DEVELOPMENT OF ELECTRICAL DISCHARGE COATING (EDC) AS CHROME-FREE

ALTERNATIVE FOR INCREASING CAMPAIGN LENGTH OF TEMPER MILL WORK ROLLS

R. Bröcking1, A. Meghwal

4, S.Melzer

1, S. Verdier

1, G. Evans

2, T. Lowbridge

2

J.-F. Vanhumbeeck3, D. Debrabandere

3 and J. Crahay

3

1 Tata Steel Research & Development, IJmuiden Technology Centre, PO Box 10000, 1970 CA

IJmuiden, The Netherlands. 2 Sarclad Ltd., Advanced Manufacturing Park, Whittle Way, Rotherham, S60 5BL, United Kingdom

3 Centre de Recherches Métallurgiques ASBL, Avenue du Bois Saint Jean 21, B-4000 Liège, Belgium

4 RWTH Aachen University, Department of Ferrous Metallurgy, Intzestr.1, D-52072 Aachen,Germany

ABSTRACT

This paper discusses the fundamentals of the EDC technology as well as several development steps

from laboratory scale up to industrial application of EDC on temper mill work rolls.

After EDC treatment on rolls, titanium carbide (TixC) and tungsten carbide (WxC) phases were present

in the surface layer up to 10wt%, depending on the chosen electrode type and EDC machine settings.

Moreover the amount of hard cementite in the surface can also be influenced by selecting suitable

EDC machine settings.

The positive influence of these hard (carbide) phases in the roll surface layer on the wear properties

was proven in a laboratory tribotest especially developed to simulate temper rolling and confirmed

(though to a somewhat lesser extent) by pilot mill endurance tests. In these pilot mill tests, work rolls

provided with a WxC- or TixC-EDC-treated surface were compared against chrome plated and

uncoated EDT-textured work rolls.

In these trials, the TiC and WC EDC variants performed quite similar and their roughness retention

was considerably superior to that of the uncoated EDT roll. However both EDC variants showed a

lower roughness retention than the chrome-plated reference roll.

Because of the promising results of these preliminary roughness retention tests, industrial testing of

EDC coated temper mill rolls has been initiated.

Roughness retention, Electrical discharge coating, chrome plating, EDT, wear resistance, roll surface

1 INTRODUCTION

Freshly ground work rolls for temper rolling mills generally undergo two additional preparation

processes before each mill campaign:

1 A surface texturing process, in order to apply the required roughness. The most widely used

technique is Electrical Discharge Texturing (EDT).

2 A hard chrome plating process, in order to enhance the wear resistance of the textured roll surface,

thus preventing too frequent work roll changes due to premature roughness loss.

Besides the rather high costs (typically €300 to €400 per roll pair per campaign) and the extra cycle

time for roll preparation, a major drawback of the hard chrome plating process is the fact that it uses

hexavalent chromium. Due to its carcinogenic properties, unauthorised use of Cr(VI) compounds will

be banned in the European Union from September 2017 under REACH regulation.

Against this background, the Electrical Discharge Coating (EDC) technology has been explored as a

Cr(VI)-free alternative preparation technique for temper mill work rolls. In the EDC technology, the

two-step procedure above is replaced by a single process: the surface roughness is applied on the roll

surface by means of electrical discharge texturing (EDT) whilst at the same time an additional surface

treatment is performed by depositing carbides or carbide forming elements into the surface layer.

For the EDC process, state-of-the-art EDT equipment is used, in which the conventional copper

electrodes have been replaced by special electrodes containing hard carbides, such as WxC or TixC.

This paper discusses the fundamentals of the EDC technology as well as several development steps

from laboratory scale up to industrial application of EDC on temper mill work rolls.

XRD analysis performed on the treated EDC surface showed the presence of titanium carbide (TixC)

and tungsten carbide (WxC) phases in the surface layer up to 10wt%, depending on the chosen

electrode type and EDC machine settings. Moreover the amount of hard cementite (Fe3C) in the

surface can also be influenced by selecting suitable EDC machine settings.

The positive influence of these hard (carbide) phases in the surface layer on the wear properties was

proven in a laboratory tribotest especially developed to simulate temper rolling and confirmed (though

to a somewhat lesser extent) by pilot mill endurance tests. In these pilot mill test, work rolls provided

with a WxC- or TixC-EDC-treated surface were compared against chrome plated and uncoated EDT-

textured work rolls.

In these trials, the TiC and WC EDC variants performed quite similar and their roughness retention

was considerably superior to that of the uncoated EDT roll. However both EDC variants showed a

lower roughness retention than the chrome-plated reference roll.

Because of the promising results of these preliminary roughness retention tests, industrial testing of

EDC coated temper mill rolls has been initiated.

2 PRINCIPLE OF ELECTRICAL DISCHARGE COATING (EDC)

Electrical discharge coating (EDC) is a surface coating/surface alloying process for making a hard and

wear resistant layer with a EDT-like surface on a metal substrate in the presence of dielectric fluid,

using tailored electrodes [1-4]. Green compact and/ or sintered metal-carbide electrodes can be used

during electrical discharge texturing to improve roll wear resistance through surface alloying [1,5].

During the EDC process electrical current is flowing through an electrode and causes ionisation of the

dielectric in the sparking gap. During ionisation temperatures of more than 8000K will occur at which

local melting and vaporisation of electrode and work-piece top surface takes place.

Figure 1: Schematic diagram of the EDC process [6]

The high electrical field between work-piece and electrode charges particles from the sintered

electrode and these particles are transferred to the work-piece material surface [6] (Figure 1).

Moreover, due to the breakdown of the dielectric fluid, carburization takes place on the coated surface

layer [1].

The characteristics of the coated surface layer depends on the composition of the electrode as well as

on the EDC machine setting applied during the process [6,7].

3 METHOD

3.1 Creating EDC Coated Steel Surfaces

Partial and fully sintered electrodes have been produced from a compacted mix of WxC and Nickel

powder as well as from a mixture of TixC and nickel powder.

Each sintered electrode has been provided with a copper holder enabling mounting into the electrical

discharge machine.

(A) (B) (C) Figure 2: Creating EDC surface bands on A2 tool steel roll utilising a cemented tungsten carbide electrode

Figure 2 shows the experimental set up in which the sintered electrode (A) has been mounted in the

electrical discharge machine (B). In the right hand image (C) the utilised electrode is surrounded by a

rubber seal between a container (filled with dielectric) and the roll surface.

On the roll several EDC bands were produced utilising several machine settings (B).

The roll (∅ 35mm, length: 550mm) used for producing the several EDC bands was an A2 tool steel

(X100CrMoV5) which was chosen because of its similarity to the steel typically used for cold mill and

temper mill work rolls.

Material analysis of this A2 steel grade is listed in Table 1.

Table 1: Material analysis of the A2 tool steel used for test roll [8]

Typical C Si Mn Cr Mo V

analysis % 1.0 0.3 0.6 5.3 1.1 0.2

Hardness 61-63 HRC

Standard AISI: A2

specifi- DIN: W.-Nr. 1.2363

cation Euro: X 100 CrMoV 5

3.2 Characterisation of the EDC surface

The phase proportions in the recast layer have been determined up to a depth of ~10µm by XRD and

subsequent Rietveld analysis. The XRD patterns were recorded in the range of 40 to 120 ° (2 theta) as

3 separate frames in reflection mode that were merged together. A fully automated Bruker D8

diffractometer (CoKa-radiation) was used equipped with an area sensitive detector system (GADDS)

and a primary graphite monochromator. Operating conditions were 30mA and 40kV.

Quantitative determination of phase proportions was performed by Rietveld analysis. Unit cell

parameters, background coefficients, preferred orientations, profile parameters and phase proportions

were obtained using the Bruker Topas software package for Rietveld refinement.

3.3 Lab scale roughness retention tests Two types of roughness retention tests have been carried out to investigate the wear resistance of EDC

coated surface on lab scale samples: the Block on Ring wear testing machine at Tata Steel R&D and

the 2-Disc & Ring wear testing machine at CRM (Figure 3).

(A) Block on Ring wear test device (B) 2-Disc & Ring wear test device

Figure 3: Block on Ring wear test device (A) and the 2-Disc & Ring wear test machine (B).

In the Block on Ring device the EDC-coated circumference of a rotating ring slides along a static steel

block. This block is from an A2 tool steel grade, quenched and ground.

Sa roughness at the circumferential of the ring has been determined with 3D confocal roughness

measurement device. Sa is the arithmetic average of the 3D roughness.

In the 2-Disc & Ring wear machine the lower disc which mimics the work roll, is made of A2 steel

grade and is quenched, ground and then surface treated. The upper disc which mimics the back-up roll

is produced from A2 steel grade and is quenched and ground. This disc is changed for each test.

The circumference speed of this disc varies from 60 to 300 m/min. The upper disc is in a rolling

contact with the lower disc. In turn this lower disc is in a slide-roll contact with a strip of uncoated

steel (drawing grade) which is clamped on the inner diameter of the ring. The ratio of circumferential

speed of the ring to the coated disc is 0.98.

Therefore, the surface treated discs experiences both the rolling contact pressure of the upper disc as

well as the roll/ slip contact with the steel strip inside the outer ring.

3.4 Pilot Mill Roughness Retention Tests

Roughness retention of EDC coated work rolls (∅140mm) has been determined at the Tata steel R&D

pilot mill (Figure 4).

Figure 4: Impression of the trial pilot mill at Tata Steel R&D

In this test the performance of EDC coated work mill rolls was compared to uncoated- and chrome

plated EDT work rolls. The used work rolls are from a X63CrMoV5 steel grade with a shell hardness

of 61-62,5 HRC.

The pilot mill work rolls were EDC coated at Sarclad using WC- and the TiC EDC electrodes (Figure

5).

Figure 5: Application of EDC coating on a pilot mill work roll at Sarclad

For this trial, 150 mm wide and 820 m long steel coils have been unidirectional rolled up to 45 passes.

Rolling was in wet condition using a solution of 5% cleaner in demineralised water. The applied

reduction on the strip was 2%.

After pass 1, 2, 5, 10, 25 and 45, the roughness of the work rolls was measured with a scanning stylus

measurement device.

3.5 EDC application on industrial rolls

Due to the promising results obtained with EDC coated samples and rolls during lab testing and pilot

mill trials, the EDC technology has been selected for further scaling up to industrial application.

Figure 6: Application of EDC coating on industrial temper mill roll at Sarclad

Figure 6 shows an industrial temper mill roll being EDC coated in an industrial EDT/ EDC machine at

Sarclad. For this 24 WC-electrodes had to be functioning simultaneously in the EDC machine.

4 RESULTS AND DISCUSSION

4.1 Thickness and integrity of the EDC recast layer

Figure 7 shows the recast layer as obtained with a tungsten carbide electrode applying three levels of

pulse current (4, 8 and 17 ampere). From this figure it can be concluded that the EDC recast layer

decreases with applied pulse current. A possible reason for this is that at lower pulse currents, longer

pulse on-times have to be applied in order to achieve the required surface roughness. At longer pulse

on-times heat is allowed to penetrate deeper into the substrate materials resulting into larger melt pools

whereas at the same time the dielectric is unable to sweep away all the molten material after every

spark. As a result the metal re-solidifies smoothly and creates a thicker recast layer [9,10].

Figure 7: Effect of applied machine settings on thickness and integrity of recast layer. Effect of applied current, 4A,

8A and 17A in image A, B and C respectively

Cracks are more prone to occur in the thicker and uniform recast layer rather than in thin recast layer

because thin recast layers have a greater ability to rapidly dissipate the heat resulting in lower

contraction stresses. Moreover, at a current of 4A more carbon ingression occurs in the recast layer

due to a longer reaction time.

Average thicknesses obtained from three random points on the coated layer at different EDC machine

settings (current and passes) are presented in Figure 8.

0

1

2

3

4

5

6

7

8

9

4 8 17

Pulse current (A)

Recast layer thickness (µm)

8 passes 16 passes

Figure 8: Recast layer thickness at different EDC machine settings

Figure 8 shows that there is no appreciable increase in the average recast layer thickness as the number

of passes increases from 8 to 16. Apparently the EDC process involves both material removal and

accretion [7].

4.2 Damage Tolerance of (micro cracked) Recast Layer

The recast layer contains micro cracks and residual stresses which may lead to failure of the substrate

material, particularly under conditions of impact and fatigue loading. The effect of loading on the

penetration of the cracks into the parent material has therefore been analysed. Figure 9 shows the

comparison between cross-sections of the EDC -WC layer before and after loading in the block-on-

ring machine.

Figure 9: Cross-sectional comparison between unloaded (left side images) and loaded (right side images) WC-EDC

coated rings after loading in a sliding contact with a static block settings)

This figure clearly shows that loading the ring does not affect the cracks in terms of widening or

penetration of cracks into the parent material. The non-occurrence of crack growth may be explained

by the presence of retained austenite in the recast layer. Two characteristics of retained austenite can

improve the rolling contact fatigue life. Firstly, the inherent ductility of the retained austenite helps in

delaying or suppressing the crack growth. Secondly, retained austenite transforms during the loading

process and increases compressive residual stress in the material which may delay a crack.

Furthermore it can also be observed that no flattening of the recast layer takes place due to the loading.

4.3 Formation of Hard Phases in the Recast Layer

Figure 10 shows the effect of pulse current and number of passes on the formation of tungsten related

phases in the recast layer.

Sum of W related phases v/s EDC machine settings

0

1

2

3

4

5

6

7

8

8 12 16 20

Number of passes

Proportion (wt%)

4A 8A 17A

Figure 10: Effect of pulse current and number of passes on the formation of tungsten related phases in the recast layer

The highest amount of hard tungsten phases is obtained at a pulse current 4A and number of passes

12, 16 and 20. The high amount of formation of W related phases at 4A can be explained by the higher

applied pulse on-time which leads to a longer reaction-time. Longer reaction-time also gives more

electrode material deposition and increases the chance of absorbing carbon from crackdown of the

dielectric [11].

Figure 11 shows the relationship between retained austenite and cementite formation in the recast

layer at 4A, 8A and 17A pulse current. Retained austenite is a quenching product and forms along

with martensite. Austenite is a stable phase of steel above austenitizing temperature but it is metastable

at other temperatures (such as at room temperature). It is clear that as austenite formation increases,

the cementite phase decreases. Pulse current 4A produces high amounts of cementite due to longer

pulse on-time as it gives more reaction-time for the melted work roll material to absorb carbon

coming from the crackdown of dielectric.

0

10

20

30

40

50

60

0 4 8 12 16 20 24 28 32

Number of pas ses

Pro

po

rtio

n (

wt%

)

Aus tentite 4A Austeni te 8A Austenite 17A

Cementite 4A Cementi te 8A Cementi te 17A

Figure 11: Relation between Austenite and Cementite formation at different machine settings

The recast layer consists primarily of martensite, cementite, retained austenite and W related hard

phases. Consequently, these hard phases make a difference in mechanical properties of the reinforced

EDC surface layer. Specifically, yield strength, hardness, wear resistance and brittleness properties are

all greater than those of the parent material.

4.4 Evolution of the Surface Topology The 3D- surface topology of the rings before and after applying 300 load cycles in the Block-on-Ring

machine under a load of 602N are depicted in Figure 12.

A: EDT Chrome plated

B: EDC-WC

C: EDC-TiC

Figure 12: 3D roughness topology of the rings as delivered and after applying 300 load cycles at a load of 602N and

corresponding visual surface aspect of the block and ring after 300 rotations

As can be seen in these images, the 3D surface topology has not changed significantly after applying

300 rotations. However, a slight wear track could be observed on the circumference of the rings, while

in contrast, the blocks show a clear scar on the contact surface.

4.5 Roughness Retention Tests

Tests have been carried out in order to determine the roughness retention of the EDC coated surfaces.

Block on Ring

Figure 13 shows the variation of Sa of EDC WC and EDC TiC coated rings relative to the chrome

plated EDT rings in the Block on Ring wear machine. Sa is the arithmetic average of the 3D

roughness.

Cr init Cr @ 300

EDC-WC init EDC-WC @ 300

EDC-TiC init EDC - TiC @300

EDC -TiC @300

cycl

EDC- WC @ 300

cycl

Cr @300 cycl

0,00

0,50

1,00

1,50

2,00

2,50

3,00

3,50

0 50 100# rotations

Sa

(µ

m)

EDT+Cr pl EDC-WC EDC-TiC

Figure 13: Sa roughness retention with number of rotations

The graphs show that the roughness of all variants hardly changes with the number of rotations.

Although as could be seen in Figure 12 large scars are formed on each block .

Note that the initial roughness of the EDC-TiC coated ring is substantially lower than the two other

variants.

Although the discriminative ability of this test may be questioned it surely indicates the reinforcement

effect of the EDC treatments on the surface wear resistance as the wear damage on the ring is minimal

whereas the damage on the static counter body is substantial.

To study the effect of further wear, the number of rotations of the EDC-WC ring and the chrome

plated ring has been increased up to 1300 rotations after which the surface of the rings have been

analysed using SEM microscopy.

SEM images and the 3D roughness profiles are depicted in Figure 14.

Sa= 2,8 Ssk=0,4 Sa= 2,7 Ssk= 0,4 Sa= 2,4; Ssk=0,5 Sa= 2,4 Ssk=0,1

Cr plated rot 1300-602N EDC WC rot 1300-602N Figure 14: SEM images and comparable 3D roughness profiles of the EDC WC and Cr plated EDT surface at the

transition of the worn and the non-worn surface after 1300 rotations

Worn

Fresh

Worn

Fresh

From these images it can be concluded that the Cr plated ring shows the lowest asperity flattening

confirmed by the unchanged Sa roughness and Ssk skewness values. However the roughness of the

EDC-WC ring also appears to be hardly affected, only the skewness reduces.

2 Disc & Ring Wear Machine

Figure 15 shows the evolution of the Ra roughness on the circumference of EDC coated discs in

comparison to chrome plated EDT and uncoated EDT discs.

0,00

0,50

1,00

1,50

2,00

2,50

3,00

3,50

0 20 40 60 80 100 120

Rolling length (km)

Ra

(µ

m)

EDT EDT Cr-plate EDC-WC EDC-TiC

Figure 15: variation of Ra roughness on EDC coated surfaces with rolling length compared to EDT Cr plated and

EDT uncoated surfaces

The graphs shows clearly that the roughness retention of the EDC-WC and the EDC-TiC coated

samples is comparable to that of the chrome plated EDT variant.

An initial roughness drop of the EDC-WC variant and to a lesser extent of the EDC-TiC can be

observed whereas the chrome plated variant has only a small roughness drop in the first part of the

test. In the continuation of the test the roughness retention of the WC variant is somewhat better than

the TiC variant. The chrome plated EDT sample performs only slightly better than the EDC variants.

In contrast the Ra roughness of the uncoated EDT decreases rapidly with rolling length.

4.6 Roughness Retention Measured at the Pilot Trial mill

The roughness variation with rolled strip length of EDC -WC and EDC TiC coated trial mill rolls as

well as a EDT chrome plated reference roll and an uncoated EDT reference roll is shown in Figure 16.

0

0,5

1

1,5

2

2,5

3

3,5

0 5 10 15 20 25 30

rolled strip length (km)

Ra

(µ

m)

uncoated EDT EDC-WC EDC-TiC EDT- Cr plated

Figure 16: Roughness variation with rolled strip length of EDC -WC and EDC TiC coated trial mill rolls compared to

EDT chrome plated and EDT uncoated EDT reference rolls

In this figure it can be observed that all variants show a significant drop in Ra roughness during the

first pass (length about 0.8 km). However the chrome plated reference roll shows the smallest

roughness drop after the first pass.

The EDT Cr plated roll possesses the best roughness retention. The Ra roughness of this roll stabilises

at a value around 2µm.

The EDC-WC and EDC TiC rolls perform quite similar, however the performance of these rolls is

significantly less than the chrome plated EDT rolls but on the other hand substantially better than the

uncoated EDT roll.

The Ra roughness of the EDC rolls stabilises at around 1.0 µm.

The uncoated EDT roll possesses the lowest roughness retention. Already after 7 km rolled strip length

the Ra value has reached a value of about 0.5 µm.

Note that due to the smaller roll diameter of the trial rolls compared to industrial rolls, a comparison to

the rolled length in industrial practice is obtained by applying a factor 4.4 to the rolled strip length in

the pilot mill trial.

5 CONCLUSIONS

Main micro structural composition of EDC coated roll surface is martensite, retained austenite,

cementite and hard W related phases such as WC, W2C etc. EDC with higher pulse current leads to

formation of lower amount of hard W related phases on coated roll surface. XRD results suggest that

after an initial build-up phase, additional passes have no significant effect on the quantitative nature of

hard W related phases. Lower EDC pulse currents give better quantitative results with respect to hard

W related phases as well as the hard cementite phase.

Cross-sectional SEM analysis of EDC coated surface shows that lower pulse current (4A) leads to

micro-cracks in the coating which always terminates at the interface of the recast layer, whereas, at

higher currents (8A and 17A) there appear to be no micro-cracks in the coating. However, recast layer

thickness is higher in the case of lower pulse current (4A). Micro-cracks occurring in the EDC layer

do not propagate during loading , the cracks terminating at the interface of the recast layer and parent

material.

After laboratory tests, the EDC-WC and the EDC-TiC coated samples show a roughness retention

comparable to that of the chrome plated EDT reference variant. In pilot mill roughness retention tests,

the TiC and WC EDC variants performed quite similar and their roughness retention was considerably

superior to that of the uncoated EDT roll. However both EDC variants showed a lower roughness

retention than the chrome-plated reference roll.

These preliminary results show that the EDC coating of temper mill rolls is a potential alternative to

chrome plating. Industrial trials are therefore planned together with the further development and

optimisation of the process.

ACKNOWLEDGEMENTS

The Authors of this paper gratefully thank the Commission of the European Communities for the

support of this work. The work is funded in part with a grant from the Research Fund for Coal and

Steel under research Grant Agreement Number: RFSR-CT-2011-00012.

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