UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

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Improving Service Performance of Piston Ring via CRTD-Bor

Alaaddin Cem Ok, Güldem Kartal Şireli, Yasin Kılıç, Servet Timur

Istanbul Technical University, Faculty of Chemistry and Metallurgy, Metallurgical and Materials Eng. Dep., 34469, İstanbul, Turkey

Abstract

The purpose of this study is to improve the service life and the performance of piston ring via cathodic reduction thermal diffusion boriding (CRTD-Bor) which is a new alternative surface hardening approach. AISI 9254 silicon-manganese steel is the common base material for piston ring applications so CRTD-Bor was applied for this steel type. The effects of process durations on boride layer thickness and composition were investigated using microscopic and X-ray diffraction (XRD) methods. Industrially desired-, app. 100 m thick-, single Fe2B phase- boride layer was achieved to grow after 90 min. of CRTD-Bor treatment (10 min of electrolysis at the current density of 200 mA/cm2 and 80 min of phase homogenization (PH) in which the polarization of electrodes were cut). Thin film XRD analysis confirmed the single phase, iron hemi-boride (Fe2B) formation. Hardness of grown boride layer was measured as 1550 0 HV. Additional Daimler-Benz Rockwell C adhesion test revealed that the formed Fe2B layer exhibited perfect adhesion to the steel substrate with HF1 quality.

1. Introduction

Piston rings located between the cylinder and the piston are essential components that provide the engine to operate efficiently. Piston rings have four major functions namely compression gas sealing, lubricating oil film control, heat transfer and support piston in the cylinder [1,2]. During their operations, the surfaces of the parts suffer from severe wear due to the frictional forces between piston and piston ring. Besides, while transferring heat from the piston crown to the cylinder, they expose high temperature approximately, 3500C. Therefore, resistance against wear and high temperature oxidation is very crucial to determine the service life and the performance. The general approaches to enhance the surface of piston rings are chrome based coatings and nitriding [1-3]. On the other hand, boriding in which boron atoms are diffused into the surface of a work piece to form complex borides (i.e. FeB/FeB2) with the base metal is the common surface hardening process for ferrous materials to produce very hard layers, i.e. 1700 HV. However, the drawbacks of conventional pack boriding which are the needs of very long process durations (i.e., eight hours and so on) and the generations of toxic gas emission

with huge amounts of solid wastes limit the wide range applications of boriding. Moreover, the dual phase (FeB+Fe2B) boride layer formations on alloy steels after pack boriding cause many functional problems due to the poor fracture toughness of FeB; hence, it can fracture or delaminate easily from the surface under high normal or tangential loading. The best performance of boride layer can only be achieved in case the grown layer on steel substrates is in the composition of single iron hemi-boride Fe2B [4,5]. Boriding process can be considered as an alternative method for piston ring applications, considering the fact that it provides minimum twice harder layers on steel surfaces compared to the currently applied surface hardening process. This is very critical to increase wear resistance. Moreover, boriding increases the high temperature oxidation resistance of ferrous materials up to 6000C which is also crucial for piston ring applications. However as mentioned above, solo Fe2B layer formation is very difficult to form on alloy steels via conventional pack boriding plus it has many negative sides. Instead, CRTD-Bor has been developed in our research group to overcome disadvantages of pack boriding with decreasing process durations (e.g. typically 1 hour) and solving environmental issues with its green nature [6,7]. In this study we applied CRTD-Bor to modify the AISI 9254- piston ring surface and accordingly, the process parameters were optimized to grow industrially desired boride layer with single phase Fe2B composition.

2. Experimental Procedure

2.1. Boriding process

4 mm thick specimens were cut from AISI 9254 steel bar with the diameters of 25 mm. They were polished up to 800-grit SiC emery paper to remove roughness on the surface before boriding, then put into acetone and cleaned the surfaces of the samples ultrasonically for 15 minutes at room temperature. CRTD-Bor experiments were carried out in a medium frequency furnace where graphite crucible was polarized as an anode and steel specimens was polarized as cathode. During experiments, the change in the electrolyte temperature was monitored continuously with N-type thermocouples that placed in the crucible. Molten salt composed of 90% borax and 10% sodium carbonate were used as the electrolyte. Boriding process was performed at

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current density of 200 mA/cm2 and the temperature of for 10 minutes. In addition to electrochemical part

of boriding, subsequently phase homogenization (PH) stages were applied in order to get rid of FeB layer. In PH, the direct current was cut off, and specimen (cathode) was left in molten bath for additional durations without polarization.

2.2 Oxidation tests

In order to examine high temperature oxidation nature of both bordided and untreated piston ring steels (AISI 9254), the samples were placed on magnesite cupels in the electrical resistance furnace (Proterm). Oxidation tests were carried out in open-air atmosphere at 350 and 50 for the total exposure time of 150 h. After holding samples for certain of times and temperatures, the samples were taken out the furnace to cool down to room temperature, then weighed and returned to the furnace. The weight gains were measured on

structures of oxidized products were identified by XRD with -sectional SEM-

EDS examinations were conducted to investigate their chemical compositions in detail.

2.3. Characterization of borided specimens

After boriding experiment, cathode part was withdrawn from the bath and allowed to cool down in air for 15 minutes then cleaned with boiling water for 30 minutes in order to remove electrolyte residues that solidified on the surface. For further cross sectional investigations for boride layer thickness measurements and morphological examinations, all samples have been prepared metallographically. Cross sectional investigation were conducted by optical microscope and SEM and EDS investigations. Phase analysis of grown boride layer was performed using the thin film X-ray method -10mA) with

The cross-sectional hardness variation of boride layers was measured by applying 50 gf in the Vikers micro hardness tester. The Daimler-Benz Rockwell-C adhesion test was utilized to assess the fracture behavior and adhesion of the boride layers produced after different CRTD-Bor treatments. The principle of this method is given in Ref [8]. A load of 1471 N was applied to cause coating damage adjacent to the boundary of the indentation. Three indentations were conducted for each borided specimen and an optical microscope was used to evaluate the test.

3. Results and Discussion

According to XRD analysis results as given in Fig.1, two phase (FeB + Fe2B) boride layer was grown after 10 min of electrolysis. In order to get rid of FeB layer, additional PH stage was applied starting from 45 min to 80 min. After 80 min PH process, FeB phase was disappeared by transformed in to Fe2B. The minimum and maximum thickness value of

single phase Fe2B boride layer was measured as 50 103 .

Figure 1. XRD pattern of borided layer produced after 10 min of electrolysis (EB) and additional PH times.

It is clearly observed from the cross-sectional optical micrograph images (Fig.2) that the presence of FeB phase was decreasing with the increasing PH time and after 10 min of electrolysis and additional 80 min of PH the produced boride layer (Fig.2-e) is homogenous without any crack formations.

(a) 10 min EB (b) 10 min EB + 45 min PH

(c) 10 min EB + 60 min PH (d) 10 min EB + 70 min PH

(e) 10 min EB + 80 min PH

Figure 2. Cross-sectional optical micrograph images with different PH times.

Cross sectional hardness test applied on the borided layers revealed (Fig.3) that, after 10 min boriding, high hardness values (1700 measured (Fig. 3-a) due to the presence of the FeB layer in the vicinity of the double phase boride layer with the thickness of 40 m. However single

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

1236 IMMC 2018 | 19th International Metallurgy & Materials Congress

phase Fe2B boride layer (Fig.3-b) exhibits the homogenous hardness variation with the range of approximately top 85 m.

(a)10 min electrolysis (b)10 min electrolysis + 80 min PH

Figure 3. Hardness distribution of borided layer after (a) 10 min electrolysis (b) 10 min electrolysis + 80 min PH

Since piston rings generally expose maximum temperature of s, the first oxidation tests were C for 150 hours. Thin film XRD analysis (Fig.4) showed that only unborided samples were oxidized to Fe2O3 on their surfaces whereas borided samples

, the minor weight increments were measured on unborided ones while borided steels kept their stabilities.

Figure 4. XRD Pattern of samples after 150 hour oxidation test at 350 oC

Figure 5. Weight change of samples after 150 hour oxidation tests at 350 oC

In order to test oxidation nature of borided and unborided AISI 9254 samples at extreme conditions, the temperature of thermal oxidation tests was increased to 500 oC. After 150 hours, there was a substantial weight increase on the surface of unborided sample (Fig 6.). Also, as seen in the thin film

XRD analysis (Fig.7), an oxide film was formed on the unborided AISI 9254 steel sample.

Figure 7. XRD Pattern of samples after 150 hour oxidation test at 500 oC

SEM cross-sectional analysis (Fig. 8-a) and elemental EDS analysis (Table 1) confirm that, unborided sample had a Fe2O3slight weight increase (0.025 mg/cm2) on the borided sample (Fig. 6), the thin film XRD analysis revealed that (Fig 7.), single phase Fe2B layer preserves its existence. Also, as seen in the cross-sectional analysis (Fig 8.-b), there is no evidence of oxide formation on borided sample.

(a) Unborided Sample (b) Borided Sample Figure 8. Cross sectional SEM analysis of samples after 150 hour oxidation tests at 500 oC

Table 1. EDS Element analysis of oxide film Element Point 1.

At.% Point 2. At.%

O 39.991 37.995 Si 0.786 5.064 Cr 0.246 0.578 Fe 58.977 56.403

Total 100.000 100.000

20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 1000

1

Borlanmamis AIS

20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 1000

1

Borlanmis AISI 9

20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 1000

1

Borlanmamis AISI 9254

20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 1000

1

Borlanmis AISI 9254

20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Figure 6. Weight change of samples after 150 hour oxidation tests at 500 oC

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Figure 8 shows the results of the Daimler-Benz Rockwell C adhesion test conducted AISI 9254 steels borided at different PH times. With increasing periods of PH durations, the adhesion quality of boride layers to the steel substrates became better due to less FeB formations on the top layers (Fig. 2). The grown boride layer after 10 min of electrolysis and subsequent 80 min of PH exhibited the perfect adhesion to the steel matrix. This result was expected due to the solo Fe2B composition of the formed boride layer.

(a) 10 min EB (b) 10 min EB + 45 min PH

(b) 10 min EB + 60 min PH (b) 10 min EB + 70 min PH

(b) 10 min EB + 80 min PH

Figure 8. Results of the Daimler-Benz Rockwell C adhesion test of AISI 9254 steel borided with different PH times.

4. Conclusion

In this study, we applied a new approach to modify piston ring surfaces in order to increase their performance and service life. The process conditions of CRTD-Bor were optimized to grow solo, industrially preferred iron hemi boride (Fe2B) layer on AISI 9254 steel. After 90 min of CRTD-Bor (10 min electrolysis and additional 80 min PH), it is possible to grow single-phase Fe2B layer with the thickness of approximately100 m. The surface hardness of ideally borided piston ring is around 155 0 HV which is 5 times harder than that of steel matrix. Moreover, this boride layer shows the perfect adhesion to the substrate without any delamination.

Acknowledgements

This work was supported by BOREN, the National Boron Research Institute of Turkey with the project number of 2016-31-07-15-004.

References

[1] , C. E. (2002). Piston ring tribology. A literature survey. VTT Tiedotteita-Research Notes, 2178(1).

[2] Mahle GMBH (Ed.) (2016). Cylinder components: Properties,applications, materials (2 ed.). Springer Vieweg

[3] Federal Mogul (2008) Piston Ring Handbook <http://korihandbook.federalmogul.com/en/section_41.htm> Dated: 15.05.2018

[4] Sinha, A.K., 1990. ASM Handbook; Heat Treating, Boriding (Boronizing), pp.437-447, ASM International, OH, USA.

[5] Hanser.

[6] --

optimizasyonu (Doctoral dissertation, Institute of Science and Technology).

[7] Kartal, G., & Timur, S. (2013). Growth kinetics of titanium borides produced by CRTD-Bor method. Surface and Coatings Technology, 215, 440-446.

[8] N. Vidakis, A. Antoniadis, N. Bilalis, The VDI 3198 indentation test evaluation of a reliable qualitative control for layered compounds, J. Mater. Process. Technol. 143 144 (2003) 481 485.