Microstructure and properties of layers on chromium steel

Microstructure and properties of layers on chromium steel

Surface & Coatings Technology 200 (2006) 6572 – 6577 www.elsevier.com/locate/surfcoat Microstructure and properties of layers on chromium steel J. Bi...

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Surface & Coatings Technology 200 (2006) 6572 – 6577 www.elsevier.com/locate/surfcoat

Microstructure and properties of layers on chromium steel J. Bielawski ⁎, J. Baranowska, K. Szczecinski Institute of Materials Science and Engineering, Szczecin University of Technology, 70-310 Szczecin, al. Piastow 19, Poland Available online 27 December 2005

Abstract Nitriding of chromium steel is surface treatment, which is used to improve hardness and wear resistance of the steel. In case of austenitic steel at low temperature nitriding (below 500 °C), it is also possible to obtain corrosion resistant layers, thanks to the presence of the so-called “expanded austenite”. The present paper shows the results of the tests carried out on the process of nitride layer formation on chromium steel with various kinds of structure (austenitic and/or ferritic). The experiment was made in temperature 400–500 °C in ammonia gas atmosphere. In order to remove the passivating layer before nitriding, ion sputtering in hydrogen was applied. The microstructure of the layers was investigated using scanning electron microscopy (SEM) and light microscopy analysis (LMA) techniques. The phase build-up was checked by XRD methods. Chemical composition was evaluated by microprobe analysis. Moreover, the thickness and microhardness of the layers were measured. It was stated that, during gas nitriding of chromium steel, it is possible to obtain layers with good mechanical properties (microhardness) and good corrosion resistance. But it was also observed that the morphology and structure of the layers formed on the ferrite seem to be different from those produced on the austenite. © 2005 Elsevier B.V. All rights reserved. Keywords: Gas nitriding; Austenitic ferritic chromium steel; Expanded austenite

1. Introduction High chromium steel belongs to corrosion resistant materials and this fact determines its practical applications. Unfortunately, this kind of steel has very low wear resistance. Therefore, a variety of surface treatments is applied to improve this property, e.g. chemical heat treatment. As a result of this process, a very hard protective layer can be obtained. However, if the process is carried out in elevated temperature (above 500 °C), the corrosion resistance of the coatings is very poor due to the formation of chromium and iron nitrides. It was found that, in the austenitic stainless steel, it is possible to obtain a corrosion resistant layer when the process temperature is lower than 500 °C, thanks to the production of a new phase called “expanded austenite” or S-phase [1–4]. Moreover, the presence of this phase increases the hardness and wear resistance of this steel. It was found that a similar kind of structure can be produced in ferritic and duplex steel [5–8], but the mechanism of its formation and the conditions of this process are not fully understood and still under investigation. The growth of the ⁎ Corresponding author. Tel.: +48 91 449 46 98; fax: +48 91 449 46 98. E-mail address: [email protected] (J. Bielawski). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.11.037

“expanded austenite” in austenitic steel is also possible during typical gas nitriding treatment [9], i.e. in much more thermodynamically stable and predictable conditions than those taking place during plasma nitriding. Therefore, the main objective of this work was to investigate the formation of gas nitrided layers on high chromium steel with various matrix structures (ferritic and ferritic–austenitic) in the conditions, which usually enable the “expanded austenite” to be formed in austenitic steel. 2. Experimental The materials chosen for the investigation were: chromium and chromium–nickel ferrite steel and ferritic–austenitic stainless steel whose compositions are presented in Table 1.

Table 1 Compositions of the steel used in the experiment (wt.%) Material

C

Cr

Ni

Mo

Mn

Si

Cu

A B C

0.09 0.07 0.03

12.44 17.1 22.09

0.35 4.3 5.22

– – 2.72

0.52 1 1.83

0.43 – 0.34

– 4.5 –

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Table 2 Treatment parameters applied during the experiments

3. Results and discussion

Activation technique—ion sputtering

Treatment—gas nitriding

Atmosphere: hydrogen, 3–5 Pa Current density: 3 mA/cm2 Voltage: 1.35 kV Time: 15 min

Atmosphere: 100% ammonia Time: 3 h Temperature: 400, 450 and 500 °C

Fig. 1 shows the typical microstructure of metallographic cross sections of a gas nitrided ferritic and austenitic–ferritic steel after treatments in various temperatures. It can be seen that all layers obtained below 500 °C are not attacked by the etchant, which could indicate their good resistance to corrosion. The etching behaviour of the samples nitrided in 500 °C varies with the samples compositions and matrix. The layer obtained on chromium–nickel ferritic steel (B) is uniformly dark after etching, while the layer on chromium ferritic steel (A) has “white” appearance with numerous dark spots on the cross section. In case of ferritic–austenitic steel, the layer changes depending on the matrix. The part obtained on the austenite is white in comparison with the one on the ferrite in which many dark lines can also be observed. In Fig. 2, the pictures of the layer surface are presented. It can be seen that, when the treatment temperature increases, the stronger relief on the surfaces could be observed. But it also should be mentioned that even for 500 °C the grain boundaries are still visible. In case of duplex steel (C), the differences between austenitic and ferritic parts are particularly well pronounced.

The size of the samples was 10 × 20 × 5 mm. They were polished to a 3 μm diamond suspension and electropolished as the ultimate method and cleaned in the ultrasonic alcohol bath. The samples were treated according to the process parameters presented in Table 2. The morphology of the nitrided layers was investigated using light microscopy (Nikon Epiphot 200) and scanning microscopy (Jeol 6100). The aqua regia was used as an etchant. The phase composition of the layers was evaluated by X-ray diffraction (CoKα, Philips) and electron back scattering diffraction (Oxford Instruments), and the nitrogen content were measured using WDX methods. Moreover, the microhardness of the layers was measured using Knoop technique (Boehler Microhardness Tester 2000) with load varying from 5 to 10 g.

a)

b)

c)

Temperature Fig. 1. Microstructure of the layers obtained on chromium steel after various nitriding temperatures.

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a)

b)

c)

Temperature Fig. 2. Surfaces of the nitrided samples after gas nitriding in various temperatures.

Austenitic grains have relief typical for this kind of steel with numerous slip lines [10], while the ferritic part remains rather smooth. The rate of the layer's growth mainly depends on temperature (Fig. 3), but the matrix and composition of the

Fig. 3. Thickness of the nitrided layers depending on the sample and the temperature of the treatment.

steel have also some influence. For ferritic matrix, including ferritic part of ferritic–austenitic steel (which has similar Nicontent—4.6 wt.%—as ferritic B steel), the layer thickness increases with the decrease of chromium content, which is an inverse effect to the one observed by Gemma at al. [11] for austenitic steel. Comparing the growth of the layer in ferritic– austenitic steel (comparing it with what?—we seem to be lacking the second element), it can also be observed that the growth of the layer on the ferritic part is quicker than on the austenite, which is particularly well visible on the sample nitrided in 500 °C (Fig. 4). In the austenitic part, the layer has a characteristic arc-like shape. The thickness of the layer is bigger close to ferrite boundary and becomes thinner in the central part of the austenite grain. The same type of layers was observed by other researchers [8,12] on plasma nitrided duplex steel. But it is worth noting that Kliauga [12] observed the opposite effect (i.e. a thicker layer for the austenite than for the ferrite) when duplex steel was gas nitrided. In Fig. 5, the nitrogen content in all samples is presented. Comparing these results with the layer thickness measurements, it can be seen that there is no direct relationship between these two parameters as it is usually observed for nitrided austenitic steel [13]. For ferric chromium–nickel (B)

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Fig. 4. A cross section of the layer of the duplex steel after nitriding in 500 °C. (a) Micrograph of the layer cross section and (b) corresponding schematic view of the layer. The arrows show the arc-like shape of the layer obtained on the austenitic matrix.

steel, the nitrogen content remains nearly at the same level for all temperatures. In 500 °C, all nitrided layers have similar nitrogen content and, for all kinds of steel, the highest nitrogen content is observed after the treatment in 450 °C. Unfortunately, it was difficult to combine these observations with phase composition of the layers, evaluated by X-ray diffraction measurements. After nitriding, a drastically drop in peaks intensity was observed. In case of ferritic steel, all the peaks related to the layer became broad and they overlapped each others (peaks marked with asterisks in Fig. 6); therefore, their plausible identification was impossible. In case of duplex steel, only peaks of γN phase were well seen (Fig. 6c) for the sample nitrided in 450 °C. This effect was also confirmed by electron back scattering diffraction. The results for duplex steel are presented in Fig. 7. In Fig. 7b and c, the phase analysis of the layer from Fig. 7a is presented for ferrite and austenite, respectively. The part of the nitrided layer, which was obtained on the austenite, was identified as austenite, which is a typical effect observed on nitrided austenitic steel when the layer is composed of “expanded austenite” [14]. The part produced on the ferritic matrix was identified neither as ferrite nor as austenite. Moreover, none of chromium or iron nitrides could be also attributed to this region. Looking at the so-called “quality image” of the layer (Fig. 7d), it can be seen that this

region is much darker than the part of the layer formed on the austenite, which means lower quality of Kikuchi's lines coming from this area. Usually, the differences in the composition or strong internal stresses could decrease this quality [15], so this effect could result from a high concentration of nitrogen in the layer. But it could also suggest that the mechanism of the layer formation in low temperature was different from a typical nitriding process where nitrides are formed on top of the surface. It could also be possible that due to the low temperature the transformations occur in small volumes and as a result very fine structure is formed, which could explain the broadening of the peaks on the diffraction patters. But these observations need to be investigated more in-depth. In Fig. 8, the microhardness profiles for nitrided samples are presented depending on the treatment temperature. For the temperature of 400 °C, the layer was too thin to obtain the valuable microhardness profiles. All the layers demonstrate very high hardness, much higher than the typical hardness for nitrided chromium steel usually below 1200 HV. It can also be observed that for steel containing nickel the microhardness drop to the matrix is more sudden than for chromium steel (A), which has profile typical for diffusion layers, gradually decreasing into the matrix. 4. Summary

Fig. 5. Nitrogen content in the samples depending on the sample and temperature of the treatment.

As a result of gas nitriding treatment, it was possible to obtain uniform layers during low temperature process. The growth of the layers and their morphology depended on the structure and chemical compositions of the matrix. According to the diffraction analysis, it was difficult to estimate if the “expanded austenite”-like phase was obtained on the ferritic matrix because of the strong broadening of the peaks. For all the samples nitrided in temperature below 500 °C, the layers remained white after etching, which could suggest their good corrosion resistance. All the layers show very good mechanical properties (high hardness) corresponding to a very high nitrogen content in the layers. Therefore, it can be stated that also during low temperature gas nitriding of ferritic chromium steel it is possible to produce hard and corrosion resistant layers but their nature seems to be different from that obtained in analogous conditions on austenite.

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K. Ichii, K. Fujimura, T. Takase, Tech. Rep. Kansai Univ. 27 (1986) 135. K. Marchev, C.V. Cooper, et al., Surf. Coat. Technol. 99 (1998) 225. P. Fewell, D.R.G. Michell, et al., Surf. Coat. Technol. 131 (2000) 300. Liang X. Bin, et al., Surf. Coat. Technol. 130 (2000) 304. C. Blawert, A. Weisheit, B.L. Mordike, F.M. Knoop, Surf. Coat. Technol. 85 (1996) 15. [6] E. Menthe, K.-T. Rie, J.W. Schultze, S. Simson, Surf. Coat. Technol. 74– 75 (1995) 412. [7] C. Blawert, B.L. Mordike, Y. Jaraskove, O. Schneeweiss, Surf. Coat. Technol. 116–119 (1999) 189. [8] B. Larisch, U. Brusky, H.-J. Spies, Surf. Coat. Technol. 116–119 (1999) 205.

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J. Baranowska, Surf. Coat. Technol. 180–181 (2004) 145. Wang Liang, Appl. Surf. Sci. 211 (2003) 308. K. Gemma, T. Tahara, M. Kawakami, J. Mater. Sci. 31 (1996) 2885. A.M. Kliauga, Doctor Thesis, Randschichtbeeinflussung von ferritischasutenitischen Chrom-Nickel-Stahlen durch Stickstoffeinsatz, Ruhr-Universitat Bochum, 1997. [13] S. Maendl, B. Rauschenbach, J. Appl. Phys. 91 (12) (2002) 9737. [14] H. He, T. Czerwiec, C. Dong, H. Michel, Surf. Coat. Technol. 163–164 (2003) 331. [15] V. Randal, O. Engler, Introduction to Texture Analysis, Macrotexture, Microtexture and Orientation Mapping, Gordon and Breach Science Publisher, Singapore, 2000, p. 148.

Fig. 6. Diffraction patterns obtained from nitrided layers for (a) ferritic chromium steel, (b) ferritic chromium–nickel steel, (c) ferritic–austenitic steel after treatment in various temperatures. (1) Non-treated sample, (2) 400 °C, (3) 450 °C, (4) 500 °C, the peaks which were not identified are marked with the asterisks.

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Fig. 7. Results of the EBSD measurements on ferritic–austenitic steel after nitriding in 450 °C. (a) SEM picture of the layer, (b) EBSD identification for ferritic phase, (c) EBSD identification for austenitic phase, (d) “quality image” from this area.

Fig. 8. Microhardness profiles for all the samples nitrided in (a) 450 °C and (b) 500 °C. For duplex steel (C), the profiles for austenite and ferrite are presented separately.