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Hard chromium plating of EDT mill work rolls
Journal of Materials Processing Technology 92±93 (1999) 281±287
Hard chromium plating of EDT mill work rolls J. SimaÄoa,*, D.K. Aspinwalla,b a School of Manufacturing and Mechanical Engineering, University of Birmingham, Birmingham B15 2TT, UK Interdisciplinary Research Centre in Materials for High Performance Applications, University of Birmingham, Birmingham, UK
b
Abstract Hard chromium plating of cold mill work rolls is common practice in order to maintain topography and increase roll service life. The paper details the effect of hard chromium plating on electrical discharge textured (EDT) roll surface topography and surface integrity. Comparative data with corresponding non-chromium plated EDT roll samples are also presented and discussed. Irrespective of the EDT roll surface topography, the coating showed a typical surface hardness in the range HV (0.025)1100±1300 against approximately 800 HV for the roll base material. The thickness of the hard chromium layers was typically 20±25 mm and cracking of the plated layers was clearly evident as a result of high residual stresses. Measurement of surface residual stress using X-ray diffraction techniques in both the longitudinal and circumferential roll directions indicated highly tensile values between 350 and 700 MPa. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Electrical discharge texturing; EDT; Roll texturing; Mill rolls; Coating; Hard-chromium plating
1. Introduction Industrial experience with hard-chromium plated mill rolls, clearly indicates that coatings applied to a particular type of roll surface texture achieved either by shot blasting (SB), electrical discharge texturing (EDT) or laser beam texturing (LBT), extend roll service life and performance. Hard chromium plated work rolls (typical plating thickness 4±15 mm) have been used for over 25 years both in tandem and temper/skin-pass rolling mills. It has been claimed that roll change times have been reduced signi®cantly when compared with similar uncoated rolls [1±3]. Electrodeposited chromium has properties similar to the solid metal, the most important are: high hardness levels (with thick deposits), low coef®cient of friction (high lubricity), good corrosion resistance, high heat resistance, low coef®cient of linear expansion and high thermal conductivity. Any differences in plating practices will inevitably cause major variations in the hardness of the electroplated material (chromium can range from 550 to 1250 HBN) [4]. There are two basic requirements which have to be met for the use of functional hard chromium coatings [2]:
*Corresponding author. Tel.: +44-121-414-3541; fax: +44-121-4143541 E-mail address: j.m.t. [email protected] (J. SimaÄo)
1. The base or substrate material must be such that it is capable of resisting externally applied forces without undergoing extensive deformation, i.e. the strength and hardness properties of the base material must be matched to some extent, to those of the hard chromium coating applied. The pairing of martensitic base materials with hard chromium coatings has proved to be a particularly successful combination. 2. The coating thickness should be restricted to a minimum where deformation or shock loading of the composite can be expected in the application concerned. The typical hardness of chromium coatings for the metalworking industries lies in the range 800±1000 HV (0.1). Coatings deposited by modern electroplating techniques may reach hardness values up to 1200 HV [2]. The achievable hardness values depend on a number of operating variables such as electrolyte bath temperature, current density and chromic acid/sulphate ratio. 2. The effect of chromium plating on the surface characteristics of the work rolls It is generally not necessary to change the type of work rolls or back up rolls in order to introduce chromium plated rolls. Similarly there is no need to change existing roll
0924-0136/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 1 2 4 - 7
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J. SimaÄo, D.K. Aspinwall / Journal of Materials Processing Technology 92±93 (1999) 281±287
grinding or texturing practices. Both bright and textured rolls will have the ``same'' ®nish characteristics after plating that they had before plating [1]. According to Wujtow and Williams [5], the electrodeposition of approximately 10 mm (0.0004 in.) of chromium to a textured work roll, has little effect on the surface topography of the roll. The peak count Pc is virtually unchanged, however, the peaks are slightly accentuated, resulting in a marginal increase in surface roughness value in the middle ranges. The most relevant and crucial effect of the hard chromium is to protect and maintain the peaks and roughness values of the work roll. The coef®cient of friction of chromium plating against steel is approximately one-half of that of steel against steel. This property is particularly advantageous under dry conditions and allows chromium plated work rolls to avoid a high proportion of damage from grit, slugs and other foreign matter that can mark the roll surface while rolling sheet metal [6]. It is also claimed that the cold-rolled product is held within tighter surface tolerances than previously obtained with plain rolls and, as long as roll damage can be avoided, the production runs can be as much as 10 times longer [5]. The use of harder rolls or, more precisely, harder roll surfaces is a key factor in extending roll life. A three to fourfold increase of roll life was obtained by deposition of a layer of hard chromium on SB textured rolls at US Steel [7]. In addition, the use of the same chromium deposition process on EDT rolls resulted in a ®ve to eight-fold increase in roll life and an increase in the EDT sheet peak count. The explanation given for this was that the chromium deposit not only increased the surface hardness of the EDT roll but also retained some ``weak'' peaks normally lost in the early stages of rolling with uncoated rolls. In 1986 the Japanese steel producer NKK installed continuous plating equipment adjacent to their roll shop. Chromium plating increased roll life by ten-fold when compared to plain EDT rolls [8]. More recently, in his review of Japanese work, Kawanami [9] reported that chromium plating on the surface of work rolls used for cold rolling greatly improved wear resistance by providing a surface hardness of 1000 HV against 800±900 HV for ordinary forged steel rolls. When chromium plated rolls were experimentally applied to tandem cold rolling [9], it was found that ®ne cracks were generated in the plated layer in the rolling direction under conditions of high load and poor lubricating conditions. Temper rolling conditions are not as severe as in the tandem mill, due to lower reduction rates (1%) and it is here that the use of chromium plated rolls has resulted in product quality improvements such as consistency of sheet surface roughness. The need to maintain the initial roll surface topography together with the extreme brittleness of the chromium, signi®cantly limits the thickness of the coating applied to mill work rolls. Fig. 1 [[10]] shows the relationship between chromium plating thickness and the
Fig. 1. Relationship between roll chromium plating thickness and length of rolled strip in temper rolling [10].
Fig. 2. Comparative performance of chromium plated and plain temper mill rolls [10].
length of temper rolled strip. It can be deduced that the optimum thickness of chromium plate ranges from 4 to 11 mm. Izushi's paper [10] also highlights the excellent wear resistance of a chromium plated roll (Fig. 2), which produced approximately seven times the length of strip achieved by a conventional temper mill roll. 3. Experimental work 3.1. Aims The aims of the present work are to investigate the effect of hard-chromium plating on EDT roll surface topography (``roughness'' and ``waviness'') and surface integrity. 3.2. Experimental equipment, materials and procedure Full details of the equipments used, including the dual voltage Spark Tec generator, the lathe and the BP 180 dielectric ¯uid/®ltration system are given in [11,12]. The DC servo quill arrangement was ®tted with a single tool electrode made from copper graphite (EC15C), supplied by Graphite Technologies. The electrode was cylindrical in shape, with a diameter of 13 mm and a length of 100 mm. Workpiece rolls were approximately 44 mm in
J. SimaÄo, D.K. Aspinwall / Journal of Materials Processing Technology 92±93 (1999) 281±287 Table 1 EDT generator test parameters EDT specimen band
Peak current [A]
On-time (ms)
Off-time (ms)
Capacitance (mF)
Electrode polarity
A B C D E F G H I J K
1 1 2 2 2 2 2 4 4 8 10
6 6 10 10 10 10 10 10 20 30 80
20 20 20 20 20 20 20 20 20 20 20
0 0 0 0 0.22 0.47 1.88 0 0 0 0
() (ÿ) () (ÿ) (ÿ) (ÿ) (ÿ) () (ÿ) (ÿ) (ÿ)
Tool electrode material: copper graphite (EC15C).
diameter and 530 mm long, made from BS 970-534A99 [(En31) (AISI E52100 alloy steel)]; 0.95±1.1% C, 1.2±1.6% Cr, 0.25±0.4% Mn, 0.04% P, 0.05% S, 0.1±0.35% Si, bal. Fe], with a measured hardness of 813 HV (63 HRC), supplied by Lee Steel Strip, Shef®eld. These were typical Sendzimir cluster mill work rolls used for the rolling of stainless steel strip and were ground prior to texturing to a surface ®nish of 0.08 mm Ra, a peak count Pc0 peaks/cm and a measured average waviness Wca0.022 mm. The work rolls were textured in 30 mm wide bands using the generator parameters shown in Table 1, in order to determine the effect of sparking conditions on both surface roughness and surface integrity. Roll rotation direction (anticlockwise viewed from the tailstock end of the workpiece), roll surfaces speed (6 m/min) and electrode axial feed rate (0.28 mm/rev), were kept constant throughout the tests, as was the servo gain (4/32). Surface roughness evaluation parameters Ra and Pc were measured using a portable Mitutoyo Surftest 201 surface pro®lometer (cut-off length of 0.8 mm, evaluation length of 4 mm and Pc level of 0.5 mm). Surface average waviness parameter Wca was measured using a Rank Taylor Hobson (RTH) Laser Form Talysurf and 3-D topographic mapping was performed using a 3-D RTH Form Talysurf-120L. Small sections for metallographical examination and 3-D mapping were ED wire cut from EDT roll surfaces using a Charmilles Technologies Robo®l 200 5 axis CNC machine. Alterations in the microstructure of the textured surface and subsurface were observed using a Leitz Wetzlar optical microscope ®tted with a JVC CCD camera, connected to a Mitsubishi DAT image recorder and colour video laser printer. SEM analysis of the samples was performed on a JEOL JSM-5410 unit. Microhardness measurements to determine the extent of the HAZ were made on a Leitz Wetzlar Miniload hardness tester using a standard load of 25 g. Residual stress measurements were performed with a Strain¯ex MSF-2M X-ray stress analyser (Rigaku, Japan). A second set of roll textured bands (A±K, Table 1) was wire cut from the work roll for hard-chromium plating. The
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coating operation was performed at Ohio Camshaft. (Roll Grinding/EDT and LBT/Chromium Plating Centre), Akron, Ohio, USA. Following on from this, both the surface topography and surface integrity of the chromium plated EDT roll specimens were assessed in the same way as the corresponding non-chromium plated EDT roll samples. Optical microscopy, SEM evaluation, microhardness and residual stress measurements, together with 2-D and 3-D topographic mapping were performed. 3.3. Results and discussion 3.3.1. Surface roughness measurements: Ra/Pc comparison of plain EDT and hard chromium plated EDT roll surfaces Data for surface roughness parameters Ra and Pc of both plain EDT and hard chromium plated EDT roll surfaces (A± K) are shown in Figs. 3 and 4. Selected 3-D topographical maps and SEM photographs of the plated textured surfaces are shown in Figs. 5 and 6.
Fig. 3. Surface roughness Ra comparison of plain EDT and Cr-plated EDT roll surfaces (±^±: plain EDT roll surface; ±}±: Cr-plated EDT roll surface).
Fig. 4. Peak count Pc comparison of plain EDT and Cr-plated EDT roll surfaces (±^±: plain EDT roll surface; ±}±: Cr-plated EDT roll surface).
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evident in the rougher roll samples, i.e. J and K, whereas the decrease in the number of peaks Pc was more noticeable in the ®ner roll samples, i.e. A, B and C (e.g. 19% in sample A and 23% in samples C). The likely reason for this is that with the ®ner EDT roll surfaces, some of the smaller peaks and valleys are covered and ``hidden'' by the chromium deposit. Under such circumstances the maximum peak-to-valley height of the chromium plated EDT sample decreased when compared to the corresponding value before the plating, i.e. from 13.29 to 12.37 mm. With the coarser EDT roll surfaces, the peaks and valleys are greater and wide enough to accommodate the thin chromium deposit, thus maintaining approximately the same Pc values while marginally increasing the average surface roughness Ra. This time a signi®cant increase in the maximum peak-to-valley value of the chromium plated EDT roll surface is evident, i.e. from 86.03 to 92.51 mm. As far as roll surface textured uniformity is concerned, the sample 3-D topographical maps shown in Fig. 5 (a) and (b) illustrate that with chromium plating, a well-de®ned surface is achievable. Successive surface roughness measurements made axially along the various roll samples and circumferentially, showed very little difference. The cracking of the plated layer (see Fig. 6) is discussed in Section 3.3.3.
Fig. 5. Sample 3-D topographical plots of the hard chromium plated EDT specimen bands: (a) A: 1.02 mm Ra; (b) K: 10.23 mm Ra.
3.3.2. Waviness measurements The average surface waviness data Wca for both the plain EDT and hard chromium plated EDT roll surfaces (A±K) are plotted in Fig. 7. The data again revealed no major differences, however, a trend was observed in that the majority of the hard chromium plated roll samples had a slightly higher Wca value. This increase was more evident in the rougher roll samples, i.e. J and K (36% and 23%, respectively). This was not a surprise since similar trends were observed when comparing the average surface roughness Ra. Fig. 8 shows the correlation between roughness and waviness for plain and chromium plated textured roll samples.
Fig. 6. Sample SEM photograph of the hard chromium plated EDT specimen band I: 3.77 mm Ra.
Comparison of the data for both surfaces, in general, revealed no major differences, other than the ability of the coating process to ``preserve'' the original workpiece surface topography. The majority of the hard chromium plated roll samples did, however, have a slightly higher average surface roughness value and also showed a slightly decrease in the peak count. This increase in the Ra value was more
Fig. 7. Average surface waviness Wca comparison of plain EDT and Crplated EDT roll surfaces (±^±: plain EDT roll surface; ±}±: Cr-plated EDT roll surface).
J. SimaÄo, D.K. Aspinwall / Journal of Materials Processing Technology 92±93 (1999) 281±287
285
Fig. 8. Relationship between the average surface roughness Ra and average surface waviness Wca for the various plain EDT and Cr-plated EDT roll specimens (^: plain EDT; }: Cr-plated EDT).
The test results clearly indicate that, in both cases, roll surface waviness Wca increases signi®cantly with surface roughness Ra. With the ®ner plain EDT/coated EDT roll surfaces, both the Ra and Wca parameter values are low. As the roll surface roughness Ra increases, the Wca also increases, maintaining a Wca/Ra ratio of approximately 0.5. These results suggest that roll surface waviness is introduced during texturing as a direct consequence of efforts to make the roll rougher. The inherent characteristics of the EDT process are responsible for the random nature of the resulting texture and also for introducing a wide range of topographic wavelengths on the roll surface. These wavelengths are not con®ned in the roughness regime (<0.8 mm), but extend further into the waviness regime. During tandem and/or temper rolling, both roughness and waviness are transferred to the steel strip. The relationship between Ra and Wca provides the steel manufacturer with a relatively simple way of producing low Wca strip (if required by the customer), i.e. by aiming at a lower Ra value. 3.3.3. Surface integrity Fig. 9 details cross-sectional photomicrographs of selected hard chromium plated EDT roll surfaces (A and K). The forces acting on the chromium layer during subsequent cold rolling operations requires that the bond between the chromium and work roll be of a high standard. The adhesion of the chromium to the base metal is dependent on atomic bonding. It is claimed [6] that the work roll surface roughness is inconsequential, thus equally good adhesion can be obtained on turned, ground, highly polished, SB or EDT surface. The major requirement is that of surface cleanliness. Therefore, before the chromium is deposited, the surface to be plated must be cleaned of all contaminants such as grease, dirt and oil. This can usually be accomplished by solvent washing, alkaline cleaning or other means. As far as
Fig. 9. Photomicrographs (cross-section) of Cr-plated EDT surfaces on specimens A and K: (a) low energy operating conditions, specimen A; (b) high energy operating conditions, specimen K.
mill rolls are concerned, the presence of scale, oxides or rust is seldom a problem. The mechanical ®nishing operations of the work rolls create modi®cations in the structure of the surface layer. The high pressures and temperatures break down and/or deform the crystal structure and lubricants or deposits of fused metallic particles are created and forced into the roll surface. In order to guarantee maximum adhesion between the chromium layer and the work roll, it is generally necessary to etch the roll surface before plating. This facilitates atomic bonding as chromium atoms deposit onto the crystalline metallic surface. However, the aim when etching a work roll is to carry out the minimum amount of etching consistent with the attainment of a bond which will guarantee that the coating can withstand the rolling forces. Increasing the degree of the etch inevitably removes greater amounts of the roll surface material and gradually destroys the initial roll surface ®nish. Under such circumstances, pitting may occur leading to higher roll surface roughness
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values and lower peak counts (surface peaks attacked preferentially) [6]. In the present work, the EDT roll samples were initially submerged and cleaned in a tank ®lled with an alkaline solution, the process took approximately 10 min. Subsequently, the samples were plated in a second tank ®lled with a chromic acid solution (i.e. chromium trioxide, CrO3) containing a speci®c amount of a catalyst (sulphuric acid), this took approximately 20 min. Controlling the amount of catalyst in the chromium plating bath is crucial. A chromium acid solution will not deposit chromium metal unless the bath contains the right amount of catalyst. If there is too little or too much, no chromium will be deposited. The ratio of the chromic acid to sulphuric acid was 100:1 (the hardest, brightest and smoothest hard chromium deposits are usually obtained at ratios of 75:1 to 100:1 in an ordinary chromic acid/sulphate bath. When ratios above 100:1 are used, the chromium deposits are softer and duller). The ®rst stage of the plating cycle was a reverse (anodic) etch which acted as the ®nal roll cleaning step. The thickness of the hard chromium layers shown in Fig. 9 (a) and (b) is 20±25 mm and cracking of the plated layers is clearly evident. This is due to the fact that chromium has little elasticity and that the layers are subject to high residual stresses. The stress increases with the layer thickness until the coating ruptures as shown by the appearance of a crack-pattern and a relief of the stress. Measurement of surface residual stress using the Rigaku X-ray stress analyser in both the longitudinal and circumferential roll directions (specimens A±K) indicated highly tensile values between 350 and 700 MPa. The chromium deposit usually contains microscopic cracks, which vary in pattern, number, size and depth. The surface of the coated layer often has a network of open cracks, with plateaus between them. By controlling the plating conditions, it is possible to produce chromium deposits that are free from cracks. The chromic acid/sulphate ratio is one of the most important factors in determining crack pattern. As the ratio is lowered, within certain limits, the number of crack lines per unit length increases, resulting in a harder plated layer. Varying the crack pattern varies the hardness of the chromium plate. Guf®e [4] states that a deposit with a crack-line pattern of around 1000 lines/inch will have a hardness of approximately 950±1025 HV, whereas at 2000 lines/inch, the hardness will be about 1100 HV. When the ratio is raised above 100:1 in a chromic acid/sulphate solution, the deposit approaches a crack free state, with hardness of about 750 HV. Although this is a reasonably hard deposit, it is relatively soft for chromium. Fig. 10 shows the microhardness depth pro®les from the surface of selected hard chromium plated EDT roll specimens with different energy input values. Hardness varied in the range 1200±1300 HV (0.025). This is considerably higher than either the average measured roll white layer hardness of approximately 600 HV (0.025) or the body of the roll approximately 820 HV (0.025). The hard coating
Fig. 10. Microhardness profiles of different hard chromium plated EDT specimens: (a) Cr-plated EDT, A; (b) Cr-plated EDT, I; (C) Cr-plated EDT, K.
microhardness values shown in Fig. 10 are in line with other published work [2,6,9]. 4. Conclusions 1. Surface topography measurements (Ra and Pc) of plain EDT and hard-chromium plated EDT roll samples, in general, revealed no major differences. 2. Roll surface waviness Wca increased significantly with surface roughness Ra for both plain and chromium plated roll samples. As the roll surface roughness Ra increased, the Wca also increased, maintaining a Wca/Ra ratio of approximately 0.5.
J. SimaÄo, D.K. Aspinwall / Journal of Materials Processing Technology 92±93 (1999) 281±287
3. The thickness of the hard chromium layers was typically 20±25 mm and cracking of the plated layers was clearly evident as a result of residual stresses. 4. The microhardness of the chromium plated samples was found to be independent of the substrate roll surface roughness values and varied in the range 1200±1300 HV (0.025). This was considerably higher than either the average measured roll white layer hardness of approximately 600 HV (0.025) or the roll body approximately 820 HV (0.025). 5. Determination of residual stresses using X-ray diffraction, showed there to be high tensile stresses induced by chromium plating at the roll surface (values between 350 and 700 MPa) 6. Measurements of roll residual stress made in the axial and circumferential directions showed little difference at the coated roll surface. Acknowledgements The authors would like to thank Prof. A.A. Ball, Head of the School of Manufacturing and Mechanical Engineering, University of Birmingham and Prof. M.H. Loretto, Director of the I.R.C. in Materials for High Performance Applications, University of Birmingham for the provision of laboratory facilities. Special thanks go to Dr. M.F. El-Menshawy at Spark Tec International, Birmingham for his technical advice and to PRAXIS XXI, JNICT, Portugal, for ®nancial support. Thanks are also extended to both Mr. John Hiler, Jr. and Ms. Judith Huber at Ohio Camshaft, Akron, Ohio, USA, for their help and use of the hard chromium plating facilities. Finally we would like to express our gratitude to Lee Steel Strip, Shef®eld, for the provision of mill work rolls.
287
References [1] B. Spencer, F.A. Teska, M. Morin, Chromium deposition of cold mill process rolls, Proc. 25th MWSP Conf., Phil, PA, 26±28 Oct. 1983, pp. 461±468. [2] H. Kleinz, H.H. Urlberger, Hard-chromed work rolls for the production of cold-rolled sheet, Metallurgical Plant Tech. Int. 6 (1992) 78±82. [3] J.P. Thomas, B.W. Morgan, A review of the development of the use of electro discharge textured and hard chromium plated work rolls within British Steel Strip Products, Proc. of Rolls 2000 Conf., Institute of Materials, Birmingham, UK, 28±29 March 1996, pp. 176±185. [4] R.K. Guffie, The Handbook of Hard Chromium Plating, Gardner Publications, USA, 1986. [5] W.Z. Wujtow, G.R. Williams, Surface treatment of rolls for special applications, Chapter 14 in: R.B. Corbett (Ed.), Rolls for the Metalworking Industries, Iron and Steel Society. Inc., Warrandale, P.A, 1990. [6] D.B. Ballinger, Chromium deposition of cold mill work rolls, Proc. 24th MWSP Conf., Houston, TX, 1982, pp. 461±472. [7] T.G. Nilan, Electric discharge cold-reduction and temper-mill-roll texturing ± improved method, improved product, vol. XIX, Proc. 23rd MWSP Conf., ISS-AIME, Pittsburgh, PA, 28±29 Oct. 1981, pp. 513±520. [8] Chrome plating work roll of cold mill, Nippon Kokan Technical Report, Overseas (51) (1987) 87±89. [9] T. Kawanami, Recent Japanese trends of work roll technologies for modern cold and temper mills, vol. 2, Proc. METEC Congress 94, DuÈsseldorf, 1994, pp. 372±378. [10] T. Izushi, Application of Cr-plated roll for work roll of cold rolling mills, J. JSTP 32(363) (1991) 458±463. [11] J. SimaÄo, D.K. Aspinwall, M.L.H. Wise, M.F. El-Menshawy, Mill roll texturing using EDT, J. Mat. Proc. Tech. 45 (1994) 207± 214. [12] J. SimaÄo, D.K. Aspinwall, M.L.H. Wise, M.F. El-Menshawy, Electrical discharge texturing of cold mill work rolls using different tool electrode materials, Iron Steel Eng. 73(3) (1996) 42± 47.
Hard chromium plating of EDT mill work rolls J. SimaÄoa,*, D.K. Aspinwalla,b a School of Manufacturing and Mechanical Engineering, University of Birmingham, Birmingham B15 2TT, UK Interdisciplinary Research Centre in Materials for High Performance Applications, University of Birmingham, Birmingham, UK
b
Abstract Hard chromium plating of cold mill work rolls is common practice in order to maintain topography and increase roll service life. The paper details the effect of hard chromium plating on electrical discharge textured (EDT) roll surface topography and surface integrity. Comparative data with corresponding non-chromium plated EDT roll samples are also presented and discussed. Irrespective of the EDT roll surface topography, the coating showed a typical surface hardness in the range HV (0.025)1100±1300 against approximately 800 HV for the roll base material. The thickness of the hard chromium layers was typically 20±25 mm and cracking of the plated layers was clearly evident as a result of high residual stresses. Measurement of surface residual stress using X-ray diffraction techniques in both the longitudinal and circumferential roll directions indicated highly tensile values between 350 and 700 MPa. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Electrical discharge texturing; EDT; Roll texturing; Mill rolls; Coating; Hard-chromium plating
1. Introduction Industrial experience with hard-chromium plated mill rolls, clearly indicates that coatings applied to a particular type of roll surface texture achieved either by shot blasting (SB), electrical discharge texturing (EDT) or laser beam texturing (LBT), extend roll service life and performance. Hard chromium plated work rolls (typical plating thickness 4±15 mm) have been used for over 25 years both in tandem and temper/skin-pass rolling mills. It has been claimed that roll change times have been reduced signi®cantly when compared with similar uncoated rolls [1±3]. Electrodeposited chromium has properties similar to the solid metal, the most important are: high hardness levels (with thick deposits), low coef®cient of friction (high lubricity), good corrosion resistance, high heat resistance, low coef®cient of linear expansion and high thermal conductivity. Any differences in plating practices will inevitably cause major variations in the hardness of the electroplated material (chromium can range from 550 to 1250 HBN) [4]. There are two basic requirements which have to be met for the use of functional hard chromium coatings [2]:
*Corresponding author. Tel.: +44-121-414-3541; fax: +44-121-4143541 E-mail address: j.m.t. [email protected] (J. SimaÄo)
1. The base or substrate material must be such that it is capable of resisting externally applied forces without undergoing extensive deformation, i.e. the strength and hardness properties of the base material must be matched to some extent, to those of the hard chromium coating applied. The pairing of martensitic base materials with hard chromium coatings has proved to be a particularly successful combination. 2. The coating thickness should be restricted to a minimum where deformation or shock loading of the composite can be expected in the application concerned. The typical hardness of chromium coatings for the metalworking industries lies in the range 800±1000 HV (0.1). Coatings deposited by modern electroplating techniques may reach hardness values up to 1200 HV [2]. The achievable hardness values depend on a number of operating variables such as electrolyte bath temperature, current density and chromic acid/sulphate ratio. 2. The effect of chromium plating on the surface characteristics of the work rolls It is generally not necessary to change the type of work rolls or back up rolls in order to introduce chromium plated rolls. Similarly there is no need to change existing roll
0924-0136/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 1 2 4 - 7
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J. SimaÄo, D.K. Aspinwall / Journal of Materials Processing Technology 92±93 (1999) 281±287
grinding or texturing practices. Both bright and textured rolls will have the ``same'' ®nish characteristics after plating that they had before plating [1]. According to Wujtow and Williams [5], the electrodeposition of approximately 10 mm (0.0004 in.) of chromium to a textured work roll, has little effect on the surface topography of the roll. The peak count Pc is virtually unchanged, however, the peaks are slightly accentuated, resulting in a marginal increase in surface roughness value in the middle ranges. The most relevant and crucial effect of the hard chromium is to protect and maintain the peaks and roughness values of the work roll. The coef®cient of friction of chromium plating against steel is approximately one-half of that of steel against steel. This property is particularly advantageous under dry conditions and allows chromium plated work rolls to avoid a high proportion of damage from grit, slugs and other foreign matter that can mark the roll surface while rolling sheet metal [6]. It is also claimed that the cold-rolled product is held within tighter surface tolerances than previously obtained with plain rolls and, as long as roll damage can be avoided, the production runs can be as much as 10 times longer [5]. The use of harder rolls or, more precisely, harder roll surfaces is a key factor in extending roll life. A three to fourfold increase of roll life was obtained by deposition of a layer of hard chromium on SB textured rolls at US Steel [7]. In addition, the use of the same chromium deposition process on EDT rolls resulted in a ®ve to eight-fold increase in roll life and an increase in the EDT sheet peak count. The explanation given for this was that the chromium deposit not only increased the surface hardness of the EDT roll but also retained some ``weak'' peaks normally lost in the early stages of rolling with uncoated rolls. In 1986 the Japanese steel producer NKK installed continuous plating equipment adjacent to their roll shop. Chromium plating increased roll life by ten-fold when compared to plain EDT rolls [8]. More recently, in his review of Japanese work, Kawanami [9] reported that chromium plating on the surface of work rolls used for cold rolling greatly improved wear resistance by providing a surface hardness of 1000 HV against 800±900 HV for ordinary forged steel rolls. When chromium plated rolls were experimentally applied to tandem cold rolling [9], it was found that ®ne cracks were generated in the plated layer in the rolling direction under conditions of high load and poor lubricating conditions. Temper rolling conditions are not as severe as in the tandem mill, due to lower reduction rates (1%) and it is here that the use of chromium plated rolls has resulted in product quality improvements such as consistency of sheet surface roughness. The need to maintain the initial roll surface topography together with the extreme brittleness of the chromium, signi®cantly limits the thickness of the coating applied to mill work rolls. Fig. 1 [[10]] shows the relationship between chromium plating thickness and the
Fig. 1. Relationship between roll chromium plating thickness and length of rolled strip in temper rolling [10].
Fig. 2. Comparative performance of chromium plated and plain temper mill rolls [10].
length of temper rolled strip. It can be deduced that the optimum thickness of chromium plate ranges from 4 to 11 mm. Izushi's paper [10] also highlights the excellent wear resistance of a chromium plated roll (Fig. 2), which produced approximately seven times the length of strip achieved by a conventional temper mill roll. 3. Experimental work 3.1. Aims The aims of the present work are to investigate the effect of hard-chromium plating on EDT roll surface topography (``roughness'' and ``waviness'') and surface integrity. 3.2. Experimental equipment, materials and procedure Full details of the equipments used, including the dual voltage Spark Tec generator, the lathe and the BP 180 dielectric ¯uid/®ltration system are given in [11,12]. The DC servo quill arrangement was ®tted with a single tool electrode made from copper graphite (EC15C), supplied by Graphite Technologies. The electrode was cylindrical in shape, with a diameter of 13 mm and a length of 100 mm. Workpiece rolls were approximately 44 mm in
J. SimaÄo, D.K. Aspinwall / Journal of Materials Processing Technology 92±93 (1999) 281±287 Table 1 EDT generator test parameters EDT specimen band
Peak current [A]
On-time (ms)
Off-time (ms)
Capacitance (mF)
Electrode polarity
A B C D E F G H I J K
1 1 2 2 2 2 2 4 4 8 10
6 6 10 10 10 10 10 10 20 30 80
20 20 20 20 20 20 20 20 20 20 20
0 0 0 0 0.22 0.47 1.88 0 0 0 0
() (ÿ) () (ÿ) (ÿ) (ÿ) (ÿ) () (ÿ) (ÿ) (ÿ)
Tool electrode material: copper graphite (EC15C).
diameter and 530 mm long, made from BS 970-534A99 [(En31) (AISI E52100 alloy steel)]; 0.95±1.1% C, 1.2±1.6% Cr, 0.25±0.4% Mn, 0.04% P, 0.05% S, 0.1±0.35% Si, bal. Fe], with a measured hardness of 813 HV (63 HRC), supplied by Lee Steel Strip, Shef®eld. These were typical Sendzimir cluster mill work rolls used for the rolling of stainless steel strip and were ground prior to texturing to a surface ®nish of 0.08 mm Ra, a peak count Pc0 peaks/cm and a measured average waviness Wca0.022 mm. The work rolls were textured in 30 mm wide bands using the generator parameters shown in Table 1, in order to determine the effect of sparking conditions on both surface roughness and surface integrity. Roll rotation direction (anticlockwise viewed from the tailstock end of the workpiece), roll surfaces speed (6 m/min) and electrode axial feed rate (0.28 mm/rev), were kept constant throughout the tests, as was the servo gain (4/32). Surface roughness evaluation parameters Ra and Pc were measured using a portable Mitutoyo Surftest 201 surface pro®lometer (cut-off length of 0.8 mm, evaluation length of 4 mm and Pc level of 0.5 mm). Surface average waviness parameter Wca was measured using a Rank Taylor Hobson (RTH) Laser Form Talysurf and 3-D topographic mapping was performed using a 3-D RTH Form Talysurf-120L. Small sections for metallographical examination and 3-D mapping were ED wire cut from EDT roll surfaces using a Charmilles Technologies Robo®l 200 5 axis CNC machine. Alterations in the microstructure of the textured surface and subsurface were observed using a Leitz Wetzlar optical microscope ®tted with a JVC CCD camera, connected to a Mitsubishi DAT image recorder and colour video laser printer. SEM analysis of the samples was performed on a JEOL JSM-5410 unit. Microhardness measurements to determine the extent of the HAZ were made on a Leitz Wetzlar Miniload hardness tester using a standard load of 25 g. Residual stress measurements were performed with a Strain¯ex MSF-2M X-ray stress analyser (Rigaku, Japan). A second set of roll textured bands (A±K, Table 1) was wire cut from the work roll for hard-chromium plating. The
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coating operation was performed at Ohio Camshaft. (Roll Grinding/EDT and LBT/Chromium Plating Centre), Akron, Ohio, USA. Following on from this, both the surface topography and surface integrity of the chromium plated EDT roll specimens were assessed in the same way as the corresponding non-chromium plated EDT roll samples. Optical microscopy, SEM evaluation, microhardness and residual stress measurements, together with 2-D and 3-D topographic mapping were performed. 3.3. Results and discussion 3.3.1. Surface roughness measurements: Ra/Pc comparison of plain EDT and hard chromium plated EDT roll surfaces Data for surface roughness parameters Ra and Pc of both plain EDT and hard chromium plated EDT roll surfaces (A± K) are shown in Figs. 3 and 4. Selected 3-D topographical maps and SEM photographs of the plated textured surfaces are shown in Figs. 5 and 6.
Fig. 3. Surface roughness Ra comparison of plain EDT and Cr-plated EDT roll surfaces (±^±: plain EDT roll surface; ±}±: Cr-plated EDT roll surface).
Fig. 4. Peak count Pc comparison of plain EDT and Cr-plated EDT roll surfaces (±^±: plain EDT roll surface; ±}±: Cr-plated EDT roll surface).
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evident in the rougher roll samples, i.e. J and K, whereas the decrease in the number of peaks Pc was more noticeable in the ®ner roll samples, i.e. A, B and C (e.g. 19% in sample A and 23% in samples C). The likely reason for this is that with the ®ner EDT roll surfaces, some of the smaller peaks and valleys are covered and ``hidden'' by the chromium deposit. Under such circumstances the maximum peak-to-valley height of the chromium plated EDT sample decreased when compared to the corresponding value before the plating, i.e. from 13.29 to 12.37 mm. With the coarser EDT roll surfaces, the peaks and valleys are greater and wide enough to accommodate the thin chromium deposit, thus maintaining approximately the same Pc values while marginally increasing the average surface roughness Ra. This time a signi®cant increase in the maximum peak-to-valley value of the chromium plated EDT roll surface is evident, i.e. from 86.03 to 92.51 mm. As far as roll surface textured uniformity is concerned, the sample 3-D topographical maps shown in Fig. 5 (a) and (b) illustrate that with chromium plating, a well-de®ned surface is achievable. Successive surface roughness measurements made axially along the various roll samples and circumferentially, showed very little difference. The cracking of the plated layer (see Fig. 6) is discussed in Section 3.3.3.
Fig. 5. Sample 3-D topographical plots of the hard chromium plated EDT specimen bands: (a) A: 1.02 mm Ra; (b) K: 10.23 mm Ra.
3.3.2. Waviness measurements The average surface waviness data Wca for both the plain EDT and hard chromium plated EDT roll surfaces (A±K) are plotted in Fig. 7. The data again revealed no major differences, however, a trend was observed in that the majority of the hard chromium plated roll samples had a slightly higher Wca value. This increase was more evident in the rougher roll samples, i.e. J and K (36% and 23%, respectively). This was not a surprise since similar trends were observed when comparing the average surface roughness Ra. Fig. 8 shows the correlation between roughness and waviness for plain and chromium plated textured roll samples.
Fig. 6. Sample SEM photograph of the hard chromium plated EDT specimen band I: 3.77 mm Ra.
Comparison of the data for both surfaces, in general, revealed no major differences, other than the ability of the coating process to ``preserve'' the original workpiece surface topography. The majority of the hard chromium plated roll samples did, however, have a slightly higher average surface roughness value and also showed a slightly decrease in the peak count. This increase in the Ra value was more
Fig. 7. Average surface waviness Wca comparison of plain EDT and Crplated EDT roll surfaces (±^±: plain EDT roll surface; ±}±: Cr-plated EDT roll surface).
J. SimaÄo, D.K. Aspinwall / Journal of Materials Processing Technology 92±93 (1999) 281±287
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Fig. 8. Relationship between the average surface roughness Ra and average surface waviness Wca for the various plain EDT and Cr-plated EDT roll specimens (^: plain EDT; }: Cr-plated EDT).
The test results clearly indicate that, in both cases, roll surface waviness Wca increases signi®cantly with surface roughness Ra. With the ®ner plain EDT/coated EDT roll surfaces, both the Ra and Wca parameter values are low. As the roll surface roughness Ra increases, the Wca also increases, maintaining a Wca/Ra ratio of approximately 0.5. These results suggest that roll surface waviness is introduced during texturing as a direct consequence of efforts to make the roll rougher. The inherent characteristics of the EDT process are responsible for the random nature of the resulting texture and also for introducing a wide range of topographic wavelengths on the roll surface. These wavelengths are not con®ned in the roughness regime (<0.8 mm), but extend further into the waviness regime. During tandem and/or temper rolling, both roughness and waviness are transferred to the steel strip. The relationship between Ra and Wca provides the steel manufacturer with a relatively simple way of producing low Wca strip (if required by the customer), i.e. by aiming at a lower Ra value. 3.3.3. Surface integrity Fig. 9 details cross-sectional photomicrographs of selected hard chromium plated EDT roll surfaces (A and K). The forces acting on the chromium layer during subsequent cold rolling operations requires that the bond between the chromium and work roll be of a high standard. The adhesion of the chromium to the base metal is dependent on atomic bonding. It is claimed [6] that the work roll surface roughness is inconsequential, thus equally good adhesion can be obtained on turned, ground, highly polished, SB or EDT surface. The major requirement is that of surface cleanliness. Therefore, before the chromium is deposited, the surface to be plated must be cleaned of all contaminants such as grease, dirt and oil. This can usually be accomplished by solvent washing, alkaline cleaning or other means. As far as
Fig. 9. Photomicrographs (cross-section) of Cr-plated EDT surfaces on specimens A and K: (a) low energy operating conditions, specimen A; (b) high energy operating conditions, specimen K.
mill rolls are concerned, the presence of scale, oxides or rust is seldom a problem. The mechanical ®nishing operations of the work rolls create modi®cations in the structure of the surface layer. The high pressures and temperatures break down and/or deform the crystal structure and lubricants or deposits of fused metallic particles are created and forced into the roll surface. In order to guarantee maximum adhesion between the chromium layer and the work roll, it is generally necessary to etch the roll surface before plating. This facilitates atomic bonding as chromium atoms deposit onto the crystalline metallic surface. However, the aim when etching a work roll is to carry out the minimum amount of etching consistent with the attainment of a bond which will guarantee that the coating can withstand the rolling forces. Increasing the degree of the etch inevitably removes greater amounts of the roll surface material and gradually destroys the initial roll surface ®nish. Under such circumstances, pitting may occur leading to higher roll surface roughness
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values and lower peak counts (surface peaks attacked preferentially) [6]. In the present work, the EDT roll samples were initially submerged and cleaned in a tank ®lled with an alkaline solution, the process took approximately 10 min. Subsequently, the samples were plated in a second tank ®lled with a chromic acid solution (i.e. chromium trioxide, CrO3) containing a speci®c amount of a catalyst (sulphuric acid), this took approximately 20 min. Controlling the amount of catalyst in the chromium plating bath is crucial. A chromium acid solution will not deposit chromium metal unless the bath contains the right amount of catalyst. If there is too little or too much, no chromium will be deposited. The ratio of the chromic acid to sulphuric acid was 100:1 (the hardest, brightest and smoothest hard chromium deposits are usually obtained at ratios of 75:1 to 100:1 in an ordinary chromic acid/sulphate bath. When ratios above 100:1 are used, the chromium deposits are softer and duller). The ®rst stage of the plating cycle was a reverse (anodic) etch which acted as the ®nal roll cleaning step. The thickness of the hard chromium layers shown in Fig. 9 (a) and (b) is 20±25 mm and cracking of the plated layers is clearly evident. This is due to the fact that chromium has little elasticity and that the layers are subject to high residual stresses. The stress increases with the layer thickness until the coating ruptures as shown by the appearance of a crack-pattern and a relief of the stress. Measurement of surface residual stress using the Rigaku X-ray stress analyser in both the longitudinal and circumferential roll directions (specimens A±K) indicated highly tensile values between 350 and 700 MPa. The chromium deposit usually contains microscopic cracks, which vary in pattern, number, size and depth. The surface of the coated layer often has a network of open cracks, with plateaus between them. By controlling the plating conditions, it is possible to produce chromium deposits that are free from cracks. The chromic acid/sulphate ratio is one of the most important factors in determining crack pattern. As the ratio is lowered, within certain limits, the number of crack lines per unit length increases, resulting in a harder plated layer. Varying the crack pattern varies the hardness of the chromium plate. Guf®e [4] states that a deposit with a crack-line pattern of around 1000 lines/inch will have a hardness of approximately 950±1025 HV, whereas at 2000 lines/inch, the hardness will be about 1100 HV. When the ratio is raised above 100:1 in a chromic acid/sulphate solution, the deposit approaches a crack free state, with hardness of about 750 HV. Although this is a reasonably hard deposit, it is relatively soft for chromium. Fig. 10 shows the microhardness depth pro®les from the surface of selected hard chromium plated EDT roll specimens with different energy input values. Hardness varied in the range 1200±1300 HV (0.025). This is considerably higher than either the average measured roll white layer hardness of approximately 600 HV (0.025) or the body of the roll approximately 820 HV (0.025). The hard coating
Fig. 10. Microhardness profiles of different hard chromium plated EDT specimens: (a) Cr-plated EDT, A; (b) Cr-plated EDT, I; (C) Cr-plated EDT, K.
microhardness values shown in Fig. 10 are in line with other published work [2,6,9]. 4. Conclusions 1. Surface topography measurements (Ra and Pc) of plain EDT and hard-chromium plated EDT roll samples, in general, revealed no major differences. 2. Roll surface waviness Wca increased significantly with surface roughness Ra for both plain and chromium plated roll samples. As the roll surface roughness Ra increased, the Wca also increased, maintaining a Wca/Ra ratio of approximately 0.5.
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3. The thickness of the hard chromium layers was typically 20±25 mm and cracking of the plated layers was clearly evident as a result of residual stresses. 4. The microhardness of the chromium plated samples was found to be independent of the substrate roll surface roughness values and varied in the range 1200±1300 HV (0.025). This was considerably higher than either the average measured roll white layer hardness of approximately 600 HV (0.025) or the roll body approximately 820 HV (0.025). 5. Determination of residual stresses using X-ray diffraction, showed there to be high tensile stresses induced by chromium plating at the roll surface (values between 350 and 700 MPa) 6. Measurements of roll residual stress made in the axial and circumferential directions showed little difference at the coated roll surface. Acknowledgements The authors would like to thank Prof. A.A. Ball, Head of the School of Manufacturing and Mechanical Engineering, University of Birmingham and Prof. M.H. Loretto, Director of the I.R.C. in Materials for High Performance Applications, University of Birmingham for the provision of laboratory facilities. Special thanks go to Dr. M.F. El-Menshawy at Spark Tec International, Birmingham for his technical advice and to PRAXIS XXI, JNICT, Portugal, for ®nancial support. Thanks are also extended to both Mr. John Hiler, Jr. and Ms. Judith Huber at Ohio Camshaft, Akron, Ohio, USA, for their help and use of the hard chromium plating facilities. Finally we would like to express our gratitude to Lee Steel Strip, Shef®eld, for the provision of mill work rolls.
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References [1] B. Spencer, F.A. Teska, M. Morin, Chromium deposition of cold mill process rolls, Proc. 25th MWSP Conf., Phil, PA, 26±28 Oct. 1983, pp. 461±468. [2] H. Kleinz, H.H. Urlberger, Hard-chromed work rolls for the production of cold-rolled sheet, Metallurgical Plant Tech. Int. 6 (1992) 78±82. [3] J.P. Thomas, B.W. Morgan, A review of the development of the use of electro discharge textured and hard chromium plated work rolls within British Steel Strip Products, Proc. of Rolls 2000 Conf., Institute of Materials, Birmingham, UK, 28±29 March 1996, pp. 176±185. [4] R.K. Guffie, The Handbook of Hard Chromium Plating, Gardner Publications, USA, 1986. [5] W.Z. Wujtow, G.R. Williams, Surface treatment of rolls for special applications, Chapter 14 in: R.B. Corbett (Ed.), Rolls for the Metalworking Industries, Iron and Steel Society. Inc., Warrandale, P.A, 1990. [6] D.B. Ballinger, Chromium deposition of cold mill work rolls, Proc. 24th MWSP Conf., Houston, TX, 1982, pp. 461±472. [7] T.G. Nilan, Electric discharge cold-reduction and temper-mill-roll texturing ± improved method, improved product, vol. XIX, Proc. 23rd MWSP Conf., ISS-AIME, Pittsburgh, PA, 28±29 Oct. 1981, pp. 513±520. [8] Chrome plating work roll of cold mill, Nippon Kokan Technical Report, Overseas (51) (1987) 87±89. [9] T. Kawanami, Recent Japanese trends of work roll technologies for modern cold and temper mills, vol. 2, Proc. METEC Congress 94, DuÈsseldorf, 1994, pp. 372±378. [10] T. Izushi, Application of Cr-plated roll for work roll of cold rolling mills, J. JSTP 32(363) (1991) 458±463. [11] J. SimaÄo, D.K. Aspinwall, M.L.H. Wise, M.F. El-Menshawy, Mill roll texturing using EDT, J. Mat. Proc. Tech. 45 (1994) 207± 214. [12] J. SimaÄo, D.K. Aspinwall, M.L.H. Wise, M.F. El-Menshawy, Electrical discharge texturing of cold mill work rolls using different tool electrode materials, Iron Steel Eng. 73(3) (1996) 42± 47.