CORROSION RESISTANCE OF PLASMA NITRIDED STRUCTURAL STEELS. David KUSMIC, Vojtech HRUBY

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1 CORROSION RESISTANCE OF PLASMA NITRIDED STRUCTURAL STEELS David KUSMIC, Vojtech HRUBY University of Defence,Faculty of Military Technology, Kounicova 65, Brno, Abstract Plasma nitriding technology, based on plasma assisted nitrogen diffusion into the steel surface, mostly used for improvement of wear resistance and fatigue strength is well known. Additional benefit of plasma nitriding is heat and corrosion resistance improvement. This study is focused on evaluation of corrosion resistance of plasma nitrided structural steels under several nitriding conditions. Experiments were performed on structural steels CSN (C45E), CSN (16MnCr5) and CSN (DIN ). Selected structural steels were manufactured for experimental specimens (size of 80x50x4 mm), heat-treated and plasma nitrided under following conditions: T = 500 C, process duration t = 10 and 20 h, p = 280 Pa, U = 510 V and variable gas mixture ratio of 3H 2 :1N 2 [l/h] and 1H 2 :3N 2 [l/h] for different compound layer phase composition. Corrosion tests were performed in the Liebisch GmbH & Co (S 400 M-TR) corrosion chamber in the mist of neutral sodium chloride dilution (NSS method), evaluated and documented in accordance to ISO 9227 standard and results compared to different plasma nitriding conditions. Under different plasma nitriding conditions were different nitride layer characteristics obtained. Nitride layers were by surface hardness (HV), metallographic testing and microhardness testing (HV 0,05 ) classified. Using the microhardness testing were different values of microhardness and nitride layer depth detected. Metallographic testing and additional concentration profiles measuring (using GDOES method) displayed different compound layers thickness, which tends to be a significant factor to corrosion resistance of plasma nitrided structural steels. Key words: Plasma nitriding, corrosion resistence, compound layer INTRODUCTION Plasma nitriding technology, based on plasma assisted nitrogen diffusion into the steel surface deels for the most effective type of nitriding proces. During the nitriding proces is the coumpound layer, so-called white layer, created on the steel surface and below this layer so-called diffusion layer, composed by stable nitrides of iron and alloying elements with high affinty to N (see Fig. 1) [1, 2]. On the nitrided surface is a compound layer, so-called white layer, created. This surface layer is characterized by increased surface hardness (up to 1200 HV and steels alloyed by Al up to 1500 HV), increased fragility and resistance to abrasion and corrosion attack [2, 3, 4]. This compound layer is composed by the nitrides of type ε-fe 2-3 N and γ-fe 4 N (and alloying elements), after the convetional gas nitriding process (see Fig. 2). Thanks the control of plasma nitriding process conditions like voltage, duration, temperature is possible to regulace the characteristics of compound layer. By nitriding atmosphere composition choise (H 2 :N 2 ratio, or by addition of C into the nitriding atmosphere) is possible to regulace the phase composition of compound layer and its properties [3, 5]. Characteristics of these surface layer is possible to change by several after-nitriding procedures [6, 7, 8, 9]. Monitoring of surface parameters Pa, Ra, Wa [ m] changes showed an increase of these parameters after plasma nitriding, markeble increase was detected on polished surfaces [10]. In this paper are corrosion resistance results of plasma nitrided steels CSN (C45E), CSN (16MnCr5) and CSN (DIN ) under different plasma nitriding conditions with different nitride layer characteristics presented.

2 Fig. 1 Typical nitrided case structure Fig. 2 Compound layer phase composition [2] 1. EXPERIMENTAL Corrosion resistance was evaluated on plasma nitrided experimental steel samples of size 80x50x4 mm. Manufactured steel samples C45E, 16MnCr5 and DIN were before plasma nitriding process heat treated to attain optimal mechanical characteristics. The chemical composition of mentioned steels was verified by the GDOES method on SA2000 LECO seen in Table 1. Tab. 1 Chemical composition/bulk Steel Standardized values [wt%] C Mn Si Cr Cu Ni V C45E max. max. max MnCr DIN GDOES/Bulk [wt%] C45E MnCr DIN Plasma nitriding procedure was performed under mostly plasma nitriding conditions: temperature 510 C, pressure of 280 Pa, in nitriding atmosphere of gas mixture H 2 :N 2 (8:24 and 24:8 l/h) for 10 and 20 hours in the RUBIG PN 60/60 plasma nitriding device. For marking of plasma nitrided steels samples see Tab. 2. Before nitriding process, the pre-cleaning by plasma was applied to remove the oxides from the steel surface under following conditions: temperature 480 C, under pressure of 100 Pa, in nitriding atmosphere of gas mixture H 2 :N 2 (20:2 l/h) for 0.5 hour. After plasma nitriding process was the surface hardness HV evaluated using the LECO AVK C2 device, the thickness of nitride layers was measured by microhardness method using the microhardness tester MH 400 at 50 g load and 10 s dwell time, compound layer thickness was evaluated and documented using the light and confocal laser microscopy OLYMPUS LEXT OLS 3000 (as the microstructure evaluation) and additionally by concentration profiles using GDOES method (SA2000 LECO device) confirmed. Corrosion testing was performed in salt spray chamber of Liebisch GmbH & Co (S 400 M-TR) by the NSS corrosion testing method (mist of neutral sodium chloride dilution) and visually

3 documented and evaluated in accordance to ISO 9227 standard. During the NSS corrosion tests different results of corrosion resistance were observed. 2. RESULTS AND DISCUSION Fig. 3 Surface hardness [HV1] Tab. 2 Compound layer thickness [μm] Steel Gas ratio H 2 :N 2 10 hours 20 hours C45E (8:24) C45E (24:8) MnCr5 (8:24) MnCr5 (24:8) DIN (8:24) DIN (24:8) Surface hardness was measured on the plasma nitrided and not nitrided steels. On all plasma nitrided steels was increased surface hardness documented as function of chemical composition of steels and plasma nitriding process parameters (see Fig. 3). The greatest increasing of surface hardness, thanks to Cr and Mn content, was documented on steel DIN after 10 hours plasma nitriding in the reverse nitriding atmosphere 1H 2 :3N 2 (8:24 l/h) and with increasing nitriding process duration is slightly decreased. In the standard ratio atmosphere 3H 2 :1N 2 (24:8 l/h) has the trend of surface hardness opposite direction, with increasing nitriding duration, increases slightly the surface hardness (as well as 16MnCr5 steel). Except the steel C45E, the surface hardness is slightly increasing after 10 and 20 hours of plasma nitriding process duration even in the case of reverse (1H 2 :3N 2-8:24 l/h) and standard (3H 2 :1N 2-24:8 l/h) gas ratio too. 2.1 Nitride layer evaluation The metallographic evaluation showed compact and pores-free compound layers created on the surface of plasma nitrided steel samples. The compound layers thickness is summarized in Table 2. Compound layers thickness was evaluated using the light and confocal laser microscopy (see Fig. 4. and 5.), the values were set from 4.9 to 8.5 μm (C45E), μm (16MnCr5) and 4.7 to 7.0 μm (DIN ). Fig. 4 Nitrided DIN (8H 2 :24N 2 l/h, 10 h) Fig. 5 Nitrided DIN (24H 2 :8N 2 l/h, 10 h)

4 Tab. 3 Nitride layer depth [mm] Steel Gas ratio H 2 :N 2 10 hours 20 hours C45E (8:24) C45E (24:8) MnCr5 (8:24) MnCr5 (24:8) DIN (8:24) DIN (24:8) After plasma nitriding in standard gas ratio (3H 2 :1N 2-24:8 l/h) was created compound layer thicker than in the case of the reverse gas ratio (1H 2 :3N 2-8:24 l/h) and with increasing nitriding duration increases the compound layer thickness in both of nitriding atmosphere ratio types. For the evaluation of nitride layer depth was the microhardness testing procedure (HV 0,05 ) used, according to DIN part 3 standard, performed on microhardness tester Leco MH 400 at 50 g load and 10 s dwell time. After plasma nitriding in standard gas ratio was greater depth of nitride layer created, in comparison to the reverse gas ratio. Depth of nitride layer increases with increasing of nitriding duration. Increasing of nitride layer depth with nitriding duration was documented in both of nitriding atmosphere types, confirmed by concentration profiles GDOES (SA2000 LECO device), see Table Corrosion resistance evaluation Corrosion testing was performed in mist of neutral sodium chloride dilution (NSS) in the Liebisch GmbH & Co (S 400 M-TR) corrosion chamber under following test condition according to ISO 9227 standard: temperature of 35 ± 2 ºC, 5 % dilution of sodium chloride, quantity of fog pollutant was 1-2 ml.h -1 to 80 cm 2, ph from 6.5 to 7.2, evaluation period was determined to 1, 2, 4, 8, 16, 24 and 48 hours (for results see Fig. 6 8). The increased corrosion resistance of plasma nitrided C45E, 16MnCr5 and DIN steels compared to not nitrided is evident. Fig. 6 Corrosion attack C45E steel Fig. 7 Corrosion attack 16MnCr5 steel Fig. 8 Corrosion attack DIN steel

5 Not nitrided 10 h, 24H 2 :8N 2 (l/h) 20 h, 24H 2 :8N 2 (l/h) Fig. 9 Corroded surface of 16MnCr5 steel after 48 hours of exposure time in NSS The most of samples were degraded after 48 hours of exposure in NSS corrosive environment, and the corrosion test was stopped. Differences of corrosion resistance between the different types of nitriding procedures are evident too. The best results were won in the case of plasma nitrided 16MnCr5 steel for 20 hours in the 24H 2 :8N 2 (l/h) nitriding atmosphere (see Fig. 9). After 48 hours of exposure time the corroded area reached 60% of steel surface (see Fig. 7). Thanks to chemical composition (presence of Cr and V), very similar results achieved the DIN steel after plasma nitriding for 10 and 20 hours in the 24H 2 :8N 2 (l/h) standard nitriding atmosphere. Minimal differences of corrosion resistance achieved plasma nitrided C45E steel (see Fig. 6). Generally for all used steels, the best results were won after 20 and 10 hours in the standard nitriding atmosphere. 3. CONCLUSION The aim of presented experimental was evaluation of corrosion resistance of plasma nitrided structural steels (C45E, 16MnCr5 and DIN ) under most widely used plasma nitriding conditions in industrial applications (see Chapter 1). Testing of corrosion resistance was performed in the mist of 5% neutral sodium chloride dilution (NSS) according to ISO 9227 standard. Before corrosion testing were created nitride layers and compound layers evaluated by surface hardness, compound layer thickness and nitride layer depth. Surface hardness of all plasma nitrided steels samples were increased (DIN steel up to three times). As effective can be the first plasma nitriding type (10 hours, 8H 2 :24N 2 (l/h)) considered. Very similar values were won after 20 hours of plasma nitriding in the 8H 2 :24N 2 (l/h) nitriding atmosphere. The thickness of compound (white) layer is on nitriding atmosphere ratio dependent and increases with nitriding duration. Even compound layers created in standard nitriding atmosphere 24H 2 :8N 2 (l/h) after 10 hours of plasma nitriding are thicker than in reverse atmosphere 8H 2 :24N 2 (l/h) after 20 hours of plasma nitriding (see Table 2). The same trend was observed in the nitride layer depth evaluation (compound and diffusion layer). The nitride layer depth increases with nitriding duration and is on nitriding atmosphere ratio dependent (see Table 3). Corrosion tests have shown, that plasma nitriding can significantly increase the corrosion resistance of structural steels. Best results were won after 20 and 10 hours in the standard nitriding atmosphere (24H 2 :8N 2 (l/h)), which is related to increased compound layer thickness.

6 ACKNOWLEDGMENT The paper was prepared with the support of the Project for the Development of the Organization of the Dep. of Mechanical Engineering, UoD "Promoting Research, Science and Inovation in the Field of Engineering". REFERENCES NIKOLUSSI, M., LEINWEBER, A. et al. Examination of phase transformations in the system Fe-N-C by means of nitrocarburising reactions and secondary annealing experiments, the α+ε two-phase equilibrium. Material Research, 98 (2007) 11, ISSN , p [1] PYE, D. Practical nitriding and ferritic nitrocarburizing. ISBN: , USA [2] HOLEMÁŘ, A., HRUBÝ, V. Plazmová nitridace v praxi. SNTL, Prague 1989, p. 90. KUSMIČ, D., HRUBÝ, V. Corrosion resistance of plasma nitrided structural steels and modern methods of testing. Advances in Military Technology, Volume 3, Issue 1, 2008, ISSN: , p [3] HIRSCH T., K., ROCHA A., da S., RAMOS F. D., STROHAECKER T. R. Residual Stress-Affected Diffusion during Plasma Nitriding of Tool Steel. Metallurgical and Materials Transactions A, vol. 35A, November 2004, p [4] SALAŠ, O. et al. Nitride nucleation and growt during plasma and post-discharge nitriding. Surface&Coatings Technology, , 2003, ISSN: , p [5] POKORNÝ, Z., HRUBÝ, V.: Plasma Nitriding of deep narrow Cavities. 6 th international conference Materials Structure & Micromechanics of Fracture, Brno 2010, p.156, ISBN [6] POKORNÝ, Z., HRUBÝ, V., KUSMIČ, D.: Plasma nitridation of Bores in Barrels of Small-bore Rifles. Surface [7] treatement, Hutnické listy No 2/2010, p , ISSN SUKÁČ, J., POSPÍCHAL, M., JOSKA, Z., POKORNÝ, Z.: Effect of Subsequent Thermal Treatment on Properties of Plasma Nitrided Low Alloyed Steel GOST 67SiCr5. 6 th international conference Materials Structure & Micromechanics of Fracture, Brno 2010, p. 179, ISBN [8] KUSMIČ, D., SVOBODA, E., HRUBÝ, V. Surface quality and the nitride layer properties. Vrstvy a povlaky Rožnov pod Radhoštěm, , ISBN: , p