**3.1 Surface modification by EDM**

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 (Kumar et al., 2009; Ho & Newman, 2003):


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. 11 is shown the typical microstructure of EDM modified steel surface.

Fig. 11. Microstructure of EDM modified steel surface.

immersed in a dielectric fluid (Asif Iqbal & Khan, 2010). It has been proven to be especially valuable in the machining of super-tough, electrically conductive materials, such as tool steels, hard 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 time-consuming preparation work must be done on the fabrication of the corresponding

The basis of EDM can be traced as far back as 1770, when English chemist Joseph Priestly discovered the erosive effect of electrical discharges (Ho & Newman, 2003). In 1943 Russian scientists Boris Lazarenko and Natalya Lazarenko (Satel, 1956) applied the destructive effect of electrical sparks for manufacturing and developed a controlled process of machining difficult-to-machine metals by vaporizing material from the surface. At the recent years the research interests and practice are directed to the novel application of electrical discharge

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 (Kumar et al., 2009; Ho &

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. 11 is shown the typical

tool electrode.

Newman, 2003): Recast layer Heat affected zone

machining in the area of surface modification.

microstructure of EDM modified steel surface.

Fig. 11. Microstructure of EDM modified steel surface.

**3.1 Surface modification by EDM** 

The recast white layer as well as the other discussed white layers 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 (Kumar et al., 2009):


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, wearand 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.

#### **3.2 Surface modification by electrical discharge treatment in electrolyte**

Such a method as EDM 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 an 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 (Krastev at al., 2009; Krastev & Yordanov, 2010). The metallic surface after electrical discharge treatment in electrolyte 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 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.

Improvement of Corrosion Resistance of Steels by Surface Modification 313

corrosion resistance which is one of the important characteristics of nanocrystalline

The electric discharges generate an enormous amount of heat, causing local melting on the workpiece surface and thereupon it is rapidly quenched from the liquid state by the electrolyte. This recast area has a specific structure which is composed of several microscopic metallurgical layers, depending of machining conditions. In Fig. 13 is shown an optical micrograph of the modified surface of high speed steel obtained by electrical

Fig. 13. Microstructure of recast layer obtained by electrical discharge treatment in

and V diffuse from the white layer to the matrix and Cr, Co in the opposite side.

Table 3. EDS analysis of modified workpiece from HS 6-5-2 steel

Chemical element Matrix of workpiece White layer Si <0.01 <0.01 Mo 5.58 4.87 V 2.30 1.63 Cr 4.25 4.52 Co <0.01 0.19 Ni <0.01 <0.01 W 8.34 5.75

The thickness and integrity of obtained recast white layer on the steel surface by electrical discharge treatment in electrolyte depend on the electrical current characteristics and time duration of the treatment. At higher voltage are observed thicker white layers by equal

The high rate of the recasting process gives opportunities for formation of metastable phases and considerable decreasing of grain size. The electrolyte type is of great importance for the chemical composition, microstructure and properties of the modified layer. By these experiments the electrolyte is on water basis and contains boron or silicon compounds. At short times of treatment it is not observed diffusion of elements from the electrolyte in the modified surface, but it is available diffusion process inside the workpiece between the white layer and the matrix – Table 3. The strong carbide-formed elements such as Mo, W,

structures.

discharge treatment in electrolyte.

electrolyte of HS 6-5-2 steel, x800.

For the electrical discharge treatment in electrolyte is developed a laboratory device, shown in Fig. 12, giving opportunities for treatment of cylindrical workpieces with diameter up to 20 mm. The 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.

The workpieces are made from high speed steel HS 6-5-2 with structure after the typical heat treatment for tools of this steel and hardness about of 950 HV. The choice of high-alloy steel is founded on the opportunity for higher effectiveness of treatment on structure and properties of modified surfaces after the nonequilibrium phase transformations from liquid state.

Fig. 12. Installation for electrical discharge treatment in electrolyte: 1 – workpiece, 2 – electrode, 3 – electrolyte, 4 – magnetic stirrer.

The principle changes that occur on the modified steel surface by the high speed quenching from liquid state in the treatment process can be described as:


Some studies at similar conditions show significant increasing of solubility of carbon in steel up to 2 % in the martensite and 3.5 % in austenite which is a precondition for high strength of the treated surface.

By the investigations were obtained layers on the workpiece surface with approximately equal thickness, depending of the electrical current characteristics and time of treatment. The modified surfaces can be observed as a light layer on the workpieces. The melted and resolidified layer during this process can be also referred as a "white layer", since generally no etching takes place in these areas at the metallographic preparation because of its high

For the electrical discharge treatment in electrolyte is developed a laboratory device, shown in Fig. 12, giving opportunities for treatment of cylindrical workpieces with diameter up to 20 mm. The 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. The workpieces are made from high speed steel HS 6-5-2 with structure after the typical heat treatment for tools of this steel and hardness about of 950 HV. The choice of high-alloy steel is founded on the opportunity for higher effectiveness of treatment on structure and properties of modified surfaces after the nonequilibrium phase transformations from liquid

> **POWER & CONTROL**

*Electrical discharges*

Fig. 12. Installation for electrical discharge treatment in electrolyte: 1 – workpiece,

Grain refinement with possibilities for obtaining of nanocrystalline structure

The principle changes that occur on the modified steel surface by the high speed quenching

Some studies at similar conditions show significant increasing of solubility of carbon in steel up to 2 % in the martensite and 3.5 % in austenite which is a precondition for high strength

By the investigations were obtained layers on the workpiece surface with approximately equal thickness, depending of the electrical current characteristics and time of treatment. The modified surfaces can be observed as a light layer on the workpieces. The melted and resolidified layer during this process can be also referred as a "white layer", since generally no etching takes place in these areas at the metallographic preparation because of its high

2 – electrode, 3 – electrolyte, 4 – magnetic stirrer.

Expansion of the solubility in solid state

High concentration of crystalline imperfections

Formation of metastable phases

*1*

*2*

*3*

*4*

of the treated surface.

from liquid state in the treatment process can be described as:

state.

corrosion resistance which is one of the important characteristics of nanocrystalline structures.

The electric discharges generate an enormous amount of heat, causing local melting on the workpiece surface and thereupon it is rapidly quenched from the liquid state by the electrolyte. This recast area has a specific structure which is composed of several microscopic metallurgical layers, depending of machining conditions. In Fig. 13 is shown an optical micrograph of the modified surface of high speed steel obtained by electrical discharge treatment in electrolyte.

Fig. 13. Microstructure of recast layer obtained by electrical discharge treatment in electrolyte of HS 6-5-2 steel, x800.

The high rate of the recasting process gives opportunities for formation of metastable phases and considerable decreasing of grain size. The electrolyte type is of great importance for the chemical composition, microstructure and properties of the modified layer. By these experiments the electrolyte is on water basis and contains boron or silicon compounds. At short times of treatment it is not observed diffusion of elements from the electrolyte in the modified surface, but it is available diffusion process inside the workpiece between the white layer and the matrix – Table 3. The strong carbide-formed elements such as Mo, W, and V diffuse from the white layer to the matrix and Cr, Co in the opposite side.


Table 3. EDS analysis of modified workpiece from HS 6-5-2 steel

The thickness and integrity of obtained recast white layer on the steel surface by electrical discharge treatment in electrolyte depend on the electrical current characteristics and time duration of the treatment. At higher voltage are observed thicker white layers by equal

Improvement of Corrosion Resistance of Steels by Surface Modification 315

cooling rate is lower a zone with dendritic structure is formed – Fig. 16a. In the other case (Fig. 16b), when the temperature is in the austenitic region and the cooling rate is higher

a b Fig. 16. SEM images of different microstructures of the modified layers on HS 6-5-2 steel surface: a - dendritic microstructure in the phase transformations zone, b – martensitic

The hardness of the modified layers can vary considerably and depend of the treatment conditions, electrolyte composition and microstructure, but in principle it is higher then the hardness of the typical microstructure of this steel. The microhardness of the modified layers is measured by Hanneman test and shows values after the different treatments up to 1600 HV which are very higher than the microhardness of HS 6-5-2 steel microstructures after the typical heat treatment. The experiments show that tools with such surface hardness

Asif Iqbal, A. K. M. & Khan A. A. (2010). Influence of Process Parameters on Electrical

ASTM Handbook. (1991). *Heat Treating, Volume 4,* ASTM International, ISBN 0-87170-379-3,

Bommi, V. C., Mohan, K. M., & Prakash, S. (2004). Surface Modification of Martensitic

Chatterjee-Fischer, R. (Ed). (1986). *Wärmebehandlung von Eisenwerkstoffen: Nitrieren und Nitrocarburieren,* Expert Verlag, ISBN 3-8169-0076-3, Sindelfingen, Germany

 http://metallurgy.iitm.ac.in/isrs/isrs04/cd/content/Papers/SE/PO-SE-6.pdf Buchkov, D. & Toshkov, V. (1990). *Ion Nitriding,* Technika, UDC 621.785.5, Sofia, Bulgaria Castelleti, L. C., Neto, A. L., & Totten G. E. (2008). Plasma Nitriding of Stainless Steels, In:

*Applied Science,* Vol. 2, No. 3, (2010), pp. 396-402, ISSN 1941-7020

December 20-22, 2004, 10.07.2011, Available from

*Industrial Heating,* 05.08. 2011, Available from

http://www.industrialheating.com/Articles/Feature\_Article/

Discharge Machined Job Surface Integrity. *American Journal of Engineering and* 

Stainless Steel Using Metal Working CO2 Laser, *Proceedings of International Symposium of Research Students on Materials Science and Engineering,* Chennai, India,

than the critical one a martensite is formed.

microstructure in the phase transformations zone.

have higher wear resistance and working capacity.

Materials Park, Ohio, USA

**4. References** 

durations of the treatment. In Fig. 14 are shown the light microscopy micrographs of steel workpiece surfaces, modified for two minutes at 80 V and 100 V. The optical measured thicknesses of the layers are 0.01 mm and 0.02 ‒ 0.03 mm respectively.

Fig. 14. Microstructure of layers, obtained for 2 min at 80 V, and for 2 min at 100 V (b) x800.

At higher voltage and longer duration of the treatment it is observed increasing of roughness of the white layer surface which is illustrated with SEM micrographs in Fig. 15.

Fig. 15. SEM micrograph of modified steel surface by duration of 2 min (a) and 3 min (b) at 100 V.

The fine structures of modified layers with specific etching are shown after SEM investigation on Fig. 16. By modification on the workpiece surface can be observed two specific zones:


The "Phase transformations zone" has different structures depending on the temperature and cooling rate. When the temperature of the steel surface is above the melting point and cooling rate is lower a zone with dendritic structure is formed – Fig. 16a. In the other case (Fig. 16b), when the temperature is in the austenitic region and the cooling rate is higher than the critical one a martensite is formed.

Fig. 16. SEM images of different microstructures of the modified layers on HS 6-5-2 steel surface: a - dendritic microstructure in the phase transformations zone, b – martensitic microstructure in the phase transformations zone.

The hardness of the modified layers can vary considerably and depend of the treatment conditions, electrolyte composition and microstructure, but in principle it is higher then the hardness of the typical microstructure of this steel. The microhardness of the modified layers is measured by Hanneman test and shows values after the different treatments up to 1600 HV which are very higher than the microhardness of HS 6-5-2 steel microstructures after the typical heat treatment. The experiments show that tools with such surface hardness have higher wear resistance and working capacity.

### **4. References**

314 Corrosion Resistance

durations of the treatment. In Fig. 14 are shown the light microscopy micrographs of steel workpiece surfaces, modified for two minutes at 80 V and 100 V. The optical measured

a b Fig. 14. Microstructure of layers, obtained for 2 min at 80 V, and for 2 min at 100 V (b) x800.

At higher voltage and longer duration of the treatment it is observed increasing of roughness of the white layer surface which is illustrated with SEM micrographs in Fig. 15.

a b Fig. 15. SEM micrograph of modified steel surface by duration of 2 min (a) and 3 min (b) at

The fine structures of modified layers with specific etching are shown after SEM investigation on Fig. 16. By modification on the workpiece surface can be observed two

The "Phase transformations zone" has different structures depending on the temperature and cooling rate. When the temperature of the steel surface is above the melting point and

100 V.

specific zones: White zone

Phase transformations zone

thicknesses of the layers are 0.01 mm and 0.02 ‒ 0.03 mm respectively.


http://metallurgy.iitm.ac.in/isrs/isrs04/cd/content/Papers/SE/PO-SE-6.pdf

Buchkov, D. & Toshkov, V. (1990). *Ion Nitriding,* Technika, UDC 621.785.5, Sofia, Bulgaria

Castelleti, L. C., Neto, A. L., & Totten G. E. (2008). Plasma Nitriding of Stainless Steels, In: *Industrial Heating,* 05.08. 2011, Available from

http://www.industrialheating.com/Articles/Feature\_Article/

Chatterjee-Fischer, R. (Ed). (1986). *Wärmebehandlung von Eisenwerkstoffen: Nitrieren und Nitrocarburieren,* Expert Verlag, ISBN 3-8169-0076-3, Sindelfingen, Germany

**14** 

**Low Temperature Thermochemical** 

Askar Triwiyanto1, Patthi Husain1, Esa Haruman2 and Mokhtar Ismail1 *1Universiti Teknologi PETRONAS,* 

*2Bakrie University,* 

*1Malaysia 2Indonesia* 

**Treatments of Austenitic Stainless Steel** 

**Without Impairing Its Corrosion Resistance** 

Austenitic stainless steel (ASS) is used applied widely owing to its very good corrosion resistance. However, the application of this material as a bearing surface is severely limited by very poor wear and friction behaviour. Consequently, Surface Engineering treatments for austenitic stainless steel are an interesting alternative way to increase the surface hardness and improve the wear resistance. For the purpose of this works, the Surface Engineering design will be classified, very broadly, into three groups : (a) those which coat the substrate: PVD, CVD, etc, (b) those which modify only the structure of the substrate, (c) those which modify the chemical composition and the structure of the substrate: thermochemical, ion implantation, plasma, etc. It is nowadays widely accepted that hard, wear and corrosion resistant surface layers can be produced on ASS by means low temperature nitriding and/or carburizing in a number of different media (salt bath,gas or plasma), each medium having its own strengths and weaknesses. In order to retain the corrosion resistance of austenitic stainless steel, these processes are typically conducted at temperatures below 450ºC and 500ºC, for nitriding and carburizing respectively. The result is a layer of precipitation free austenite, supersaturated with nitrogen and/or carbon, which is usually referred to as S-

Starting from the mid of 1980's, investigations have been performed to improve surface hardness of ASS and thus enlarging their possibility of wider application, but led significant loss of its corrosion resistance. This tendency occur due to the sensitivity effect. Sensitization is a common problem in austenitic steel where precipitation of chromium carbides (Cr23C6) occurs at the grain boundaries at elevated temperatures, typically between 450 to 850oC; diffusional reaction in forming chromium nitride/carbide leads to the depletion of Cr in the austenitic solid solution and consequently unable to produce Cr2O3 passive layer to make stainless feature. As a result, it reduces the corrosion resistance property of the stainless

**1. Introduction** 

phase or expanded austenite.

**2. Enlarging application of Austenitic Stainless Steel** 


http://www.sciencedirect.com/science/article/pii/S0890695503001627

