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Characterization of complex (B + C) diffusion layers formed on chromium and nickel-based low-carbon steel
Applied Surface Science 202 (2002) 252–260
Characterization of complex (B þ C) diffusion layers formed on chromium and nickel-based low-carbon steel A. Pertek, M. Kulka* Poznan University of Technology, Institute of Materials Science and Engineering, Pl. M.Sklodowskiej-Curie 5, 60-965 Poznan, Poland Received 14 February 2002; received in revised form 14 February 2002; accepted 25 August 2002
Abstract Combined surface hardening with boron and carbon was used for low-carbon chromium and nickel-based steels. The microstructure, boron contents, carbon profiles and chosen properties of borided layers produced on the carburized steels have been examined. These complex (B þ C) layers are termed borocarburized layers. The microhardness profiles and wear resistance of these layers have been studied. In the microstructure of the borocarburized layer two zones have been observed: iron borides (FeB þ Fe2 B) and a carburized layer. The depth (70–125 mm) and microhardness (1500–1800 HV) of iron borides zone have been found. The carbon content (1.2–1.94 wt.%) and microhardness (700–950 HV) beneath iron borides zone have been determined. The microhardness gradient in borocarburized layer has been reduced in comparison with the only borided layer. An increase of distance from the surface is accompanied by a decrease of carbon content and microhardness in the carburized zone. The carbon and microhardness profiles of borided, carburized and borocarburized layers have been presented. A positive influence of complex layers (B þ C) on the wear resistance was determined. The wear resistance of the borocarburized layer was determined to be greater in comparison with that for only borided or only carburized layers. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Diffusion; Carburizing; Boriding; Borocarburizing; Wear resistance
1. Introduction Diffusion boriding is a thermochemical treatment that permits boride layers of good performance properties to be produced on steels [1–5]. The borided steel surfaces exhibit extreme hardness, high wear resistance and strong corrosion resistance. A prospective line for future development of this technique is the production of multicomponent and composite borided layers. These layers can be formed * Corresponding author. Tel.: þ48-616-653573; fax: þ48-616-653576. E-mail addresses: [email protected] (A. Pertek), [email protected] (M. Kulka).
using fixed methods. During B–C-nitriding process [6] boron, carbon and nitrogen atoms were diffused into the surface of 1045 steel. This reduces the hardness gradient of the boride layer to the substrate and improves the mechanical and anticorrosion properties in comparison with only borided layer. In the other process [3,7,8] borided layers were produced using, prior to boriding, electroless (autocatalytic) nickel plating of the steel parts to be borided. The layers thus obtained show a good resistance to frictional wear and corrosion. The effect of precarburizing treatment on the morphology of the boride layer has been studied [9]. In this paper, the microstructure, boron content at the surface, carbon profiles and chosen properties of borided layers produced on the previously carburized
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 9 4 0 - 6
A. Pertek, M. Kulka / Applied Surface Science 202 (2002) 252–260
chromium and nickel-based low-carbon steels have been investigated. These complex diffusion layers were termed borocarburized layers.
253
Table 1 Chemical composition of used steels Material
C
Cr
Ni
Mn
Si
P
S
Steel 1 Steel 2
0.15 0.13
1.69 0.76
1.53 3.03
0.57 0.50
0.22 0.32
0.035 0.025
0.035 0.030
2. Experimental procedure Chemical compositions of two low-carbon chromium and nickel-based steels used by experiments is presented in Table 1. The carburized layers have been formed in controlled carburizing atmosphere [10] at 930 8C. The arrangements are shown and described in Fig. 1. The atmosphere of a fixed composition (cracked methanol with propane–butane gas) has been used. The specimens were put into the quartz tube (24) and carburized at high carbon potential (1.25% C) for 6 or 20 h. Carbon potential Cp was controlled by means of dew-point measuring system (16) and by pure Fe–C foils carburized until equilibrium with atmosphere was obtained. Carbon content in pure Fe–C alloy corresponds to carbon potential of atmosphere. After carburizing, the specimens were slowly cooled in used atmosphere.
Gas boriding [11] was carried out by means of the apparatus shown in Fig. 2. The specimens were put into the quartz tube (2). Before boriding the system was checked to see by vacuummeter (7) that the air was removed by vacuum pump (5). A gas mixture of hydrogen and BCl3 (up to 5 vol.%) was fed through at a flow rate of 50 l/h after the furnace (1) had reached a temperature of 950 8C. Then the furnace was turned up so that the specimens reached the required temperature. After heating for 3 h the furnace was moved down and the specimens were cooled in used atmosphere. The parameters of the all the thermochemical processes were selected as in Table 2. Two types of specimens were investigated. The specimens in the shape of cylinders of about 20 mm dia and 100 mm long were used in the chemical composition analysis of diffusion layers. Carbon profiles
Fig. 1. The arrangements used by carburizing: (1) methanol; (2) peristaltic miniflow pump; (3) generator of atmosphere; (4) power supply system; (5) water cooler; (6) manostat; (7) rotameter; (8) valve; (9) ethyl acetate; (10) thermostat; (11) bottle with propane–butane; (12) manometer; (13) pressure-reducing valve; (14) U-tube manometer; (15) ruff; (16) dew-point measuring system; (17) temperature measuring system; (18) power supply system and temperature regulation; (19) carburizing furnace; (20) regulating thermoelement; (21) gas input; (22) gas output; (23) measuring thermoelement; (24) internal and external quartz tubes; (25) ventilator; (26) motor of ventilator.
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Fig. 2. The arrangements used by boriding: (1) boriding furnace; (2) quartz tube; (3) power supply system and temperature regulation; (4) temperature measuring system; (5) vacuum pump; (6) vacuum measuring probe; (7) vacuummeter; (8) bottle with hydrogen; (9) bottle with nitrogen; (10) BCl3; (11) cryostat; (12) temperature measuring system; (13) Dewar flask with liquid nitrogen; (14) rotameters; (15) manostat; (16) device for rinsing outlet gases. Table 2 The parameters of thermochemical processes Type of process
Carburizing parameters Cpa
Carburizing (1) Carburizing (2) Carburizing (3) Boriding Borocarburizing (1) Borocarburizing (2) a
(wt.%)
1.25 1.25 0.80 – 1.25 1.25
Boriding parameters
Temperature (8C)
Time (h)
Temperature (8C)
Time (h)
930 930 930 – 930 930
6 20 3.5 – 6 20
– – – 950 950 950
– – – 3 3 3
Carbon potential.
through carburized and borocarburized layers were determined by chemical analysis of successive layers. The polished cross-sections were observed by use of a SEM Philips XL 30. Boron contents in iron borides and carbon contents underneath the iron borides after boriding and borocarburizing were measured by an EDAX X-ray microanalyzer equipped with EDS, using a 358 take-off angle. The accelerating voltage was 12 kV. The detector Si (Li) with ultra thin window, standardless quantitative analysis, matrix correction algorithms ZAF for SEM bulk analysis have been applied. The second type of specimens was used to research the functional properties. To examine frictional wear resistance of the diffusion layers a research
apparatus, in which a roller constitutes a sample and sintered carbide plate a countersample, has been applied (Fig. 3). The frictional tests have been made under the loading P ¼ 147N and the sample speed of
Fig. 3. Scheme of wear. P ¼ 147N, rotational speed n ¼ 250 min1.
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255
0.26 m/s, under conditions of dry friction (unlubricated sliding contact). Wear resistance has been defined as specimen mass loss per friction surface and unit of time. The factor of mass wear intensity Izw was determined as: Izw ¼ Dm=ðFtÞ, where Dm is the mass loss (mg); F the friction surface (cm2); t the friction time (h). In second case, wear resistance has been determined on the base of the dia measurements of samples. The factor of linear wear intensivity was defined as: Izl ¼ Dl=t, where Dl is the radius loss (mm); t the friction time (h). To determine microhardness profiles (Vickers method), the apparatus ZWICK 3212 B has been applied. Before the microhardness and wear resistance tests, carburized, borided, and borocarburized specimens were oil quenched at 850 8C and tempered at 150 8C.
[14] and show that the two-phase iron boride layers (FeB þ Fe2 B) were formed after borocarburizing of used steels. Microstructures of cross-sections of borocarburized and hardened layers are shown in Figs. 4–7. Two zones in borocarburized layer are visible: an iron
3. Results and discussion
Fig. 4. SEM microstructure of borocarburized (1) and hardened chromium and nickel-based steel 1 (1.69% Cr, 1.53% Ni).
3.1. Microstructure of borocarburized layers The two types of borided layers can be produced on steels: the two-phase iron boride layer (FeB þ Fe2 B) and the layer with only Fe2B. The results of boron measurements at the surface of borocarburized layers are visible in Table 3. The boron contents obtained at the surface of formed diffusion layers are caused by the presence of iron borides FeB. The results are in agreement with theoretical boron content 16.23 wt.% Table 3 The results of boron content measurements at the surface a
Type of process
Material
B (wt.%)
Borocarburizing (1)
Steel 1
15.84 1.21 16.05 1.30 8.80 0.85
Steel 2
16.54 1.37 16.02 1.64 14.85 1.27
Steel 1
16.81 1.40 15.39 1.45 16.14 1.38
Steel 2
16.86 1.27 15.86 1.54 15.50 1.30
Borocarburizing (2)
a
Boron content.
Fig. 5. SEM microstructure of borocarburized (2) and hardened chromium and nickel-based steel 1 (1.69% Cr, 1.53% Ni).
Fig. 6. SEM microstructure of borocarburized (1) and hardened chromium and nickel-based steel 1 (0.76% Cr, 3.03% Ni).
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A. Pertek, M. Kulka / Applied Surface Science 202 (2002) 252–260 Table 4 The results of carbon content measurements
Fig. 7. SEM microstructure of borocarburized (2) and hardened chromium and nickel-based steel 1 (0.76% Cr, 3.03% Ni).
borides zone (FeB þ Fe2 B) and a carburized zone. The iron borides zone after borocarburizing (1) exhibits higher case depth than that obtained after borocarburizing (2). The iron borides zone (FeB þ Fe2 B) formed on steel 1 after first process reached a thickness of about 120 mm (Fig. 4), and after second process, about 110 mm (Fig. 5). It is an effect of fixed carbon content at the surface before boriding and it is in agreement with other results [9]. In case of borocarburized layers formed on steel 2 the difference is more visible. After borocarburizing (1) the iron borides zone reached a thickness of about 150 mm (Fig. 6) and after borocarburizing (2), 70 mm (Fig. 7). The higher cementite content beneath the iron borides were observed after borocarburizing (2). It is an effect of higher carbon content presence. The research results of carbon content at the surface after carburizing (Cs) and beneath iron borides after boriding (Cb) are presented in Table 4. The essential influence of chromium and nickel contents in steels on the carbon activity in austenite during carburizing [10,12,13] was demonstrated. The activity of carbon is diminished by chromium and increased by nickel. The influence of chromium is higher than that of nickel. The carbon content at the surface of steel 1 (1.69% Cr, 1.53% Ni) obtained after carburizing is higher than that determined from gas composition of the carburizing atmosphere (carbon potential 1.25% C). The carburizing (2) during the longer time (20 h) is accompanied by higher carbon content at the surface (2.72% Cs). After boriding of previously carburized steel 1 (borocarburizing (2)), carbon content 1.94% Cb was determined beneath iron borides. In the second case (borocarburizing (1)), the
Type of process
Material
Csa (wt.%)
Cbb (wt.%)
Carburizing (1)
Steel 1 Steel 2
2.35 1.38
–
Carburizing (2)
Steel 1 Steel 2
2.72 1.43
–
Carburizing (3)
Steel 1
0.85
– c
0.53
Boriding
Steel 1
0.15
Borocarburizing (1)
Steel 1 Steel 2
2.35 1.38
1.50 1.23
Borocarburizing (2)
Steel 1 Steel 2
2.72 1.43
1.94 1.20
a
Carbon content at the surface after carburizing. Carbon content beneath the iron borides after boriding. c Carbon content in steel 1 before boriding. b
carbon content 2.35% Cs and 1.5% Cb were obtained, respectively. The carbon contents at the surface of steel 2 (0.76% Cr, 3.03% Ni) after carburizing are slightly higher than carbon potential of atmosphere (1.25% C). It is a result of much higher nickel content in comparison with chromium content in this steel. The carburizing during the used times (6 and 20 h) is accompanied by nearly the same carbon content at the surface (1.38 and 1.43% Cs). After boriding of previously carburized steel 2 beneath iron borides carbon contents 1.23 and 1.20% Cb were obtained, respectively. 3.2. Carbon profiles after carburizing and borocarburizing The carbon profiles after carburizing and beneath the iron borides zone in borocarburized layers are shown in Figs. 8–11. The time of carburizing has an influence on carbon profiles. After carburizing (2) for a longer time (20 h), the higher carbon contents at the surface and the higher case depth were obtained (Figs. 10 and 11) than after carburizing (1) (Figs. 8 and 9). During the boriding of carburized layer the carbon is moved in a core direction by following the boron diffusion front. This is also caused by the difference in carbon activity between surface area and core of steel. In effect the decrease of carbon content under iron borides and increase in the core of steel have been observed.
A. Pertek, M. Kulka / Applied Surface Science 202 (2002) 252–260
Fig. 8. Carbon profiles in carburized (1) and borocarburized (1) layers formed on steel 1.
Fig. 9. Carbon profiles in carburized (1) and borocarburized (1) layers formed on steel 2.
257
Fig. 11. Carbon profiles in carburized (2) and borocarburized (2) layers formed on steel 2.
Fig. 12. Carbon profile in carburized (3) layer formed on steel 1.
In Fig. 12, the carbon profile in steel 1 after typical carburizing (3) has been shown. The wear resistance of this carburized layer have been compared with properties of complex diffusion layers (after borocarburizing). 3.3. Microhardness of borided, carburized and borocarburized layers
Fig. 10. Carbon profiles in carburized (2) and borocarburized (2) layers formed on steel 1.
The microhardness profiles in hardened diffusion layers are presented in Figs. 13–16. The highest microhardness at the surface after boriding and borocarburizing was obtained. The microhardness of the iron borides zone (FeB þ Fe2 B) 1500–1800 HV has been determined. The higher microhardness (about 1800 HV) is caused by the presence of iron borides FeB in the surface zone. The lower values (about 1500 HV) are
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Fig. 13. Microhardness profiles in hardened diffusion layers after boriding, carburizing (1) and borocarburizing (1).
Fig. 14. Microhardness profiles in hardened diffusion layers after carburizing (1) and borocarburizing (1).
Fig. 16. Microhardness profiles in hardened diffusion layers after carburizing (2) and borocarburizing (2).
accompanied by iron borides Fe2B. The highest microhardness of iron borides (1800 HV) has been obtained only in case of borided layers (Figs. 13 and 15). The boron contents obtained at the surface of hardened borocarburized layers (Table 4) are caused by the presence of iron borides FeB, too. The microhardness of hardened carburizing layer at the surface has been determined. After carburizing of steel 1 (Figs. 13 and 15) the microhardness obtained at the surface (about 950 HV) is higher than that (700– 800 HV) after carburizing of steel 2 (Figs. 14 and 16). In case of borocarburized layers the microhardness below the iron borides zone is the same as that obtained at the surface of only carburized layers. An increase of the distance from the surface is accompanied by a decrease in the microhardness, but higher values near a core of steel were observed in case of borocarburized layers. It is related to the appearance of higher carbon contents in comparison with only carburized layers. A lower gradient of microhardness of the iron boride zone to the substrate in borocarburized layers in comparison with the only borided layer has been observed (Figs. 13 and 15). The microhardness of the hardened borocarburized layers below iron borides (700–950 HV) is higher than that obtained by boriding and hardening processes (450 HV). 3.4. Resistance to wear
Fig. 15. Microhardness profiles in hardened diffusion layers after boriding, carburizing (2) and borocarburizing (2).
The results of wear resistance research are presented in Table 5. The lowest values of mass and
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259
Table 5 The results of wear resistance tests Type of process
Material Cba (wt.%)
Izwb Izlc (mg/(cm2 h)) (mm/h)
Carburizing (3)
Steel 1
–
1.67
2.17
Boriding
Steel 1
0.53
1.70
2.51
Borocarburizing (1)
Steel 1 Steel 2
1.50 1.23
1.35 1.25
2.00 1.86
a
Carbon content beneath the iron borides after boriding. Mass wear intensity factor. c Linear wear intensity factor. b
linear wear intensity factors have been observed in the case of borocarburized layers. Complex diffusion layers (B þ C) have a positive influence on the wear resistance. The results of specimen mass loss on a friction surface unit in a function of friction time are shown in Fig. 17 (hardened carburized and borided layers) and Fig. 18 (hardened borocarburized layers). The research of specimen radius loss in a function of friction time is presented in Figs. 19 and 20, respectively. The wear intensity factors are defined as the directional coefficients of linear wear equations. When comparing the resistance to frictional wear of the borided layers on steel 1 previously subjected or not subjected to carburization, we can see that the borided layers formed on carburized steel 1 show a higher resistance to frictional wear. The higher wear resistance of borocarburized layer has been shown in comparison with only carburized layer, too. The lowest wear intensity factors and highest wear resistance
Fig. 17. Weight wear of the borided and carburized (3) layers formed on steel 1.
Fig. 18. Weight wear of the borocarburized (1) layers formed on used steels.
Fig. 19. Linear wear of the borided and carburized (3) layers formed on steel 1.
Fig. 20. Linear wear of the borocarburized (1) layers formed on used steels.
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have been obtained in case of borocarburized layer produced on steel 2. Probably, it is related to the influence of carbon contents beneath iron borides (Table 5).
been found. The essentially higher wear resistance of borocarburized layer has been obtained in comparison with only borided or only carburized layers. References
4. Conclusions Combined surface hardening with boron and carbon was proposed for chromium and nickel-based lowcarbon steels. Gas boriding applied to parts that have been previously carburized enables the production of complex borocarburized layers. In the microstructure of these layers two zones have been observed: iron borides and carburized zone. The depth (70–150 mm) and microhardness (1500–1800 HV) of iron borides zone have been found. The depth of this zone and carbon profile underneath can be controlled by carburizing parameters before boriding. The boron contents obtained at the surface of formed diffusion layers are caused by the presence of two-phase iron borides zone (FeB þ Fe2 B). The gradient of microhardness of the iron borides zone to the substrate in borocarburized layers has been reduced in comparison with that of an only borided layer. The carbon contents (1.2– 1.94 wt.%) and microhardness (700–950 HV) below iron borides zone have been determined. An increase of distance from the surface is accompanied by a gradual decrease of carbon content and microhardness in carburized zone. An advantageous influence of complex (B þ C) layers on the frictional wear resistance has
[1] A. Graf Von Matuschka, Borieren, Hanse´ r (Carl), Munchen, Wien, 1977. [2] B. Venkataraman, G. Sundararajan, Surf. Coat. Technol. 73 (1995) 177–184. [3] T. Wierzchon˜ , P. Bielin˜ ski, Mater. Manuf. Process. 10 (1995) 121. [4] H.J. Hunger, G. Trute, Heat Treat. Met. 21 (2) (1994) 31–39. [5] C.H. Xu, J.K. Xi, W. Gao, J. Mater. Process. Technol. 65 (1997) 94–98. [6] P.X. Yan, Y.C. Su, Mater. Chem. Phys. 39 (1995) 304–308. [7] T. Wierzchon˜ , P. Bielin˜ ski, K. Sikorski, Surf. Coat. Technol. 73 (1995) 121. [8] K. Sikorski, T. Wierzchon˜ , P. Bielin˜ ski, J. Mater. Sci. 33 (1998) 811. [9] M.R. Toroghinezhad, M. Salehi, F. Ashrafizadeh, Mater. Manuf. Process. 12 (1) (1997) 117–123. [10] M. Kulka, M. Przylecka, W. Gestwa, Materials Science Forum, Trans. Tech. Publications, Switzerland, vol. 163–165, 1994, pp. 87–92. [11] A. Pertek, Materials Science Forum, Trans. Tech. Publications, Switzerland, vol. 163–165, 1994, pp. 323–328. [12] T. Wada, H. Wada, J.F. Elliott, J. Chipman, Met. Trans. 2 (1971) 2199–2208. [13] M. Kulka, M. Przylecka, W. Gestwa, in: Proceedings of the Second International Conference on Carburizing and Nitriding with Atmospheres, Cleveland Marriott Society Center, Ohio (USA), 6–8 December 1995, p. 185–188. [14] O. Kubaschewski, Iron—Binary Phase Diagrams, Springer, Berlin, 1982.
Characterization of complex (B þ C) diffusion layers formed on chromium and nickel-based low-carbon steel A. Pertek, M. Kulka* Poznan University of Technology, Institute of Materials Science and Engineering, Pl. M.Sklodowskiej-Curie 5, 60-965 Poznan, Poland Received 14 February 2002; received in revised form 14 February 2002; accepted 25 August 2002
Abstract Combined surface hardening with boron and carbon was used for low-carbon chromium and nickel-based steels. The microstructure, boron contents, carbon profiles and chosen properties of borided layers produced on the carburized steels have been examined. These complex (B þ C) layers are termed borocarburized layers. The microhardness profiles and wear resistance of these layers have been studied. In the microstructure of the borocarburized layer two zones have been observed: iron borides (FeB þ Fe2 B) and a carburized layer. The depth (70–125 mm) and microhardness (1500–1800 HV) of iron borides zone have been found. The carbon content (1.2–1.94 wt.%) and microhardness (700–950 HV) beneath iron borides zone have been determined. The microhardness gradient in borocarburized layer has been reduced in comparison with the only borided layer. An increase of distance from the surface is accompanied by a decrease of carbon content and microhardness in the carburized zone. The carbon and microhardness profiles of borided, carburized and borocarburized layers have been presented. A positive influence of complex layers (B þ C) on the wear resistance was determined. The wear resistance of the borocarburized layer was determined to be greater in comparison with that for only borided or only carburized layers. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Diffusion; Carburizing; Boriding; Borocarburizing; Wear resistance
1. Introduction Diffusion boriding is a thermochemical treatment that permits boride layers of good performance properties to be produced on steels [1–5]. The borided steel surfaces exhibit extreme hardness, high wear resistance and strong corrosion resistance. A prospective line for future development of this technique is the production of multicomponent and composite borided layers. These layers can be formed * Corresponding author. Tel.: þ48-616-653573; fax: þ48-616-653576. E-mail addresses: [email protected] (A. Pertek), [email protected] (M. Kulka).
using fixed methods. During B–C-nitriding process [6] boron, carbon and nitrogen atoms were diffused into the surface of 1045 steel. This reduces the hardness gradient of the boride layer to the substrate and improves the mechanical and anticorrosion properties in comparison with only borided layer. In the other process [3,7,8] borided layers were produced using, prior to boriding, electroless (autocatalytic) nickel plating of the steel parts to be borided. The layers thus obtained show a good resistance to frictional wear and corrosion. The effect of precarburizing treatment on the morphology of the boride layer has been studied [9]. In this paper, the microstructure, boron content at the surface, carbon profiles and chosen properties of borided layers produced on the previously carburized
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 9 4 0 - 6
A. Pertek, M. Kulka / Applied Surface Science 202 (2002) 252–260
chromium and nickel-based low-carbon steels have been investigated. These complex diffusion layers were termed borocarburized layers.
253
Table 1 Chemical composition of used steels Material
C
Cr
Ni
Mn
Si
P
S
Steel 1 Steel 2
0.15 0.13
1.69 0.76
1.53 3.03
0.57 0.50
0.22 0.32
0.035 0.025
0.035 0.030
2. Experimental procedure Chemical compositions of two low-carbon chromium and nickel-based steels used by experiments is presented in Table 1. The carburized layers have been formed in controlled carburizing atmosphere [10] at 930 8C. The arrangements are shown and described in Fig. 1. The atmosphere of a fixed composition (cracked methanol with propane–butane gas) has been used. The specimens were put into the quartz tube (24) and carburized at high carbon potential (1.25% C) for 6 or 20 h. Carbon potential Cp was controlled by means of dew-point measuring system (16) and by pure Fe–C foils carburized until equilibrium with atmosphere was obtained. Carbon content in pure Fe–C alloy corresponds to carbon potential of atmosphere. After carburizing, the specimens were slowly cooled in used atmosphere.
Gas boriding [11] was carried out by means of the apparatus shown in Fig. 2. The specimens were put into the quartz tube (2). Before boriding the system was checked to see by vacuummeter (7) that the air was removed by vacuum pump (5). A gas mixture of hydrogen and BCl3 (up to 5 vol.%) was fed through at a flow rate of 50 l/h after the furnace (1) had reached a temperature of 950 8C. Then the furnace was turned up so that the specimens reached the required temperature. After heating for 3 h the furnace was moved down and the specimens were cooled in used atmosphere. The parameters of the all the thermochemical processes were selected as in Table 2. Two types of specimens were investigated. The specimens in the shape of cylinders of about 20 mm dia and 100 mm long were used in the chemical composition analysis of diffusion layers. Carbon profiles
Fig. 1. The arrangements used by carburizing: (1) methanol; (2) peristaltic miniflow pump; (3) generator of atmosphere; (4) power supply system; (5) water cooler; (6) manostat; (7) rotameter; (8) valve; (9) ethyl acetate; (10) thermostat; (11) bottle with propane–butane; (12) manometer; (13) pressure-reducing valve; (14) U-tube manometer; (15) ruff; (16) dew-point measuring system; (17) temperature measuring system; (18) power supply system and temperature regulation; (19) carburizing furnace; (20) regulating thermoelement; (21) gas input; (22) gas output; (23) measuring thermoelement; (24) internal and external quartz tubes; (25) ventilator; (26) motor of ventilator.
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Fig. 2. The arrangements used by boriding: (1) boriding furnace; (2) quartz tube; (3) power supply system and temperature regulation; (4) temperature measuring system; (5) vacuum pump; (6) vacuum measuring probe; (7) vacuummeter; (8) bottle with hydrogen; (9) bottle with nitrogen; (10) BCl3; (11) cryostat; (12) temperature measuring system; (13) Dewar flask with liquid nitrogen; (14) rotameters; (15) manostat; (16) device for rinsing outlet gases. Table 2 The parameters of thermochemical processes Type of process
Carburizing parameters Cpa
Carburizing (1) Carburizing (2) Carburizing (3) Boriding Borocarburizing (1) Borocarburizing (2) a
(wt.%)
1.25 1.25 0.80 – 1.25 1.25
Boriding parameters
Temperature (8C)
Time (h)
Temperature (8C)
Time (h)
930 930 930 – 930 930
6 20 3.5 – 6 20
– – – 950 950 950
– – – 3 3 3
Carbon potential.
through carburized and borocarburized layers were determined by chemical analysis of successive layers. The polished cross-sections were observed by use of a SEM Philips XL 30. Boron contents in iron borides and carbon contents underneath the iron borides after boriding and borocarburizing were measured by an EDAX X-ray microanalyzer equipped with EDS, using a 358 take-off angle. The accelerating voltage was 12 kV. The detector Si (Li) with ultra thin window, standardless quantitative analysis, matrix correction algorithms ZAF for SEM bulk analysis have been applied. The second type of specimens was used to research the functional properties. To examine frictional wear resistance of the diffusion layers a research
apparatus, in which a roller constitutes a sample and sintered carbide plate a countersample, has been applied (Fig. 3). The frictional tests have been made under the loading P ¼ 147N and the sample speed of
Fig. 3. Scheme of wear. P ¼ 147N, rotational speed n ¼ 250 min1.
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0.26 m/s, under conditions of dry friction (unlubricated sliding contact). Wear resistance has been defined as specimen mass loss per friction surface and unit of time. The factor of mass wear intensity Izw was determined as: Izw ¼ Dm=ðFtÞ, where Dm is the mass loss (mg); F the friction surface (cm2); t the friction time (h). In second case, wear resistance has been determined on the base of the dia measurements of samples. The factor of linear wear intensivity was defined as: Izl ¼ Dl=t, where Dl is the radius loss (mm); t the friction time (h). To determine microhardness profiles (Vickers method), the apparatus ZWICK 3212 B has been applied. Before the microhardness and wear resistance tests, carburized, borided, and borocarburized specimens were oil quenched at 850 8C and tempered at 150 8C.
[14] and show that the two-phase iron boride layers (FeB þ Fe2 B) were formed after borocarburizing of used steels. Microstructures of cross-sections of borocarburized and hardened layers are shown in Figs. 4–7. Two zones in borocarburized layer are visible: an iron
3. Results and discussion
Fig. 4. SEM microstructure of borocarburized (1) and hardened chromium and nickel-based steel 1 (1.69% Cr, 1.53% Ni).
3.1. Microstructure of borocarburized layers The two types of borided layers can be produced on steels: the two-phase iron boride layer (FeB þ Fe2 B) and the layer with only Fe2B. The results of boron measurements at the surface of borocarburized layers are visible in Table 3. The boron contents obtained at the surface of formed diffusion layers are caused by the presence of iron borides FeB. The results are in agreement with theoretical boron content 16.23 wt.% Table 3 The results of boron content measurements at the surface a
Type of process
Material
B (wt.%)
Borocarburizing (1)
Steel 1
15.84 1.21 16.05 1.30 8.80 0.85
Steel 2
16.54 1.37 16.02 1.64 14.85 1.27
Steel 1
16.81 1.40 15.39 1.45 16.14 1.38
Steel 2
16.86 1.27 15.86 1.54 15.50 1.30
Borocarburizing (2)
a
Boron content.
Fig. 5. SEM microstructure of borocarburized (2) and hardened chromium and nickel-based steel 1 (1.69% Cr, 1.53% Ni).
Fig. 6. SEM microstructure of borocarburized (1) and hardened chromium and nickel-based steel 1 (0.76% Cr, 3.03% Ni).
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Fig. 7. SEM microstructure of borocarburized (2) and hardened chromium and nickel-based steel 1 (0.76% Cr, 3.03% Ni).
borides zone (FeB þ Fe2 B) and a carburized zone. The iron borides zone after borocarburizing (1) exhibits higher case depth than that obtained after borocarburizing (2). The iron borides zone (FeB þ Fe2 B) formed on steel 1 after first process reached a thickness of about 120 mm (Fig. 4), and after second process, about 110 mm (Fig. 5). It is an effect of fixed carbon content at the surface before boriding and it is in agreement with other results [9]. In case of borocarburized layers formed on steel 2 the difference is more visible. After borocarburizing (1) the iron borides zone reached a thickness of about 150 mm (Fig. 6) and after borocarburizing (2), 70 mm (Fig. 7). The higher cementite content beneath the iron borides were observed after borocarburizing (2). It is an effect of higher carbon content presence. The research results of carbon content at the surface after carburizing (Cs) and beneath iron borides after boriding (Cb) are presented in Table 4. The essential influence of chromium and nickel contents in steels on the carbon activity in austenite during carburizing [10,12,13] was demonstrated. The activity of carbon is diminished by chromium and increased by nickel. The influence of chromium is higher than that of nickel. The carbon content at the surface of steel 1 (1.69% Cr, 1.53% Ni) obtained after carburizing is higher than that determined from gas composition of the carburizing atmosphere (carbon potential 1.25% C). The carburizing (2) during the longer time (20 h) is accompanied by higher carbon content at the surface (2.72% Cs). After boriding of previously carburized steel 1 (borocarburizing (2)), carbon content 1.94% Cb was determined beneath iron borides. In the second case (borocarburizing (1)), the
Type of process
Material
Csa (wt.%)
Cbb (wt.%)
Carburizing (1)
Steel 1 Steel 2
2.35 1.38
–
Carburizing (2)
Steel 1 Steel 2
2.72 1.43
–
Carburizing (3)
Steel 1
0.85
– c
0.53
Boriding
Steel 1
0.15
Borocarburizing (1)
Steel 1 Steel 2
2.35 1.38
1.50 1.23
Borocarburizing (2)
Steel 1 Steel 2
2.72 1.43
1.94 1.20
a
Carbon content at the surface after carburizing. Carbon content beneath the iron borides after boriding. c Carbon content in steel 1 before boriding. b
carbon content 2.35% Cs and 1.5% Cb were obtained, respectively. The carbon contents at the surface of steel 2 (0.76% Cr, 3.03% Ni) after carburizing are slightly higher than carbon potential of atmosphere (1.25% C). It is a result of much higher nickel content in comparison with chromium content in this steel. The carburizing during the used times (6 and 20 h) is accompanied by nearly the same carbon content at the surface (1.38 and 1.43% Cs). After boriding of previously carburized steel 2 beneath iron borides carbon contents 1.23 and 1.20% Cb were obtained, respectively. 3.2. Carbon profiles after carburizing and borocarburizing The carbon profiles after carburizing and beneath the iron borides zone in borocarburized layers are shown in Figs. 8–11. The time of carburizing has an influence on carbon profiles. After carburizing (2) for a longer time (20 h), the higher carbon contents at the surface and the higher case depth were obtained (Figs. 10 and 11) than after carburizing (1) (Figs. 8 and 9). During the boriding of carburized layer the carbon is moved in a core direction by following the boron diffusion front. This is also caused by the difference in carbon activity between surface area and core of steel. In effect the decrease of carbon content under iron borides and increase in the core of steel have been observed.
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Fig. 8. Carbon profiles in carburized (1) and borocarburized (1) layers formed on steel 1.
Fig. 9. Carbon profiles in carburized (1) and borocarburized (1) layers formed on steel 2.
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Fig. 11. Carbon profiles in carburized (2) and borocarburized (2) layers formed on steel 2.
Fig. 12. Carbon profile in carburized (3) layer formed on steel 1.
In Fig. 12, the carbon profile in steel 1 after typical carburizing (3) has been shown. The wear resistance of this carburized layer have been compared with properties of complex diffusion layers (after borocarburizing). 3.3. Microhardness of borided, carburized and borocarburized layers
Fig. 10. Carbon profiles in carburized (2) and borocarburized (2) layers formed on steel 1.
The microhardness profiles in hardened diffusion layers are presented in Figs. 13–16. The highest microhardness at the surface after boriding and borocarburizing was obtained. The microhardness of the iron borides zone (FeB þ Fe2 B) 1500–1800 HV has been determined. The higher microhardness (about 1800 HV) is caused by the presence of iron borides FeB in the surface zone. The lower values (about 1500 HV) are
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Fig. 13. Microhardness profiles in hardened diffusion layers after boriding, carburizing (1) and borocarburizing (1).
Fig. 14. Microhardness profiles in hardened diffusion layers after carburizing (1) and borocarburizing (1).
Fig. 16. Microhardness profiles in hardened diffusion layers after carburizing (2) and borocarburizing (2).
accompanied by iron borides Fe2B. The highest microhardness of iron borides (1800 HV) has been obtained only in case of borided layers (Figs. 13 and 15). The boron contents obtained at the surface of hardened borocarburized layers (Table 4) are caused by the presence of iron borides FeB, too. The microhardness of hardened carburizing layer at the surface has been determined. After carburizing of steel 1 (Figs. 13 and 15) the microhardness obtained at the surface (about 950 HV) is higher than that (700– 800 HV) after carburizing of steel 2 (Figs. 14 and 16). In case of borocarburized layers the microhardness below the iron borides zone is the same as that obtained at the surface of only carburized layers. An increase of the distance from the surface is accompanied by a decrease in the microhardness, but higher values near a core of steel were observed in case of borocarburized layers. It is related to the appearance of higher carbon contents in comparison with only carburized layers. A lower gradient of microhardness of the iron boride zone to the substrate in borocarburized layers in comparison with the only borided layer has been observed (Figs. 13 and 15). The microhardness of the hardened borocarburized layers below iron borides (700–950 HV) is higher than that obtained by boriding and hardening processes (450 HV). 3.4. Resistance to wear
Fig. 15. Microhardness profiles in hardened diffusion layers after boriding, carburizing (2) and borocarburizing (2).
The results of wear resistance research are presented in Table 5. The lowest values of mass and
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Table 5 The results of wear resistance tests Type of process
Material Cba (wt.%)
Izwb Izlc (mg/(cm2 h)) (mm/h)
Carburizing (3)
Steel 1
–
1.67
2.17
Boriding
Steel 1
0.53
1.70
2.51
Borocarburizing (1)
Steel 1 Steel 2
1.50 1.23
1.35 1.25
2.00 1.86
a
Carbon content beneath the iron borides after boriding. Mass wear intensity factor. c Linear wear intensity factor. b
linear wear intensity factors have been observed in the case of borocarburized layers. Complex diffusion layers (B þ C) have a positive influence on the wear resistance. The results of specimen mass loss on a friction surface unit in a function of friction time are shown in Fig. 17 (hardened carburized and borided layers) and Fig. 18 (hardened borocarburized layers). The research of specimen radius loss in a function of friction time is presented in Figs. 19 and 20, respectively. The wear intensity factors are defined as the directional coefficients of linear wear equations. When comparing the resistance to frictional wear of the borided layers on steel 1 previously subjected or not subjected to carburization, we can see that the borided layers formed on carburized steel 1 show a higher resistance to frictional wear. The higher wear resistance of borocarburized layer has been shown in comparison with only carburized layer, too. The lowest wear intensity factors and highest wear resistance
Fig. 17. Weight wear of the borided and carburized (3) layers formed on steel 1.
Fig. 18. Weight wear of the borocarburized (1) layers formed on used steels.
Fig. 19. Linear wear of the borided and carburized (3) layers formed on steel 1.
Fig. 20. Linear wear of the borocarburized (1) layers formed on used steels.
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have been obtained in case of borocarburized layer produced on steel 2. Probably, it is related to the influence of carbon contents beneath iron borides (Table 5).
been found. The essentially higher wear resistance of borocarburized layer has been obtained in comparison with only borided or only carburized layers. References
4. Conclusions Combined surface hardening with boron and carbon was proposed for chromium and nickel-based lowcarbon steels. Gas boriding applied to parts that have been previously carburized enables the production of complex borocarburized layers. In the microstructure of these layers two zones have been observed: iron borides and carburized zone. The depth (70–150 mm) and microhardness (1500–1800 HV) of iron borides zone have been found. The depth of this zone and carbon profile underneath can be controlled by carburizing parameters before boriding. The boron contents obtained at the surface of formed diffusion layers are caused by the presence of two-phase iron borides zone (FeB þ Fe2 B). The gradient of microhardness of the iron borides zone to the substrate in borocarburized layers has been reduced in comparison with that of an only borided layer. The carbon contents (1.2– 1.94 wt.%) and microhardness (700–950 HV) below iron borides zone have been determined. An increase of distance from the surface is accompanied by a gradual decrease of carbon content and microhardness in carburized zone. An advantageous influence of complex (B þ C) layers on the frictional wear resistance has
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