silveira, I.L. May, Mater. Charact. 29 (1992) 387 – 396. [10] T.A. DeBold, J. Met. 41 (1989) 12 – 15. [11] P.J. Mutton, J.D. Watson, Wear 48 (1978) 385 – 398.
The effect of niobium and homogenization on the wear resistance and some mechanical properties of ferritic stainless steel containing 17–18 wt.% chromium M. Aksoy a,*, V. Kuzucu b, M.H. Korkut c, M.M. Yildirim c a
Uni6ersity of Fırat, Faculty of Engineering, Department of Metallurgical Engineering, Elazig˘, Turkey b Uni6ersity of Firat, Faculty of Sciences and Arts, Department of Physics, Elazig˘, Turkey c Uni6ersity of Firat, Faculty of Technical Education, Department of Metallurgy, Elazig˘, Turkey Received 14 January 1998
Abstract The effect of niobium (Nb) content and homogenization on the abrasive wear resistance, surface hardness, microhardness and toughness of ferritic stainless steels containing 17–18 wt.% Cr was investigated. Abrasive wear resistance was related to surface hardness, microhardness and toughness. It was seen that there is approximately a linear relationship between the wear resistance and microhardness of both homogenized and unhomogenized samples: the harder the sample, the more resistant it is to abrasive wear. A similar linear relationship between the wear resistance and surface hardness of the homogenized samples was also seen, but a deviation from this linear relationship occurred between the wear resistance and surface hardness of the unhomogenized samples. In addition, it was also found that the resistance to abrasive wear decreases with increasing toughness of the sample. It was concluded that Nb has no a significant effect on abrasion resistance and toughness for the unhomogenized samples. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Ferritic stainless steel; Wear resistance; Surface hardness; Microhardness; Toughness
1. Introduction Ferritic stainless steels remain ferritic at all temperatures, they are not allotropic and they cannot be hardened by heating and quenching [1]. Their microstructure consists of ferrite (a-Fe), sigma phase and M23C6 carbide, where M indicates a mixture of metal atoms, but other phases may form after exposure to high temperatures [2]. The sigma phase, which is an intermetallic phase, increases hardness, which is sometimes useful, but it decreases ductility, notch toughness, and corrosion resistance[1]. It is dissolved at temperatures above 900°C [3]. To avoid M23C6 particles, the alloy is cooled rapidly after having been annealed at temperatures of between 1050 and 1100°C for a time sufficient to dissolve these carbides [2]. In their an-
* Corresponding author. Fax: +90-424-233-0062.
nealed condition, i.e. after rapid cooling from elevated temperatures, ferritic stainless steels gain maximum softness, ductility, and corrosion resistance, since the sigma phase and M23C6 particles are absent [1]. It is well known that wear resistant materials can be obtained by reinforcing soft phases with harder phases [4,5]. Therefore, it is the purpose of this work to obtain a hard phase in the soft ferrite phase by adding various amounts of Nb as a strong carbide maker into the ferritic stainless steels, and to combine the advantage of energy absorption of the softer phase and greater wear resistance of the harder phase. With this aim, the authors have investigated the microstructures, the wear properties and some mechanical properties such as surface hardness, microhardness and toughness, of ferritic stainless steels containing different amounts of niobium (Nb), using scanning electron microscopy (SEM) and applying surface hardness, microhardness, Charpy V-notch impact and abrasive wear tests.
0924-0136/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 8 ) 0 0 4 4 6 - 4
M. Aksoy et al. / Journal of Materials Processing Technology 91 (1999) 172–177
173
2. Experimental The alloys (chemical compositions are shown in Table 1) were cast and forged into the form of bars of 20 mm diameter. Some samples cut from these bars were rapidly cooled after homogenizing at 1100°C for 1/2 h. After being polished, all samples were etched electrolitically by holding them in 50 ml HNO3 and 50 ml pure water solution at 1.5 volt for 1.5 min. Micrographs of the etched surfaces were taken by SEM. The surface hardness of the samples were measured in the range of 93 error band with HB hardness scale under 612.5 N load. The microhardness of the samples were measured in the range of 95 error band with HV hardness scale under 0.5 N load. In addition, the toughness of the samples was evaluated in the range of 90.6 error band using Charpy V-notch specimens. A pin-ondisk apparatus was used for evaluating the abrasive wear resistance. The wear samples for the pin-on-disk tests were machined to cylinders of 10 mm diameter and 30 mm length. These samples were polished metallographicaly. All of the wear tests were performed under a load of 10 N in a dry air atmosphere at room temperature. SiC abrasive paper of 80 mesh size was used. The mass loss of these samples was measured with a precision balance and the averages of these results were taken. The wear surface was observed using SEM. The wear resistance was determined in the range of 9 0.2 error band for each sample using the equation [6,7]: Wr =d·F·S/DG where DG, d, S and F are mass loss (mg), density (g cm − 3), length of wear path (cm) and load (N), respectively.
3. Results and discussion The microstructure of the samples considered here was examined in previous paper [8]. Therefore, only the results are summarised here. The microstructure of the Nb-free sample consists of ferrite, sigma phase and M23C6 carbide. Additionally, NbC forms in the samples Table 1 The chemical compositions of the samples Element (wt.%) Sample no.
C
Mn
Si
Cr
Nb
P
S
1 2 3 4 5
0.048 0.054 0.052 0.055 0.053
0.25 0.26 0.30 0.30 0.35
0.28 0.26 0.30 0.33 0.47
18.21 17.95 17.75 17.59 17.07
— 0.5 1.0 1.5 3.0
0.020 0.020 0.020 0.020 0.020
0.010 0.010 0.010 0.010 0.010
Fig. 1. Scanning electron micrographs ( × 500) of the homogenized samples 2 (a) and 5 (b).
containing Nb: it also forms Nb2C in sample 5, being different from the other samples. However, Nb2C is dissolved during heating at 1100 °C. After homogenization, the microstructure of samples 2,3,4 and 5 consist of only two phases, ferrite and NbC. Both the amount and size of the NbC particles increase with increasing Nb content in the samples, as seen in Fig. 1. The white phases seen in SEM micrographs are NbC particles. The surface hardness of the unhomogenized samples decreases with increasing Nb content (Fig. 2(a)). However, the increase in the surface hardness of sample 5 is a result of the existence of Nb2C particles in this sample [8]. Homogenization has decreased the surface hardness of samples 1 and 5, although it has increased the surface hardness of samples 2 and 3 (Fig. 2(b)). However, the surface hardness of sample 4 has not been influenced by homogenization. The reason for the increase in the surface hardness of sample 2 is the formation of new NbC in the form of very small particles both in the matrix and at the grain boundaries, during homogenization [9]. At the same time, the decrease in the surface hardness of samples 1 and 5 is the partial
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Fig. 2. Surface hardness and wear resistance versus Nb ratio for: (a) the unhomogenized; and (b) the homogenized samples.
dissolution of M23C6 and the completely dissolution of Nb2C, respectively. In addition, dissolution of the sigma phase is also an important factor in decreasing of the surface hardness of these samples. In sample 1, a very small part of the M23C6 particles was absent,
Fig. 3. Scanning electron micrograph ( × 500) of the homogenized sample 1.
Fig. 4. Microhardness and wear resistance versus Nb ratio for: (a) the unhomogenized; and (b) the homogenized samples.
because the annealing time was insufficient (see Fig. 3). If M23C6 particles were absent completely, then it could be expected that the surface hardness of sample 1 would be less. The microhardness of the samples decreased with increase in the Nb content, and the microhardnesses of the samples that contain 1.5–3.0 wt.% Nb became equal to that of the Nb-free sample (Fig. 4(a)). The reason for the decreasing in the microhardness with increasing Nb content may be NbC particles becoming longer, as seen in SEM micrographs. Homogenization has increased the microhardness of all samples (Fig. 4(b)). This increase in the microhardness can have originated from both the diffusion of the elements that are components of M23C6 particles and of the sigma phase dissolved due to the homogenization into the matrix, and the formation of new NbC particles in the matrix during annealing. Since M23C6 particles have been formed to a lesser extent in samples 4 and 5 [8], the dissolution of these carbides has contributed very little to the microhardness of these samples.
M. Aksoy et al. / Journal of Materials Processing Technology 91 (1999) 172–177
Nb has not affected significantly the toughness of the unhomogenized samples (Fig. 5(a)). However, the toughness of samples 4 and 5 has been affected somewhat by the homogenization process (Fig. 5(b)). This situation leads to the result that a Nb content of greater than 1.5% incites the formation of the sigma phase, which is a hard, brittle intermetallic phase [10] in unhomogenized samples. Since the sigma phase that is heavily formed in samples 4 and 5 has been rendered absent by homogenization [8], the toughness of these samples has increased considerably. Nb has also not affected the wear resistance considerably for the unhomogenized samples, as seen in Figs. 2, 4 and 5. After being homogenized, the wear resistance of all samples has increased. This increase has originated from the dissolution of the sigma phase during homogenization. With regard to the other samples, the much greater increase in the wear resistance of sample 2 can also be associated with high hardness values besides the dissolution of the sigma phase. In addition, both the amount and the size of the NbC particles has increased with increase in the Nb ratio, as can be seen
Fig. 5. Toughness and wear resistance versus Nb ratio for: (a) the unhomogenized; and (b) homogenized samples.
175
Fig. 6. Scanning electron micrograph ( × 500) of wear scars of the homogenized sample 2 subjected to the pin-on-disk wear test.
in Fig. 1. SEM micrograph of wear scars of the homogenized sample 2 having the greatest wear resistance amongst all of the samples is shown in Fig. 6. The relationships of the surface hardness, microhardness, toughness and wear resistance to the Nb ratio are given in Figs. 2, 4 and 5. There is a correlation between wear resistance and surface hardness for the unhomogenized samples, except for1 and 5, as seen in Fig. 2(a): as the surface hardness decreases, the wear resistance decreases. Although the sample 1 has the greatest surface hardness, it has the lowest wear resistance. The reason for this contradiction may be a result of the formation of the sigma phase in band form around the M23C6 particles as seen in Fig. 7. The brittle sigma phase is easily broken or cracked by abrasive particles during the wear test and it partially empties around M23C6 particles, thus the support of M23C6 particles becomes weak. Hence, this phenomenon causes a decrease in the
Fig. 7. Optic microscope micrograph ( × 1000) of the unhomogenized sample 1 [8]. The sigma phase has formed in a band form around M23C6 particles.
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Fig. 8. Optic microscope micrograph ( × 1000) of the unhomogenized sample 3. The sigma phase has formed in individual and large block form, not in band form, around the M23C6 particles.
Fig. 9. The variation of the wear resistance versus the surface hardness.
contribution of the M23C6 particles to the wear resistance. This situation is not the case for the other samples. Because the sigma phase in these samples, as seen in Fig. 8, is in individual and large block form, it is not in band form around M23C6 particles as in sample 1. Sample 5 has greater surface hardness than that of the other samples containing Nb. The reason for this is the formation of Nb2C particles in this sample besides the other phases [8]. However, these carbides haven’t contributed to the wear resistance. This situation can be explained by the hypothesis that Nb2C particles with hexagonal close packed structure (hcp) creates a weak bond with the matrix with body centered cubic (bcc) structure, and these carbides are easily pulled from their sites during the wear test. The contradiction between surface hardness and wear resistance of samples 1 and 5 has disappeared after homogenization because the sigma phase formed around the M23C6 particles in sample 1 and the Nb2C particles formed in sample 5 have dissolved during homogenization (Fig. 2(b)). As seen in Fig. 9, there is an approximately linear relationship between the wear resistance and surface hardness of both homogenized and unhomogenized samples containing Nb, except for the unhomogenized sample 5. As seen in Fig. 10, there is also a linear relationship between the wear resistance and microhardness of the unhomogenized samples containing Nb: the harder the sample, the more resistant it is to abrasive wear. However, this linear relationship for the homogenized samples is not perfect. If Fig. 2(a) is compared with Fig. 4(a), it is apparent that the main factor that affects the wear resistance is the microhardness of the matrix. Other investigators have also done detailed studies on the effect of hardness and its
relationship to wear resistance [11], during these studies, it being found that the wear resistance increases with increase in the microhardness. As seen in Fig. 5, there is also a correlation between wear resistance and toughness: the wear resistance decreases with increase in toughness.
4. Conclusions 1. Nb does not have a significant effect on wear resistance and toughness of the unhomogenized samples. 2. Homogenization has increased the wear resistance of all samples. The greatest ratio of increase in the wear resistance due to homogenization has occurred in the sample containing 0.5 wt.% Nb. The homoge-
Fig. 10. The variation of the wear resistance versus the microhardness.
M. Aksoy et al. / Journal of Materials Processing Technology 91 (1999) 172–177
nization has also increased considerably the toughness of the samples containing 1.5 – 3.0 wt.% Nb. 3. There is an approximately linear relationship between the wear resistance and the microhardness of both homogenized and unhomogenized samples containing Nb, as was seen between the wear resistance and the surface hardness of the homogenized samples: however a deviation from this relationship between the wear resistance and the surface hardness occurred for the unhomogenized samples. 4. It was established that there is a correlation between the abrasive wear resistance and the toughness: the greater the toughness of the sample, the less resistant it is to abrasive wear. 5. For wear applications, homogenization should be done at such temperature and for such time as to enable the dissolution of the sigma phase.
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References [1] P.M. Unterweiser, H.E. Boyer, J.J. Kubbs (Eds.), Heat Treater’s Guide: Standard Practices and Procedures for Steel, ASM, USA, 1993 p. 418. [2] G.F.V. Voort, H.M. James, K. Mills, J.R. Davis, J.D. Destefani, D.A. Dieterich, G.M. Crankovic, M.J. Frissell (Eds.), ASM Handbook: Metallography and Microstructures, Vol. 9, USA: ASM International, 1992, p. 284. [3] R.A. Lula, Stainless Steel, 5th ed., American Society for Metals, Metals Park, Ohio, USA, 1993, pp. 10 – 57. [4] M. Aksoy, B. Karamis¸, E. Evin, Wear 193 (1996) 248 –252. [5] A. Wang, H.J. Rach, Wear 147 (1991) 355 – 424. [6] S. Seomantri, A.C. Mc. Gee, I. Finnie, Wear 104 (1985) 77–91. [7] DIN 50320, Verschleiss, December, 1979, pp. 38 – 51. [8] V. Kuzucu, M. Aksoy, M.H. Korkut, M.M. Yildirim, Mater. Sci. Eng. A 230 (1997) 75 – 80. [9] G.D.A. Soares, L.H. Almeida, T.L. Silveira, I.L. May, Mater. Charact. 29 (1992) 387 – 396. [10] T.A. DeBold, J. Met. 41 (1989) 12 – 15. [11] P.J. Mutton, J.D. Watson, Wear 48 (1978) 385 – 398.