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Investigation of compound layer formed during ion nitriding of AISI 4140 steel
COATIN6$
ELSEVIER
Surfaceand Coatings Technology80 (1996)283-286
Htil#OLO /
Investigation of compound layer formed during ion nitriding of AISI 4140 steel A. (~elik a, S. Karadeniz b " Department of Mechanical Engineering, University of Atatiirk, Erzurum, Turkey b Department of Mechanical Engineering, University ofDokuz Eyl~il, Izmir, Turkey Received 31 January 1995; accepted in final form 10 April 1995
Abstract Low alloy AISI 4140 steel was ion nitrided at temperatures of 500, 550 and 600 °C for various times in a 50%N2-50%H2 gas mixture. The nature of the compound layer formed was studied by X-ray diffraction and optical metallography. The X-ray analysis shows the existence of a 7-Fe4N compound zone on the surface at all three temperatures. The thickness of the compound layer increases with increasing time at 500 and 550 °C; however, it begins to decrease with increasing time at 600 °C. Keywords: Ion nitriding; Compound layer
1. Introduction Ion nitriding of steel has recently received considerable industrial interest owing to its beneficial surface property enhancement compared with other commercial nitriding processes. It is more economical because it introduces faster nitrogen diffusion, which in turn allows for lower nitriding temperatures or shorter treatment times. Fig. 4 (see Section 3) shows the microstructure of a crosssection of ion-nitrided steel. It can be seen that the layer on the ion-nitrided steel specimen consists of two parts. The outermost layer, which is very thin and consists mainly of iron nitrides (Fe4N), is not attacked by an alcoholic nitric acid etch, so that it is white on the micrograph. For this reason the layer is usually referred to as the "white layer". Because of the structure and special character of this layer, the more recent term is the "compound layer". Beneath the compound layer is the so-called diffusion zone, where the nitrogen has mainly been incorporated into the existing iron lattice as interstitial atoms or as a finely dispersed alloy nitride precipitate [ 1 ]. The compound layer provides the material with good physical characteristics against wearing and rolling friction. However, in the case of a very hard and brittle layer in a construction subjected to stresses, a thinner white layer or no white layer is necessary, since when the material is subjected to stresses, cracking begins in this layer. The thickness of this layer should therefore be determined according to the use of the material. 0257-8972/96/$15.00© 1996ElsevierScienceS.A. All rights reserved SSDI 0257-8972(95)02471-9
The process has been successfully applied to alloy steels, tool steels and stainless steels [2,3]. Cho and Lee [4] and Keller [5] observed that carbon has a strong influence on the nature of the compound layer. Soccorsy and Ebihara [6] subjected samples of AISI 4140 which had been gas nitrided for 12 h at 524 °C to an additional 18 h treatment time under ionizing conditions in carbonfree plasmas and observed a significant decrease in the thickness of the compound layer. Kurny [7] studied the iron nitriding of pure iron in carbon-free plasmas and obtained a mixture of 7 and e nitrides at higher partial pressures of nitrogen in the treatment gas. In this study we investigate the compound layer formed during ion nitriding of alloy steel AISI 4140 (whose composition is given in Table 1) in the temperature range 500-600 °C for treatment times up to 10h in a 5 0 % N 2 - 5 0 % H 2 gas mixture.
2. Experimental details The samples were discs 10 mm in diameter and 3 mm thick, made from AISI 4140 steel, cleaned mechanically and degreased in trichloroethylene before being put in the furnace. The equipment used in the experiments is shown in Fig. 1. It consists essentially of a glass bell-jar in which an insulated control electrode is situated which holds the specimen. The cleaned and degreased sample was placed on the cathode and the chamber was evacu-
A. ~elik, S. Karadeniz/Surface and Coatings Technology 80 (1996) 283-286
284
Table 1 Chemical composition of AISI 4140 steel Element C
Mn
Si
Cr
Ni
Mo
V
Cu
P
Amount 0.4 0,69 0.33 1.12 0.08 0.17 0.0023 0.087 0.017
(%) 550
(o)
Gas inlet J
Sample
~s
o-,
Belt Jar
E )o
In e~U
Anode
. .
Ca' a0
I!
X-Detoil Vocuum Gouge
Throttl e
a5
50
28 Fig. 2. X-Ray analysis of AISI 4140 steel ion nitrided at 550 °C for (a) 1 and (b) 10 h.
voIve
To VQC UU rn p u m p
G
9
Ni-Cr-Ni
2,02
T her mocouple
/
Gos
5
mixer 2,18
Leak v o l v e
.^,
(o)
600 ~h
Needle valves
E
=
Fig. 1. Schematic diagram of ion-nitriding apparatus.
/', ated to 3 x 10 -2 mbar. The glow discharge was generated in a mixture of nitrogen and hydrogen at 10 mbar consisting of 50% N2 and 50% H 2. The premixed gas mixture was admitted into the chamber through a needle valve. The system was pumped continuously with a double-stage rotary pump during ion nitriding. The temperature was measured at the bottom of the specimens using an N i - C r - N i thermocouple 0.5 mm in diameter. Parameters such as the composition of the gas mixture inside the vacuum chamber, the desired temperature and the distance between cathode and anode were kept constant. Specimens were ion nitrided for times of 1, 4 and 10 h at temperatures of 500, 550 and 600 °C. After ion nitriding, the specimens were cooled to room temperature in the vacuum chamber gas. The surface phase structures of the ion-nitrided samples were investigated by X-ray diffraction and optical metallography.
3. Results and discussion
X-Ray diffraction patterns of the ion-nitrided specimens were recorded using Cu K~ radiation in a JSDx-100 $4 X-ray diffractometer. Figs. 2 and 3 show
I \ ~.o~
E
¢"
rhl
£nn /.~
2,02
A 2'~8
/I 9
~ 40
a5
(c)
600 }Oh
50
2g Fig. 3. X-Ray analysis of AISI 4140 steeliron nitrided at 600 °C for (a) I, (b) 4 and (c) 10 h.
the diffraction patterns obtained for ion-nitrided AISI 4140 low alloy steel. The X-ray analysis shows the existence of a 7 - F e 4 N compound zone on the surface at all temperatures. Fig. 2 shows that the intensity of the diffraction line belonging to the 7 phase increases with increasing time at 550 °C. However, at 600 °C the compound layer thickness is larger after 4 h than after 10 h (Fig. 3). At 500 °C the compound layer thickness is very small after 1 h but increases with increasing time. No
A. ~elik, S. Karadeniz/Surface and Coatings Technology 80 (1996) 283-286
saturation of the white layer thickness was observed in the time range investigated. After ion nitriding for various lengths of time at various temperatures, the samples were taken out and metallographically prepared by etching in 2% Nital. They were observed and photographed in an optical microscope. Fig. 4 shows the metallographic structure of specimens ion nitrided at 600 °C for 4 and 10 h. Changes in the compound layer thickness with increasing time at various temperatures are shown in Fig. 5. It can be seen that the compound layer thickness of the 600 °C specimen increases up to 4 h and then decreases. For example, the compound layer thickness is 4 tam after 1 h, 9 gm after 4h and 3 ~tm after 10h at 600 °C. Increasing time and temperature cause a decrease in thickness. Sputtering can be another possible reason for this decrease in thickness of the compound layer [7]. The compound layer thickness will probably decrease
(a)
285
14
E=_12 ~) 10 O
.~
8
_~6 E
62 2
,4
6
8
1o
12
Time (h)
Fig. 5. Variation in thickness of compound layer with treatment time at various temperatures: • 600 °C; O 500 °C; O 550 °C.
after further treatment (longer than 10h) at 500 and 550 °C; however, no saturation of the compound layer thickness is found up to 10h at 500 and 550 °C in this study. The occurrence of sputtering requires a sufficiently high temperature and a long enough treatment time. Sputtering causes a decrease in the compound layer thickness. In the treatment at 600 °C for 10 h the compound layer thickness decreases owing to sputtering; however, in the treatment at 600 °C for 4 h the thickness of the compound layer was larger than that after 10 h owing to the insufficient treatment time for reduction of the compound layer. In addition, some workers [8] have shown that denitriding results in a reduction of the compound layer thickness. Denitriding possibly increases the amount of hydrogen in the plasma. It is possible that the denitriding reaction is faster at higher temperatures, causing a greater decrease in thickness. Vickers microhardness profiles of treated samples have been measured using a microhardness tester with a load of 80 gf. From Table 2 it can be seen that the highest surface hardness is obtained for a treatment time of 10 h Table 2 Dependence of surface hardness and compound layer thickness on nitriding time and temperature Process parameters T
(b) SO Jam
Fig. 4. Microstructure of AISI 4140 steel ion nitrided at 600 °C for (a) 4 and (b) 10 h.
(°C)
Surface hardness (HV)
Compound layer thickness (gm)
750-820 850 900 900 980 720-770 640-700 600-650 610-650 540-600 460-510 400
1-2 3 4 5 6 5 6 6-8 12-14 3-4 8-10 2 3
t (h)
500 1 500 4 500 10 550 1 550 4 550 10 600 1 600 4 600 10 Un-nitrided material
286
A. ~elik, S. Karadeniz/Surface and Coatings Technology 80 (1996) 283-286
at 500 °C. The surface hardness of the material is reduced as the temperature increases; the surface hardness at 600 °C is decreased appreciably compared with that at 500 °C.
out at high temperatures, because this results in a decrease in surface hardness.
References 4. Conclusions A larger thickness of the compound layer has been obtained at 550 °C than at 500 and 600 °C. The thickness of the compound layer increases up to a certain time and then decreases at 600 °C. The 7-Fe4N phase is formed at all temperatures. Higher temperature treatment, sputtering and denitriding cause the compound layer thickness to decrease. At high temperatures and long treatment times a thinner white layer was obtained; however, the ion-nitriding process should not be carried
[ 1] B. Edenhofer, Physical and metallurgical aspects of ion nitriding, Heat Treat. Met., 2 (1974) 59-67. [2] C.V. Robino and O.T. Inal, Mater. Sci. Eng., 59 (1982) 79. [3] K. Ozbaysal and O.T. Inal, J. Mater. Sci., 21 (1986) 4318. [4] K.S. Cho and C.O. Lee, J. Mater. Technol., 102 (1980) 231. [5] K. Keller, Hart. Tech. Mitt., 26 (1971) 125. [6] W.D. Soccorsy and W.T. Ebihara, Trans. Metall. Soc. AIME, (1970) 3. I-7] A.S.W. Kurny, Ph.D. Thesis, Indian Institute of Science, Bangalore, 1982. [8] A.S.W.Kurny, R.M. Mallya and M. Mohan Rao, A study on the nature of the compound layer formed during the ion nitriding of En40B steel, Mater. Sci. Eng., 78 (1986) 95-100.
ELSEVIER
Surfaceand Coatings Technology80 (1996)283-286
Htil#OLO /
Investigation of compound layer formed during ion nitriding of AISI 4140 steel A. (~elik a, S. Karadeniz b " Department of Mechanical Engineering, University of Atatiirk, Erzurum, Turkey b Department of Mechanical Engineering, University ofDokuz Eyl~il, Izmir, Turkey Received 31 January 1995; accepted in final form 10 April 1995
Abstract Low alloy AISI 4140 steel was ion nitrided at temperatures of 500, 550 and 600 °C for various times in a 50%N2-50%H2 gas mixture. The nature of the compound layer formed was studied by X-ray diffraction and optical metallography. The X-ray analysis shows the existence of a 7-Fe4N compound zone on the surface at all three temperatures. The thickness of the compound layer increases with increasing time at 500 and 550 °C; however, it begins to decrease with increasing time at 600 °C. Keywords: Ion nitriding; Compound layer
1. Introduction Ion nitriding of steel has recently received considerable industrial interest owing to its beneficial surface property enhancement compared with other commercial nitriding processes. It is more economical because it introduces faster nitrogen diffusion, which in turn allows for lower nitriding temperatures or shorter treatment times. Fig. 4 (see Section 3) shows the microstructure of a crosssection of ion-nitrided steel. It can be seen that the layer on the ion-nitrided steel specimen consists of two parts. The outermost layer, which is very thin and consists mainly of iron nitrides (Fe4N), is not attacked by an alcoholic nitric acid etch, so that it is white on the micrograph. For this reason the layer is usually referred to as the "white layer". Because of the structure and special character of this layer, the more recent term is the "compound layer". Beneath the compound layer is the so-called diffusion zone, where the nitrogen has mainly been incorporated into the existing iron lattice as interstitial atoms or as a finely dispersed alloy nitride precipitate [ 1 ]. The compound layer provides the material with good physical characteristics against wearing and rolling friction. However, in the case of a very hard and brittle layer in a construction subjected to stresses, a thinner white layer or no white layer is necessary, since when the material is subjected to stresses, cracking begins in this layer. The thickness of this layer should therefore be determined according to the use of the material. 0257-8972/96/$15.00© 1996ElsevierScienceS.A. All rights reserved SSDI 0257-8972(95)02471-9
The process has been successfully applied to alloy steels, tool steels and stainless steels [2,3]. Cho and Lee [4] and Keller [5] observed that carbon has a strong influence on the nature of the compound layer. Soccorsy and Ebihara [6] subjected samples of AISI 4140 which had been gas nitrided for 12 h at 524 °C to an additional 18 h treatment time under ionizing conditions in carbonfree plasmas and observed a significant decrease in the thickness of the compound layer. Kurny [7] studied the iron nitriding of pure iron in carbon-free plasmas and obtained a mixture of 7 and e nitrides at higher partial pressures of nitrogen in the treatment gas. In this study we investigate the compound layer formed during ion nitriding of alloy steel AISI 4140 (whose composition is given in Table 1) in the temperature range 500-600 °C for treatment times up to 10h in a 5 0 % N 2 - 5 0 % H 2 gas mixture.
2. Experimental details The samples were discs 10 mm in diameter and 3 mm thick, made from AISI 4140 steel, cleaned mechanically and degreased in trichloroethylene before being put in the furnace. The equipment used in the experiments is shown in Fig. 1. It consists essentially of a glass bell-jar in which an insulated control electrode is situated which holds the specimen. The cleaned and degreased sample was placed on the cathode and the chamber was evacu-
A. ~elik, S. Karadeniz/Surface and Coatings Technology 80 (1996) 283-286
284
Table 1 Chemical composition of AISI 4140 steel Element C
Mn
Si
Cr
Ni
Mo
V
Cu
P
Amount 0.4 0,69 0.33 1.12 0.08 0.17 0.0023 0.087 0.017
(%) 550
(o)
Gas inlet J
Sample
~s
o-,
Belt Jar
E )o
In e~U
Anode
. .
Ca' a0
I!
X-Detoil Vocuum Gouge
Throttl e
a5
50
28 Fig. 2. X-Ray analysis of AISI 4140 steel ion nitrided at 550 °C for (a) 1 and (b) 10 h.
voIve
To VQC UU rn p u m p
G
9
Ni-Cr-Ni
2,02
T her mocouple
/
Gos
5
mixer 2,18
Leak v o l v e
.^,
(o)
600 ~h
Needle valves
E
=
Fig. 1. Schematic diagram of ion-nitriding apparatus.
/', ated to 3 x 10 -2 mbar. The glow discharge was generated in a mixture of nitrogen and hydrogen at 10 mbar consisting of 50% N2 and 50% H 2. The premixed gas mixture was admitted into the chamber through a needle valve. The system was pumped continuously with a double-stage rotary pump during ion nitriding. The temperature was measured at the bottom of the specimens using an N i - C r - N i thermocouple 0.5 mm in diameter. Parameters such as the composition of the gas mixture inside the vacuum chamber, the desired temperature and the distance between cathode and anode were kept constant. Specimens were ion nitrided for times of 1, 4 and 10 h at temperatures of 500, 550 and 600 °C. After ion nitriding, the specimens were cooled to room temperature in the vacuum chamber gas. The surface phase structures of the ion-nitrided samples were investigated by X-ray diffraction and optical metallography.
3. Results and discussion
X-Ray diffraction patterns of the ion-nitrided specimens were recorded using Cu K~ radiation in a JSDx-100 $4 X-ray diffractometer. Figs. 2 and 3 show
I \ ~.o~
E
¢"
rhl
£nn /.~
2,02
A 2'~8
/I 9
~ 40
a5
(c)
600 }Oh
50
2g Fig. 3. X-Ray analysis of AISI 4140 steeliron nitrided at 600 °C for (a) I, (b) 4 and (c) 10 h.
the diffraction patterns obtained for ion-nitrided AISI 4140 low alloy steel. The X-ray analysis shows the existence of a 7 - F e 4 N compound zone on the surface at all temperatures. Fig. 2 shows that the intensity of the diffraction line belonging to the 7 phase increases with increasing time at 550 °C. However, at 600 °C the compound layer thickness is larger after 4 h than after 10 h (Fig. 3). At 500 °C the compound layer thickness is very small after 1 h but increases with increasing time. No
A. ~elik, S. Karadeniz/Surface and Coatings Technology 80 (1996) 283-286
saturation of the white layer thickness was observed in the time range investigated. After ion nitriding for various lengths of time at various temperatures, the samples were taken out and metallographically prepared by etching in 2% Nital. They were observed and photographed in an optical microscope. Fig. 4 shows the metallographic structure of specimens ion nitrided at 600 °C for 4 and 10 h. Changes in the compound layer thickness with increasing time at various temperatures are shown in Fig. 5. It can be seen that the compound layer thickness of the 600 °C specimen increases up to 4 h and then decreases. For example, the compound layer thickness is 4 tam after 1 h, 9 gm after 4h and 3 ~tm after 10h at 600 °C. Increasing time and temperature cause a decrease in thickness. Sputtering can be another possible reason for this decrease in thickness of the compound layer [7]. The compound layer thickness will probably decrease
(a)
285
14
E=_12 ~) 10 O
.~
8
_~6 E
62 2
,4
6
8
1o
12
Time (h)
Fig. 5. Variation in thickness of compound layer with treatment time at various temperatures: • 600 °C; O 500 °C; O 550 °C.
after further treatment (longer than 10h) at 500 and 550 °C; however, no saturation of the compound layer thickness is found up to 10h at 500 and 550 °C in this study. The occurrence of sputtering requires a sufficiently high temperature and a long enough treatment time. Sputtering causes a decrease in the compound layer thickness. In the treatment at 600 °C for 10 h the compound layer thickness decreases owing to sputtering; however, in the treatment at 600 °C for 4 h the thickness of the compound layer was larger than that after 10 h owing to the insufficient treatment time for reduction of the compound layer. In addition, some workers [8] have shown that denitriding results in a reduction of the compound layer thickness. Denitriding possibly increases the amount of hydrogen in the plasma. It is possible that the denitriding reaction is faster at higher temperatures, causing a greater decrease in thickness. Vickers microhardness profiles of treated samples have been measured using a microhardness tester with a load of 80 gf. From Table 2 it can be seen that the highest surface hardness is obtained for a treatment time of 10 h Table 2 Dependence of surface hardness and compound layer thickness on nitriding time and temperature Process parameters T
(b) SO Jam
Fig. 4. Microstructure of AISI 4140 steel ion nitrided at 600 °C for (a) 4 and (b) 10 h.
(°C)
Surface hardness (HV)
Compound layer thickness (gm)
750-820 850 900 900 980 720-770 640-700 600-650 610-650 540-600 460-510 400
1-2 3 4 5 6 5 6 6-8 12-14 3-4 8-10 2 3
t (h)
500 1 500 4 500 10 550 1 550 4 550 10 600 1 600 4 600 10 Un-nitrided material
286
A. ~elik, S. Karadeniz/Surface and Coatings Technology 80 (1996) 283-286
at 500 °C. The surface hardness of the material is reduced as the temperature increases; the surface hardness at 600 °C is decreased appreciably compared with that at 500 °C.
out at high temperatures, because this results in a decrease in surface hardness.
References 4. Conclusions A larger thickness of the compound layer has been obtained at 550 °C than at 500 and 600 °C. The thickness of the compound layer increases up to a certain time and then decreases at 600 °C. The 7-Fe4N phase is formed at all temperatures. Higher temperature treatment, sputtering and denitriding cause the compound layer thickness to decrease. At high temperatures and long treatment times a thinner white layer was obtained; however, the ion-nitriding process should not be carried
[ 1] B. Edenhofer, Physical and metallurgical aspects of ion nitriding, Heat Treat. Met., 2 (1974) 59-67. [2] C.V. Robino and O.T. Inal, Mater. Sci. Eng., 59 (1982) 79. [3] K. Ozbaysal and O.T. Inal, J. Mater. Sci., 21 (1986) 4318. [4] K.S. Cho and C.O. Lee, J. Mater. Technol., 102 (1980) 231. [5] K. Keller, Hart. Tech. Mitt., 26 (1971) 125. [6] W.D. Soccorsy and W.T. Ebihara, Trans. Metall. Soc. AIME, (1970) 3. I-7] A.S.W. Kurny, Ph.D. Thesis, Indian Institute of Science, Bangalore, 1982. [8] A.S.W.Kurny, R.M. Mallya and M. Mohan Rao, A study on the nature of the compound layer formed during the ion nitriding of En40B steel, Mater. Sci. Eng., 78 (1986) 95-100.