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Microstructure of two centrifugal cast high speed steels for hot strip mills applications
Materials and Design 34 (2012) 372–378
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microstructure of two centrifugal cast high speed steels for hot strip mills applications V. Vitry a,⇑, S. Nardone b, J.-P. Breyer c, M. Sinnaeve c, F. Delaunois a a
Service de Métallurgie, Université de Mons, Belgium Laborelec, Linkebeek, Belgium c Marichal Ketin, Liège, Belgium b
a r t i c l e
i n f o
Article history: Received 6 May 2011 Accepted 17 July 2011 Available online 3 August 2011 Keywords: A. Ferrous metals and alloys C. Casting G. Metallography
a b s t r a c t High speed steels (HSS) present excellent hardness, wear resistance and high-temperature properties. These mechanical properties are due to the presence of a great amount of hard carbides in the martensitic matrix. In the last 10 years, Japanese rollmakers have developed HSS grades and introduced them into hot strip mills. The Marichal Ketin society (Liège, Belgium) has developed two grades of HSS: Kosmos and Aurora. Both grades present interesting properties but Aurora shows overall better performance than Kosmos, mainly because of a better distribution of harder (MC and M2C) carbides in the martensitic matrix. Moreover, the hardness of the Aurora grade stays constant in depth and can be strongly improved by heat treatment, due to secondary hardening. The purpose of this work is to describe the microstructure and the mechanical properties of the Kosmos and Aurora grades by various techniques such as optical microscopy, scanning electron microscopy (SEM), and macrohardness measurements. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Hot strip mill rolls are submitted to extreme wear and high temperature conditions through their use [1–3]. For this reason, development efforts are made to improve their performance by the use of alloys with the best possible properties under those conditions. In this scope, high speed steels (HSS) have been developed by Japanese rollmakers since the 1980s and introduced in hot strip mills. HSS are complex multi-component alloys whose microstructure is composed of blocks of primary carbides within a matrix of tempered martensite, with the presence of fine secondary carbides [4–9]. This structure is mainly formed during solidification and can only be modified in a limited manner by heat treatment or hot working [4], and is strongly influenced by alloy composition and solidification rate. Due to this particular structure, HSS grades exhibit excellent wear resistance in high temperature operation [5–8]. They have been and still are the object of several studies [4–18]. The main alloying elements in HSS are carbon, vanadium, chromium, tungsten and molybdenum. Their role in the structure of
⇑ Corresponding author. Tel.: +32 65 37 44 38. E-mail address: [email protected] (V. Vitry). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.07.041
HSS is the following: Carbon is of course needed for the formation of carbides. Moreover, the effects of carbon in unalloyed steel, which is well known, can also be observed in alloyed steels, such as HSS. Vanadium is a strong carbide-forming element. The formula of vanadium carbide is typically V8C7. They are usually called MC. Chromium is also a carbide-forming element. The chromium carbides are usually of the M7C3 or M23C6 type. Tungsten forms very hard carbides and allows secondary hardening of HSS. However, the high specific mass of its carbides is the origin of segregation problems. Molybdenum has a behaviour similar to tungsten but is less sensible to segregation. Both of those elements form carbides of the M2C or M6C type. The various carbides present in HSS can be identified by their morphologies and their localization in the steel, as summarized on Fig. 1. Marichal Ketin (Liège, Belgium) is a company specialized in the production of hot strip mills rolls. They have developed two HSS grade called Kosmos and Aurora, that are the object of the present work. Kosmos was first developed and lead to the replacement of the previously used High Chrome rolls by bi-metallic rolls with an external layer of Kosmos grade and a core of nodular cast iron. The Aurora grade was developed later as a mean to improve and optimise the mechanical properties of HSS for the use in hot strip mills. Both grades are used in bi-metallic rolls that are produced by centrifugal casting. The external metal is poured first, then the core metal.
V. Vitry et al. / Materials and Design 34 (2012) 372–378
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Fig. 1. Summary of carbide morphology and localization in HSS.
This study focuses on the determination of the microstructure and the mechanical properties of the Kosmos and Aurora grades by various techniques such as optical microscopy, chemical etching, scanning electron microscope (SEM) and macrohardness measurements. 2. Experimental details 2.1. Samples preparation Two HSS grades were studied in this work. They are named Kosmos and Aurora. Both grades are variations on the AISI M2 HSS grade. Their chemical composition is presented in Table 1. The Aurora grade differs from the Kosmos one by its higher Mo content, lower Cr content and by absence of W. The microstructure of both grades was studied on 43 different samples obtained from bars (in as-cast and heat-treated conditions) cut in the shell of centrifugally cast rolls. These samples were divided into four batches: batches 1, 2 and 4 were cut at depth of 10 mm from the roll surface; batch 3 was cut along the diameter of the roll in order to study the microstructure as a function of the depth and to observe the influence of the cooling rate (Fig. 1).
Table 1 Chemical composition of Kosmos and Aurora grades (wt.%). Grade
C
Cr
Mo
V
W
Kosmos Aurora
1.5–2 1.5–2
5.0–7.0 3.0–5.0
3.0–4.0 5.0–7.0
4.0–6.0 4.0–6.0
1.5–2.5 –
Some of the specimens were heat treated as follow: the Kosmos grade was tempered to guarantee a shore hardness of 77/83. The Aurora grade needed a previous austenization flowed by air quenching before the tempering could be implemented. Specimens were mounted, ground with an abrasive SiC paper (up to grade 4000) and finally polished with a diamond paste (1/4 lm). Several etchants were used to determine the microstructure of Kosmos and Aurora grades by differential coloration of the carbides dispersed in the martensitic matrix [19–21]. Nital (10 wt.% HNO3 in ethanol [22]) was used to reveal the grain boundaries in the martensitic matrix. Murakami etchant (3 g K3Fe(CN)6; 10 g NaOH;100 ml H2O [8,22]) was used to colour the chromium containing M2C carbides in black. Groesbeck etchant (4 g KMnO4, 4 g NaOH, and 100 ml H2O) enables to identify on a single sample M7C3 carbides (coloured in light brown), MC carbides (coloured in pink) and M2C (coloured in black). Another etchant (5 g FeCl3; 10 ml HNO3; 3 ml HCl; 87 ml ethyl alcohol [8]) was used to dissolve the martensitic matrix without any degradation of the carbides localized on the surface of the sample. 2.2. Analysis of the samples Optical microscopy, coupled with image analysis was used to study the microstructure of the samples. A contrast image analysis of the etched specimens was carried out using the Paint Shop Pro Version 8.10 software, in order to evaluate the volume fraction of each type of carbides. The microstructure was also studied using a Jeol JSM 5900 LV scanning electron microscope. Hardness of the samples was
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investigated at 3 different scales: Vickers Macrohardness was measured with a EMCO Test M4U-025Hardness tester under a load of 30 kg. Depth profile Macrohardness measurements were carried out under a load of 30 kg. To avoid mutual influence of the indents, the depth profile was carried out using a lateral offset: five parallel lines were determined on the sample, with spacing sufficient to avoid influence between the lines. Depth profile was carried out on each line with a spacing of 10 mm between successive indents. An offset of n 2 mm (n = 0–4) was used to ensure every distance in the depth profile was measured. As this method is not the most used for depth profile, reproducibility was ensured by repeating the experiment three times for each experimental condition. Microhardness was obtained with a LECO M-400-A hardness tester, under a load of 50gf, with a Vickers indenter. Lastly, the nanohardness of the samples was investigated using a MTS nanoindenter XP with a Berkovitch (tetrahedron shaped) indenter, under a load of 4 mN. This technique is usually used for the investigation of mechanical properties of coatings [23–25]. 3. Results and discussion 3.1. Optical examination of unetched and etched samples The presence of large amounts of carbide-former alloying elements such as chromium, molybdenum, vanadium and tungsten in the Kosmos grade leads to the formation of hard carbides in the material. The Aurora grade contains less chromium, more molybdenum and is free of tungsten. This leads to considerable modifications of the alloys microstructure, mainly of the amounts, distribution and morphology of the carbides. Microscopic examination of etched samples of the Kosmos and Aurora grades shows the presence of various carbides with different morphologies and colouration, which form a dense network in the martensitic matrix. The carbides responsible for the hardness of the Kosmos grade [26–34] are mainly M7C3 but also MC and M2C hard carbides (Fig. 2). The Aurora grade [35] contains two different carbides (MC and M2C (Fig. 3)) dispersed in the matrix: and is exempt of M7C3. The volume fractions of the various carbides were calculated by image contrast analysis for both HSS grades. They are listed in Table 2. The amount of carbides is slightly higher in the Kosmos grade than in the Aurora, which is expected from the composition of the alloy. The tempering heat treatment increases the primary carbide content for both grades. Heat treatment of the Aurora grade (Fig. 4) induces secondary hardening by precipitation of small (1 lm) chromium-rich M23C6 carbides in the tempered martensitic matrix and by the decomposition of the M2C carbides in MC and M6C [35–36]. The secondary
Fig. 2. Samples selection.
Fig. 3. Kosmos grade microsctructure (Groesbeck etchant).
hardening effect is less important for the Kosmos grade, because this grade contains less MC and M2C carbides (see Fig. 5). 3.2. SEM observation Both grades were observed by SEM after matrix dissolution, in the as-cast and heat-treated state. The distribution of M7C3, MC and M2C carbides at the grain boundaries of the martensitic matrix of the as-cast Kosmos grade can be observed on Fig. 6: Number 1 indicates the presence of elongated MC vanadium carbides (with a chemistry close to V8C7). Number 2 indicates other MC carbides with a globular morphology and a similar chemistry as the previous ones. Number 5 shows the presence of chromium-rich M7C3 carbides, with 20 at.% Cr, 20 at.% Fe and 40 at.% C . Their morphology is better seen on Fig. 7. They present a fish-bone shape with large lamellae of constant thickness and can be easily differentiated from the M6C carbides (Fig. 8). Those carbides are usually observed for very high cooling rates (near the roll surface). Number 7 in Fig. 6 indicates stick shaped M2C carbides with a high molybdenum content. They are present near the M7C3 carbides. Fig. 9 brings some indications about carbide forming elements diffusion: the martensitic matrix (marked 1) becomes progressively enriched in Cr and Mo in the areas next to M7C3 and M2C carbides (marked 2). Diffusion of the carbide-forming elements occurs when carbides are close together: some MC carbides (marked 3) become enriched in Mo from the neighboring M7C3 carbides (marked 5). Some undefined carbides are also present (marked 4). The microstructure of the heat-treated Kosmos grade presents an higher number of globular MC carbides that are the result of the decomposition of the M2C carbides. The MC carbides are present at the grain boundaries and close to the M2C carbides and are smaller that in the as-cast Kosmos grade (see Fig. 10). Small (1 lm) M23C6 carbides with an high iron content are formed during the heat treatment (see Fig. 11). The matrix of the as-cast Aurora grade is constituted of acicular martensite (Fig. 12) with lamellar M2C carbides (marked 1, 3 and 6) that form a continuous network at the grain boundaries and small, homogeneously dispersed MC carbides (marked 2 and 4) at the grain boundaries and in the center of grains. There are no M7C3 carbides in the as-cast Aurora grade due to the lower chromium content of the alloy. In the heat treated Aurora grade, the amount of M2C carbides decreases and the continuous network is broken. Those present a
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V. Vitry et al. / Materials and Design 34 (2012) 372–378 Table 2 Carbide fractions (%) of Kosmos and Aurora grades in the as-cast and heat treated state. Etchant
Carbides fraction (%)
As-cast Kosmos
Heat-treated Kosmos
As-cast Aurora
Heat-treated Aurora
Matrix dissolution Matrix dissolution followed by Groesbeck
Primary carbides (%) Primary carbides (%) MC carbides (%)
8.8 8.8 5.5
10.1 15.3 5.8
7.7 7 4.2
8.3 12.9 4.2
Fig. 4. Aurora grade microsctructure (Groesbeck etchant). Fig. 7. Morphology of M7C3 carbides in as-cast Kosmos grade (matrix dissolution).
Fig. 5. Secondary hardening in Aurora grade (Groesbeck etchant). Fig. 8. Morphology of M6C carbides in as-cast Kosmos grade (matrix dissolution).
lamellar morphology (Fig. 13). More small MC carbides, dispersed in the matrix, can be observed.
3.3. Hardness measurements
Fig. 6. SEM image of as-cast Kosmos grade (matrix dissolution).
The macro and microhardness was measured on samples cut 10 mm below the surface. The use of a 50gf load allows measuring the hardness of the martensitic matrix without influence of the carbides. As can be seen on Table 3, in the as-cast state, the macrohardness of the Kosmos grade is slightly higher than that of the Aurora. However microhardness of the Aurora grade (i.e. hardness of the martensitic matrix) is higher. This suggests, as the carbide fractions are similar, that the prima ry MC and M2C carbides, which are reputed to be the harder ones with a hardness value of 2400–3000 HV against less than 1500 HV for the M7C3 and
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Fig. 9. SEM image of as-cast Kosmos grade (matrix dissolution) : diffusion of carbide-forming elements.
Fig. 12. SEM image of as-cast Aurora grade (matrix dissolution).
Fig. 13. SEM image of heat treated Aurora grade (matrix dissolution). Fig. 10. SEM image of heat treated Kosmos grade (matrix dissolution).
Fig. 11. Morphology of M23C6 carbides in heat treated Kosmos grade (matrix dissolution).
1500–2000 HV for M2C [37–39], have a lesser hardening effect. However, more recent results suggest a higher hardness (2380 hv10) for M7C3 and M2C (2230 hv10) and lower values (2740 hv10) for MC [8].
Neither of those values can explain the lower hardness of ascast Aurora and the contradictory values found in the literature made us question their validity. We tried thus to measure the hardness of individual carbides by the nanoindentation technique, which should be more accurate than the usual microhardness values given for them as they can be obtained without matrix effect due the very small size of the indents. The lamellar morphology of the M2C and M7C3 is not well suited for accurate hardness measurements and the results obtained for those carbides were not exploitable. However, the globular primary MC carbides are well adapted for this technique. Their hardness was found to be 2970 ± 240 HV under a load of 4 mN. This value is close to the previously published one [8,37–39]. However, as the load used is quite smaller in the present case, we can affirm than the published values were probably overvalued, except for the results of Hwang et al. [8] and Belzunce et al. [39]. After heat treatment, the secondary hardening due to the precipitation of small MC carbides in the Aurora grade is much more important than the hardening of the Kosmos grade, which is indicated by higher macrohardness values. The lowering of the microhardness values for both grades is due to the tempering of the martensitic matrix. Vickers macrohardness values were converted to HRC using the table in ASM handbook [40] to ease comparison with published results.
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V. Vitry et al. / Materials and Design 34 (2012) 372–378 Table 3 Vickers macro and microhardness of Kosmos and Aurora high speed steels. Sample
As-cast Kosmos
As-cast Aurora
Heat treated Kosmos
Heat treated Aurora
Vickers Macrohardness (HV30) Macrohardness (converted to HRC) Vickers microhardness (hv50)
643 ± 14 57.3 603 ± 21
599 ± 6 55.2 635 ± 19
685 ± 9 59.5 548 ± 10
740 ± 20 61.8 607 ± 13
is significantly higher, it decreases with depth, while the Aurora grade Rolls have a constant hardness up to the core metal is reached. After heat treatment, both grades present a stable hardness in depth (see Fig. 15), showing the beneficial effects of the treatment. The hardness of the Aurora grade is consistently higher than that of the Kosmos, as is expected from the macrohardness values. 4. Conclusion
Fig. 14. Depth evolution of hardness on as-cast Kosmos and Aurora.
Two high speed steels grades, Kosmos and Aurora were investigated. The Aurora grade presents better mechanical properties than the Kosmos, due to an optimal distribution of MC and M2C carbides in the martensitic matrix. These carbides are both smaller and harder that the M7C3 which are observed in the Kosmos grade. Heat treating of both grades shows the precipitation of M23C6 carbides in the matrix and the decomposition of M2C carbide, which leads to secondary hardening. The main feature of the Aurora grade, apart for the greater amount of secondary hardening, is the consistency of its hardness in depth in the as-cast state: the hardness of the Kosmos grade decreases significantly with depth. Further development research could lead to a further increase in the mechanical properties of Aurora grade by refining of the chemical composition and heat treatments. Hot hardness testing is envisaged to study the mechanical behaviour of both HSS at elevated temperature and thus to assess the material in working conditions. Acknowledgments
Fig. 15. Depth evolution of hardness on heat treated Kosmos and Aurora.
The macro hardness values of Table 3 were then compared with published values [8,16–17,41]: – Pelizzari et al. measured the Rockweel hardness of austenized and tempered HSS grades with a tungsten equivalent (Weq = W + 2 Mo) varying between 3 and 12 (ours are comprised between 6 and 10.5). They obtained values ranging from 57 up to 62 HRC, which is comparable to ours [29,41]. – Xu et al. worked on austenized and tempered high vanadium HSS with 10.3 wt.% V and a Weq close to 6. They obtained an overall hardness of only 65.5 HRC. – Hwang et al. obtained, on as cast HSS, with Weq in the range 5– 15, hardness values between 588 (for Weq = 5) to 722 (for Weq = 15) HV30. Once more, those values are comparable to ours. The Kosmos and Aurora grade present thus good mechanical properties, compared to others HSS grades. The cooling rate during casting influences strongly the microstructure, and thus the macrohardness, of the alloy, as shown in Fig. 14. While the superficial hardness of the Kosmos grade rolls
The authors wish to thank the Materials Science’s Service (Materia Nova) of the University of Mons (UMONS) (Belgium) and the INISMa (Mons, Belgium) for their technical support, respectively for the nanoindenter and for the SEM. References [1] Lévêque R. Aciers à outils: classification et évolution. Techniques de l’ingénieur. Ref. M4588; 2004. [2] Bagur F. Matériaux pour outils de coupe. Techniques de l’ingénieur. Ref. MB7080; 1999. [3] Lienard P, Pacque C, Breyer JP. Solutions modernes pour améliorer la qualité des cylindres de laminoirs moulés. Hommes et fonderie; Novembre 1995. [4] Rodenburg C, Rainforth WM. A quantitative analysis of the influence of carbides size distributions on wear behaviour of high-speed steel in dry rolling/sliding contact. Acta Mater 2007;55:2443–54. [5] Boccalini M, Goldstein H. Solidification of high speed steel. Int Mater Rev 2001;46(2). [6] Lecomte-Beckers J, Tchoufang Tchuindjang J, Pierard E, Breyer JP. Physical metallurgy of a high speed steel material for hot rolling mill rolls. In: Proceedings of the 14th rolling conference – 1st conference on uses of steel, San Nicolas; 5–7 November 2002. [7] Lecomte-Beckers J, Tchoufang J, Ernst Tchuindjang R, Breyer JP. Structural investigations of high speed steel rolls for hot strip mill. In: 41st Rolling seminar processes, rolled and coated product; 26–28 October 2004. [8] Hwang Keun Chul, Lee Sunghar, Lee Hui Choon. Effects of allowing elements on microstructure and fracture properties of cast high speed steel rolls, part I: microstructural analysis. Mater Sci Eng 1998;A254:282–95. [9] Karagoz S, Andren H-O. Secondary hardening in high speed steels. Z Metallkd 1992;83:386–94.
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Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microstructure of two centrifugal cast high speed steels for hot strip mills applications V. Vitry a,⇑, S. Nardone b, J.-P. Breyer c, M. Sinnaeve c, F. Delaunois a a
Service de Métallurgie, Université de Mons, Belgium Laborelec, Linkebeek, Belgium c Marichal Ketin, Liège, Belgium b
a r t i c l e
i n f o
Article history: Received 6 May 2011 Accepted 17 July 2011 Available online 3 August 2011 Keywords: A. Ferrous metals and alloys C. Casting G. Metallography
a b s t r a c t High speed steels (HSS) present excellent hardness, wear resistance and high-temperature properties. These mechanical properties are due to the presence of a great amount of hard carbides in the martensitic matrix. In the last 10 years, Japanese rollmakers have developed HSS grades and introduced them into hot strip mills. The Marichal Ketin society (Liège, Belgium) has developed two grades of HSS: Kosmos and Aurora. Both grades present interesting properties but Aurora shows overall better performance than Kosmos, mainly because of a better distribution of harder (MC and M2C) carbides in the martensitic matrix. Moreover, the hardness of the Aurora grade stays constant in depth and can be strongly improved by heat treatment, due to secondary hardening. The purpose of this work is to describe the microstructure and the mechanical properties of the Kosmos and Aurora grades by various techniques such as optical microscopy, scanning electron microscopy (SEM), and macrohardness measurements. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Hot strip mill rolls are submitted to extreme wear and high temperature conditions through their use [1–3]. For this reason, development efforts are made to improve their performance by the use of alloys with the best possible properties under those conditions. In this scope, high speed steels (HSS) have been developed by Japanese rollmakers since the 1980s and introduced in hot strip mills. HSS are complex multi-component alloys whose microstructure is composed of blocks of primary carbides within a matrix of tempered martensite, with the presence of fine secondary carbides [4–9]. This structure is mainly formed during solidification and can only be modified in a limited manner by heat treatment or hot working [4], and is strongly influenced by alloy composition and solidification rate. Due to this particular structure, HSS grades exhibit excellent wear resistance in high temperature operation [5–8]. They have been and still are the object of several studies [4–18]. The main alloying elements in HSS are carbon, vanadium, chromium, tungsten and molybdenum. Their role in the structure of
⇑ Corresponding author. Tel.: +32 65 37 44 38. E-mail address: [email protected] (V. Vitry). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.07.041
HSS is the following: Carbon is of course needed for the formation of carbides. Moreover, the effects of carbon in unalloyed steel, which is well known, can also be observed in alloyed steels, such as HSS. Vanadium is a strong carbide-forming element. The formula of vanadium carbide is typically V8C7. They are usually called MC. Chromium is also a carbide-forming element. The chromium carbides are usually of the M7C3 or M23C6 type. Tungsten forms very hard carbides and allows secondary hardening of HSS. However, the high specific mass of its carbides is the origin of segregation problems. Molybdenum has a behaviour similar to tungsten but is less sensible to segregation. Both of those elements form carbides of the M2C or M6C type. The various carbides present in HSS can be identified by their morphologies and their localization in the steel, as summarized on Fig. 1. Marichal Ketin (Liège, Belgium) is a company specialized in the production of hot strip mills rolls. They have developed two HSS grade called Kosmos and Aurora, that are the object of the present work. Kosmos was first developed and lead to the replacement of the previously used High Chrome rolls by bi-metallic rolls with an external layer of Kosmos grade and a core of nodular cast iron. The Aurora grade was developed later as a mean to improve and optimise the mechanical properties of HSS for the use in hot strip mills. Both grades are used in bi-metallic rolls that are produced by centrifugal casting. The external metal is poured first, then the core metal.
V. Vitry et al. / Materials and Design 34 (2012) 372–378
373
Fig. 1. Summary of carbide morphology and localization in HSS.
This study focuses on the determination of the microstructure and the mechanical properties of the Kosmos and Aurora grades by various techniques such as optical microscopy, chemical etching, scanning electron microscope (SEM) and macrohardness measurements. 2. Experimental details 2.1. Samples preparation Two HSS grades were studied in this work. They are named Kosmos and Aurora. Both grades are variations on the AISI M2 HSS grade. Their chemical composition is presented in Table 1. The Aurora grade differs from the Kosmos one by its higher Mo content, lower Cr content and by absence of W. The microstructure of both grades was studied on 43 different samples obtained from bars (in as-cast and heat-treated conditions) cut in the shell of centrifugally cast rolls. These samples were divided into four batches: batches 1, 2 and 4 were cut at depth of 10 mm from the roll surface; batch 3 was cut along the diameter of the roll in order to study the microstructure as a function of the depth and to observe the influence of the cooling rate (Fig. 1).
Table 1 Chemical composition of Kosmos and Aurora grades (wt.%). Grade
C
Cr
Mo
V
W
Kosmos Aurora
1.5–2 1.5–2
5.0–7.0 3.0–5.0
3.0–4.0 5.0–7.0
4.0–6.0 4.0–6.0
1.5–2.5 –
Some of the specimens were heat treated as follow: the Kosmos grade was tempered to guarantee a shore hardness of 77/83. The Aurora grade needed a previous austenization flowed by air quenching before the tempering could be implemented. Specimens were mounted, ground with an abrasive SiC paper (up to grade 4000) and finally polished with a diamond paste (1/4 lm). Several etchants were used to determine the microstructure of Kosmos and Aurora grades by differential coloration of the carbides dispersed in the martensitic matrix [19–21]. Nital (10 wt.% HNO3 in ethanol [22]) was used to reveal the grain boundaries in the martensitic matrix. Murakami etchant (3 g K3Fe(CN)6; 10 g NaOH;100 ml H2O [8,22]) was used to colour the chromium containing M2C carbides in black. Groesbeck etchant (4 g KMnO4, 4 g NaOH, and 100 ml H2O) enables to identify on a single sample M7C3 carbides (coloured in light brown), MC carbides (coloured in pink) and M2C (coloured in black). Another etchant (5 g FeCl3; 10 ml HNO3; 3 ml HCl; 87 ml ethyl alcohol [8]) was used to dissolve the martensitic matrix without any degradation of the carbides localized on the surface of the sample. 2.2. Analysis of the samples Optical microscopy, coupled with image analysis was used to study the microstructure of the samples. A contrast image analysis of the etched specimens was carried out using the Paint Shop Pro Version 8.10 software, in order to evaluate the volume fraction of each type of carbides. The microstructure was also studied using a Jeol JSM 5900 LV scanning electron microscope. Hardness of the samples was
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investigated at 3 different scales: Vickers Macrohardness was measured with a EMCO Test M4U-025Hardness tester under a load of 30 kg. Depth profile Macrohardness measurements were carried out under a load of 30 kg. To avoid mutual influence of the indents, the depth profile was carried out using a lateral offset: five parallel lines were determined on the sample, with spacing sufficient to avoid influence between the lines. Depth profile was carried out on each line with a spacing of 10 mm between successive indents. An offset of n 2 mm (n = 0–4) was used to ensure every distance in the depth profile was measured. As this method is not the most used for depth profile, reproducibility was ensured by repeating the experiment three times for each experimental condition. Microhardness was obtained with a LECO M-400-A hardness tester, under a load of 50gf, with a Vickers indenter. Lastly, the nanohardness of the samples was investigated using a MTS nanoindenter XP with a Berkovitch (tetrahedron shaped) indenter, under a load of 4 mN. This technique is usually used for the investigation of mechanical properties of coatings [23–25]. 3. Results and discussion 3.1. Optical examination of unetched and etched samples The presence of large amounts of carbide-former alloying elements such as chromium, molybdenum, vanadium and tungsten in the Kosmos grade leads to the formation of hard carbides in the material. The Aurora grade contains less chromium, more molybdenum and is free of tungsten. This leads to considerable modifications of the alloys microstructure, mainly of the amounts, distribution and morphology of the carbides. Microscopic examination of etched samples of the Kosmos and Aurora grades shows the presence of various carbides with different morphologies and colouration, which form a dense network in the martensitic matrix. The carbides responsible for the hardness of the Kosmos grade [26–34] are mainly M7C3 but also MC and M2C hard carbides (Fig. 2). The Aurora grade [35] contains two different carbides (MC and M2C (Fig. 3)) dispersed in the matrix: and is exempt of M7C3. The volume fractions of the various carbides were calculated by image contrast analysis for both HSS grades. They are listed in Table 2. The amount of carbides is slightly higher in the Kosmos grade than in the Aurora, which is expected from the composition of the alloy. The tempering heat treatment increases the primary carbide content for both grades. Heat treatment of the Aurora grade (Fig. 4) induces secondary hardening by precipitation of small (1 lm) chromium-rich M23C6 carbides in the tempered martensitic matrix and by the decomposition of the M2C carbides in MC and M6C [35–36]. The secondary
Fig. 2. Samples selection.
Fig. 3. Kosmos grade microsctructure (Groesbeck etchant).
hardening effect is less important for the Kosmos grade, because this grade contains less MC and M2C carbides (see Fig. 5). 3.2. SEM observation Both grades were observed by SEM after matrix dissolution, in the as-cast and heat-treated state. The distribution of M7C3, MC and M2C carbides at the grain boundaries of the martensitic matrix of the as-cast Kosmos grade can be observed on Fig. 6: Number 1 indicates the presence of elongated MC vanadium carbides (with a chemistry close to V8C7). Number 2 indicates other MC carbides with a globular morphology and a similar chemistry as the previous ones. Number 5 shows the presence of chromium-rich M7C3 carbides, with 20 at.% Cr, 20 at.% Fe and 40 at.% C . Their morphology is better seen on Fig. 7. They present a fish-bone shape with large lamellae of constant thickness and can be easily differentiated from the M6C carbides (Fig. 8). Those carbides are usually observed for very high cooling rates (near the roll surface). Number 7 in Fig. 6 indicates stick shaped M2C carbides with a high molybdenum content. They are present near the M7C3 carbides. Fig. 9 brings some indications about carbide forming elements diffusion: the martensitic matrix (marked 1) becomes progressively enriched in Cr and Mo in the areas next to M7C3 and M2C carbides (marked 2). Diffusion of the carbide-forming elements occurs when carbides are close together: some MC carbides (marked 3) become enriched in Mo from the neighboring M7C3 carbides (marked 5). Some undefined carbides are also present (marked 4). The microstructure of the heat-treated Kosmos grade presents an higher number of globular MC carbides that are the result of the decomposition of the M2C carbides. The MC carbides are present at the grain boundaries and close to the M2C carbides and are smaller that in the as-cast Kosmos grade (see Fig. 10). Small (1 lm) M23C6 carbides with an high iron content are formed during the heat treatment (see Fig. 11). The matrix of the as-cast Aurora grade is constituted of acicular martensite (Fig. 12) with lamellar M2C carbides (marked 1, 3 and 6) that form a continuous network at the grain boundaries and small, homogeneously dispersed MC carbides (marked 2 and 4) at the grain boundaries and in the center of grains. There are no M7C3 carbides in the as-cast Aurora grade due to the lower chromium content of the alloy. In the heat treated Aurora grade, the amount of M2C carbides decreases and the continuous network is broken. Those present a
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V. Vitry et al. / Materials and Design 34 (2012) 372–378 Table 2 Carbide fractions (%) of Kosmos and Aurora grades in the as-cast and heat treated state. Etchant
Carbides fraction (%)
As-cast Kosmos
Heat-treated Kosmos
As-cast Aurora
Heat-treated Aurora
Matrix dissolution Matrix dissolution followed by Groesbeck
Primary carbides (%) Primary carbides (%) MC carbides (%)
8.8 8.8 5.5
10.1 15.3 5.8
7.7 7 4.2
8.3 12.9 4.2
Fig. 4. Aurora grade microsctructure (Groesbeck etchant). Fig. 7. Morphology of M7C3 carbides in as-cast Kosmos grade (matrix dissolution).
Fig. 5. Secondary hardening in Aurora grade (Groesbeck etchant). Fig. 8. Morphology of M6C carbides in as-cast Kosmos grade (matrix dissolution).
lamellar morphology (Fig. 13). More small MC carbides, dispersed in the matrix, can be observed.
3.3. Hardness measurements
Fig. 6. SEM image of as-cast Kosmos grade (matrix dissolution).
The macro and microhardness was measured on samples cut 10 mm below the surface. The use of a 50gf load allows measuring the hardness of the martensitic matrix without influence of the carbides. As can be seen on Table 3, in the as-cast state, the macrohardness of the Kosmos grade is slightly higher than that of the Aurora. However microhardness of the Aurora grade (i.e. hardness of the martensitic matrix) is higher. This suggests, as the carbide fractions are similar, that the prima ry MC and M2C carbides, which are reputed to be the harder ones with a hardness value of 2400–3000 HV against less than 1500 HV for the M7C3 and
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Fig. 9. SEM image of as-cast Kosmos grade (matrix dissolution) : diffusion of carbide-forming elements.
Fig. 12. SEM image of as-cast Aurora grade (matrix dissolution).
Fig. 13. SEM image of heat treated Aurora grade (matrix dissolution). Fig. 10. SEM image of heat treated Kosmos grade (matrix dissolution).
Fig. 11. Morphology of M23C6 carbides in heat treated Kosmos grade (matrix dissolution).
1500–2000 HV for M2C [37–39], have a lesser hardening effect. However, more recent results suggest a higher hardness (2380 hv10) for M7C3 and M2C (2230 hv10) and lower values (2740 hv10) for MC [8].
Neither of those values can explain the lower hardness of ascast Aurora and the contradictory values found in the literature made us question their validity. We tried thus to measure the hardness of individual carbides by the nanoindentation technique, which should be more accurate than the usual microhardness values given for them as they can be obtained without matrix effect due the very small size of the indents. The lamellar morphology of the M2C and M7C3 is not well suited for accurate hardness measurements and the results obtained for those carbides were not exploitable. However, the globular primary MC carbides are well adapted for this technique. Their hardness was found to be 2970 ± 240 HV under a load of 4 mN. This value is close to the previously published one [8,37–39]. However, as the load used is quite smaller in the present case, we can affirm than the published values were probably overvalued, except for the results of Hwang et al. [8] and Belzunce et al. [39]. After heat treatment, the secondary hardening due to the precipitation of small MC carbides in the Aurora grade is much more important than the hardening of the Kosmos grade, which is indicated by higher macrohardness values. The lowering of the microhardness values for both grades is due to the tempering of the martensitic matrix. Vickers macrohardness values were converted to HRC using the table in ASM handbook [40] to ease comparison with published results.
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V. Vitry et al. / Materials and Design 34 (2012) 372–378 Table 3 Vickers macro and microhardness of Kosmos and Aurora high speed steels. Sample
As-cast Kosmos
As-cast Aurora
Heat treated Kosmos
Heat treated Aurora
Vickers Macrohardness (HV30) Macrohardness (converted to HRC) Vickers microhardness (hv50)
643 ± 14 57.3 603 ± 21
599 ± 6 55.2 635 ± 19
685 ± 9 59.5 548 ± 10
740 ± 20 61.8 607 ± 13
is significantly higher, it decreases with depth, while the Aurora grade Rolls have a constant hardness up to the core metal is reached. After heat treatment, both grades present a stable hardness in depth (see Fig. 15), showing the beneficial effects of the treatment. The hardness of the Aurora grade is consistently higher than that of the Kosmos, as is expected from the macrohardness values. 4. Conclusion
Fig. 14. Depth evolution of hardness on as-cast Kosmos and Aurora.
Two high speed steels grades, Kosmos and Aurora were investigated. The Aurora grade presents better mechanical properties than the Kosmos, due to an optimal distribution of MC and M2C carbides in the martensitic matrix. These carbides are both smaller and harder that the M7C3 which are observed in the Kosmos grade. Heat treating of both grades shows the precipitation of M23C6 carbides in the matrix and the decomposition of M2C carbide, which leads to secondary hardening. The main feature of the Aurora grade, apart for the greater amount of secondary hardening, is the consistency of its hardness in depth in the as-cast state: the hardness of the Kosmos grade decreases significantly with depth. Further development research could lead to a further increase in the mechanical properties of Aurora grade by refining of the chemical composition and heat treatments. Hot hardness testing is envisaged to study the mechanical behaviour of both HSS at elevated temperature and thus to assess the material in working conditions. Acknowledgments
Fig. 15. Depth evolution of hardness on heat treated Kosmos and Aurora.
The macro hardness values of Table 3 were then compared with published values [8,16–17,41]: – Pelizzari et al. measured the Rockweel hardness of austenized and tempered HSS grades with a tungsten equivalent (Weq = W + 2 Mo) varying between 3 and 12 (ours are comprised between 6 and 10.5). They obtained values ranging from 57 up to 62 HRC, which is comparable to ours [29,41]. – Xu et al. worked on austenized and tempered high vanadium HSS with 10.3 wt.% V and a Weq close to 6. They obtained an overall hardness of only 65.5 HRC. – Hwang et al. obtained, on as cast HSS, with Weq in the range 5– 15, hardness values between 588 (for Weq = 5) to 722 (for Weq = 15) HV30. Once more, those values are comparable to ours. The Kosmos and Aurora grade present thus good mechanical properties, compared to others HSS grades. The cooling rate during casting influences strongly the microstructure, and thus the macrohardness, of the alloy, as shown in Fig. 14. While the superficial hardness of the Kosmos grade rolls
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