Effect of nitrogen on the plasma (ion)-carburized layer of high nitrogen austenitic stainless steel

Effect of nitrogen on the plasma (ion)-carburized layer of high nitrogen austenitic stainless steel

Surface & Coatings Technology 200 (2005) 521 – 524 www.elsevier.com/locate/surfcoat Effect of nitrogen on the plasma (ion)-carburized layer of high n...

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Surface & Coatings Technology 200 (2005) 521 – 524 www.elsevier.com/locate/surfcoat

Effect of nitrogen on the plasma (ion)-carburized layer of high nitrogen austenitic stainless steel Y. Uedaa,*, N. Kanayamaa, K. Ichiib, T. Oishib, H. Miyakeb a

Shimane Institute for Industrial Technology, 1 Hokuryou-cho, Matsue, Shimane, 690-0816, Japan b Faculty of Engineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka, 564-8680, Japan Available online 17 May 2005

Abstract Austenitic stainless steel or high nitrogen austenitic steel are used extensively as structural non-magnetic materials. The former, because of their low surface hardness, have low wear resistance. Recently developed plasma (ion)-carburizing improves the surface of austenitic stainless steel by removing the oxide scale. In contrast, high nitrogen austenitic stainless steel are expected as wear-resistance material because the hardness is higher than that of conventional austenitic stainless steel. In this study, two kinds of austenitic stainless steel were plasma (ion)-carburized in an atmosphere of CH4 + H2 plasma at 1303 K for 14.4 ks. The metallurgical characteristics were then investigated using XRD (X-ray diffraction method), OM (optical microscopy), hardness testing, glow discharge optical emission spectroscopy (GDOES) and a scanning electron microscopy (SEM). The results show that the plasma (ion)-carburized low nitrogen austenitic stainless steel had the layer with structure similar to the common austenitic stainless steel. The plasma (ion)-carburized high nitrogen austenitic stainless steel had the carbide layer at the top surface, carbide dispersed layer in the matrix, and then lamellar structure of carbides toward the inner layer. D 2005 Elsevier B.V. All rights reserved. Keywords: Plasma carburizing; Austenitic stainless steel; GDOES; Carbide

1. Introduction Austenitic stainless steel and high nitrogen austenitic stainless steel are used extensively as structural non-magnetic materials. They show low surface hardness, low wear resistance, low fatigue resistance, and weak anti-seizure properties. Attempts have been made to develop surface engineering techniques to improve these properties [1]. Surface modification of austenitic stainless steel is usually difficult because of the formation of an oxide scale (Cr2O3) on the steel surface owing to the strong affinity of chromium to oxygen [2]. This oxide scale obstructs the penetration of carbon into the steel. A recently developed plasma (ion)-carburizing technique improves the surface of austenitic stainless steel by using ion bombarding to remove the oxide scale. In contrast, high nitrogen austenitic stainless steel are expected to be wear-resistance materials because their * Corresponding author. Tel.: +81 852 60 5119; fax: +81 852 60 5109. E-mail address: [email protected] (Y. Ueda). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.191

hardness is higher than that of conventional austenitic stainless steel. In this paper, the surface metallurgical properties of high nitrogen austenitic stainless steel were investigated by plasma (ion)-carburizing in an atmosphere of CH4 + H2 plasma at 1303 K.

2. Experiment In this experiment, two kinds of austenitic stainless steel were used for test pieces. One is the low nitrogen austenitic stainless steel (specimen A), the other is the high nitrogen austenitic stainless steel (specimen B) [3]. Table 1 lists the Table 1 Chemical composition (mass %) of experimental material Elements

C

Si

Mn

Ni

Cr

Mo

N

Fe

Specimen A Specimen B

0.037 0.050

0.89 0.97

16.64 16.66

16.58 0.12

15.68 15.62

4.84 3.83

0.04 0.81

Bal. Bal.

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Y. Ueda et al. / Surface & Coatings Technology 200 (2005) 521 – 524

In Plasma Treatment temperature : 1303K Heating

Carburizing

Sputter cleaning

50%Ar-H2

33%CH4-67%H2 320Pa

Furnace cooling

100Pa

3.6

1.8

14.4

100µm

100µm

(a)

(b)

Time / ks Fig. 1. Plasma (ion)-carburizing process.

3. Results and discussion Fig. 2 shows the result of the OM examination of the specimens which were plasma (ion)-carburized for 14.4 ks Table 2 Parameters for plasma (ion)-carburizing treatment Process Gas mixtures and volumes (m3/s) Treatment time (ks) Total pressure (Pa) Voltage (V) Current density (A/m2)

Sputter cleaning Ar: 5.0  10 H2: 5.0  10 1.8 100 350 0.042

6 6

Plasma (ion) carburizing CH4: 5.0  10 6 H2: 1.0  10 5 14.4 320 500 0.008

Fig. 2. Microstructure of the plasma (ion)-carburized layer treated at 1303 K for 14.4 ks: (a) specimen A and (b) specimen B.

Intensity / a.u.

• M7C3



Specimen B

• • •

γ - Fe



• ••

• •

••



• •

••







Specimen A

• 30

35

• •

40



• •• 45



50

55

60

65

70

75

80

2Theta / degree Fig. 3. X-ray diffraction profile of the specimen that was plasma (ion)carburized at 1303 K for 14.4 ks.

at 1303 K. In the specimen A, as shown in Fig. 2(a), carbides precipitated both in the grain matrix and the grain boundary from the surface to a depth of 0.25 mm. This behavior is similar to the carburized layer of a common austenitic stainless steel [4]. In the specimen B, white layer 700

Vickers hardness / HV

chemical compositions of these specimens. They were cut into 15  25 mm and a thickness of 5 mm pieces, and polished by using emery paper. A diagram of the plasma (ion)-carburizing process using dc plasma and cold wall furnace is shown in Fig. 1. After the temperature was raised to 1303 K in 3.6 ks, Ar + H2 mixed gas was introduced into the chamber and the specimen ware etched for 1.8 ks with sputtering. After the etching was completed, the chamber was evacuated, and then CH4 + H2 mixed gas was passed into the chamber. The plasma (ion)-carburizing treatment was carried out in the stream of the mixed gas for 14.4 ks. After plasma (ion)carburizing was completed; the specimen was cooled in the furnace. Table 2 lists the parameters for the plasma (ion)carburizing treatments investigated in this study. After the completion of plasma (ion)-carburizing, the microstructure of a cross section was examined using optical microscopy (OM). An X-ray diffraction method (XRD) was used to identify the structure of the carburized layer in the treated specimen. The hardness distribution profile of the carburized layer was measured using Vickers micro-hardness tester with a load of 0.1 kg. The element distribution profile of the carburized layer was measured using grow discharge optical emission spectroscopy (GDOES) and the morphology of the carburizing layer was examined using scanning electron microscope (SEM).

600 500 400

Specimen B

300 200

Specimen A

100

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Distance from the surface / mm Fig. 4. Hardness distribution curve of the specimen that was plasma (ion)carburized at 1303 K for 14.4 ks.

Y. Ueda et al. / Surface & Coatings Technology 200 (2005) 521 – 524

6

Intensity / a.u.

Cr 4

C

Specimen A

Ni Fe

Mo Mn

2

N 0

Si 0

100

200

300

400

Sputtering time / s 6

Specimen B

Fe

Intensity / a.u.

Cr 4

C Mo

2

Mn 0

N

Si Ni

0

100

200

300

400

Sputtering time / s Fig. 5. Intensity distribution profiles of elements of the specimen that was plasma (ion)-carburized at 1303 K for 14.4 ks using GDOES.

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was formed from the surface to a depth of 30 Am, and black layer was followed. Fig. 3 shows the results of the XRD analysis of the specimen treated by plasma (ion)-carburizing for 14.4 ks at 1303 K. Before the carburizing, only g phase was detected in XRD profiling of specimen A and B. After the carburizing, M7C3 [5] and g phase were detected in specimen A, thereby confirming that the structure located adjacent to the surface consisted of M7C3 and g phase, while M7C3 and (a phase) were detected in specimen B, because the Mn at the surface in specimen B decreased by vaporizing during the low pressure plasma process [6]. Fig. 4 shows the hardness distribution curves of the specimens that were plasma (ion)-carburized for 14.4 ks at 1303 K from the surface to a depth of about 2.0 mm. The hardness was about HV500 on the surface of the specimen A, and about HV600 on the surface of the specimen B. The hardness of specimen B was higher than that of specimen A in both the carburized layer and the matrix, which indicates that the matrix hardness is affected by nitrogen. Fig. 5 shows the intensity distribution profiles of elements from the surface to the inner layer of the specimen after carburizing. Carbon and chromium tended to behave similarly, exhibiting an increase in the intensity first at the top surface, reaching then a minimum with the increasing sputtering time, and finally increasing towards the inner layer. In contrast, the intensity of iron (iron and nickel at the specimen A) behaved in an opposite manner, increasing

Specimen A

Specimen B

15µm

15µm

(a)

(b)

5µm

5µm

(c)

(d)

Fig. 6. SEM micrograph of cross section of the specimen that was plasma (ion)-carburized at 1303 K for 14.4 ks: (a) and (c) specimen A and (b) and (d) specimen B.

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with sputtering time and reaching peak value at 100 s as shown in Fig. 5. Fig. 6 shows the results of the SEM micrograph of a cross section of these specimens after carburizing. As shown in Fig. 6(a), carbides precipitated both in the grain matrix and the grain boundary in specimen A. In specimen B, as shown in Fig. 6(b), the structure in which carbides disperse in the matrix was observed, and in inner area, lamellar structure of carbides was formed in the grain matrix. As shown in Fig. 6(c) and (d), a carbide layer with a thickness of several micrometers was formed on the surface of the specimens. This carbide layer indicates that the intensities of C and Cr are due to the Cr carbides that have formed on the top surface as shown in Fig. 5.

carbide layer at the top surface, carbide dispersed layer in the matrix following a lamellar structure as the inner layer.

Acknowledgements The authors would like to thank Dr. T. Sone and Dr. N. Ueda of the Technology Research Institute of Osaka Prefecture for the GDOES measurements. This research was financed in part by the Kansai University Grant-in-Aid for the Promotion of Advanced Research in Graduate Courses, 2004.

References 4. Conclusions Two kinds of austenitic stainless steel were plasma (ion)carburized at a temperature of 1303 K for 14.4 ks. Using GDOES for the intensity measurements and SEM for the morphology observations, the following results were obtained: The plasma (ion)-carburized low nitrogen austenitic stainless steel (specimen A) had the layer with structure similar to the common austenitic stainless steel. In contrast, it is observed that the plasma (ion)-carburized high nitrogen austenitic stainless steel (specimen B) had the layers;

[1] J.R. Davis, ASM Handbook, ASM International, Materials Park, OH, 1994, p. 741. [2] Y. Sun, X. Li, T. Bell, Stainless Steel 2000, Thermochemical Surface Engineering of Stainless Steel, 2000, p. 51. [3] K. Ichii, K. Oku, T. Morikawa, Current Advance in Materials and Processes, Report of ISIJ Meeting, vol. 17, p. 1184 (in Japanese). [4] Y. Ueda, N. Kanayama, K. Ichii, T. Oishi, H. Miyake, Report of 57th JSHT Meeting, 2003, p. 35 (in Japanese). [5] L.R. Woodyatt, G. Krauss, Met. Trans. A, vol. 7A, 1976, p. 983. [6] Y. Ueda, N. Kanayama, K. Ichii, T. Oishi, H. Miyake, Abstracts Book of 4th Asian – European International Conference on Plasma Surface Engineering, 2003, p. 172.