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A study on the nature of the compound layer formed during the ion nitriding of En40B steel
Materials Science and Engineering, 78 (1986) 95-100
95
A Study on the Nature of the Compound Layer Formed during the Ion Nitriding of En40B Steel A. S. W. KURNY
Department of Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka (Bangladesh) R. M. MALLYA and M. MOHAN RAO
Department of Metallurgy, Indian Institute of Science, Bangalore (India) (Received March 12, 1985; in revised form August 13, 1985)
ABSTRACT
Commercial-grade E n 4 0 B steel has been ion nitrided in the temperature range 4 7 5 550 °C in a 2 5 % N 2 - 7 5 % H 2 gas mixture. The nature o f the c o m p o u n d layer formed was studied by the X-ray diffraction technique and optical metallography. It was observed that the structure o f the c o m p o u n d layer gradually transforms from a predominantly e nitride to a predominantly ~[' nitride structure with increasing treatment time. Optical metallography studies on sections orthogonal to the nitrided surface showed that, after about 5 h o f treatment, the thickness o f the c o m p o u n d layer decreases with further increase in treatment time.
1. INTRODUCTION Nitriding in salt baths and in ammonia gas atmospheres are well known processes and are widely utilized around the world. In recent years a new technique called ion nitriding has gained equal industrial importance and is being studied widely. The nature o f the compound layers produced by ion nitriding has been studied by a number of investigators [1-3]. Edenhofer [1] contends that, during ion nitriding in a t r e a t m e n t gas free of carbon, only 7' nitride forms. Cho and Lee [2] and also Keller [3] observed that carbon has a strong influence on the nature of the compound layer formed during ion nitriding. The thickness o f the c o m p o u n d layers also were f o u n d to be dependent on t r e a t m e n t conditions [4]. Kurny [5], however, studied the ion nitriding of pure i r o n i n carbon-free 0025-5416/86/$3.50
plasmas and obtained a mixture of ~/' and e nitrides at higher partial pressures of nitrogen in the t r e a t m e n t gas. The present objective was to investigate the nature of the c o m p o u n d layers formed during the ion nitriding of commercial-grade En40B steel (0.2-0.3 wt.% C, 0.4-0.65 wt.% Mn, 0.40 wt.% Ni, 2.9-3.5 wt.% Cr, 0.4-0.7 wt.% Mo, 0.10-0.35 wt.% Si, 0.05 wt.% S and 0.05 wt.% P) in the temperature range 475550 °C and for treatment times up to 10 h in a 25%N2-75%H 2 gas mixture.
2. EXPERIMENTAL DETAILS Specimens measuring 2.5 cm X 2 cm X 0.1 cm were cut from cold-rolled blocks of commercially available grade En40B steel. These were then oil quenched from 900 °C, tempered at 600 °C for 0.5 h, cleaned mechanically and degreased in trichloroethylene before nitriding. The equipment used in the experiments is shown in Fig. 1 and details can be found elsewhere [5]. The gas premixed to controlled proportion of 25% N 2 a~d 75% H 2 was admitted into the chamber through a gas distribution valve. The ion-nitriding process was carried out in the abnormal glow region of the discharge current-voltage ( I - V ) characteristic curve shown in Fig. 2. The heat-treated, cleaned and degreased sample was placed on the cathode and the chamber was evacuated to 1 X 10 -4 Tort. A purified mixture of 25% N2 and 75% H 2 was then leaked in through the needle valve. The pressure inside the chamber, measured with the manometer, was maintained at © Elsevier Sequoia/Printed in The Netherlands
96 ,nod~ Valve
Resistance 500 mA, 2K
Purification system Needle
eoction Chamber
D,C. power supply # IcY 17_0 mA
Flow Meter Cothode pie
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Meter Mixing Chomber
.5
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c
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Temperature k ~ Indicator High Vacuum Pumping System Oil Manometer
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Fig. 1. Experimental set-up for ion nitriding.
ANOMALOUS GLOW
TOWNSEND
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E~ i~ 12
Id 4
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Fig. 2. I - V characteristics for different types of discharge.
(this includes the time to reach the nitriding temperature, which was usually 5-6 min) at the desired temperature, the power supply was switched off and the specimen cooled to room temperature in a stream of the treatment gas. Specimens were nitrided for times of 0.5, 2, 5 and 10 h at temperatures of 4 7 5 , 5 0 0 , 525 and 550 °C. The effects were followed using X-ray diffraction techniques and optical metallography.
3. RESULTS AND DISCUSSION
7.25 Tort by balancing the leak rate against the mechanical pump. The power supply was switched on and the voltage gradually increased to strike a glow. The voltage was adjusted and maintained at 500 V, and the current was adjusted to maintain a constant nitriding temperature. The temperature was read o f f on a temperature indicator, using a protected chromel-alumel thermocouple introduced through the cathode (a stainless steel tube 10 mm in diameter was silver brazed to a stainless steel disc 25 mm in diameter and 3 mm thick to form the cathode) and embedded in the steel disc (as illustrated in Fig. 1). After nitriding for a specified time
3.1. X-ray diffraction X-ray diffraction patterns of the nitrided specimens were recorded using Fe Kc~ radiation in a Philips X-ray diffractometer. A scanning speed of 1 ° min -1 with a chart speed of 1 cm min -1 was used and 0 values could be read with an accuracy of +0.05 ° . These patterns, which were obtained on the nitrided specimens w i t h o u t any further treatment, were used to identify the phases present and to index the diffraction lines. Figures 3 and 4 show the diffraction patterns obtained on ion-nitrided En40B steel. An analysis of these patterns shows
97
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Fig. 4. X-ray diffraction patterns of En40B steel ion nitrided at 550 °C (500 V; 7.25 Torr; 25%N 275%H2) for (a) 0.5 h, (b) 2 h, (c) 5 h and (d) 10 h.
Fig. 3. X-ray diffraction patterns of En40B steel ion nitrided at 475 °C (500 V; 7.25 Torr; 25%N 275%H2) for (a) 0.5 h, (b) 2 h, (c) 5 h and (d) 10 h.
t h a t at all t h e t e m p e r a t u r e s u n d e r s t u d y ( 4 7 5 , 5 0 0 , 5 2 5 and 550 °C) a m i x t u r e o f 3" nitride and e nitride is f o r m e d . A careful e x a m i n a t i o n o f these p a t t e r n s also shows t h a t t h e i n t e n s i t y o f t h e d i f f r a c t i o n lines belonging t o e nitride decreases with t i m e , whereas t h a t belonging t o 3" nitride increases with time. A s e m i q u a n t i t a t i v e idea o f t h e variation in t h e i r relative a m o u n t s was o b t a i n e d b y c o m p a r i n g t h e intensities o f t h e d i f f r a c t i o n lines w h i c h d o n o t overlap b u t are sufficiently
intense. T h e d i f f r a c t i o n lines used for t h e c o m p a r i s o n are t h e (100) and ( 2 0 0 ) lines f r o m t h e e and t h e ~" nitrides respectively. It s h o u l d be n o t e d t h a t , a l t h o u g h t h e intensities o f d i f f r a c t i o n lines o f a given phase are n o t a precise measure o f t h e q u a n t i t y o f t h a t phase in a n y given m i x t u r e o f phases, t h e i n t e n s i t y values can be used as a measure o f t h e relative a m o u n t s o f each phase. Table 1 shows t h a t t h e i n t e n s i t y o f t h e d i f f r a c t i o n line belonging t o t h e e phase gradually decreases with increasing t r e a t m e n t t i m e at a n y t e m p e r a t u r e , indicating a decrease in t h e q u a n t i t y o f this phase. In c o n t r a s t , t h e i n t e n s i t y o f t h e d i f f r a c t i o n line belonging to
98 TABLE 1 Summary of results of X-ray diffraction studies on ion-nitrided En40B steel (500 V; 7.25 Torr; 25%N2-75%H2) Temperature
Time
(°C)
(h)
Phase detected a
Lattice parameter
"YP2OO
elOO
"//{~
a~'~oo
475 475 475 475
0.5 2 5 10
88 33 56 93
25 67 44 7
0.3 0.5 1.3 13
3.809 3.831 3.831 3.816
500 500 500 500
0.5 2 5 10
20 25 92 94
80 75 8 6
0.25 0.33 11.5 13.6
3.816 3.816 3.822 3.828
525 525 525 525
0.5 2 5 10
23 68.5 87.5 93
77 31.5 12.5 7
0.31 2.2 7 12
3.816 3.822 3.822 3.828
550 550 550 550
0.5 2 5 10
40 70 85 96
60 30 15 4
0.67 2.3 5.7 24
3.809 3.816 3.787 3.831
aThe values indicate the areas in square millimetres under the diffraction lines.
7 ' nitride increases w i t h increasing t r e a t m e n t t i m e at t e m p e r a t u r e . F o r a t r e a t m e n t t i m e o f 10 h at all t e m p e r a t u r e s , t h e s t r u c t u r e o f t h e c o m p o u n d l a y e r is p r e d o m i n a n t l y V' nitride. This gradual c h a n g e - o v e r f r o m a mixed nitride structure to a predominantly single-nitride s t r u c t u r e is p o s s i b l y t e m p e r a t u r e d e p e n d e n t . With r e f e r e n c e t o Fig. 3 it can b e seen t h a t t h e t i m e r e q u i r e d f o r t r a n s f o r m a t i o n t o a p r e d o m i n a n t l y 7 ' nitride s t r u c t u r e at 475 °C is a b o u t 10 h, w h e r e a s f r o m Fig. 4 it can be seen t h a t this t r a n s f o r m a t i o n t a k e s a b o u t 6 - 7 h at 550 °C. To detect the structure of the compound l a y e r d u r i n g t h e initial stages o f ion nitriding, o n e s p e c i m e n was n i t r i d e d f o r 10 rain at 500 °C. T h e X - r a y p a t t e r n o b t a i n e d (Fig. 5) s h o w s t h a t t h e s t r u c t u r e is p r e d o m i n a n t l y e nitride. T h u s , d u r i n g t h e ion nitriding o f E n 4 0 B in 2 5 % N 2 - 7 5 % H 2 gas m i x t u r e s , t h e s t r u c t u r e o f t h e c o m p o u n d l a y e r is pred o m i n a n t l y e nitride; this g r a d u a l l y t r a n s f o r m s t o a p o l y p h a s e s t r u c t u r e a n d finally t o a p r e d o m i n a n t l y single-phase (7' nitride) structure. It s h o u l d b e m e n t i o n e d t h a t , d u r i n g t h e ion nitriding o f p u r e i r o n in a m i x t u r e o f 2 5 % N 275%H2, o n l y 7 ' n i t r i d e f o r m e d [5]. In c o n t r a s t , f o r E n 4 0 B steel a m i x t u r e o f V' n i t r i d e a n d e
o
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J
i
I
2o
Fig. 5. X-ray diffraction patterns of En40B steel ion nitrided at 500 °C (500 V; 7.25 Tort; 25%N 275%H2) for 10 min.
nitride was o b t a i n e d . This is p o s s i b l y b e c a u s e o f t h e c a r b o n c o n t e n t in t h e steel. E d e n h o f e r [6] o b s e r v e d t h a t in t h e p r e s e n c e o f c a r b o n t h e stability range o f t h e e nitride increases. T h e gradual d e c r e a s e in t h e a m o u n t o f e nitride in t h e c o m p o u n d l a y e r c o u l d be a s c r i b e d t o t h e decrease in c a r b o n c o n t e n t o f t h e steel d u e t o s p u t t e r i n g . E d e n h o f e r [6] has r e p o r t e d t h a t , d u r i n g ion nitriding, sputtering removes carbon together with o t h e r e l e m e n t s f r o m t h e w o r k p i e c e surface. H e p r e s e n t e d m i c r o s t r u c t u r e s a n d also t h e
99 nitrogen and carbon profiles to confirm that the carbon content decreases towards the surface of the sample during ion nitriding in a carbon-free plasma. In these experiments, the system pressure was maintained at 7.25 Tort by balancing the leak rate against the pumping rate. As a result, the carbon content both in the material (because of sputtering) and in the plasma (because of the pumping) was continuously decreasing with increasing treatment time, leading to transformation in the structure of the c o m p o u n d layers.
3.2. Optical metallography Sections orthogonal to the nitrided surface were m o u n t e d in a cold-setting resin, prepared using standard techniques and etched in 2% Nital; t h e y were observed and photographed on a Neophot optical metallograph. Optical micrographs (Fig. 6) of En40B steel ion nitrided in 25%N2-75%H 2 show that the etchant used neither reveals the depth of nitrogen penetration nor etches the c o m p o u n d layer. It can also be seen t h a t the core of the specimen remains unaltered by the nitriding. The thickness of the c o m p o u n d layers on the ion-nitrided samples, measured on the screen of the Neophot optical metallograph under a magnification of 1000 diameters, was found to increase with time, at all temperatures, to a certain m a x i m u m value and then to decrease. The time to reach the m a x i m u m thickness in this steel is 2 - 5 h (Fig. 7) depending on temperature. The m a x i m u m thickness is also higher at higher temperatures, that at 550 °C being almost twice that at 475 °C. It has been mentioned earlier that the compound layer has been identified as a mixture of ~,' nitride and e nitride for treatment times of less than 5 h at all temperatures. At longer treatment times, the layer structure transforms to single-phase ~/' nitride, at all temperatures. The time for total transformation is less at higher treatment temperatures. A calculation o f the unit cell volumes of the ~,' nitride and e nitride showed t h a t the difference between the volumes of the two nitrides cannot account for the decrease in the thickness o f the c o m p o u n d layer at longer treatment times. However, the variation in the thickness of the c o m p o u n d layer may be related to the nature of the nitrides in the c o m p o u n d layer.
Fig. 6. Optical micrographs of En40B steel ion nitrided (500 V; 7.25 Torr; 25%N2-75%H2) for (a) 10 hat 500°C and (b) 5 hat 550°C (etch, 2% Nital). (Magnifications, 1688 ×.) Studies [5, 6] have shown that 7' nitride grows to a m a x i m u m thickness of about 6 - 8 pm while e nitride can grow to about 50 pm or more in thickness. Thus, as long as
100
24
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16
8
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4
1 2
I t,
I 6 Time
I 8
I ~0
I 12
(h )
Fig. 7. Variation in the thickness of the compound layer with treatment time at different temperatures for En40B steel (500 V; 7.25 Torr; 25%N2-75%H2): v, 550 °C;o, 525 °C;o, 500°C; X, 475 °C.
e nitride is present, the thickness of the layer increases. At lower temperatures, e nitride is present for a longer time and continues to grow. Since the growth of the layer is diffusion controlled, the maximum thickness at lower temperatures is less. Moreover, the transformation to e nitride takes longer. At higher temperatures, however, the growth of the layer is faster and a thick c o m p o u n d layer is formed in a short treatment time. The possible reasons for the decrease in thickness of the c o m p o u n d layer could be that, in the absence of carbon, the layer transforms to a lower nitride and more hydrogen is available for denitriding. As a result the denitriding rate possibly increases. It is possible that the denitriding reaction is faster at higher temperatures, causing a greater decrease in thickness. Sputtering could be another possible reason for this decrease in thickness of the c o m p o u n d layer [6]. Soccorsy and Ebihara [4] subjected samples of AISI 4140 steel (0.40 wt.% C, 0.90 wt.%Mn, 0.30 wt.%Si, 0.95 wt.%Cr and 0.20 wt.%Mo) which had been gas
nitrided for 12 h at 524 °C to an additional 18 h treatment under ionizing conditions in carbon-free plasmas and observed a significant decrease in the thickness o f the c o m p o u n d layer (the thickness decreased to less than one-half of the original thickness). They ascribed this decrease in thickness to denitriding caused by the hydrogen ions. However, they did not identify the structures of the nitrides but, since they used only 25%N2-75%H2 as the treatment gas, it is possible that decarburization occurred, caused transformations in the structure of the c o m p o u n d layer and enhanced the rate of denitriding, leading to the decrease in the thickness of the c o m p o u n d layer. It should be noted that, because the growth of the layer is diffusion controlled, it is slower at longer times at any given temperature, leading to a more pronounced effect of sputtering towards the end o f the (10 h) treatment.
4. CONCLUSIONS
(1) The structure of the c o m p o u n d layer formed during the ion nitriding of. En40B steel shows a gradual transformation from e nitride to 7' nitride with increasing treatment time. The thickness o f the c o m p o u n d layer also shows a variation with treatment time. (2) The variation in the structure and in the thickness of the c o m p o u n d layer may be related to the decarburization, denitriding and sputtering that take place during ion nitriding.
REFERENCES 1 B. Edenhofer, Heat Treat. Met., 2 (1974) 60. 2 K. S. Cho and C. O. Lee, J. Eng. Mater. Technol., 102 (1980) 231. 3 K. Keller, Ha'rterei-Tech. Mitt., 26 (1971) 125. 4 W. D. Soccorsy and W. T. Ebihara, Trans. Metall. Soc. AIME, (1970), Paper A70-61, 3. 5 A. S. W. Kurny, Ph.D. Thesis, Indian Institute of Science, Bangalore, 1982. 6 B. Edenhofer, Heat Treat. Met., 2 (1974) 62.
95
A Study on the Nature of the Compound Layer Formed during the Ion Nitriding of En40B Steel A. S. W. KURNY
Department of Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka (Bangladesh) R. M. MALLYA and M. MOHAN RAO
Department of Metallurgy, Indian Institute of Science, Bangalore (India) (Received March 12, 1985; in revised form August 13, 1985)
ABSTRACT
Commercial-grade E n 4 0 B steel has been ion nitrided in the temperature range 4 7 5 550 °C in a 2 5 % N 2 - 7 5 % H 2 gas mixture. The nature o f the c o m p o u n d layer formed was studied by the X-ray diffraction technique and optical metallography. It was observed that the structure o f the c o m p o u n d layer gradually transforms from a predominantly e nitride to a predominantly ~[' nitride structure with increasing treatment time. Optical metallography studies on sections orthogonal to the nitrided surface showed that, after about 5 h o f treatment, the thickness o f the c o m p o u n d layer decreases with further increase in treatment time.
1. INTRODUCTION Nitriding in salt baths and in ammonia gas atmospheres are well known processes and are widely utilized around the world. In recent years a new technique called ion nitriding has gained equal industrial importance and is being studied widely. The nature o f the compound layers produced by ion nitriding has been studied by a number of investigators [1-3]. Edenhofer [1] contends that, during ion nitriding in a t r e a t m e n t gas free of carbon, only 7' nitride forms. Cho and Lee [2] and also Keller [3] observed that carbon has a strong influence on the nature of the compound layer formed during ion nitriding. The thickness o f the c o m p o u n d layers also were f o u n d to be dependent on t r e a t m e n t conditions [4]. Kurny [5], however, studied the ion nitriding of pure i r o n i n carbon-free 0025-5416/86/$3.50
plasmas and obtained a mixture of ~/' and e nitrides at higher partial pressures of nitrogen in the t r e a t m e n t gas. The present objective was to investigate the nature of the c o m p o u n d layers formed during the ion nitriding of commercial-grade En40B steel (0.2-0.3 wt.% C, 0.4-0.65 wt.% Mn, 0.40 wt.% Ni, 2.9-3.5 wt.% Cr, 0.4-0.7 wt.% Mo, 0.10-0.35 wt.% Si, 0.05 wt.% S and 0.05 wt.% P) in the temperature range 475550 °C and for treatment times up to 10 h in a 25%N2-75%H 2 gas mixture.
2. EXPERIMENTAL DETAILS Specimens measuring 2.5 cm X 2 cm X 0.1 cm were cut from cold-rolled blocks of commercially available grade En40B steel. These were then oil quenched from 900 °C, tempered at 600 °C for 0.5 h, cleaned mechanically and degreased in trichloroethylene before nitriding. The equipment used in the experiments is shown in Fig. 1 and details can be found elsewhere [5]. The gas premixed to controlled proportion of 25% N 2 a~d 75% H 2 was admitted into the chamber through a gas distribution valve. The ion-nitriding process was carried out in the abnormal glow region of the discharge current-voltage ( I - V ) characteristic curve shown in Fig. 2. The heat-treated, cleaned and degreased sample was placed on the cathode and the chamber was evacuated to 1 X 10 -4 Tort. A purified mixture of 25% N2 and 75% H 2 was then leaked in through the needle valve. The pressure inside the chamber, measured with the manometer, was maintained at © Elsevier Sequoia/Printed in The Netherlands
96 ,nod~ Valve
Resistance 500 mA, 2K
Purification system Needle
eoction Chamber
D,C. power supply # IcY 17_0 mA
Flow Meter Cothode pie
Qc-~
Valve
Meter Mixing Chomber
.5
g
c
g
"o
£ ~
Temperature k ~ Indicator High Vacuum Pumping System Oil Manometer
z
Fig. 1. Experimental set-up for ion nitriding.
ANOMALOUS GLOW
TOWNSEND
Z......
f
i ~
~
J INORMAL GLOW I
E~ i~ 12
Id 4
/
~F
ARC DISCHARGE
1o"I
CURRENT I (A)
Fig. 2. I - V characteristics for different types of discharge.
(this includes the time to reach the nitriding temperature, which was usually 5-6 min) at the desired temperature, the power supply was switched off and the specimen cooled to room temperature in a stream of the treatment gas. Specimens were nitrided for times of 0.5, 2, 5 and 10 h at temperatures of 4 7 5 , 5 0 0 , 525 and 550 °C. The effects were followed using X-ray diffraction techniques and optical metallography.
3. RESULTS AND DISCUSSION
7.25 Tort by balancing the leak rate against the mechanical pump. The power supply was switched on and the voltage gradually increased to strike a glow. The voltage was adjusted and maintained at 500 V, and the current was adjusted to maintain a constant nitriding temperature. The temperature was read o f f on a temperature indicator, using a protected chromel-alumel thermocouple introduced through the cathode (a stainless steel tube 10 mm in diameter was silver brazed to a stainless steel disc 25 mm in diameter and 3 mm thick to form the cathode) and embedded in the steel disc (as illustrated in Fig. 1). After nitriding for a specified time
3.1. X-ray diffraction X-ray diffraction patterns of the nitrided specimens were recorded using Fe Kc~ radiation in a Philips X-ray diffractometer. A scanning speed of 1 ° min -1 with a chart speed of 1 cm min -1 was used and 0 values could be read with an accuracy of +0.05 ° . These patterns, which were obtained on the nitrided specimens w i t h o u t any further treatment, were used to identify the phases present and to index the diffraction lines. Figures 3 and 4 show the diffraction patterns obtained on ion-nitrided En40B steel. An analysis of these patterns shows
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Fig. 4. X-ray diffraction patterns of En40B steel ion nitrided at 550 °C (500 V; 7.25 Torr; 25%N 275%H2) for (a) 0.5 h, (b) 2 h, (c) 5 h and (d) 10 h.
Fig. 3. X-ray diffraction patterns of En40B steel ion nitrided at 475 °C (500 V; 7.25 Torr; 25%N 275%H2) for (a) 0.5 h, (b) 2 h, (c) 5 h and (d) 10 h.
t h a t at all t h e t e m p e r a t u r e s u n d e r s t u d y ( 4 7 5 , 5 0 0 , 5 2 5 and 550 °C) a m i x t u r e o f 3" nitride and e nitride is f o r m e d . A careful e x a m i n a t i o n o f these p a t t e r n s also shows t h a t t h e i n t e n s i t y o f t h e d i f f r a c t i o n lines belonging t o e nitride decreases with t i m e , whereas t h a t belonging t o 3" nitride increases with time. A s e m i q u a n t i t a t i v e idea o f t h e variation in t h e i r relative a m o u n t s was o b t a i n e d b y c o m p a r i n g t h e intensities o f t h e d i f f r a c t i o n lines w h i c h d o n o t overlap b u t are sufficiently
intense. T h e d i f f r a c t i o n lines used for t h e c o m p a r i s o n are t h e (100) and ( 2 0 0 ) lines f r o m t h e e and t h e ~" nitrides respectively. It s h o u l d be n o t e d t h a t , a l t h o u g h t h e intensities o f d i f f r a c t i o n lines o f a given phase are n o t a precise measure o f t h e q u a n t i t y o f t h a t phase in a n y given m i x t u r e o f phases, t h e i n t e n s i t y values can be used as a measure o f t h e relative a m o u n t s o f each phase. Table 1 shows t h a t t h e i n t e n s i t y o f t h e d i f f r a c t i o n line belonging t o t h e e phase gradually decreases with increasing t r e a t m e n t t i m e at a n y t e m p e r a t u r e , indicating a decrease in t h e q u a n t i t y o f this phase. In c o n t r a s t , t h e i n t e n s i t y o f t h e d i f f r a c t i o n line belonging to
98 TABLE 1 Summary of results of X-ray diffraction studies on ion-nitrided En40B steel (500 V; 7.25 Torr; 25%N2-75%H2) Temperature
Time
(°C)
(h)
Phase detected a
Lattice parameter
"YP2OO
elOO
"//{~
a~'~oo
475 475 475 475
0.5 2 5 10
88 33 56 93
25 67 44 7
0.3 0.5 1.3 13
3.809 3.831 3.831 3.816
500 500 500 500
0.5 2 5 10
20 25 92 94
80 75 8 6
0.25 0.33 11.5 13.6
3.816 3.816 3.822 3.828
525 525 525 525
0.5 2 5 10
23 68.5 87.5 93
77 31.5 12.5 7
0.31 2.2 7 12
3.816 3.822 3.822 3.828
550 550 550 550
0.5 2 5 10
40 70 85 96
60 30 15 4
0.67 2.3 5.7 24
3.809 3.816 3.787 3.831
aThe values indicate the areas in square millimetres under the diffraction lines.
7 ' nitride increases w i t h increasing t r e a t m e n t t i m e at t e m p e r a t u r e . F o r a t r e a t m e n t t i m e o f 10 h at all t e m p e r a t u r e s , t h e s t r u c t u r e o f t h e c o m p o u n d l a y e r is p r e d o m i n a n t l y V' nitride. This gradual c h a n g e - o v e r f r o m a mixed nitride structure to a predominantly single-nitride s t r u c t u r e is p o s s i b l y t e m p e r a t u r e d e p e n d e n t . With r e f e r e n c e t o Fig. 3 it can b e seen t h a t t h e t i m e r e q u i r e d f o r t r a n s f o r m a t i o n t o a p r e d o m i n a n t l y 7 ' nitride s t r u c t u r e at 475 °C is a b o u t 10 h, w h e r e a s f r o m Fig. 4 it can be seen t h a t this t r a n s f o r m a t i o n t a k e s a b o u t 6 - 7 h at 550 °C. To detect the structure of the compound l a y e r d u r i n g t h e initial stages o f ion nitriding, o n e s p e c i m e n was n i t r i d e d f o r 10 rain at 500 °C. T h e X - r a y p a t t e r n o b t a i n e d (Fig. 5) s h o w s t h a t t h e s t r u c t u r e is p r e d o m i n a n t l y e nitride. T h u s , d u r i n g t h e ion nitriding o f E n 4 0 B in 2 5 % N 2 - 7 5 % H 2 gas m i x t u r e s , t h e s t r u c t u r e o f t h e c o m p o u n d l a y e r is pred o m i n a n t l y e nitride; this g r a d u a l l y t r a n s f o r m s t o a p o l y p h a s e s t r u c t u r e a n d finally t o a p r e d o m i n a n t l y single-phase (7' nitride) structure. It s h o u l d b e m e n t i o n e d t h a t , d u r i n g t h e ion nitriding o f p u r e i r o n in a m i x t u r e o f 2 5 % N 275%H2, o n l y 7 ' n i t r i d e f o r m e d [5]. In c o n t r a s t , f o r E n 4 0 B steel a m i x t u r e o f V' n i t r i d e a n d e
o
J
J
i
I
2o
Fig. 5. X-ray diffraction patterns of En40B steel ion nitrided at 500 °C (500 V; 7.25 Tort; 25%N 275%H2) for 10 min.
nitride was o b t a i n e d . This is p o s s i b l y b e c a u s e o f t h e c a r b o n c o n t e n t in t h e steel. E d e n h o f e r [6] o b s e r v e d t h a t in t h e p r e s e n c e o f c a r b o n t h e stability range o f t h e e nitride increases. T h e gradual d e c r e a s e in t h e a m o u n t o f e nitride in t h e c o m p o u n d l a y e r c o u l d be a s c r i b e d t o t h e decrease in c a r b o n c o n t e n t o f t h e steel d u e t o s p u t t e r i n g . E d e n h o f e r [6] has r e p o r t e d t h a t , d u r i n g ion nitriding, sputtering removes carbon together with o t h e r e l e m e n t s f r o m t h e w o r k p i e c e surface. H e p r e s e n t e d m i c r o s t r u c t u r e s a n d also t h e
99 nitrogen and carbon profiles to confirm that the carbon content decreases towards the surface of the sample during ion nitriding in a carbon-free plasma. In these experiments, the system pressure was maintained at 7.25 Tort by balancing the leak rate against the pumping rate. As a result, the carbon content both in the material (because of sputtering) and in the plasma (because of the pumping) was continuously decreasing with increasing treatment time, leading to transformation in the structure of the c o m p o u n d layers.
3.2. Optical metallography Sections orthogonal to the nitrided surface were m o u n t e d in a cold-setting resin, prepared using standard techniques and etched in 2% Nital; t h e y were observed and photographed on a Neophot optical metallograph. Optical micrographs (Fig. 6) of En40B steel ion nitrided in 25%N2-75%H 2 show that the etchant used neither reveals the depth of nitrogen penetration nor etches the c o m p o u n d layer. It can also be seen t h a t the core of the specimen remains unaltered by the nitriding. The thickness of the c o m p o u n d layers on the ion-nitrided samples, measured on the screen of the Neophot optical metallograph under a magnification of 1000 diameters, was found to increase with time, at all temperatures, to a certain m a x i m u m value and then to decrease. The time to reach the m a x i m u m thickness in this steel is 2 - 5 h (Fig. 7) depending on temperature. The m a x i m u m thickness is also higher at higher temperatures, that at 550 °C being almost twice that at 475 °C. It has been mentioned earlier that the compound layer has been identified as a mixture of ~,' nitride and e nitride for treatment times of less than 5 h at all temperatures. At longer treatment times, the layer structure transforms to single-phase ~/' nitride, at all temperatures. The time for total transformation is less at higher treatment temperatures. A calculation o f the unit cell volumes of the ~,' nitride and e nitride showed t h a t the difference between the volumes of the two nitrides cannot account for the decrease in the thickness o f the c o m p o u n d layer at longer treatment times. However, the variation in the thickness of the c o m p o u n d layer may be related to the nature of the nitrides in the c o m p o u n d layer.
Fig. 6. Optical micrographs of En40B steel ion nitrided (500 V; 7.25 Torr; 25%N2-75%H2) for (a) 10 hat 500°C and (b) 5 hat 550°C (etch, 2% Nital). (Magnifications, 1688 ×.) Studies [5, 6] have shown that 7' nitride grows to a m a x i m u m thickness of about 6 - 8 pm while e nitride can grow to about 50 pm or more in thickness. Thus, as long as
100
24
2O E
x
16
8
o: oE
4
1 2
I t,
I 6 Time
I 8
I ~0
I 12
(h )
Fig. 7. Variation in the thickness of the compound layer with treatment time at different temperatures for En40B steel (500 V; 7.25 Torr; 25%N2-75%H2): v, 550 °C;o, 525 °C;o, 500°C; X, 475 °C.
e nitride is present, the thickness of the layer increases. At lower temperatures, e nitride is present for a longer time and continues to grow. Since the growth of the layer is diffusion controlled, the maximum thickness at lower temperatures is less. Moreover, the transformation to e nitride takes longer. At higher temperatures, however, the growth of the layer is faster and a thick c o m p o u n d layer is formed in a short treatment time. The possible reasons for the decrease in thickness of the c o m p o u n d layer could be that, in the absence of carbon, the layer transforms to a lower nitride and more hydrogen is available for denitriding. As a result the denitriding rate possibly increases. It is possible that the denitriding reaction is faster at higher temperatures, causing a greater decrease in thickness. Sputtering could be another possible reason for this decrease in thickness of the c o m p o u n d layer [6]. Soccorsy and Ebihara [4] subjected samples of AISI 4140 steel (0.40 wt.% C, 0.90 wt.%Mn, 0.30 wt.%Si, 0.95 wt.%Cr and 0.20 wt.%Mo) which had been gas
nitrided for 12 h at 524 °C to an additional 18 h treatment under ionizing conditions in carbon-free plasmas and observed a significant decrease in the thickness o f the c o m p o u n d layer (the thickness decreased to less than one-half of the original thickness). They ascribed this decrease in thickness to denitriding caused by the hydrogen ions. However, they did not identify the structures of the nitrides but, since they used only 25%N2-75%H2 as the treatment gas, it is possible that decarburization occurred, caused transformations in the structure of the c o m p o u n d layer and enhanced the rate of denitriding, leading to the decrease in the thickness of the c o m p o u n d layer. It should be noted that, because the growth of the layer is diffusion controlled, it is slower at longer times at any given temperature, leading to a more pronounced effect of sputtering towards the end o f the (10 h) treatment.
4. CONCLUSIONS
(1) The structure of the c o m p o u n d layer formed during the ion nitriding of. En40B steel shows a gradual transformation from e nitride to 7' nitride with increasing treatment time. The thickness o f the c o m p o u n d layer also shows a variation with treatment time. (2) The variation in the structure and in the thickness of the c o m p o u n d layer may be related to the decarburization, denitriding and sputtering that take place during ion nitriding.
REFERENCES 1 B. Edenhofer, Heat Treat. Met., 2 (1974) 60. 2 K. S. Cho and C. O. Lee, J. Eng. Mater. Technol., 102 (1980) 231. 3 K. Keller, Ha'rterei-Tech. Mitt., 26 (1971) 125. 4 W. D. Soccorsy and W. T. Ebihara, Trans. Metall. Soc. AIME, (1970), Paper A70-61, 3. 5 A. S. W. Kurny, Ph.D. Thesis, Indian Institute of Science, Bangalore, 1982. 6 B. Edenhofer, Heat Treat. Met., 2 (1974) 62.