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Structure of the carbon layer deposited on the steel surface after low pressure carburizing
Vacuum 85 (2010) 429e433
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Structure of the carbon layer deposited on the steel surface after low pressure carburizing ski b R. Gorockiewicz a, *, A. qapin a b
Faculty of Mechanical Engineering of the University of Zielona Góra, Podgórna 50, Zielona Góra, Poland , Poland Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, Poznan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 July 2007 Received in revised form 29 July 2010 Accepted 12 August 2010
During low pressure carburizing a carbon layer precipitates on the surface of steel parts. The structure of the carbon layer was tested by means of optical and electron microscopy and Raman spectroscopy as well. It was found out that the carbon layer is composed of fine-crystalline graphite. A sample was carburized (boost step), and was subsequently observed with an optical microscope. The dominant shade of the previously existing austenite grains was gray with few brighter areas (observation with a scanning microscope showed that these areas were darker). Within the gray areas the size of graphite crystallites was 7e20 nm, and within the brighter areas 1e7 nm. The diffusion step led to a change in the grain shade and to a decrease in the size of graphite crystallites. Gray-shaded grains were made of a mixture of gray and dark areas. The areas of homogenous brightness contained graphite made of crystallites of size 1e2 nm, and the areas where the gray shade was not homogenous contained grains of size up to 4 nm. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Low pressure carburizing Steel Structure Raman spectroscopy
1. Introduction Low pressure carburizing of steel means a diffusive flow of carbon mass from carburizing atmosphere to a surface layer of the items carburized. The process continues below the temperature of austenite transformation, usually within the range of 850e1000 C and comprises two steps: a boost stage and diffusion. In the boost step the carburizing atmosphere, consisting of aliphatic hydrocarbons (acetylene, ethylene, propane) additionally diluted with hydrogen or nitrogen [1e3], is let into the process chamber under lowered pressure (within the range of several or even several hundred Pa). The carburized layer after the boost step contains a high level of carbon on the surface. The diffusion steps aim to transport the carbon mass deeper into the surface layer, which results in thickening of the carburized layer. The diffusion step enables the diffusion of carbon inward from the carburized surface, resulting in a lower surface carbon content and a more gradual surface layerecore transition. The temperature, number and duration of the boost and diffusion steps are crucial for the carbon profile in the carburized layer of a particular type of steel. It is thought that the carburizing process in the boost step is a result of a catalytic reaction of the carburizing atmosphere with
the charge surface, which leads to carbon release in the form of atoms, the absorption of it by the surface and diffusive transport deeper into the material. On the surface of the carburized parts, the limit of carbon solubility in austenite is reached in a very short time (according to Ref. [4] only after 5 min under the temperature of 950 C) which is then followed by precipitation of carbon deposit [5] and, in the case of alloy steel carburizing, also by formation of carbides. The work described in Ref. [6] shows that the carbon deposit forms on the surface of the carburized parts at the moment of contact with the carburizing atmosphere. According to the research by Kula and co-workers [5] the carbon deposit formed is composed mainly of carbon atoms and of hydrocarbons of different saturation degree (hydrocarbon radicals) and does not constitute an additional source of carbon atoms. On the other hand, according to the work described in Refs. [6,7], the carbon deposit is an active carbon layer, composed of hydrocarbon radicals participating in carbon transport from the atmosphere to the austenite and constitutes an additional carbon source during the diffusion step. The aim of the this work was to use microscope observations and Raman Scattering to investigate the structure of the carbon layer, especially that precipitated on the surface of steel after low pressure carburizing. 2. Experimental details
* Corresponding author. E-mail addresses: [email protected], [email protected] (R. Gorockiewicz). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.08.005
Low pressure carburizing (LPC) was conducted in a vacuum furnace manufactured by Seco/Warwick by means of the FineCarbÒ
430
ski / Vacuum 85 (2010) 429e433 R. Gorockiewicz, A. Łapin
method [8]. The FineCarbÒ method is based on a fixed number of boost and diffusion steps. During the boost step a carburizing atmosphere consisting of a mixture of acetylene, ethylene and hydrogen is let into the process chamber. The boost step was carried out at a temperature of 950 C and using a pressure fluctuation of 6e7 hPa. The charge consisted of the ballast, comprising low-carbon steel sheets with a specified surface area, and samples of C15 steel. The samples were of 20 10 mm dia. with a polished flat face surface. The carburizing parameters were a low pressure (4 Pa), heating up to 950 C at a speed of 10 C/min, heating under low pressure conditions for 30 min at a temperature of 950 C, performing a specified number of boost and diffusion steps at this temperature and then cooling down to the room temperature in a nitrogen stream under a pressure of 0.5 MPa. The study included two processes: the 506 process 3n_15d_2 n (3 min of boost þ 15 min of diffusion þ 2 min of boost); the 505 process e 3n_15d_2n_1d (as process 506 þ 1 min of diffusion). The carburized face surface of the samples was subsequently observed in the polarized light of the optical microscope and the electron microscope (JEOL 5600JV). Moreover, Raman scattering of the light was investigated with a LabRAM HR 800 spectrometer (HORIBA Jobin Yvon) with different laser excitations: 633, 514, and 488 nm. The spectra were recorded from different places on the sample. Using the optical microscope connected to the spectrometer it was possible to choose a region on the sample. 3. Results and discussion The results of observations, measurements, and calculations of the samples under investigation are presented in Figs. 1e4 and in the Tables 1e3. Fig. 1 presents photographs of the surfaces taken with the optical microscope in the polarized light and with the
scanning microscope. It is possible to see the grains of the previously existing austenite as different shades outlined on the surface of both samples. In the case of the 506 process, the sample shade of the grain surface is gray (Fig. 1a and c) with outlined boundaries of grains with the settled carbon deposit (Fig. 1c), in the background of which one can also observe a small number of brighter areas (Fig. 1a). In the scanning microscope photographs these are shown as darker areas (Fig. 1c). The shade of the grain surface for the 505 process sample differs from the sample 506, which is especially visible in the photograph taken with the scanning microscope (Fig. 1d). The photograph presents grains with a gray shade and grains with a mixture of gray and dark areas (Fig. 1d) In the photograph taken with the optical microscope (Fig. 1b) the dark spots correspond to the dark areas in the scanning microscope photograph. The recorded Raman spectra of both samples as well as characteristic areas of the surface analyzed are illustrated in Figs. 2e4. These spectra show well-shaped bands of 1250e1650 cm1, showing lines typical for carbon materials [9e13]. It is well known that Raman scattering is a powerful tool for understanding the chemical structure of polymorph carbon varieties. Generally, in Raman spectroscopy analysis of carbon, the “G” and “D” lines are used to characterize the carbon sample. For instance, for single crystals of graphite only one line can be observed at 1580 5 cm1 (G line), which is assigned to the E2g variety of the infinite crystal. For other materials like stress-annealed pyrolitic graphite, commercial graphites, activated charcoal, lampblack, and vitreous carbon another line can be found at 1355 cm1 (D line) [9]. This band is attributed to the A1g variety of the small crystallites or boundaries of larger crystallites of graphite; it becomes Raman active due to the finite crystal size. The Raman intensity of this band is inversely proportional to the crystallite size and is caused by
Fig. 1. Examples of the structure of C15 steel surface: a) c) process 506; b) d) process 505. a), b) optical microscope e polarized light; c), d) scanning microscope e picture SEI.
ski / Vacuum 85 (2010) 429e433 R. Gorockiewicz, A. Łapin
431
Fig. 2. Examples of C15 steel structure with indicated measuring places of the Raman spectra. a), c) e process 506; b), d), e) e process 505.
a breakdown of the k-selection rule [9]. Moreover, the intensity of this band allows assessment of the crystallite size in the surface layer of carbon samples. The intensity of the 1355 cm1 line increases as one goes from stress-annealed pyrolitic graphite through commercial graphite to carbon black. This increase
corresponds to that of the “unorganized” carbon quantity in the samples and gives evidence to a decrease in the graphite crystal size. The D to G intensity ratio is used as a convenient measure of crystalline order and in-plane crystal size. It is well known that there is a linear relationship (La (nm) ¼ 4.4(ID/IG)1) between IG/ID
Fig. 3. Examples of spectra obtained on the surface of C15 steel e process 506. The spectra recorded on the areas shown in Fig. 2 a) and c).
Fig. 4. Examples of spectra obtained on the surface of C15 steel e process 505. The spectra recorded on the areas shown in Fig. 2 e), b) and d).
ski / Vacuum 85 (2010) 429e433 R. Gorockiewicz, A. Łapin
432
Table 1 Results of measurements and calculations for D and G bands recorded from the light areas of the 506 sample. Line
Center, cm1
Measurement 1 (506_7) D 1328.7 G 1583.9 Measurement 2 (506_8) D 1331.2 G 1582.2 Measurement 3 (506_6) D 1327.0 G 1581.5 Measurement 4 (506_5) D 1326.8 G 1581.5
Area F
Half width
FD/FG
Crystallite size La, nm
11930 2972
36.1 25.9
4.0
1.1
54680 92140
37.7 20.9
0.597
7.4
9765 8869
32.4 23.8
1.101
4.0
9554 8796
31.9 23.7
1.09
4.0
and the in-plane crystal size La obtained from x-ray data [9,10]. The linear relationship shows that the Raman intensity is proportional to the percentage of “boundary regions” in the sample. Moreover, the intensity ratio provides a quantitative description of the carbon microstructure, and is easily evaluated for each sample through a Gaussian curve-fitting algorithm. For example, for laser Raman studies of carbon films, intensity ratios are between 0.4 and 2.5 and correspond to in-plane crystal sizes between roughly 1.8 and 11.0 nm [11]. Diamond can be characterized by a narrow band at 1332 cm1, corresponding to three times degenerated vibration of the symmetry of T2g. Intermediate phases DLC (Diamond-Like Carbon) and amorphous carbon are represented by wide D and G bands [12]. The bands presented in Figs. 3 and 4 coming from the range of 1250e1650 cm1are narrow, which may suggest the good order of the carbon layers. The location and intensity of the bands suggest the occurrence of fine-crystalline graphite with a different crystallite size. In the brighter areas of the 506 sample (Figs. 1a, 2c, and 3 e 506_7) the G band (range approx. 1580 cm1) has a smaller intensity in comparison with the D band (range of 1350 cm1). This indicates the presence of graphite with crystallite size of 1.0e7.0 nm (Table 1). On the other hand, in darker areas of the sample 506 (Figs. 1a, 2a, and 3 e 506_3) the G band is better shaped than the D band and the calculated crystallite size falls within the range of 7.0e20.0 nm (Table 2). On the surface of the 505 sample in the areas of homogenous brightness (Figs. 1b,d, 2e and 4 e 505_4) graphite composed of crystallites of size 1.0e2.0 nm (Table 1) was found and, in the areas where the gray shade was not homogenous (the area with spots), (Figs. 1b,d, 2b and 4 e 505_1) the graphite had crystallites of size 4.0 nm (Table 4).
Table 2 Results of measurements and calculations for D and G bands recorded from the dark areas of the 506 sample. Line
Center, cm1
Measurement 1 (506_1) D 1329.0 G 1580.0 Measurement 2 (506_3) D 1330.0 G 1580.0 Measurement 3 (506_4) D 1330.0 G 1582.0 Measurement 4 (506_2) D 1329.3 G 1580.4
Area F
Half width
FD/FG
Crystallite size La, nm
10260 17650
38.9 18.4
0.582
7.6
5680 26000
36.2 15.2
0.218
20.2
35490 93720
37.1 20.8
0.379
11.6
9418 17680
36.8 17.6
0.533
8.3
Table 3 Results of measurements and calculations for D and G bands recorded from the spotty areas of the 505 sample. Line
Center, cm1
Measurement1 (505_4) D 1330.9 G 1583.9 Measurement 2 (505_6) D 1327.2 G 1584.4 Measurement 3 (505_2) D 1327.7 G 1584.2
Area F
Half width
FD/FG
Crystallite size La, nm
28360 10170
29.9 22.5
2.788
1.6
4297 1075
36.2 15.2
4.04
1.1
4975 1147
39.3 35.2
4.337
1.1
Table 4 Results of measurements and calculations for D and G bands recorded from the light areas of the 505 sample. Line
Center, cm1
Area F
Measurement 1 (505_1) D 1328.7 3224 G 1583.0 3404 Measurement 2 (505_5) D 1328.5 3304 G 1583.1 3622
Half width
FD/FG
Crystallite size La, nm
22.2 35.2
0.947
4.7
34.0 21.7
0.912
4.8
4. Conclusions Microscope observations (with the optical microscope-polarized light and the scanning microscope) of the C15 steel sample surfaces carburized with the use of The FineCarbÒ method revealed the occurrence of the outlined grains of previously existing austenite of different shade. For a sample after the boost steps only the dominant shade of the grains was gray with a few brighter areas (optical microscope). In the scanning microscope image of the grains these areas were darker. The diffusion steps led to a change in the shade, which was especially visible when observing the samples with the scanning microscope, which showed grains as a gray shade. The Raman investigation of the samples showed the existence of a carbon layer consisting of fine-crystalline graphite. In the brighter areas of the boost step only sample the size of graphite crystallites was 1.0e7.0 nm, and in the darker areas of 7.0e20.0 nm. The diffusion step led to a decrease in the size of the crystallites. The graphite that occurred in the areas with homogenous brightness had crystallites of size 1.0e2.0 nm, whereas, in the areas where the gray shade was not homogenous, the crystallites were 4.0 nm. This indicates that carbon settles in the form of fine-crystalline graphite during the boost steps. This graphite is subjected to further refinement in the diffusion steps. Taking into consideration the results of papers [5e7] it can be assumed that, during LPC of charges consisting of steel parts, an active carbon layer settles in the form of fine-crystalline graphite and hydrocarbon radicals on the surface of the parts. The layer mediates in transporting carbon from the gas atmosphere to the austenite and provides an additional source of carbon in the diffusion steps. Experience has shown that a carbon layer (carbon deposit) is formed during LPC. The layer is appropriately thin when the supply of carbon from the atmosphere slightly exceeds the amount of carbon which can be taken up by the charge. In this case the carburizing process runs properly. In the case of a carbon supply
ski / Vacuum 85 (2010) 429e433 R. Gorockiewicz, A. Łapin
largely exceeding the demand, the excess of carbon results in precipitaion of a considerable quantity of carbon deposit. In an extreme case this occurs in the form of a tarry substance or soot, not only hampering the control of the carburizing process, but also destroying the process chamber and the pump system of the furnace [14,15]. When the supply of carbon is lower than the demand, uncontrolled local decarburization of the charge surface takes place. For specified parameters of the carburizing process, i.e., the pressure and components of the gas atmosphere, the amount of the atmosphere let into the chamber must correspond to the size of the charge surface, so that the supply will only slightly exceed the demand, which results in formation of a thin carbon layer on the charge surface so that the carburizing process is controlled and runs without disruption. Acknowledgements The research was carried out under the programme R&N of Seco/Warwick Poland.
433
References [1] Gräfen W, Edenhofër B. Heat Treating of Metals 1999;4:79e85. [2] Preisser F, Seman N, Zenker WR. In: Proc. of The 1st International Automotive Heat treating Conference. Puerto Vallarta, Mexico 1998; pp. 135e147. [3] Kula P, Olejnik J, Heilman P. Mieszanina do nawe˛ glania w podcisnieniu. Patent P e 356754. _ [4] Dybowski K, Kula P. Inzynieria Materia1owa 2005;5(147):391e3. _ [5] Pietrasik R. Inzynieria Powierzchniowa 2005;5(147):437e9. _ [6] Gorockiewicz R. Inzynieria Materia1owa 2006;5(152):981e4. _ [7] Kula P, Siniarski D, Pietrasik R, Kaczmarek q, Korecki M, Adamek A. Inzynieria Materia1owa 2006;5(152):1092e6. [8] Kula P, Olejnik J, Kowalewski J. Heat treating & hardening of gears Chicago; 2004 March 16. p. 1. [9] Tuinstra F, Koenig JL. Journal of Chemical Physics 1970;53:1126e30. [10] Knight DS, White WB. Journal of Materials Research 1989;4:385e93. [11] Taylor CA, Wayne MF, Chiu WKS. Surface and Coatings Technology 2004;182:131e7. [12] Nistor LC, Van Landuyt J, Ralchenko VG, Obraztsova ED, Smolin AA. Diamond and Related Materials 1977;6:159e68. [13] Brunetto R, Baratta GA, Strazzula G. Journal of Applied Physics 2004;1 (96):380e6. [14] Osterman WM. Development experience in low-torr range vacuum carburizing. Industrial Heating; September, 2005. s. 95e99. [15] Kula P, Olejnik J. Industrial Heating; September, 2003. s. 99e102.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Structure of the carbon layer deposited on the steel surface after low pressure carburizing ski b R. Gorockiewicz a, *, A. qapin a b
Faculty of Mechanical Engineering of the University of Zielona Góra, Podgórna 50, Zielona Góra, Poland , Poland Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, Poznan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 July 2007 Received in revised form 29 July 2010 Accepted 12 August 2010
During low pressure carburizing a carbon layer precipitates on the surface of steel parts. The structure of the carbon layer was tested by means of optical and electron microscopy and Raman spectroscopy as well. It was found out that the carbon layer is composed of fine-crystalline graphite. A sample was carburized (boost step), and was subsequently observed with an optical microscope. The dominant shade of the previously existing austenite grains was gray with few brighter areas (observation with a scanning microscope showed that these areas were darker). Within the gray areas the size of graphite crystallites was 7e20 nm, and within the brighter areas 1e7 nm. The diffusion step led to a change in the grain shade and to a decrease in the size of graphite crystallites. Gray-shaded grains were made of a mixture of gray and dark areas. The areas of homogenous brightness contained graphite made of crystallites of size 1e2 nm, and the areas where the gray shade was not homogenous contained grains of size up to 4 nm. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Low pressure carburizing Steel Structure Raman spectroscopy
1. Introduction Low pressure carburizing of steel means a diffusive flow of carbon mass from carburizing atmosphere to a surface layer of the items carburized. The process continues below the temperature of austenite transformation, usually within the range of 850e1000 C and comprises two steps: a boost stage and diffusion. In the boost step the carburizing atmosphere, consisting of aliphatic hydrocarbons (acetylene, ethylene, propane) additionally diluted with hydrogen or nitrogen [1e3], is let into the process chamber under lowered pressure (within the range of several or even several hundred Pa). The carburized layer after the boost step contains a high level of carbon on the surface. The diffusion steps aim to transport the carbon mass deeper into the surface layer, which results in thickening of the carburized layer. The diffusion step enables the diffusion of carbon inward from the carburized surface, resulting in a lower surface carbon content and a more gradual surface layerecore transition. The temperature, number and duration of the boost and diffusion steps are crucial for the carbon profile in the carburized layer of a particular type of steel. It is thought that the carburizing process in the boost step is a result of a catalytic reaction of the carburizing atmosphere with
the charge surface, which leads to carbon release in the form of atoms, the absorption of it by the surface and diffusive transport deeper into the material. On the surface of the carburized parts, the limit of carbon solubility in austenite is reached in a very short time (according to Ref. [4] only after 5 min under the temperature of 950 C) which is then followed by precipitation of carbon deposit [5] and, in the case of alloy steel carburizing, also by formation of carbides. The work described in Ref. [6] shows that the carbon deposit forms on the surface of the carburized parts at the moment of contact with the carburizing atmosphere. According to the research by Kula and co-workers [5] the carbon deposit formed is composed mainly of carbon atoms and of hydrocarbons of different saturation degree (hydrocarbon radicals) and does not constitute an additional source of carbon atoms. On the other hand, according to the work described in Refs. [6,7], the carbon deposit is an active carbon layer, composed of hydrocarbon radicals participating in carbon transport from the atmosphere to the austenite and constitutes an additional carbon source during the diffusion step. The aim of the this work was to use microscope observations and Raman Scattering to investigate the structure of the carbon layer, especially that precipitated on the surface of steel after low pressure carburizing. 2. Experimental details
* Corresponding author. E-mail addresses: [email protected], [email protected] (R. Gorockiewicz). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.08.005
Low pressure carburizing (LPC) was conducted in a vacuum furnace manufactured by Seco/Warwick by means of the FineCarbÒ
430
ski / Vacuum 85 (2010) 429e433 R. Gorockiewicz, A. Łapin
method [8]. The FineCarbÒ method is based on a fixed number of boost and diffusion steps. During the boost step a carburizing atmosphere consisting of a mixture of acetylene, ethylene and hydrogen is let into the process chamber. The boost step was carried out at a temperature of 950 C and using a pressure fluctuation of 6e7 hPa. The charge consisted of the ballast, comprising low-carbon steel sheets with a specified surface area, and samples of C15 steel. The samples were of 20 10 mm dia. with a polished flat face surface. The carburizing parameters were a low pressure (4 Pa), heating up to 950 C at a speed of 10 C/min, heating under low pressure conditions for 30 min at a temperature of 950 C, performing a specified number of boost and diffusion steps at this temperature and then cooling down to the room temperature in a nitrogen stream under a pressure of 0.5 MPa. The study included two processes: the 506 process 3n_15d_2 n (3 min of boost þ 15 min of diffusion þ 2 min of boost); the 505 process e 3n_15d_2n_1d (as process 506 þ 1 min of diffusion). The carburized face surface of the samples was subsequently observed in the polarized light of the optical microscope and the electron microscope (JEOL 5600JV). Moreover, Raman scattering of the light was investigated with a LabRAM HR 800 spectrometer (HORIBA Jobin Yvon) with different laser excitations: 633, 514, and 488 nm. The spectra were recorded from different places on the sample. Using the optical microscope connected to the spectrometer it was possible to choose a region on the sample. 3. Results and discussion The results of observations, measurements, and calculations of the samples under investigation are presented in Figs. 1e4 and in the Tables 1e3. Fig. 1 presents photographs of the surfaces taken with the optical microscope in the polarized light and with the
scanning microscope. It is possible to see the grains of the previously existing austenite as different shades outlined on the surface of both samples. In the case of the 506 process, the sample shade of the grain surface is gray (Fig. 1a and c) with outlined boundaries of grains with the settled carbon deposit (Fig. 1c), in the background of which one can also observe a small number of brighter areas (Fig. 1a). In the scanning microscope photographs these are shown as darker areas (Fig. 1c). The shade of the grain surface for the 505 process sample differs from the sample 506, which is especially visible in the photograph taken with the scanning microscope (Fig. 1d). The photograph presents grains with a gray shade and grains with a mixture of gray and dark areas (Fig. 1d) In the photograph taken with the optical microscope (Fig. 1b) the dark spots correspond to the dark areas in the scanning microscope photograph. The recorded Raman spectra of both samples as well as characteristic areas of the surface analyzed are illustrated in Figs. 2e4. These spectra show well-shaped bands of 1250e1650 cm1, showing lines typical for carbon materials [9e13]. It is well known that Raman scattering is a powerful tool for understanding the chemical structure of polymorph carbon varieties. Generally, in Raman spectroscopy analysis of carbon, the “G” and “D” lines are used to characterize the carbon sample. For instance, for single crystals of graphite only one line can be observed at 1580 5 cm1 (G line), which is assigned to the E2g variety of the infinite crystal. For other materials like stress-annealed pyrolitic graphite, commercial graphites, activated charcoal, lampblack, and vitreous carbon another line can be found at 1355 cm1 (D line) [9]. This band is attributed to the A1g variety of the small crystallites or boundaries of larger crystallites of graphite; it becomes Raman active due to the finite crystal size. The Raman intensity of this band is inversely proportional to the crystallite size and is caused by
Fig. 1. Examples of the structure of C15 steel surface: a) c) process 506; b) d) process 505. a), b) optical microscope e polarized light; c), d) scanning microscope e picture SEI.
ski / Vacuum 85 (2010) 429e433 R. Gorockiewicz, A. Łapin
431
Fig. 2. Examples of C15 steel structure with indicated measuring places of the Raman spectra. a), c) e process 506; b), d), e) e process 505.
a breakdown of the k-selection rule [9]. Moreover, the intensity of this band allows assessment of the crystallite size in the surface layer of carbon samples. The intensity of the 1355 cm1 line increases as one goes from stress-annealed pyrolitic graphite through commercial graphite to carbon black. This increase
corresponds to that of the “unorganized” carbon quantity in the samples and gives evidence to a decrease in the graphite crystal size. The D to G intensity ratio is used as a convenient measure of crystalline order and in-plane crystal size. It is well known that there is a linear relationship (La (nm) ¼ 4.4(ID/IG)1) between IG/ID
Fig. 3. Examples of spectra obtained on the surface of C15 steel e process 506. The spectra recorded on the areas shown in Fig. 2 a) and c).
Fig. 4. Examples of spectra obtained on the surface of C15 steel e process 505. The spectra recorded on the areas shown in Fig. 2 e), b) and d).
ski / Vacuum 85 (2010) 429e433 R. Gorockiewicz, A. Łapin
432
Table 1 Results of measurements and calculations for D and G bands recorded from the light areas of the 506 sample. Line
Center, cm1
Measurement 1 (506_7) D 1328.7 G 1583.9 Measurement 2 (506_8) D 1331.2 G 1582.2 Measurement 3 (506_6) D 1327.0 G 1581.5 Measurement 4 (506_5) D 1326.8 G 1581.5
Area F
Half width
FD/FG
Crystallite size La, nm
11930 2972
36.1 25.9
4.0
1.1
54680 92140
37.7 20.9
0.597
7.4
9765 8869
32.4 23.8
1.101
4.0
9554 8796
31.9 23.7
1.09
4.0
and the in-plane crystal size La obtained from x-ray data [9,10]. The linear relationship shows that the Raman intensity is proportional to the percentage of “boundary regions” in the sample. Moreover, the intensity ratio provides a quantitative description of the carbon microstructure, and is easily evaluated for each sample through a Gaussian curve-fitting algorithm. For example, for laser Raman studies of carbon films, intensity ratios are between 0.4 and 2.5 and correspond to in-plane crystal sizes between roughly 1.8 and 11.0 nm [11]. Diamond can be characterized by a narrow band at 1332 cm1, corresponding to three times degenerated vibration of the symmetry of T2g. Intermediate phases DLC (Diamond-Like Carbon) and amorphous carbon are represented by wide D and G bands [12]. The bands presented in Figs. 3 and 4 coming from the range of 1250e1650 cm1are narrow, which may suggest the good order of the carbon layers. The location and intensity of the bands suggest the occurrence of fine-crystalline graphite with a different crystallite size. In the brighter areas of the 506 sample (Figs. 1a, 2c, and 3 e 506_7) the G band (range approx. 1580 cm1) has a smaller intensity in comparison with the D band (range of 1350 cm1). This indicates the presence of graphite with crystallite size of 1.0e7.0 nm (Table 1). On the other hand, in darker areas of the sample 506 (Figs. 1a, 2a, and 3 e 506_3) the G band is better shaped than the D band and the calculated crystallite size falls within the range of 7.0e20.0 nm (Table 2). On the surface of the 505 sample in the areas of homogenous brightness (Figs. 1b,d, 2e and 4 e 505_4) graphite composed of crystallites of size 1.0e2.0 nm (Table 1) was found and, in the areas where the gray shade was not homogenous (the area with spots), (Figs. 1b,d, 2b and 4 e 505_1) the graphite had crystallites of size 4.0 nm (Table 4).
Table 2 Results of measurements and calculations for D and G bands recorded from the dark areas of the 506 sample. Line
Center, cm1
Measurement 1 (506_1) D 1329.0 G 1580.0 Measurement 2 (506_3) D 1330.0 G 1580.0 Measurement 3 (506_4) D 1330.0 G 1582.0 Measurement 4 (506_2) D 1329.3 G 1580.4
Area F
Half width
FD/FG
Crystallite size La, nm
10260 17650
38.9 18.4
0.582
7.6
5680 26000
36.2 15.2
0.218
20.2
35490 93720
37.1 20.8
0.379
11.6
9418 17680
36.8 17.6
0.533
8.3
Table 3 Results of measurements and calculations for D and G bands recorded from the spotty areas of the 505 sample. Line
Center, cm1
Measurement1 (505_4) D 1330.9 G 1583.9 Measurement 2 (505_6) D 1327.2 G 1584.4 Measurement 3 (505_2) D 1327.7 G 1584.2
Area F
Half width
FD/FG
Crystallite size La, nm
28360 10170
29.9 22.5
2.788
1.6
4297 1075
36.2 15.2
4.04
1.1
4975 1147
39.3 35.2
4.337
1.1
Table 4 Results of measurements and calculations for D and G bands recorded from the light areas of the 505 sample. Line
Center, cm1
Area F
Measurement 1 (505_1) D 1328.7 3224 G 1583.0 3404 Measurement 2 (505_5) D 1328.5 3304 G 1583.1 3622
Half width
FD/FG
Crystallite size La, nm
22.2 35.2
0.947
4.7
34.0 21.7
0.912
4.8
4. Conclusions Microscope observations (with the optical microscope-polarized light and the scanning microscope) of the C15 steel sample surfaces carburized with the use of The FineCarbÒ method revealed the occurrence of the outlined grains of previously existing austenite of different shade. For a sample after the boost steps only the dominant shade of the grains was gray with a few brighter areas (optical microscope). In the scanning microscope image of the grains these areas were darker. The diffusion steps led to a change in the shade, which was especially visible when observing the samples with the scanning microscope, which showed grains as a gray shade. The Raman investigation of the samples showed the existence of a carbon layer consisting of fine-crystalline graphite. In the brighter areas of the boost step only sample the size of graphite crystallites was 1.0e7.0 nm, and in the darker areas of 7.0e20.0 nm. The diffusion step led to a decrease in the size of the crystallites. The graphite that occurred in the areas with homogenous brightness had crystallites of size 1.0e2.0 nm, whereas, in the areas where the gray shade was not homogenous, the crystallites were 4.0 nm. This indicates that carbon settles in the form of fine-crystalline graphite during the boost steps. This graphite is subjected to further refinement in the diffusion steps. Taking into consideration the results of papers [5e7] it can be assumed that, during LPC of charges consisting of steel parts, an active carbon layer settles in the form of fine-crystalline graphite and hydrocarbon radicals on the surface of the parts. The layer mediates in transporting carbon from the gas atmosphere to the austenite and provides an additional source of carbon in the diffusion steps. Experience has shown that a carbon layer (carbon deposit) is formed during LPC. The layer is appropriately thin when the supply of carbon from the atmosphere slightly exceeds the amount of carbon which can be taken up by the charge. In this case the carburizing process runs properly. In the case of a carbon supply
ski / Vacuum 85 (2010) 429e433 R. Gorockiewicz, A. Łapin
largely exceeding the demand, the excess of carbon results in precipitaion of a considerable quantity of carbon deposit. In an extreme case this occurs in the form of a tarry substance or soot, not only hampering the control of the carburizing process, but also destroying the process chamber and the pump system of the furnace [14,15]. When the supply of carbon is lower than the demand, uncontrolled local decarburization of the charge surface takes place. For specified parameters of the carburizing process, i.e., the pressure and components of the gas atmosphere, the amount of the atmosphere let into the chamber must correspond to the size of the charge surface, so that the supply will only slightly exceed the demand, which results in formation of a thin carbon layer on the charge surface so that the carburizing process is controlled and runs without disruption. Acknowledgements The research was carried out under the programme R&N of Seco/Warwick Poland.
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