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VIIth International Metallurgical Congress, Ohrid 2016

ABOUT SOME NONTRADITIONAL PROCESSES FOR SURFACE MODIFICATION OF STEELS

Dimitar Krastev University of Chemical Technology and Metallurgy, Sofia, Bulgaria [email protected] Abstract Surface modification in a wider sense includes all types of surface treatments and coatings that result in change of surface layers composition and microstructure. There are different processes for modifying the surfaces of structural alloys, dictated by the performance requirements of the alloy in its service environment. One of approaches, traditional for the steels, is to modify the surface region of engineering alloys via diffusion of different elements and forming a layer with determinate chemical composition, microstructure and properties. These are the commonly used in practice methods for thermochemical treatment of metals, which extended with the methods for physical vapor deposition and chemical vapor deposition, form the basic modern techniques for surface engineering regarding to metals. In recent years a particular attention is directed to the advanced methods for surface modification of metals such as laser surface treatment, electrical discharge machining (EDM) and plasma electrolysis, which give a modified surface with specific combination of properties based on nonequilibrium microstructural characteristics. The metallic surface modification by EDM is of great scientific interest and opportunity for practical application. Normally used for manufacturing dies, moulds and aerospace components, many researchers are adapting the technique to enable surface hardening of workpieces to create, in some cases, new harder alloys on the surface of components to increase their working life and wear resistance. Such a process, which will be the main object of this report, is the electrical discharge treatment in electrolyte, where the modification goes by a high energy thermal process in a very small volume on the metallic surface, involving melting, vaporisation, activation and alloying in electrical discharges and after that cooling of this surface with high rate in the electrolyte. The high energy process put together with the nonequilibrium phase transformations in the metallic system causes considerable modifications of the metallic surface and obtaining of layers with finecrystalline and nanocrystalline structure. The investigations show that obtained on tool steels layers have higher hardness, wear resistance and corrosion resistance, which give considerable increasing of working life and wide opportunities for industrial application. Key words surface modification, steels, electrical discharge treatment in electrolyte

Introduction In operating conditions the steel parts surfaces are subjected in most cases to high stress, wear, corrosion, aggressive impact at elevated temperatures, etc. Moreover, very often the requirements for high strength and wear resistance of the surface are accompanied by a need of high toughness or plasticity in the core. Practice has proved that any such specific operational requirements very well could be solved with the methods of surface modification. The combination of hard surface and softer interior, made possible, for example, by using of case hardening processes, which have wide application in modern surface engineering. Surface modification, because of its efficiency, has in recent years an intensive development and along with the traditional processes some novel techniques try to find their place in practice. The major type of processes that are traditionally used for surface modification of steels are classified as [1]: ● Thermal ● Thermochemical ● Coatings and thin films Basic thermal processes are flame and induction hardening, which change the surface structure after nonequilibrium phase transformations and produce a surface layer with high hardness and wear resistance. Thermochemical or diffusion methods modify the surface of steels via diffusion process of different elements, such as carbon, nitrogen, boron and chromium, which change the chemical composition and structure of steel surface and give as a result higher hardness, wear- and corrosion resistance of obtained layers. Coatings and thin films have very wide application and include along

VIIth International Metallurgical Congress, Ohrid 2016

with conventional processes for hard chromium plating or high corrosion resistant nickel plating and advanced PVD or CVD thin films obtaining. However, today completely different technologies for surface modification have been applied to and developed for steels. The objective remains the same, that is, enhanced surface performance, but technologies that incorporate highenergy beams, plasmas, magnetic and electric fields, and lasers have been applied. By these processes can be obtained the same as by the traditional methods for surface modification diffusion layers and coatings, but also so called “recast layers” in result of high energy attack on the metal surface. The recast layers on metals and alloys are created by treating the surface with high energy stream such as laser, ion beam or electrical discharge for a very short time and pulse characteristics. The high energy attack on the surface involves local melting and in many cases vaporizing of metal microvolumes. After the cooling, on the treated metal surface a recast layer with different structure and properties from the substrate is formed. This recast layer can be with the same chemical composition as the substrate or with different one if in the thermal process suitable conditions for surface alloying are created. When the recast process is not controlled there are on the surface microcracks and pores which have negative influence on the surface properties and the recast layer must be removed. In the controlled recast processes it is possible to produce surface layer with determinate chemical composition, thickness, structural characteristics and properties, which are unique for the material with the very high hardness, corrosion- and wear resistance. The basic techniques that give opportunities in this direction are laser surface treatment and electrical discharge machining. Laser surface treatment is widely used to recast and modify localized areas of metallic components. The heat generated by the adsorption of the laser light provides a local melting and after controlled cooling is obtained a recast layer on the metal surface with high hardness, wear resistance and corrosion resistance. The laser surface melting is based on rapid scanning of the surface with a beam focused to a power density scale of 104 W/cm2 to 107 W/cm2. Quench rates up to 108 - 1010 K/sec provide the formation of fine structures, the homogenization of microstructures, the extension of solid solubility limits, formation of nonequilibrium phases and amorphous phases or metallic glasses, with corrosion resistance 10–100 time higher compared to crystalline [2]. Laser surface melting is a simple technique as no additional materials are introduced, and it is especially effective for processing ferrous alloys with grain refinement and increase of the alloying elements content in solid solution. In fact the process has been employed for improving the hardness, cavitation erosion and corrosion resistance of a number of ferrous alloys. When high density laser beam is irradiated on AISI H13 steel surface, the structure reaches homogeneous austenite region in a very short time. After a short hold in the homogeneous austenite state the carbides can be dissolved and at higher temperature the surface will be melted. After rapid cooling it is formed martensitic structure with high hardness and strength. Melted zone structure is observed as white strips with high hardness of about HV 500 in comparison with HV 240 for the base structure – Fig. 1-I [3]. White Layer Heat Affected Zone

I II III Fig. 1. Microstructure of modified steels surfaces by different nontraditional processes: I - Laser melted zones on AISI H13 steel at different scan rates: (a) 73.2 mm/s; (b) 146.4 mm/s; (c) 219.6 mm/s; 366 mm/s II - SEM micrograph of nitrocarburized by plasma electrolysis 41CrAlNi7 steel III - Microstructure of EDM modified steel surface The laser surface melting can be combined with a simultaneous controlled addition of alloying elements. These alloying elements diffuse rapidly into the melt pool, and the desired depth of alloying can be obtained in a short period of time. By this means, a desired alloy chemistry and microstructure can be generated on the sample surface and the degree of microstructural refinement will depend on the solidification rate. The surface of a low-cost alloy, such as low carbon steels, can be selectively alloyed to enhance properties, such as resistance to wear and corrosion [4]. Electrical discharge machining is a thermoelectric process that erodes workpiece material by series of discrete but controlled electrical sparks between the workpiece and electrode immersed in a dielectric fluid [5]. It has been proven to be especially valuable in the machining of super-tough, electrically conductive materials, such as tool steels, hard

VIIth International Metallurgical Congress, Ohrid 2016

metals and space-age alloys. These materials would have been difficult to machine by conventional methods, but EDM has made it relatively simple to machine intricate shapes that would be impossible to produce with conventional cutting tools. In EDM process, the shapes of mold cavities are directly copied from that of the tool electrode, so timeconsuming preparation work must be done on the fabrication of the corresponding tool electrode. The electrical discharge machining uses electrical discharges to remove material from the workpiece, with each spark producing temperature of about 8000-20000 ºC. This causes melting and vaporizing of small volumes of the metal surface and after cooling in the dielectric fluid the melted zones are transformed in recast layer with specific structure. The EDM modified surface consists from two distinctive zones – recast layer and heat affected zone [6,7]. The recast layer is also named white layer and it crystallizes from the liquid metal cooled at high rate in the dielectric fluid. The depth of this top melted zone depends on the pulse energy and duration. Below the top white layer is the heat affected zone with changes in the average chemical composition and possible phase changes. In Fig. 1-III is shown the typical microstructure of EDM modified steel surface. The recast white layer can not be etched and has very high hardness, corrosion resistance and wear resistance. The phenomenon of surface modification by EDM has been observed for over four decades. Under the high temperature of the discharge column, the white layer can dissolve carbon from the gases formed in the discharge column from the hydrocarbon dielectric and receives higher carbon content than the base material and hence show increased resistance to abrasion and corrosion. Moreover electrode material has been found in the workpiece surface after machining with conventional electrode. Better surface properties have been obtained by machining with powder metallurgy electrodes containing alloying elements which diffuse in the workpiece surface. Fine powders mixed in the dielectric offer another way for achieving desirable surface modification. All this determines the three main directions for surface modification by electrical discharge machining - surface modification by conventional electrode materials; surface modification by powder metallurgy electrodes and surface modification by powder-mixed dielectric [6]. In the EDM process with conventional electrode has been observed material transfer from the electrode to workpiece surface which is a function of the various electrical parameters of the circuit. The high energy machining results in lower surface deposition, but there is more diffusion in depth. Also it is found that the negative polarity is desirable for increase of material transfer from the tool electrode. The improvement of the surface integrity, wear- and corrosion resistance of the workpiece material can be realized by surface alloying during sparking, using sintered powder metallurgy electrode. With the alloying there is a potential to increase workpiece hardness from two to five times and significant enhance the corrosion resistance that of the bulk material. It is possible remarkable to increase the corrosion resistance of carbon steel by using of composite electrodes containing cooper, aluminium, tungsten carbide and titanium. The material from the electrode is transferred to the workpiece and the characteristics of the surface layer can be changed significantly. The same results can be achieved with the addition of metallic and compound powders in the dielectric. In this case are used Ni, Co, Fe, Al, Cr, Cu, Ti, C (graphite), etc. At Plasma Electrolysis the processes are of similar nature as EDM and in some cases can be also obtained recast layers with specific characteristics. Electrolytic plasma technologies are an effective method for metals surface modification which can combine different processes such as cleaning of metal surfaces, formation of diffusion layers, metal deposition, plasma electrolytic oxidation, obtaining of ceramic and composite coatings [8-10]. For the plasma electrolysis process DC voltage is applied to the electrodes in aqueous electrolyte, which produces plasma on the surface of workpiece. The thermal, electrical, chemical and mechanical impact on the workpiece can create different surface characteristics, depending on process parameters. Electrolytic plasma technologies enable to obtain coatings on metal surface with crystalline on amorphous structure and a wide spectrum of functionality on ferrous and nonferrous metals and alloys. The investigations show that after cleaning process in electrolytic plasma the surface microstructure has been characterized by the presence of layer on AISI 1010 steel with thickness 150 – 200 nm and grain size 10 – 20 nm. This nanostructured layer is in a result of high rate quenching in surrounded electrolyte of melted in plasma bubbles steel surface. The plasma electrolysis gives a similar effect during a coating process. Microhardness testing of Zn coatings on AISI 1010 steel shows higher values of about 0.97 GPa in comparison with approximately 0.8 GPa for the bulk Zn, which is also determined from the rapid cooling of localized molten surface layers and grain size in the nanoscale [11]. Surface alloying is also a modern direction of plasma electrolysis application. It has been used to harden steels by surface alloying using diffusion process of carbon and nitrogen. In this case, it must be note, that the diffusion rate of different elements in the metal surface during plasma electrolysis processing is significantly higher as compared to the conventional thermochemical treatment. Nitriding and nitrocarburizing processes are among the most discussed as an application of plasma electrolysis for thermochemical treatment. When the process is carried out in water based electrolyte contained urea and sodium carbonate on the surface of DIN 41CrAlNi7 steel are obtained layers consist of ε-Fe2-3N and γ′-Fe4N [12]. SEM micrographs show (Fig. 1-II) that after the nitrocarburizing process a white layer and diffusion layer are formed on the workpiece surface. Such a method as EDM and Plasma Electrolysis is the electrical discharge treatment in electrolyte based suspension, where the modification goes by a high energy thermal process in a very small volume on the metallic surface, involving melting, vaporisation, activation and alloying in electrical discharges and after that cooling of this surface with high rate

VIIth International Metallurgical Congress, Ohrid 2016

in the electrolyte. The high energy process put together with the nonequilibrium phase transformations in the metallic system causes considerable modifications of the metallic surface and obtaining of layers with finecrystalline and nanocrystalline structure [13-15]. The metallic surface after electrical discharge treatment has a different structure in comparison with the metal matrix which determines different properties. It is observed remarkable increasing of hardness, strength and corrosion resistance related to the nonequilibrium phase transformations and to the obtained finecrystalline microstructure. The investigations show that obtained on tools layers have higher hardness, wear resistance, tribocorrosion resistance and corrosion resistance, which give better performance, considerable increasing of working life and wide opportunities for industrial application.

Experimental For the electrical discharge treatment in electrolyte based suspension is developed a laboratory device, shown in Fig. 2, giving opportunities for treatment of workpieces with diameter up to 20 mm. The suspension of boron carbide in electrolyte 3 is in active movement by mixing from a magnetic stirrer 4. After passing of electric current with determinate characteristics through the electrolyte between the workpiece 1 and electrode 2 starts an active sparking on the workpiece surface. The sparking characteristics depend on different factors such as parameters of the electric current, type and composition of the electrolyte, movement of the workpiece and electrolyte. POWER & CONTROL

1 Electrical discharges

2 3 4

a

b

Fig. 2. Electrical discharge treatment in electrolyte based suspension: a – schematic layout (1 – workpiece, electrode, 3 – electrolyte, 4 – magnetic stirrer); b – active sparking around the workpiece at 200 – 240 V

2 –

The workpieces for the discussed investigations are made from high-speed steel HS 6-5-2 with structure after the typical heat treatment for tools of this steel. The chemical composition of steel is given in Table 1.

Steel HS 6-5-2

C 0.89

Table 1. Chemical composition of investigated steel Chemical composition, % Si Mn S P Cr W 0.31 0.50 0.018 0.022 3.30 6.50

V 2.30

Mo 5.40

The electrolyte composition and its characteristics are of great importance for the process parameters and for the microstructure and properties of the modified layers. By these experiments the electrolyte is on water basis and in it are dissolved glycerol and sodium carbonate. In the electrolyte is suspended fine sized B4C. The boron content of the electrolyte and the opportunity for its accelerated diffusion by activation in electrical discharges on the metallic surface increases the tendency for borides formation and grain size refinement in the modified layers. The specific properties of the recast white layer in a case of high-speed tool steels are the remarkable high hardness, strength and corrosion resistance related to the nonequilibrium phase transformations in the high alloyed metallic system. For the experiments, between the electrodes is applied direct current with voltage from 80 to 240 V. The time of treatment is from 1 to 10 minutes. The investigations show that the optimal time is 3 minutes. Obtained layers have been investigated by light microscopy, SEM, XRD and Hanneman microhardness testing.

Results and Discussion The metallic surface after electrical discharge treatment in electrolyte based suspension receives a different structure in comparison with the metal matrix which determines different properties. In most cases on the surface is formed a recast

VIIth International Metallurgical Congress, Ohrid 2016

white layer with fine- or nanocrystalline structure. It is a result of high energy local heating and melting by electrical discharge impact on the metallic surface and following quenching in the electrolyte with high rate from the liquid phase. The white layer characteristics, its homogenous structure and thickness, are of dependence on the electric current parameters and on the duration of treatment. This white layer is typical for the high alloy high-speed steel after electrical discharge treatment in electrolyte. By lower voltage the obtained recast white layer is inhomogeneous and local deposited on the metal surface. The treatment time increasing in this case shows an insignificant influence on the process. In Fig. 3 a is shown an optical micrograph of a high-speed tool steel surface microstructure after electrical discharge treatment at 150 V for 3 minutes. The thickness of the obtained layer is under 0.01 mm. The investigations show that it is possible to obtain a white layer on the tool steel surface at voltages of about 100 V – Fig. 3 b, but the electrical discharges energy is insufficient for carbides dissolving and the hardness of the modified surface can not receive the expected high level. The voltages over 200 V give a very high intensity of sparking on the metallic surface with enough energy for melting and dissolving of the carbides and because of that it is obtained an approximately compact recast layer with homogeneous structure. By the high speed quenching from liquid state the solubility of alloying elements remains very high in a supersaturated solid solution and after the nonequilibrium phase transformations it is formed metastable structures with high hardness and wear resistance. In Fig. 3 c is shown optical micrographs of obtained recast layers for 3 minutes at 200 V. The thickness of the obtained white layers is about 0.05 – 0.06 mm.

b

a

c

Fig. 3. Microstructure of modified HS 6-5-2 steel surfaces after different parameters of treatment: a – 150 V for 3 min, 800x; b – 100 V for 2 min, 750x; c – 200 V for 3 min, 800x The XRD investigation of the modified by electrical discharge treatment in boron carbide containing electrolyte highspeed steel surface shows a significant difference with the bulk material which can be observed in Fig. 4 a and b. The modified surface has the typical diffraction patterns for the nanocrystalline structures. The XRD analysis also proves the mass transfer of boron from the electrolyte and its reactive diffusion in the metallic surface. In the modified steel surface along with typically for high-speed steel structure carbides also presents Me2B.

a

b

Fig. 4. XRD patterns of HS 6-5-2 steel surface: a - before and b - after electrical discharge modification in electrolyte Homogeneous structure of the recast layer can be observed by a higher magnification on SEM micrograph in Fig. 5 a. The modified surface has a very high corrosion resistance and can not be etched. The microhardness of the white layer

VIIth International Metallurgical Congress, Ohrid 2016

in these cases reaches HV 1500–1600 compared to HV 780–820 of the core. The fine structures of modified layers with specific etching are shown after SEM investigation on Fig. 5 b and c.

a

c b Fig. 5. SEM micrographs of obtained recast layers on HS 6-5-2 steel surfaces

After deeper etching on the workpiece surface can be observed the two specific zones: ● “White zone” ● “Phase transformations zone” The “Phase transformations zone” has different structures depending on the temperature and cooling rate. When the temperature has risen above the melting point and cooling rate is lower a zone with dendritic structure is formed – Fig. 5 b. In the other case (Fig. 5 c), when the temperature is in the austenitic region and the cooling rate is higher than the critical, martensite is formed.

References 1. ASM Handbook.. Heat Treating, Volume 4, ASTM International, Materials Park, Ohio, (1991). 2. Bommi, V. C., Mohan, K. M., & Prakash, S. Surface Modification of Martensitic Stainless Steel Using Metal Working CO2 Laser, Proceedings of International Symposium of Research Students on Materials Science and Engineering, Chennai, India, December 20-22, (2004). 3. Lee J, Jang J, Joo B, Son Y, Moon Y. Laser surface hardening of AISI H13 tool steel. Transactions of Nonferrous Metals Society of China, 19 (2009) pp. 917-920. 4. Davis, J. R., Surface Engineering for Corrosion and Wear Resistance, ASM International, Materials Park, Ohio, (2001). 5. Asif Iqbal, A. K. M. & Khan A. A., Influence of Process Parameters on Electrical Discharge Machined Job Surface Integrity. American Journal of Engineering and Applied Science, Vol. 2, No. 3, (2010), pp. 396-402. 6. Kumar, S., Singh, R., Singh, T. P., Sethi, B. L., Surface Modification by Electrical Discharge Machining: A Review. Journal of Materials Processing Technology, Vol. 209, (2009), pp. 3675-3687. 7. Ho, K. H., Newman, S. T.. State of the Art Electrical Discharge Machining (EDM). International Journal of Machine Tools & Manufacture, Vol. 43, (2003), pp. 1287-1300. 8. Yerokhin A. L., X. Nie, A. Leyland, A. Mathews, S. J. Dowey, Plasma electrolysis for surface engineering, Surface and Coatings Technology, Vol. 122, 1999, pp.73-93. 9. Gupta P., G. Tenhundfeld, E.O. Diagle, D. Ryabkov, Electrolytic plasma technology: Science and engineering – An overview. Surface & Coatings Technology, 201 (2007) pp. 8746-8760. 10. Pogrebnyak A. D., A. Sh. Kaverina and M. K. Kylyshkanov, Electrolytic Plasma Processing for Plating Coatings and Treating Metals and Alloys. Protection of Metals and Physical Chemistry of Surfaces, vol. 50, No 1, (2014), pp. 72-87. 11. Meletis E. I., X. Nie, F. L. Wang, j. C. Jang, Electrolytic plasma processing for cleaning and metal-coating of steel surfaces, Surface and Coatings Technology, 150 (2002) pp. 246-256. 12. Tavakoli H., S. M. Mousavi Khoie, S. P. H. Marashi and O. Balhasani, Effect of Electrolyte Composition on Characteristics of Plasma Electrolysis Nitrocarburizing, Journal of Materials Engineering and Performance, 8 (2013) pp. 2351-2358. 13. Krastev, D.; Yordanov, B. About the Surface Hardening of Tool Steels by Electrical Discharge Treatment in Electrolyte, Solid State Phenomena, 159 (2010) pp. 137-140. 14. D. Krastev, V. Paunov, B. Yordanov, V. Lazarova, Surface modification of steels by electrical discharge treatment in electrolyte, Journal of Chemical Technology and Metallurgy, 49 (2014) pp. 35-39. 15. Krastev D., B. Yordanov, V. Paunov, Surface Modification of Tools by Electrical Discharge Treatment in Electrolyte, Micro and Nanosystems, 6 (2014) pp. 21-25.

VIIth International Metallurgical Congress, Ohrid 2016

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