Effect of plasma nitriding on the surface properties of the chromium diffusion coating layer in iron-base alloys

Effect of plasma nitriding on the surface properties of the chromium diffusion coating layer in iron-base alloys

Surface and Coatings Technology 116–119 (1999) 391–397 www.elsevier.nl/locate/surfcoat Effect of plasma nitriding on the surface properties of the ch...

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Surface and Coatings Technology 116–119 (1999) 391–397 www.elsevier.nl/locate/surfcoat

Effect of plasma nitriding on the surface properties of the chromium diffusion coating layer in iron-base alloys Do Yon Chang a, Sang Yul Lee b, *, Sung-Goon Kang c a Electrochemical Processing Group, Korea Institute of Machinery and Materials, Changwon 641-010, South Korea b Department of Materials Engineering, Materials Processing Laboratory, Hankuk Aviation University, KoYang, KyungKi-Do 412-791, South Korea c Department of Materials Engineering, HanYang University, Seoul 133-791, South Korea

Abstract Duplex plasma surface treatments of chromizing and plasma nitriding on various alloys such as mild steel (AISI 1020), AISI H13, and 1Cr–0.5Mo steel (ASTM A213) were carried out. Specimens were chromized at 1200–1300°C for 5 h and were subsequently plasma nitrided at 530°C for 1 h. Effects of plasma nitriding on the Cr diffusion coating and the characteristics of the final duplex-treated specimens were analyzed using SEM, EDS, XRD, and microhardness tester. The chromizing for 5 h by pack cementation process had created a Cr diffusion layer of approximately 150–300 mm thickness. Subsequent plasma nitriding on the Cr-diffused layer induced formation of a duplex-treated surface layer with a largely improved microhardness up to approximately 1500 Hv (50 gf ). The main cause of the large improvement in surface hardness was due to the fact that CrN and Fe N phases were created by chromizing and plasma nitriding treatment. Also being studied was the high temperature wear x resistance of the duplex-treated specimens at 600°C. Comparing the duplex-treated specimens with the specimens treated only by chromizing, the wear volume of the duplex-treated AISI 1020 and H13 steels after a wear test at 600°C were reduced by a factor of 8 and 3, respectively. Also the examination of the wear tracks after wear test at 600°C on chromized and duplex-treated specimens indicated that regardless of the alloy compositions, the wear tracks of the duplex-treated specimen showed an abrasive type wear while the chromized specimens showed an adhesive type wear. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Chromizing; CrN; Iron nitride; Plasma nitriding

1. Introduction Since the late 1980s and early 1990s, extensive research has been carried out on the development of surface treatment processes to improve the service life of mold and high speed tool steels for high temperature applications, and to develop new coating materials in order to improve the tribological properties of existing coating materials such as TiN for machine parts and tool applications. Promising coatings for tribological use as an alternative to TiN are chromium based nitrides such as CrN, Cr N [1–4]. Chromium nitride coatings 2 have been successfully made using various types of physical vapor deposition (PVD) processes, such as cathodic arc deposition [5,6 ], reactive ion plating [7,8], hollow cathode discharge [9,10], and magnetron sputtering [11–13]. * Tel.: +82-2-300-0166; fax: +82-2-3158-3770. E-mail address: [email protected] (S.Yul Lee)

In this study, as one of many attempts to develop a CrN coating to improve the service life of mold and high speed tool steels for high temperature applications, the duplex treatment of chromizing and plasma nitriding was performed. The structural characteristics as well as the wear properties at 600°C were investigated.

2. Experimental In this work AISI 1020, AISI H13, and ASTM A213 were studied. Cleaned and degreased specimens were chromized using pack cementation for 5 h at 1200– 1300°C. The pack composition was 25 wt% Cr (+200 mesh), 69 wt% Al O (+250 mesh), and 6 wt% NH Cl. 2 3 4 Sputter cleaning was performed prior to plasma nitriding for 30 min under H atmosphere and plasma nitriding 2 was performed in a 50% N –50% H atmosphere for 2 2 1 h at 530°C. Specimens were cooled in the nitriding

0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 23 4 - 0

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a)

AISI 1020

b)

AISI 1020

c)

AISI H13

d)

AISI H13

e)

ASTM A213

Fig. 1. Cross-sectional SEM micrographs of chromized layer of (a, b) AISI 1020, (c, d) AISI H13, (e) ASTM A213.

chamber under N . The microstructure was studied by 2 SEM and EDS analysis using Oxford Link ISIS with Li-doped Si window at an accelerating voltage of 20 kV. The quantitative analysis was made using ZAF method. Micro-Vickers’ hardness was measured using a load of 0.05 kgf and an average of seven readings was taken. A

diffractometer using Cu Ka radiation was used for XRD analysis. Ball-on-disk type wear tests were performed at 600°C. The applied load was 0.4 kgf and the sliding distance was 2 km with a linear velocity of 0.3 m/s. An alumina ball 8 mm in diameter was used as counterface ball and the relative humidity was 50–60%.

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3. Results and discussion 3.1. Microstructural analysis Fig. 1 shows the cross-sectional micrographs of the specimens after chromizing treatment. For the AISI 1020 specimen, a ferrite–pearlite microstructure, typical of low carbon mild steel, along with a thick and featureless chromized layer was observed as shown in Fig. 1(a). The thickness of the chromized layer was of the order of 180–190 mm. A higher magnification photograph of the chromium diffusion layer on the surface of the chromized AISI 1020 specimen in Fig. 1(b) shows that surface carbide as well as grain boundary carbide have formed, as confirmed by XRD analysis later in this work (refer to Fig. 2). Also found was a layer of decarburized zone between the chromized layer and the matrix (approximately 220 mm thick), which was believed to be a source of carbon to form the surface as well as grain boundary carbides during chromizing. The white area at the end of the chromium diffusion layer towards the matrix in Fig. 1(a) was confirmed to be fine pearlite. Having approximately 0.37 wt% C, AISI H13 tool steel was expected to have an extensive formation of carbide during chromizing and the cross-sectional micrographs of this steel in Fig. 1(c) confirm this expectation. After chromizing under the same processing conditions, the thickness of the chromized layer in the AISI H13 specimen was measured to be of the order of 150 mm, which was much less than that in the AISI 1020 steel. This could be attributed to the amount of carbon content in the matrix. A high amount of carbon in the matrix leads to an extensive formation of carbides as shown in Fig. 1(d ), and this in turn hinders or decelerates chromium diffusion into the matrix, resulting in a thin chromized layer. This suggestion that the carbon content in the matrix does greatly affect the thickness of the chromized layer was confirmed by examining the cross-sectional micrographs of ASTM A213 tube steel

Fig. 2. XRD patterns of chromized specimens and XRD patterns after polishing the surface chromium carbide away.

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in Fig. 1(e). The thickness of the chromized layer in ASTM A213 with approximately 0.15 wt% C was approximately of the order of 200–210 mm, slightly higher than that of AISI 1020 with approximately 0.2 wt% C. Also, close examination of the surface microstructure in Fig. 1(e) reveals that only surface carbides without any grain boundary carbide in the chromized layer were observed. Chromizing on Inconel 718, Fe–Ni base superalloy with approximately 0.04 wt% C and 17 wt% Cr under the same processing conditions was done and the thickness of the chromized layer was measured to be approximately 300 mm [14], confirming the previous suggestion about the effect of carbon content on the thickness of the chromized layer. AISI H13, ASTM A213, Inconel 718 contain 5 wt% Cr, 1 wt% Cr, 17 wt% Cr, respectively and the thicknesses of the chromized layer after the same processing conditions are of the order of 150 mm, 200–210 mm, and 300 mm, respectively. Based on this observation it could be concluded that the effect of other alloying elements such as Cr, Mo, etc. on the thickness of the chromized layer is not as strong as that of the carbon content in the matrix. The results from the XRD analysis on the chromized surface of the specimens in Fig. 2 revealed that all three specimens showed similar XRD patterns, and the types of carbides were identified to be Cr C and Cr C . 23 6 7 3 After removing the surface carbides by mechanical polishing, only diffracted peaks from a solid solution of Fe–Cr ferrite remained, as shown in Fig. 2. Plasma nitriding on the chromized specimens was done for 1 h at 530°C. The cross-sectional microstructure after the duplex treatment is shown in Fig. 3. AISI 1020 steel is generally considered to be difficult to nitride since it does not contain any elements which have a strong affinity to nitrogen to form nitride [15]. But in the chromized 1020 steel it was possible to form a nitrogen diffusion layer on the order of 70–80 mm. This could be attributed to the fact that chromium in the chromized layer has promoted the nitrogen diffusion by forming chromium nitride, which was later confirmed by XRD analysis. A higher magnification photograph of the plasma nitrided layer on the surface of the chromized AISI 1020 specimen in Fig. 3(b) shows that a thin white layer, typical of plasma nitrided surface, was identified. Also, the grain boundary carbide formed during the chromizing process seems to disappear. Results from the EDS line profile analysis of Cr and N in the chromized and plasma nitrided area in Fig. 4 show the Cr concentration at the surface to be more than 25 wt% Cr, decreasing continuously towards the interface between the chromized layer and the matrix. Also from the EDS results the high concentration of nitrogen in the duplex-treated layer was confirmed. After plasma nitriding the chromized AISI 1020 steel,

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a)

AISI 1020

b)

AISI 1020

c)

AISI H13

d)

AISI H13

e)

ASTM A213

Fig. 3. Cross-sectional SEM micrographs of chromized and plasma nitrided layer of (a, b) AISI 1020, (c, d ) AISI H13, (e) ASTM A213.

the surface of the specimen was analyzed using XRD and the results are shown in Fig. 5. It shows that not only the CrN phase is present in the layer, but c∞(Fe N ) nitride as well as e( Fe N ) nitride coexist. So 4 2–3 it is noted that CrN layer mixed with iron nitrides can be formed by a duplex treatment of first chromizing and

then plasma nitriding on a mild steel. A similar tendency of the formation of chromium nitride and iron nitride layers and a thickness of approximately 70–80 mm were observed in AISI H13 and ASTM A213 steels as shown in Fig. 3(c) and (e), except that large carbides formed in the chromized layer in the AISI H13 steel still

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Fig. 4. EDS line profiles of Cr, N in the chromized and nitrided specimens of AISI 1020.

remained in the plasma nitrided layer [large carbides were recognized by deep etching in Fig. 3(d )]. The results from the XRD analysis in Fig. 5 show that a similar diffraction was obtained independent of the alloy types. This is understandable because all three specimens have a similar microstructure before plasma nitriding, with the exception of the size of grain boundary carbides (refer to Fig. 1). 3.2. Microhardness analysis The microhardness profiles of the specimens after duplex treatment of chromizing and plasma nitriding are shown in Fig. 6. The surface microhardness of the duplex-treated AISI 1020, i.e. chromized and plasma nitrided AISI 1020, was measured to 1400–1500 Hv and decreases to approximately 1300 Hv at the interface

Fig. 5. XRD patterns of chromized and nitrided specimens.

between the duplex-treated layer and the chromized layer as the amount of nitrogen decreases towards the matrix. It is noted that the microhardness in the duplextreated layer is slightly lower than the Cr N coating x layer by PVD process, which is approximately 1600– 2000 Hv [16 ]. This could be attributed to the fact that there are not only CrN phase, but c∞(Fe N ) and 4 e( Fe N ) phases present in the chromized and plasma 2–3 nitrided layer, compared with the single CrN or Cr N x phase obtained by PVD process. Generally it is well known that iron nitrides have slightly lower hardness than chromium nitrides [17]. In the duplex-treated layer in AISI 1020 a relatively high microhardness of more than 1300 Hv could be obtained and AISI H13 and ASTM A213 steels show a similar microhardness distri-

Fig. 6. Cross-sectional micro-Vickers’ hardness profiles of chromized and nitrided specimens.

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Fig. 7. (a) Wear volume and (b) weight change after the wear test at 600°C on chromized AISI 1020 and AISI H13 and chromized and nitrided AISI 1020 and AISI H13; (c, d) SEM micrographs of wear track on chromized AISI H13 specimen, chromized and nitrided AISI H13 specimen, respectively.

bution to AISI 1020 with the exception of the thickness of the duplex-treated layer.

3.3. Wear behaviour at 600°C The results from the ball-on-disk type wear test at 600°C are summarized in Fig. 7. The wear volume in Fig. 7 was calculated from the wear track using a profilometer. The chromized AISI 1020 and H13 steel showed a poorer wear resistance than the duplex-treated specimens as shown in Fig. 7(a) and (b). The duplextreated specimen showed the lowest friction coefficient and this agrees with the results from the wear volume measurements. The high surface hardness and deep case in the duplex-treated specimen provided an excellent 600°C wear resistance. The wear tracks after wear test at 600°C on chromized and duplex-treated specimens AISI H13 are shown in Fig. 7(c) and (d ). Regardless of the alloy compositions the wear tracks of the duplex-treated specimen in Fig. 7(d ) showed an abrasive type wear while the chromized specimens showed an adhesive type wear.

4. Conclusions

1. Using a duplex surface treatment of chromizing for 5 h at 1200–1300°C and plasma nitriding for 1 h at 530°C, CrN coating mixed with iron nitrides, with a thickness more than 70–80 mm was successfully produced on several iron-based alloys. 2. Excellent surface hardness and hardness profiles could be obtained by duplex treatment of chromizing followed by plasma nitriding, resulting in an excellent high temperature wear resistance. This indicates the duplex treatment by chromizing and plasma nitriding to be a potential process for improving elevated temperature surface properties of steels.

Acknowledgement This work was funded by the Korea Ministry of Education through the 1997 Advanced Materials Program.

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