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SEM and TEM microstructural investigation of high-speed tool steel after laser melting
Materials Chemistry and Physics 81 (2003) 510–512
SEM and TEM microstructural investigation of high-speed tool steel after laser melting S. K˛ac∗ , J. Kusi´nski Faculty of Metallurgy and Materials Science, University of Mining and Metallurgy, Mickiewicza 30 Ave., 30-059 Cracow, Poland
Abstract The microstructure of a continuous CO2 laser-melted high-speed steel, namely M2, has been studied. The formation of the microstructure under rapid solidification conditions is described by means of scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. A very fine dendritic structure is characteristic for the microstructure formed under such rapid solidification conditions like laser melting. This structure generally consists of fine dendrites of austenite crossed by a very fine carbide network or the eutectic without the primary large carbides. The structure obtained in the surface layer after laser melting permits to get a high level of hardness and shows an improved wear resistance. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Tool steel; Laser melting; Rapid solidification
1. Introduction The application of laser melting and directional solidification of surface layers permits to obtain structures and properties of practical interest. This technology permits to supply an enormous density of energy precisely to the treated surface during a very short time [1–3]. Laser radiation of tool steels causes changes of the microstructure and properties different from those produced by a conventional heat treatment. The laser-treated tool steels have generally a high level of hardness and wear, corrosive, erosive and fatigue resistance. These advantages are brought by extremely high heating and cooling rates during the laser treatment and by high stresses produced by the high temperature gradient that cause elastic and plastic strain of the material [4–8].
2. Experimental procedure The chemical composition of the M2 steel is listed in Table 1. The laser treatment was performed on steel coupons using a continuous wave CO2 laser with a generated beam power of 1.33–1.85 kW. A scanning speed of 13.3–26.6 mm s−1 was applied during laser melting. The laser beam with a Gaussian energy density redistribution was defocused on the surface to a spot of 2 and 3 mm in di∗ Corresponding author. Fax: +48-12-633-3673. E-mail address: [email protected] (S. K˛ac).
ameter. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations have been used to reveal the microstructural details of the laser-melted zone. In addition, energy-dispersive X-ray spectroscopy (EDS) has been applied for chemical microanalysis of the precipitates.
3. Results and discussion The bottom of the melted zone has a cell structure. A continuous carbide network is located in the cell boundaries. The center of these cells is composed of martensite and retained austenite formed during rapid cooling of the laser-melted surface layer (Fig. 1). The central and surface part of the melted zone has a dendritic structure. The dendritic crystals are surrounded by a eutectic. The orientation of the main axes of the dendrites corresponds to the direction of heat transport (Fig. 2). In the central and surface part of the melted zone, large martensite needles are observed. These needles are parallel and they cut across a few dendrites (Fig. 3). Inside the dendrites, martensitic plates are also visible, but they are considerably smaller. Their size is limited by the width of the dendrite branches. Between the melted zone and heat-affected zone there is a transient zone (partially melted zone), in which a cell structure is observed. The martensite needles are situated inside the cells. At the cell boundaries a continuous carbide network is located. In this area, partially melted primary
0254-0584/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00062-2
S. K˛a c, J. Kusi´nski / Materials Chemistry and Physics 81 (2003) 510–512
511
Table 1 Chemical composition of the M2 steel (wt.%) C
Mn
P
S
Cr
W
V
Mo
Co
0.91
0.23
0.03
0.012
3.92
6.51
1.86
4.99
0.22
Fig. 4. TEM microstructure of the laser-melted zone of M2 steel.
Fig. 1. SEM microstructure of the transient zone after laser melting.
carbides are visible. The dissolution processes of the primary carbides and the creation of a eutectic around these carbides can be seen in this area. The analysis of the microstructure by means of TEM shows that the microstructure after laser melting consists of austenite which was transformed to martensite during rapid
cooling of the steel after solidification of the liquid pool and relatively regular oval-shaped carbides (Fig. 4). The measurement of the chemical composition and maps of the elemental distribution in the microstructure were realized by means of an EDS analyzer. Different size and chemical composition of the carbides are observed in the melted zone. Fine carbides (about 250 m in diameter) and larger ones (1–3 m in diameter) are visible as dark precipitates on the micrograph shown in Fig. 5. Around these precipitates a fine shell is present. The analysis of the chemical composition shows that these carbides are rich in tungsten and contain chromium and molybdenum. The shell contains mostly iron and this was identified to be austenite. The analysis of electron diffraction pattern shows that these precipitates are M6 C carbides. Probably, these carbides were not dissolved completely during laser melting, becoming the nuclei for crystallization of the austenite rich in iron and cobalt. During the crystallization, the alloying elements are pushed by the crystallization front (solid/liquid interface) enriching the liquid solution, which crystallizes as austenite, transforming to martensite during cooling to room temperature. Fig. 6 presents the maps, which show the redistribution of elements
Fig. 2. SEM microstructure of the laser-melted and heat-affected zone.
Fig. 3. SEM microstructure of the laser-melted zone of M2 steel.
Fig. 5. TEM microstructure of the laser-melted zone and electron diffraction pattern of the marked area in the laser-melted zone.
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S. K˛a c, J. Kusi´nski / Materials Chemistry and Physics 81 (2003) 510–512
in the laser-melted zone in which fine (about 1 m in diameter) gray precipitates can be observed (Fig. 7). The solution of the electron diffraction patterns show that they are MC carbides. Probably they are the primary carbides precipitated from the liquid solution during solidification after laser melting. The investigation of the chemical composition indicates that these carbides contain vanadium.
4. Conclusions 1. Laser melting of M2 steel permits to obtain very fine structures. 2. The melted zone has a dendritic structure, only at the bottom a cellular structure is observed. 3. The eutectic is situated in the interdendritic areas. 4. A continuous carbide network is located in the grain boundaries at the bottom of the melted zone. 5. Large martensite needles cut across a few dendrites while the smaller martensitic needles are situated inside the dendrites. 6. The structure of the melted zone consists of martensitic matrix and M6 C particles, which contain tungsten, chromium and molybdenum, as well as MC particles, which contain mainly vanadium.
Fig. 6. TEM microstructure of the laser-melted zone and maps of the elemental distribution.
Acknowledgements This work was supported by the Polish State Committee for Scientific Research, grant no. 7T08C 001 17. References
Fig. 7. TEM microstructure and electron diffraction pattern of the marked area of the laser-melted zone.
[1] N.B. Dahotre, Lasers in Surface Engineering, ASM International, Ont., Canada, 1998. [2] J. Kusi´nski, Lasery i ich zastosowanie w in˙zynierii materiałowej, WN Akapit, Kraków, 2000. [3] J. Mazumder, J. Met. 35 (5) (1983) 18. [4] S. K˛ac, J. Kusi´nski, S. Hnat, A. Woldan, In˙zynieria Materiałowa 112 (1999) 365. [5] S. K˛ac, J. Kusi´nski, A. Woldan, In˙zynieria Materiałowa 123 (2001) 442. [6] S. K˛ac, J. Kusi´nski, A. Woldan, In˙zynieria Materiałowa 119 (2000) 319. [7] B. Major, R. Ebner, In˙zynieria Powierzchni 1 (1996) 53. [8] A. Bylica, A. Dziedzic, Krzepni˛ecie Metali i Stopów 42 (2000) 275.
SEM and TEM microstructural investigation of high-speed tool steel after laser melting S. K˛ac∗ , J. Kusi´nski Faculty of Metallurgy and Materials Science, University of Mining and Metallurgy, Mickiewicza 30 Ave., 30-059 Cracow, Poland
Abstract The microstructure of a continuous CO2 laser-melted high-speed steel, namely M2, has been studied. The formation of the microstructure under rapid solidification conditions is described by means of scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. A very fine dendritic structure is characteristic for the microstructure formed under such rapid solidification conditions like laser melting. This structure generally consists of fine dendrites of austenite crossed by a very fine carbide network or the eutectic without the primary large carbides. The structure obtained in the surface layer after laser melting permits to get a high level of hardness and shows an improved wear resistance. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Tool steel; Laser melting; Rapid solidification
1. Introduction The application of laser melting and directional solidification of surface layers permits to obtain structures and properties of practical interest. This technology permits to supply an enormous density of energy precisely to the treated surface during a very short time [1–3]. Laser radiation of tool steels causes changes of the microstructure and properties different from those produced by a conventional heat treatment. The laser-treated tool steels have generally a high level of hardness and wear, corrosive, erosive and fatigue resistance. These advantages are brought by extremely high heating and cooling rates during the laser treatment and by high stresses produced by the high temperature gradient that cause elastic and plastic strain of the material [4–8].
2. Experimental procedure The chemical composition of the M2 steel is listed in Table 1. The laser treatment was performed on steel coupons using a continuous wave CO2 laser with a generated beam power of 1.33–1.85 kW. A scanning speed of 13.3–26.6 mm s−1 was applied during laser melting. The laser beam with a Gaussian energy density redistribution was defocused on the surface to a spot of 2 and 3 mm in di∗ Corresponding author. Fax: +48-12-633-3673. E-mail address: [email protected] (S. K˛ac).
ameter. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations have been used to reveal the microstructural details of the laser-melted zone. In addition, energy-dispersive X-ray spectroscopy (EDS) has been applied for chemical microanalysis of the precipitates.
3. Results and discussion The bottom of the melted zone has a cell structure. A continuous carbide network is located in the cell boundaries. The center of these cells is composed of martensite and retained austenite formed during rapid cooling of the laser-melted surface layer (Fig. 1). The central and surface part of the melted zone has a dendritic structure. The dendritic crystals are surrounded by a eutectic. The orientation of the main axes of the dendrites corresponds to the direction of heat transport (Fig. 2). In the central and surface part of the melted zone, large martensite needles are observed. These needles are parallel and they cut across a few dendrites (Fig. 3). Inside the dendrites, martensitic plates are also visible, but they are considerably smaller. Their size is limited by the width of the dendrite branches. Between the melted zone and heat-affected zone there is a transient zone (partially melted zone), in which a cell structure is observed. The martensite needles are situated inside the cells. At the cell boundaries a continuous carbide network is located. In this area, partially melted primary
0254-0584/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00062-2
S. K˛a c, J. Kusi´nski / Materials Chemistry and Physics 81 (2003) 510–512
511
Table 1 Chemical composition of the M2 steel (wt.%) C
Mn
P
S
Cr
W
V
Mo
Co
0.91
0.23
0.03
0.012
3.92
6.51
1.86
4.99
0.22
Fig. 4. TEM microstructure of the laser-melted zone of M2 steel.
Fig. 1. SEM microstructure of the transient zone after laser melting.
carbides are visible. The dissolution processes of the primary carbides and the creation of a eutectic around these carbides can be seen in this area. The analysis of the microstructure by means of TEM shows that the microstructure after laser melting consists of austenite which was transformed to martensite during rapid
cooling of the steel after solidification of the liquid pool and relatively regular oval-shaped carbides (Fig. 4). The measurement of the chemical composition and maps of the elemental distribution in the microstructure were realized by means of an EDS analyzer. Different size and chemical composition of the carbides are observed in the melted zone. Fine carbides (about 250 m in diameter) and larger ones (1–3 m in diameter) are visible as dark precipitates on the micrograph shown in Fig. 5. Around these precipitates a fine shell is present. The analysis of the chemical composition shows that these carbides are rich in tungsten and contain chromium and molybdenum. The shell contains mostly iron and this was identified to be austenite. The analysis of electron diffraction pattern shows that these precipitates are M6 C carbides. Probably, these carbides were not dissolved completely during laser melting, becoming the nuclei for crystallization of the austenite rich in iron and cobalt. During the crystallization, the alloying elements are pushed by the crystallization front (solid/liquid interface) enriching the liquid solution, which crystallizes as austenite, transforming to martensite during cooling to room temperature. Fig. 6 presents the maps, which show the redistribution of elements
Fig. 2. SEM microstructure of the laser-melted and heat-affected zone.
Fig. 3. SEM microstructure of the laser-melted zone of M2 steel.
Fig. 5. TEM microstructure of the laser-melted zone and electron diffraction pattern of the marked area in the laser-melted zone.
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S. K˛a c, J. Kusi´nski / Materials Chemistry and Physics 81 (2003) 510–512
in the laser-melted zone in which fine (about 1 m in diameter) gray precipitates can be observed (Fig. 7). The solution of the electron diffraction patterns show that they are MC carbides. Probably they are the primary carbides precipitated from the liquid solution during solidification after laser melting. The investigation of the chemical composition indicates that these carbides contain vanadium.
4. Conclusions 1. Laser melting of M2 steel permits to obtain very fine structures. 2. The melted zone has a dendritic structure, only at the bottom a cellular structure is observed. 3. The eutectic is situated in the interdendritic areas. 4. A continuous carbide network is located in the grain boundaries at the bottom of the melted zone. 5. Large martensite needles cut across a few dendrites while the smaller martensitic needles are situated inside the dendrites. 6. The structure of the melted zone consists of martensitic matrix and M6 C particles, which contain tungsten, chromium and molybdenum, as well as MC particles, which contain mainly vanadium.
Fig. 6. TEM microstructure of the laser-melted zone and maps of the elemental distribution.
Acknowledgements This work was supported by the Polish State Committee for Scientific Research, grant no. 7T08C 001 17. References
Fig. 7. TEM microstructure and electron diffraction pattern of the marked area of the laser-melted zone.
[1] N.B. Dahotre, Lasers in Surface Engineering, ASM International, Ont., Canada, 1998. [2] J. Kusi´nski, Lasery i ich zastosowanie w in˙zynierii materiałowej, WN Akapit, Kraków, 2000. [3] J. Mazumder, J. Met. 35 (5) (1983) 18. [4] S. K˛ac, J. Kusi´nski, S. Hnat, A. Woldan, In˙zynieria Materiałowa 112 (1999) 365. [5] S. K˛ac, J. Kusi´nski, A. Woldan, In˙zynieria Materiałowa 123 (2001) 442. [6] S. K˛ac, J. Kusi´nski, A. Woldan, In˙zynieria Materiałowa 119 (2000) 319. [7] B. Major, R. Ebner, In˙zynieria Powierzchni 1 (1996) 53. [8] A. Bylica, A. Dziedzic, Krzepni˛ecie Metali i Stopów 42 (2000) 275.