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Tribological behavior of coatings for continuous casting of steel
Surface and Coatings Technology 146 – 147 (2001) 55–64
Tribological behavior of coatings for continuous casting of steel Alejandro Sanz* Danieli & C. SpA, Centro Research and Development (CRD), Via Nazionale 41, 33042 Buttrio (Ud), Italy
Abstract There are a large number of steel making processes in which great demands are made on the surface behavior of several components that come into direct contact with steel under various conditions. Continuous casting is mainly a heat-extraction process. The mold must rapidly transfer heat from the steel to the cooling water. In continuous casting, steel solidification starts when it comes in contact with the mold liner’s interior surface. The key job of every mold consists in cooling the molten steel in a controlled way. The mold is a major element in the overall economics of a continuous casting plant which explains the number of innovative approaches to increase the working life (length of time during which the mold shows acceptable dimensional stability to meet the quality standards) or to satisfy the new demands to be met by the mold liners. Coating the mold inner is a firmly established practice, in particular with electrolytic surface modification treatments, to cope with the various operating needs including low wettability, high hardness, good wear resistance and low cost. Several pin-on-disk tests were carried out to determine the friction and wear behavior of different coatings. The friction partner for all coatings was a K30 (WC–Co 9%) chip, the sliding speed was 10 cmys at a temperature of 2508C and a load of 5 N. The sliding time was 200 h (720 000 revolutions for a radius of friction 16 mm). Additional tests for shorter times allowed verification of the morphological evolution of the wear track. All coatings were also evaluated using a scratch test. This test introduces stresses at the interface between the coating and the substrate as the sample is displaced at constant speed. The critical load (Lc ) recorded from the scratch test translates the complex intrinsic properties of a specific coating into a very reproducible figure of great practical significance. This paper presents a tribological characterization of conventional electrolytic coatings, bare copper alloys and some new surface solutions for continuous steel casting molds. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Continuous casting; Mold inner coatings; Pin-on-disk tests; Scratch tests; Electrolytic deposits; Thermal spray coatings; Chemically modified layers
1. Introduction Continuous casting is mainly a heat-extraction process. The mold must rapidly transfer heat from the steel to the cooling water. The function of the mold liner is to shape the strand and to produce a sufficiently strong, sound, low stressed shell that contains the liquid core as it continues to solidify that can withstand the pulling stresses. The conditions for creating a homogeneous shell are an efficient and uniform heat transfer. The requirement for high thermal conductivity leads to the natural choice of copper and its alloys as the base materials for mold substrates. Heat transfer along the * Tel.: q39-0432-97-1821; fax: q39-0432-97-1821. E-mail address: [email protected] (A. Sanz).
mold wall is not uniform with the maximum flux located 20–50 mm below the meniscus. More than 60% of the heat exchange by the mold occurs in the mold top-half. Achieving improved heat conduction is at least as important as reducing friction for achieving high casting speeds w1–4x. Friction phenomena occur when two moving surfaces are rubbed directly against each other. The traditional solution (only applicable if casting with a submerged nozzle) is the use of the casting fluxes (if not, casting oils are used as lubricants). Friction between the solidifying steel and the mold is basically sliding (with a small fraction of sticky friction). The frictional force between the strand and the mold can be analyzed by calculating the thermal shrinkage and the shearing strength of the meniscus. The frictional force between
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 4 7 5 - X
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Table 1 Materials and coatings under study with main relevant properties Materials (abbreviation used) Cu–Cr–Zr (Cu) Hard chromium Nickel Ytria stabilized zirconia (YSZ) YSZqchemical hardening (YSZqCH) Al2O3qCr3C2 (FGM) CoqWCqchemical slurry hardening (WC–CoqCS) CoqCr3C2 (CoqCrC) Amorphous A Amorphous B
Deposition method
Thickness (mm)
Hardness (HV)
Thermal conductivity (WymK)
Electrolytic Electrolytic Super D-gun
80 120 200
120 1000 220 960
340 67 55 3.3
Super D-gun D-gun Plasma Chemical slurry qheating (several cycles) Electrolytic
230 250
1268 580
4.5 9
100 80
1500 500
15 17.5
HVOF HVOF
120 120
1200 850
4.5 4.5
steel and the copper mold increases with increasing temperature. The mold inner coating is a firmly established practice, in particular with electrolytic surface modification treatments (mainly hard-Cr, Ni and their combination). Under the new Environmental Protection Agency (EPA) rules, hexavalent chromium compounds are increasingly subjected to maximum exposure limits leading to increasing costs. This trend will consistently continue upward, and, therefore, there is a focus on hard chrome replacement w5–7x. Several interesting options exist, but in most cases their line-of-sight characteristic does not make them feasible for tubular molds. The requirements for the highest possible standards in terms of working life, operational safety and low maintenance leave the possibility of some localized antiwear applications. The surface solutions for increasing the wear resistance are normally not compatible with the high thermal conductivity required for the mold’s primary function. However, in some areas of the mold, the heat extraction capacity is in excess of that required or the wear-prone areas are meaningless in the global heat extraction process which allow protection of some local critical areas w7–9x. 2. Experimental Nine different types of coatings were tribologically evaluated under conditions close to those found in a high-speed bloom caster at 6 mymin. The coating range covers those already in service in several melt shops as well as several innovative coatings aimed at future applications. Different deposition and hardening techniques have been explored in the present work. Tables 1 and 2 give a summary of the tribological experimental set-up covering the materials tested as well as the geometry and features of the equipment used.
Copper based alloy was used as a reference for bare molds. This same alloy was used as a substrate for all coatings with the exception of the electrolytic CoqCrC coating. Three electrolytic coatings were evaluated. The electroplated cobalt was co-deposited with a fine chromium carbide powder to enhance wear properties. This Coq CrC coating was deposited on an AISI 316 stainless steel to avoid bath poisoning with copper ions. Hard chromium is the most commonly used coating for billets and blooms (long products). Electrolytic nickel is used for slab and thin slab mould (flat products). The thermal sprayed materials included thermal barriers, amorphous coatings and functionally graded materials. Ytria stabilized zirconia is a thermal barrier that could be used to limit the heat flux peak at the meniscus level in a casting mold. The amorphous coatings produce smooth, hard surfaces with high abrasive and erosive wear resistance. The difference between the amorphous A coating and the amorphous B coating resides in a higher chromium content in the latter which leads to a very low porosity and an increased corrosion resistance. The functionally graded material is a 150-mm-thick series of layers grading an Al2O3™Cr2C3 transition. The FGM was deposited on a 50-mm-thick, Co-base bond coat. Two types of chemically hardened coatings were explored. Chemically hardened YSZ was obtained by heating to 2608C and quenching into phosphoric acid (left immersed for several hours), then slowly heated to 4808C followed by a slow cooling. The WC–CoqCS is a thermochemically formed composite coating. After applying a WC–17% Co coating, a water-based chemical slurry with chromium oxide, silicon dioxide and aluminum oxide was applied on the surface. The component
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Table 2 Tribological testing set-up: nature of the tests and sensibility of the measuring devices Wear test
Adhesion tests
Test configuration Pin chip material Load Sliding speed Mean estimated pressure (assuming perfect flat contact) Chamber temperature Radius of friction Sliding distance
Friction sensor Sensor sensitivity Sensor resolution
Pin-on-disk WC–Co 9% 5N 10 cmys 5Ny8.4 mm2 s 0.6 Nymm2 (6 bars)
Test configuration Indenter Indenter radius Indenter material
Scratch tests Rockwell 200 mm Diamond
Speed
10 mmymin
2508C 16 mm 200 h or 720 000 rev. (100%) 160 h or 576 000 rev. (80%) 100 h or 360 000 rev. (50%) 60 h or 216 000 rev. (30%) LVDT up to 10 N 0.1 mVyVymm 0.02 mm
Loading rate Scratch length Load range
100 Nymin 10 mm 0–100 N
Lc1 and Lc2 determination method Acoustic sensor
Optical microscopy Average of 3 scratches Vallen type
Penetration monitoring Fn, Dx, AE sampling rate Fn, Dx, AE channel filtering Fn: frictional force, Dx: displacement, AE: acoustic emission
LVDT sensor
was dried and heated to 5008C. By repeating this procedure several times, a totally dense, pore-free coating was produced that was chemically bonded to the substrate. The coated surfaces were very smooth (Ras 0.5 mm). Tribological tests were performed on a high temperature pin-on-disk tribometer with a chip-on-disk configuration. A pin (chip slider) is pressed with a certain load on a disk, which is in rotation under well-defined conditions. During the rotation of the pin, friction forces apply a force on the arm where the slider is fixed. Friction force is determined by direct measurement (with an inductive sensor) of the tangential force applied on the flexible arm. The friction coefficient is given by the ratio between the friction force and the normal force (applied load). The pin-on-disk tests were run for a maximum of 200 h which is considered the reference (100%) and for three shorter times (160, 100 and 60 h equivalent to 80, 50 and 30%) to clearly understand the evolution of the interfacial phenomena at the contact. Disk wear was calculated at the end of the test with a profilometer. Chip wear was measured by optical microscopy. The wear rate, t (see equation below), was determined for the disk and was calculated according to the standard DIN 50324. This calculation includes volume loss, applied load, and the total sliding distance of the test. ts
V FnØl
wm3yNmx
(1)
1000 Hz 10 Hz
where V (m3) is the worn volume, Fn (N) is the normal force applied and l (m) is the total sliding distance. Three wear measurements were performed with a Taylor–Hobson profilometer to get an average section of the track. The worn volume was determined by integration of the section. The worn volume of the disk was calculated with the following equation: Vdisks2prS
Žmm3.
(2)
where S (mm2) is the worn area calculated with the program ‘sillon’ of the Talysurf and r (mm) is the radius of the wear track w10–13x. Scratch testing consists of introducing stresses at the interface between the coating and the substrate. This is achieved by pressing a diamond stylus on the sample surface with a normal load FN. As the sample is displaced at constant speed, the resulting stresses at the interface cause flaking or chipping of the coating. The smallest load at which a specific failure event is recorded is called the critical load (Lc). Lc translates the complex intrinsic properties of a specific coating system into a very reproducible figure of great practical significance. The scratch tester provides cross-referenced data on Lc by simultaneously recording three different effects: (a) tangential force variations; (b) acoustic emission fluctuations; and (c) microscopic deformations w14–17x. A series of six scratches were performed on the samples before and after annealing at 2508C for 200 h. Three scratches were made perpendicular (H) and the other three scratches parallel (≤) to the grinding direction
A. Sanz / Surface and Coatings Technology 146 – 147 (2001) 55–64
58 Table 3 Pin-on-disk wear test results Material
Test duration (h)
Disk wear rate (m2yN)
Chip wear wworn surface (mm2) yworn thickness (mm)x
mmin
mmean
mmax
Cu Hard Cr
200 h 200 h 160 h 100 h 60 h Few
6.14 1.20 9.50 2.10 1.70
E-14 E-13 E-14 E-13 E-13
8.41 (max)y0.60 10.8y0.65 6.8y0.35 9.6y0.58 4.72y0.22
0.61 0.60 0.60 0.53 0.53
0.80 0.82 0.69 0.68 0.65 )4
0.99 0.99 0.79 0.83 0.77
200 h 160 h 100 h 60 h 200 h 160 h 100 h 60 h 200 h 160 h 100 h 60 h
2.30 2.13 6.51 6.40 1.14 3.10 2.20 1.50 5.63 2.29 1.36 3.79
E-14 E-14 E-14 E-14 E-14 E-14 E-14 E-14 E-16 E-15 E-15 E-15
0.47 0.36 0.44 0.72 0.20 0.46 0.50 0.46 0.34 0.56 0.48 0.46
0.70 0.73 0.73 0.88 0.77 0.80 0.80 0.60 0.80 0.75 0.65 0.75
1.04 0.98 0.95 1.04 1.1 1.02 1.00 0.80 1.04 0.94 0.85 0.82
200 h 160 h 277.7 200 h 160 h 100 h 60 h 200 h 160 h 100 h 60 h 200 h 160 h 100 h 60 h
Not measurable by profilometry
3y0.34 2.8y0.38 5.8y0.33 2.5y0.13 5.2y0.2 6y0.27 4.9y0.17 3y0.1 4y0.11 3.9y0.135 2.8y0.1 4.1y0.16 (scratches) Not measurable by optical microscopy 7.4y0.51 4.9y0.49 4.5y0.34 4.0y0.27 Not measurable by optical microscopy
0.35 0.36 0.42 0.30 0.20 0.30 0.34 0.42 0.40 0.28 0.42 0.40 0.44 0.36 0.36
0.45 0.39 0.47 0.60 0.50 0.70 0.70 0.52 0.51 0.50 0.52 0.65 0.65 0.40 0.41
0.49 0.43 0.54 0.84 0.81 0.90 0.89 0.60 0.65 0.66 0.63 0.81 0.79 0.56 0.58
Ni YSZ
YSZqCH
FGM
WC–Coq CS
CoqCrC
Amorphous A
Amorphous B
3.89 2.53 3.00 3.98 5.33 5.21 5.00 5.93 2.89 3.11 1.60 1.62
E-14 E-14 E-14 E-14 E-14 E-14 E-14 E-14 E-13 E-13 E-13 E-13
of the substrates. The critical loads Lc1 (H) and Lc2 (≤) are determined by optical observations which are more reliable than acoustic emission. 3. Results and discussion Tables 3 and 4 present a general summary of the results of the tribological evaluation of the different coatings. Fig. 1 shows the microstructure cross-section of an Al2O3™Cr3C2 functionally graded material, which is considered as a potential candidate for future mold surfaces. From the data of the copper alloy disk wear rate as a function of test duration, it can be noted that chip wear is very important generating significant quantities of debris particles. A thick black oxide layer, probably CuO, was formed on the disk. It seems that the high solubility between Co (from WC) and Cu plays an important role of wear and increases the wear of the two partners.
1.6y0.065 1.3y0.055 Not measurable Not measurable
The hard-chromium wear rate was similar for 200 and 160 h, and higher for 100 and 60 h. This behavior was certainly due to a high wear rate at the beginning of the test because of the high pressure of the chip on the coating (small contact area). Optical and microscopic observations showed the cracked nature of the Crlayer (mostly after the 200-h at 2508C annealing). There was a preferential wear of the chip in the sliding direction (Fig. 2). The chips exhibited wear that increased with the test duration except for the test at 100 h that exhibited very high wear. There was a significant difference in the friction behavior between the test at 200 h and the three other tests. The mean estimated pressures (at the end of the test) were 4.6 bar for the 200-h test, 7.3 bars for the 100-h, 5.2 bar for the test at 50% and 10 bar for the shortest test. The scratch test samples showed some little cracks in the coating outside the track without any important delamination and several transverse cracks through the track. The scratch tests confirmed the brittle nature of the
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Table 4 Scratch test results Material Cu Hard Cr Ni YSZ YSZqCH FGM WC–CoqCS CoqCrC Amorphous A Amorphous B
Sample
Lc1: 1st small cohesive failure (H)
Lc2:1st big cohesive failure (≤)
Reference Annealed Reference Annealed Reference Annealed Reference Annealed Reference Annealed Reference Annealed Reference Annealed Reference Annealed Reference Annealed
60 N 22 N 11 N 6N 48 N 48 N 66 N 47 N 43.5 N 42 N 68 N 64 N Not measurable Not measurable 35 N 35 N 63 N 48 N
50 N 25 N 13 N 16 N 64 N 72 N )100 N 57 N 48.5 N 53 N Not measurable Not measurable Not measurable Not measurable 23 N 35 N 60 N 33 N
coating. There were no significant differences between the two tests (before and after 2508C) and between the two grinding directions. The scratch behavior of the coating before and after 200 h at 2508C does not seem to be very different even though the critical load was higher before the heating procedure. The cracks were very small, comparable between the two tests, and difficult to quantify with accuracy. The other often-used coating in steel casting molds is electrolytic nickel. Three tests were performed and stopped after a few revolutions because of very bad wear and friction behavior. Deep scratches were
observed on the coating during the initial test cycle. The friction force increased very significantly and saturated the electrical signal (friction coefficient )4). Under these conditions, it was impossible to continue the tests for the expected duration. Therefore the test series has been stopped. The scratch tests showed important deformation of the coating due to the applied load. The coating seems to be very soft with no cracks observed. It was difficult to determine exactly the critical load of the coating because no important failures were observed, just a large plastic deformation of the coating. Nevertheless, there were no significant differences in failures,
Fig. 1. Metallographic cross-section of the FGM coating.
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Fig. 2. Pin-on-disk wear tracks on a hard-chromium coating after 720 000 revolutions (200 h). Temperature 2508C, speed: 10 cmys, load: 5 N. (a) Wear of the chip; (b) wear track of the coating; and (c) detail of the wear track.
on one hand between the two grinding directions, and on the other hand before heating and after annealing (200 h) of the coating at 2508C. The influence of the annealing was relatively weak. The wear and friction behavior of the electrolytic Ni coating and hard Cr coating were very different. The first is very soft and is plastically deformed with the pin-on-disk tests, whereas the second is brittle and exhibit numerous cracks at high temperature and under high stresses. The hardness of the electroplated Cr falls steadily at the mold working temperatures leading to hardness values of approximately 750 HV after 1 h at 3008C. The average thickness of a hard chromium coating at the mold level ranges from 35 to 120 mm. In some cases, hard chromium plating is preceded by a nickel plating. The hard chromium provides a wear and chemical resistant layer, and, nickel provides a layer with a high strain-to-fracture feature and with a thermal expansion coefficient similar to that of copper. The latter aspect allows the deposition of relatively thick Ni-layers (1–5 mm) with different thickness profiles. In some cases Ni is used alone (without any external Cr layer) in thick layers, but it normally shows some chemical reactivity with the mold fluxes. It is more common to see Ni as an intermediate layer or applied in the final 2y3 of the mold length so as to prevent direct contact with molten slag. Thermal barriers YSZ and YSZqCH are similar to those chemically hardened at a later stage. For the YSZ there is a remarkable difference between the two longterm tests and the two short-term ones. This is probably
due to the fact that the pressure is more important at the beginning of the test (i.e. the contact area is smaller), and the asperities of the coating are sharper, so the abrasion effect and scratches are more important. There was no significant difference in friction, except the test at 30% of the total time where the friction is slightly higher than for the other tests (ms0.88). Mean estimated pressure: 5 Ny3.5 mm2s1.43 Nymm2 (14.3 bar). The scratch behavior of the YSZ coating before and after the test at 2508C does not seem to be very different. The difference in wear rate of the YSZqCH coating between the beginning and the end of the test was less important than for YSZ coating. Moreover, the wear rate was a factor 2–3 lower for the YSZqCH coating. This better behavior is probably due to the chemical hardening and the associated densification of the coating. The friction coefficient of the YSZqCH system became less and less stable when increasing the test duration. The YSZqCH scratch resistance decreases after the thermal treatment. Optical micrographs showed that delamination appears in the track at a lower load for the test after 2508C. The wear rate of the functionally graded material (FGM) was more important during the first part of the test (60 h) and decreased during the test duration until 200 h. Concerning optical microscopy, the wear mechanism seemed to be the same all along the test. The track was worn progressively without large scratches and without thick oxide layers. There was just a difference for 200 h where a small oxide layer in the middle of the track was observed. Important friction at the
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Fig. 3. Pin-on-disk wear tracks on a WC–CoqCS coating after 1 000 000 revolutions (277.7 h). Temperature 2508C, speed: 10 cmys, load: 5 N. (a) Wear of the chip; and (b) wear track of the coating.
beginning of the test was responsible for the wear appearance at first in the sliding direction on the edge of the chip. The contact area remained relatively constant if compared to different tests (except for the 200-h test). The wear behavior seems to be the same for all chips except for the 200-h test (scratches are observed). There was no significant difference in friction behavior for the different tests, except in the first part of the curve of the 200-h test where the friction coefficient was a little higher than for the other tests. Pressure was relatively constant during the tests. Contact area was the same for all tests (from 30 to 100% of the total duration), and the average contact area was 3.7 mm2. The estimated pressure was 5 Ny3.7 mm2s1.35 Nymm2 (13.5 bar). The wear behavior of the WC–CoqCS coating was very good because no wear tracks were visible with optical microscopy. There was only a little polishing effect of the surface, which was perceptible to the eyes. Wear of the chip could not be measured. Nevertheless, some scratches were visible in the sliding direction of the test (Fig. 3). The wear behavior was so good, that the general arrangement of the test for this material was shifted to 1 000 000 revolutions under the pin-on-disk test (equivalent to 277.7 continuous test hours). There was no significant difference in the friction behavior if compared with the other three tests. The friction was very stable and relatively low. Mean estimated pressure was 5 Ny9 mm2s0.55 Nymm2 (5.6 bar). The scratch behavior of the coating before and after the test at 2508C did not seem to be very different. Optical microscopy analysis of the scratch tests showed that the failure is
not measurable outside the track. Some small cohesive cracks were found. Cracks in the track were measurable. The critical loads LC1 and LC2 were determined by optical observations. The CoqCrC coating contain 15–25% chromium carbide with an average size of 2–5 mm. At the working temperature, the carbides resist wear by supporting loads (and in case of glaze formation, they help to anchor it). Some carbide changes at high temperature (Cr3C2™ M7C3) allow doubling the carbide fraction volume and therefore hardness is increased up to 650 HV. The test for pin-on-disk test for 200 h showed a wear behavior which was worse than for the other tests. This behavior could have resulted from a change in the wear mechanism, which induced a higher wear. Indeed, the friction curve showed a behavior totally different after 300 000 revolutions. Friction was more stable, but higher (m;0.6) than at the beginning of the test. There was no significant difference in the friction behavior among the different tests. The first part of the curve shows low friction, followed by an unstable part where the friction increases and decreases quickly, then followed by a more stable but higher friction. The mean estimated pressure was 5 Ny5.2 mm2 s0.96 Nymm2 (9.6 bar). A series of three scratches were performed on the substrate before and after heating at 2508C for 200 h. Only plastic deformation of the coating without any delaminations or important cracks was observed (Fig. 4). The general form of the microstructure of the two analyzed amorphous deposits is a two-phase solid with dendritic chromium borides in a matrix of iron–chro-
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Fig. 4. Scratch test on a CoqCrC coating.
mium or nickel–chromium (depending on the type of coating) which was predominantly amorphous (40– 50%). Fully amorphous structures are moderately stable (crystallizing under service conditions and not providing significant wear improvements). There was no significant difference in the friction behavior of the different tests for the amorphous A coating. The curve was very stable and the mean friction coefficient was approximately 0.5. The wear of the chip was not measurable ( just a few scratches). Some small cracks in the coating outside the track without any important delaminations and a high density of transverse cracks through the track were observed. The coating seems to be very brittle. There was no significant difference between the two tests of the metallic amorphous A coating, before and after 2508C, and between the two grinding directions. The scratch behavior of the coating before and after the test at 2508C seemed quite similar. The wear behavior of amorphous coating B is different from what was obtained for the other coating. The friction curves for the two longest tests showed several periods of high and stable friction followed by short periods of low friction. This high friction would explain the high wear rate, whereas friction was more stable and lower for the two other tests. There was a preferential wear of the chip in the sliding direction for the two longest tests and not measurable wear for the tests at 60 and 100 h. This was due to the fact that the wear mechanism was different along the tests. The mean estimated pressure for the longest test (200 and 160 h) was: 5 Ny1.5 mm2s3.3 Nymm2 (33 bars). The mean estimated pressure for the shorter tests (100 and 60 h) is: 5 Ny 8.41 mm2s0.6 Nymm2 (6 bars). The scratch test track of metallic glass B (Fig. 5) showed some small cracks in the coating outside the track without any important delaminations and a lot of transverse cracks through the
track. The wear behavior of amorphous coating A was much better than amorphous B coating, and the friction behavior was more stable. The information obtained with scratch tests showed that type A was more brittle than type B (radial cracks through the coating were numerous). 4. Conclusions Chromium plating is widely used as a confirmed solution to cope with the various operating needs including low wettability, high hardness, good wear resistance and low cost. The as-deposited Cr-hard plating has a hardness of approximately 1000 HV. The deposit is very brittle and the high degree of internal tensile residual stresses causes it to be microcracked spontaneously. The requirements for the highest possible standards in terms of working life, operational safety and low maintenance leave the possibility for some, localized, antiwear applications. The surface solutions for increasing wear resistance are normally not compatible with the high thermal conductivity required for the mold’s primary function. In some areas of the mold, the heat extraction capacity is in excess to what is desired or wear-prone areas are meaningless in the global heat extraction. Among these areas that can be improved, could be cited the lateral plates of slab and thin slab casters (also called end-plates), the bottom end of the mold (where the air-gap is already formed and heat transfer is limited to radiation) and guiding devices (grip plates or foot-rollers) at the exit of the mold. The reduction in porosity plus the thermal treatment associated with the chemical slurry treatment leads to fully dense coating chemically bonded to the substrate. Chemically hardened metal based coatings additionally show friction coefficients lower than those of electroplated
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Fig. 5. Scratch test on an amorphous coating (amorphous B). (a) Scratch perpendicular to the roughness direction; and (b) scratch parallel to the roughness direction.
hard chromium. The WC–CoqWS materials have now been introduced in the above mentioned areas of the mold with an excess of heat extraction capacity and under severe wear conditions. Ceramic based chemically hardened coatings show better wear behavior than conventional hard-chromium layers with similar friction coefficients. Materials like the YSZqCH have a simpler processing schedule (single chemical slurryqannealing at the end) than the WC–CoqWS materials but cannot match the wear resistance of the latter. The high thermal resistance of both the YSZ and YSZqCH does not allow their use on the whole mold surface but just some selected areas. Some new electrodeposited Co alloys are harder than Ni deposits and are becoming increasingly interesting for casting purposes. Co rich plating leads to the formation of wear resistant surfaces able to promote the formation of adherent cobalt oxide in sufficient quantity to produce a glaze and to prevent metal-to-metal contact. Electrochemical solutions for depositing CoqCrC coatings are very sensitive to bath contamination by metals (Cu in particular). This family of coatings could be used on top of a nickel bond coat with proper masking of the areas not to be coated (a similar procedure to the one followed with the NiqCr electrolytic coatings in continuous casting molds). The amorphous coatings family is limited to application where the working temperature of the surface is kept below 6008C (in order to avoid crystallization). The electroplated amorphous coating candidate themselves as hard-Cr plating alternatives, but not for high-
speed caster where maximum peak temperature could be harmful for the coating. The working life of a mold depends on several different variables (steel grade, quality standards, mold fluxes, casting parameters, etc.), but coatings can give a significant impulse toward high wear resistance, chemical inertness and higher productivity through increased machine availability and through strand retention of nominal dimensions. Vitae Alejandro Sanz: Materials Science Engineering Degree at the Simon Bolivar University. Master Science ´ (DEA) at the Ecole Nationale Superieure de Chimie (Institut National Politechnique). Ph.D. at Ecole Nation´ ´ al Superieure de l’Aeronautique et de l’Espace. Current position executive manager of materials development for the Danieli & C group. References w1x A. Sanz, Revue de Metallurgie-CIT, ´ March (2000) 353. w2x D.A Breslin, A. Hetherrington, P.N. Walker, MPT Int. March (1999) 68. w3x K.C. Mills, A review of the ECSC-under research on mould fluxes, Commission of the European Communities: Technical Steel Research, EUR13177 1991. w4x S. Chandra, I.V. Samarasekera, J.K. Brimacombe, A. Bakshi, B.N. Walker, Ironmaking Steelmaking 23 (6) (1996) 512. w5x Y.M. Won, T.-J. Yeo, K.H. Oh, J.-K Park, J. Choi, C.H. Yim, ISIJ Int. 38 (1) (1998) 53. w6x G. Xu, J. Cui, X. Na, J. Iron Steel Res. Int. 5 (1) (1998) 13. w7x K.J. Sorimachi, S. Nabeshima, ISIJ Int. 84 (2) (1998) 103.
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w8x M. Yao, D.C. Fang., Ironmaking Steelmaking 23 (6) (1996) 522. w9x H. Tani, S. Tatsuguchi, T. Tsuzawa, T. Ushio, United States Patent no. 4.688.320, August, 1984. w10x J. von Stebut, Surf. Coat. Technol. 68y69 (1994) 591. w11x J.P. Celis, Surf. Coat. Technol. 74y75 (1995) 15. ´ w12x E.A. Steinmann, Y. Tardy, H.E. Hintermann, Thin Solid Films 154 (1987) 333.
w13x P.J. Burnett, D.S. Rickerby, Thin Solid Films 157 (1988) 233. w14x S.J. Bull, D.S. Rickerby, Thin Solid Films 181 (1989) 545. w15x J. Sekler, P.A. Steinmann, H.S. Hintermann, Surf. Coat. Technol. 36 (1988) 519. w16x D. Muller, E. Fromm, Thin Solid Films 270 (1995) 411. w17x S. Bennett, A. Matthews, Surf. Coat. Technol. 74y75 (1995) 869.
Tribological behavior of coatings for continuous casting of steel Alejandro Sanz* Danieli & C. SpA, Centro Research and Development (CRD), Via Nazionale 41, 33042 Buttrio (Ud), Italy
Abstract There are a large number of steel making processes in which great demands are made on the surface behavior of several components that come into direct contact with steel under various conditions. Continuous casting is mainly a heat-extraction process. The mold must rapidly transfer heat from the steel to the cooling water. In continuous casting, steel solidification starts when it comes in contact with the mold liner’s interior surface. The key job of every mold consists in cooling the molten steel in a controlled way. The mold is a major element in the overall economics of a continuous casting plant which explains the number of innovative approaches to increase the working life (length of time during which the mold shows acceptable dimensional stability to meet the quality standards) or to satisfy the new demands to be met by the mold liners. Coating the mold inner is a firmly established practice, in particular with electrolytic surface modification treatments, to cope with the various operating needs including low wettability, high hardness, good wear resistance and low cost. Several pin-on-disk tests were carried out to determine the friction and wear behavior of different coatings. The friction partner for all coatings was a K30 (WC–Co 9%) chip, the sliding speed was 10 cmys at a temperature of 2508C and a load of 5 N. The sliding time was 200 h (720 000 revolutions for a radius of friction 16 mm). Additional tests for shorter times allowed verification of the morphological evolution of the wear track. All coatings were also evaluated using a scratch test. This test introduces stresses at the interface between the coating and the substrate as the sample is displaced at constant speed. The critical load (Lc ) recorded from the scratch test translates the complex intrinsic properties of a specific coating into a very reproducible figure of great practical significance. This paper presents a tribological characterization of conventional electrolytic coatings, bare copper alloys and some new surface solutions for continuous steel casting molds. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Continuous casting; Mold inner coatings; Pin-on-disk tests; Scratch tests; Electrolytic deposits; Thermal spray coatings; Chemically modified layers
1. Introduction Continuous casting is mainly a heat-extraction process. The mold must rapidly transfer heat from the steel to the cooling water. The function of the mold liner is to shape the strand and to produce a sufficiently strong, sound, low stressed shell that contains the liquid core as it continues to solidify that can withstand the pulling stresses. The conditions for creating a homogeneous shell are an efficient and uniform heat transfer. The requirement for high thermal conductivity leads to the natural choice of copper and its alloys as the base materials for mold substrates. Heat transfer along the * Tel.: q39-0432-97-1821; fax: q39-0432-97-1821. E-mail address: [email protected] (A. Sanz).
mold wall is not uniform with the maximum flux located 20–50 mm below the meniscus. More than 60% of the heat exchange by the mold occurs in the mold top-half. Achieving improved heat conduction is at least as important as reducing friction for achieving high casting speeds w1–4x. Friction phenomena occur when two moving surfaces are rubbed directly against each other. The traditional solution (only applicable if casting with a submerged nozzle) is the use of the casting fluxes (if not, casting oils are used as lubricants). Friction between the solidifying steel and the mold is basically sliding (with a small fraction of sticky friction). The frictional force between the strand and the mold can be analyzed by calculating the thermal shrinkage and the shearing strength of the meniscus. The frictional force between
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 4 7 5 - X
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Table 1 Materials and coatings under study with main relevant properties Materials (abbreviation used) Cu–Cr–Zr (Cu) Hard chromium Nickel Ytria stabilized zirconia (YSZ) YSZqchemical hardening (YSZqCH) Al2O3qCr3C2 (FGM) CoqWCqchemical slurry hardening (WC–CoqCS) CoqCr3C2 (CoqCrC) Amorphous A Amorphous B
Deposition method
Thickness (mm)
Hardness (HV)
Thermal conductivity (WymK)
Electrolytic Electrolytic Super D-gun
80 120 200
120 1000 220 960
340 67 55 3.3
Super D-gun D-gun Plasma Chemical slurry qheating (several cycles) Electrolytic
230 250
1268 580
4.5 9
100 80
1500 500
15 17.5
HVOF HVOF
120 120
1200 850
4.5 4.5
steel and the copper mold increases with increasing temperature. The mold inner coating is a firmly established practice, in particular with electrolytic surface modification treatments (mainly hard-Cr, Ni and their combination). Under the new Environmental Protection Agency (EPA) rules, hexavalent chromium compounds are increasingly subjected to maximum exposure limits leading to increasing costs. This trend will consistently continue upward, and, therefore, there is a focus on hard chrome replacement w5–7x. Several interesting options exist, but in most cases their line-of-sight characteristic does not make them feasible for tubular molds. The requirements for the highest possible standards in terms of working life, operational safety and low maintenance leave the possibility of some localized antiwear applications. The surface solutions for increasing the wear resistance are normally not compatible with the high thermal conductivity required for the mold’s primary function. However, in some areas of the mold, the heat extraction capacity is in excess of that required or the wear-prone areas are meaningless in the global heat extraction process which allow protection of some local critical areas w7–9x. 2. Experimental Nine different types of coatings were tribologically evaluated under conditions close to those found in a high-speed bloom caster at 6 mymin. The coating range covers those already in service in several melt shops as well as several innovative coatings aimed at future applications. Different deposition and hardening techniques have been explored in the present work. Tables 1 and 2 give a summary of the tribological experimental set-up covering the materials tested as well as the geometry and features of the equipment used.
Copper based alloy was used as a reference for bare molds. This same alloy was used as a substrate for all coatings with the exception of the electrolytic CoqCrC coating. Three electrolytic coatings were evaluated. The electroplated cobalt was co-deposited with a fine chromium carbide powder to enhance wear properties. This Coq CrC coating was deposited on an AISI 316 stainless steel to avoid bath poisoning with copper ions. Hard chromium is the most commonly used coating for billets and blooms (long products). Electrolytic nickel is used for slab and thin slab mould (flat products). The thermal sprayed materials included thermal barriers, amorphous coatings and functionally graded materials. Ytria stabilized zirconia is a thermal barrier that could be used to limit the heat flux peak at the meniscus level in a casting mold. The amorphous coatings produce smooth, hard surfaces with high abrasive and erosive wear resistance. The difference between the amorphous A coating and the amorphous B coating resides in a higher chromium content in the latter which leads to a very low porosity and an increased corrosion resistance. The functionally graded material is a 150-mm-thick series of layers grading an Al2O3™Cr2C3 transition. The FGM was deposited on a 50-mm-thick, Co-base bond coat. Two types of chemically hardened coatings were explored. Chemically hardened YSZ was obtained by heating to 2608C and quenching into phosphoric acid (left immersed for several hours), then slowly heated to 4808C followed by a slow cooling. The WC–CoqCS is a thermochemically formed composite coating. After applying a WC–17% Co coating, a water-based chemical slurry with chromium oxide, silicon dioxide and aluminum oxide was applied on the surface. The component
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Table 2 Tribological testing set-up: nature of the tests and sensibility of the measuring devices Wear test
Adhesion tests
Test configuration Pin chip material Load Sliding speed Mean estimated pressure (assuming perfect flat contact) Chamber temperature Radius of friction Sliding distance
Friction sensor Sensor sensitivity Sensor resolution
Pin-on-disk WC–Co 9% 5N 10 cmys 5Ny8.4 mm2 s 0.6 Nymm2 (6 bars)
Test configuration Indenter Indenter radius Indenter material
Scratch tests Rockwell 200 mm Diamond
Speed
10 mmymin
2508C 16 mm 200 h or 720 000 rev. (100%) 160 h or 576 000 rev. (80%) 100 h or 360 000 rev. (50%) 60 h or 216 000 rev. (30%) LVDT up to 10 N 0.1 mVyVymm 0.02 mm
Loading rate Scratch length Load range
100 Nymin 10 mm 0–100 N
Lc1 and Lc2 determination method Acoustic sensor
Optical microscopy Average of 3 scratches Vallen type
Penetration monitoring Fn, Dx, AE sampling rate Fn, Dx, AE channel filtering Fn: frictional force, Dx: displacement, AE: acoustic emission
LVDT sensor
was dried and heated to 5008C. By repeating this procedure several times, a totally dense, pore-free coating was produced that was chemically bonded to the substrate. The coated surfaces were very smooth (Ras 0.5 mm). Tribological tests were performed on a high temperature pin-on-disk tribometer with a chip-on-disk configuration. A pin (chip slider) is pressed with a certain load on a disk, which is in rotation under well-defined conditions. During the rotation of the pin, friction forces apply a force on the arm where the slider is fixed. Friction force is determined by direct measurement (with an inductive sensor) of the tangential force applied on the flexible arm. The friction coefficient is given by the ratio between the friction force and the normal force (applied load). The pin-on-disk tests were run for a maximum of 200 h which is considered the reference (100%) and for three shorter times (160, 100 and 60 h equivalent to 80, 50 and 30%) to clearly understand the evolution of the interfacial phenomena at the contact. Disk wear was calculated at the end of the test with a profilometer. Chip wear was measured by optical microscopy. The wear rate, t (see equation below), was determined for the disk and was calculated according to the standard DIN 50324. This calculation includes volume loss, applied load, and the total sliding distance of the test. ts
V FnØl
wm3yNmx
(1)
1000 Hz 10 Hz
where V (m3) is the worn volume, Fn (N) is the normal force applied and l (m) is the total sliding distance. Three wear measurements were performed with a Taylor–Hobson profilometer to get an average section of the track. The worn volume was determined by integration of the section. The worn volume of the disk was calculated with the following equation: Vdisks2prS
Žmm3.
(2)
where S (mm2) is the worn area calculated with the program ‘sillon’ of the Talysurf and r (mm) is the radius of the wear track w10–13x. Scratch testing consists of introducing stresses at the interface between the coating and the substrate. This is achieved by pressing a diamond stylus on the sample surface with a normal load FN. As the sample is displaced at constant speed, the resulting stresses at the interface cause flaking or chipping of the coating. The smallest load at which a specific failure event is recorded is called the critical load (Lc). Lc translates the complex intrinsic properties of a specific coating system into a very reproducible figure of great practical significance. The scratch tester provides cross-referenced data on Lc by simultaneously recording three different effects: (a) tangential force variations; (b) acoustic emission fluctuations; and (c) microscopic deformations w14–17x. A series of six scratches were performed on the samples before and after annealing at 2508C for 200 h. Three scratches were made perpendicular (H) and the other three scratches parallel (≤) to the grinding direction
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58 Table 3 Pin-on-disk wear test results Material
Test duration (h)
Disk wear rate (m2yN)
Chip wear wworn surface (mm2) yworn thickness (mm)x
mmin
mmean
mmax
Cu Hard Cr
200 h 200 h 160 h 100 h 60 h Few
6.14 1.20 9.50 2.10 1.70
E-14 E-13 E-14 E-13 E-13
8.41 (max)y0.60 10.8y0.65 6.8y0.35 9.6y0.58 4.72y0.22
0.61 0.60 0.60 0.53 0.53
0.80 0.82 0.69 0.68 0.65 )4
0.99 0.99 0.79 0.83 0.77
200 h 160 h 100 h 60 h 200 h 160 h 100 h 60 h 200 h 160 h 100 h 60 h
2.30 2.13 6.51 6.40 1.14 3.10 2.20 1.50 5.63 2.29 1.36 3.79
E-14 E-14 E-14 E-14 E-14 E-14 E-14 E-14 E-16 E-15 E-15 E-15
0.47 0.36 0.44 0.72 0.20 0.46 0.50 0.46 0.34 0.56 0.48 0.46
0.70 0.73 0.73 0.88 0.77 0.80 0.80 0.60 0.80 0.75 0.65 0.75
1.04 0.98 0.95 1.04 1.1 1.02 1.00 0.80 1.04 0.94 0.85 0.82
200 h 160 h 277.7 200 h 160 h 100 h 60 h 200 h 160 h 100 h 60 h 200 h 160 h 100 h 60 h
Not measurable by profilometry
3y0.34 2.8y0.38 5.8y0.33 2.5y0.13 5.2y0.2 6y0.27 4.9y0.17 3y0.1 4y0.11 3.9y0.135 2.8y0.1 4.1y0.16 (scratches) Not measurable by optical microscopy 7.4y0.51 4.9y0.49 4.5y0.34 4.0y0.27 Not measurable by optical microscopy
0.35 0.36 0.42 0.30 0.20 0.30 0.34 0.42 0.40 0.28 0.42 0.40 0.44 0.36 0.36
0.45 0.39 0.47 0.60 0.50 0.70 0.70 0.52 0.51 0.50 0.52 0.65 0.65 0.40 0.41
0.49 0.43 0.54 0.84 0.81 0.90 0.89 0.60 0.65 0.66 0.63 0.81 0.79 0.56 0.58
Ni YSZ
YSZqCH
FGM
WC–Coq CS
CoqCrC
Amorphous A
Amorphous B
3.89 2.53 3.00 3.98 5.33 5.21 5.00 5.93 2.89 3.11 1.60 1.62
E-14 E-14 E-14 E-14 E-14 E-14 E-14 E-14 E-13 E-13 E-13 E-13
of the substrates. The critical loads Lc1 (H) and Lc2 (≤) are determined by optical observations which are more reliable than acoustic emission. 3. Results and discussion Tables 3 and 4 present a general summary of the results of the tribological evaluation of the different coatings. Fig. 1 shows the microstructure cross-section of an Al2O3™Cr3C2 functionally graded material, which is considered as a potential candidate for future mold surfaces. From the data of the copper alloy disk wear rate as a function of test duration, it can be noted that chip wear is very important generating significant quantities of debris particles. A thick black oxide layer, probably CuO, was formed on the disk. It seems that the high solubility between Co (from WC) and Cu plays an important role of wear and increases the wear of the two partners.
1.6y0.065 1.3y0.055 Not measurable Not measurable
The hard-chromium wear rate was similar for 200 and 160 h, and higher for 100 and 60 h. This behavior was certainly due to a high wear rate at the beginning of the test because of the high pressure of the chip on the coating (small contact area). Optical and microscopic observations showed the cracked nature of the Crlayer (mostly after the 200-h at 2508C annealing). There was a preferential wear of the chip in the sliding direction (Fig. 2). The chips exhibited wear that increased with the test duration except for the test at 100 h that exhibited very high wear. There was a significant difference in the friction behavior between the test at 200 h and the three other tests. The mean estimated pressures (at the end of the test) were 4.6 bar for the 200-h test, 7.3 bars for the 100-h, 5.2 bar for the test at 50% and 10 bar for the shortest test. The scratch test samples showed some little cracks in the coating outside the track without any important delamination and several transverse cracks through the track. The scratch tests confirmed the brittle nature of the
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Table 4 Scratch test results Material Cu Hard Cr Ni YSZ YSZqCH FGM WC–CoqCS CoqCrC Amorphous A Amorphous B
Sample
Lc1: 1st small cohesive failure (H)
Lc2:1st big cohesive failure (≤)
Reference Annealed Reference Annealed Reference Annealed Reference Annealed Reference Annealed Reference Annealed Reference Annealed Reference Annealed Reference Annealed
60 N 22 N 11 N 6N 48 N 48 N 66 N 47 N 43.5 N 42 N 68 N 64 N Not measurable Not measurable 35 N 35 N 63 N 48 N
50 N 25 N 13 N 16 N 64 N 72 N )100 N 57 N 48.5 N 53 N Not measurable Not measurable Not measurable Not measurable 23 N 35 N 60 N 33 N
coating. There were no significant differences between the two tests (before and after 2508C) and between the two grinding directions. The scratch behavior of the coating before and after 200 h at 2508C does not seem to be very different even though the critical load was higher before the heating procedure. The cracks were very small, comparable between the two tests, and difficult to quantify with accuracy. The other often-used coating in steel casting molds is electrolytic nickel. Three tests were performed and stopped after a few revolutions because of very bad wear and friction behavior. Deep scratches were
observed on the coating during the initial test cycle. The friction force increased very significantly and saturated the electrical signal (friction coefficient )4). Under these conditions, it was impossible to continue the tests for the expected duration. Therefore the test series has been stopped. The scratch tests showed important deformation of the coating due to the applied load. The coating seems to be very soft with no cracks observed. It was difficult to determine exactly the critical load of the coating because no important failures were observed, just a large plastic deformation of the coating. Nevertheless, there were no significant differences in failures,
Fig. 1. Metallographic cross-section of the FGM coating.
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Fig. 2. Pin-on-disk wear tracks on a hard-chromium coating after 720 000 revolutions (200 h). Temperature 2508C, speed: 10 cmys, load: 5 N. (a) Wear of the chip; (b) wear track of the coating; and (c) detail of the wear track.
on one hand between the two grinding directions, and on the other hand before heating and after annealing (200 h) of the coating at 2508C. The influence of the annealing was relatively weak. The wear and friction behavior of the electrolytic Ni coating and hard Cr coating were very different. The first is very soft and is plastically deformed with the pin-on-disk tests, whereas the second is brittle and exhibit numerous cracks at high temperature and under high stresses. The hardness of the electroplated Cr falls steadily at the mold working temperatures leading to hardness values of approximately 750 HV after 1 h at 3008C. The average thickness of a hard chromium coating at the mold level ranges from 35 to 120 mm. In some cases, hard chromium plating is preceded by a nickel plating. The hard chromium provides a wear and chemical resistant layer, and, nickel provides a layer with a high strain-to-fracture feature and with a thermal expansion coefficient similar to that of copper. The latter aspect allows the deposition of relatively thick Ni-layers (1–5 mm) with different thickness profiles. In some cases Ni is used alone (without any external Cr layer) in thick layers, but it normally shows some chemical reactivity with the mold fluxes. It is more common to see Ni as an intermediate layer or applied in the final 2y3 of the mold length so as to prevent direct contact with molten slag. Thermal barriers YSZ and YSZqCH are similar to those chemically hardened at a later stage. For the YSZ there is a remarkable difference between the two longterm tests and the two short-term ones. This is probably
due to the fact that the pressure is more important at the beginning of the test (i.e. the contact area is smaller), and the asperities of the coating are sharper, so the abrasion effect and scratches are more important. There was no significant difference in friction, except the test at 30% of the total time where the friction is slightly higher than for the other tests (ms0.88). Mean estimated pressure: 5 Ny3.5 mm2s1.43 Nymm2 (14.3 bar). The scratch behavior of the YSZ coating before and after the test at 2508C does not seem to be very different. The difference in wear rate of the YSZqCH coating between the beginning and the end of the test was less important than for YSZ coating. Moreover, the wear rate was a factor 2–3 lower for the YSZqCH coating. This better behavior is probably due to the chemical hardening and the associated densification of the coating. The friction coefficient of the YSZqCH system became less and less stable when increasing the test duration. The YSZqCH scratch resistance decreases after the thermal treatment. Optical micrographs showed that delamination appears in the track at a lower load for the test after 2508C. The wear rate of the functionally graded material (FGM) was more important during the first part of the test (60 h) and decreased during the test duration until 200 h. Concerning optical microscopy, the wear mechanism seemed to be the same all along the test. The track was worn progressively without large scratches and without thick oxide layers. There was just a difference for 200 h where a small oxide layer in the middle of the track was observed. Important friction at the
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61
Fig. 3. Pin-on-disk wear tracks on a WC–CoqCS coating after 1 000 000 revolutions (277.7 h). Temperature 2508C, speed: 10 cmys, load: 5 N. (a) Wear of the chip; and (b) wear track of the coating.
beginning of the test was responsible for the wear appearance at first in the sliding direction on the edge of the chip. The contact area remained relatively constant if compared to different tests (except for the 200-h test). The wear behavior seems to be the same for all chips except for the 200-h test (scratches are observed). There was no significant difference in friction behavior for the different tests, except in the first part of the curve of the 200-h test where the friction coefficient was a little higher than for the other tests. Pressure was relatively constant during the tests. Contact area was the same for all tests (from 30 to 100% of the total duration), and the average contact area was 3.7 mm2. The estimated pressure was 5 Ny3.7 mm2s1.35 Nymm2 (13.5 bar). The wear behavior of the WC–CoqCS coating was very good because no wear tracks were visible with optical microscopy. There was only a little polishing effect of the surface, which was perceptible to the eyes. Wear of the chip could not be measured. Nevertheless, some scratches were visible in the sliding direction of the test (Fig. 3). The wear behavior was so good, that the general arrangement of the test for this material was shifted to 1 000 000 revolutions under the pin-on-disk test (equivalent to 277.7 continuous test hours). There was no significant difference in the friction behavior if compared with the other three tests. The friction was very stable and relatively low. Mean estimated pressure was 5 Ny9 mm2s0.55 Nymm2 (5.6 bar). The scratch behavior of the coating before and after the test at 2508C did not seem to be very different. Optical microscopy analysis of the scratch tests showed that the failure is
not measurable outside the track. Some small cohesive cracks were found. Cracks in the track were measurable. The critical loads LC1 and LC2 were determined by optical observations. The CoqCrC coating contain 15–25% chromium carbide with an average size of 2–5 mm. At the working temperature, the carbides resist wear by supporting loads (and in case of glaze formation, they help to anchor it). Some carbide changes at high temperature (Cr3C2™ M7C3) allow doubling the carbide fraction volume and therefore hardness is increased up to 650 HV. The test for pin-on-disk test for 200 h showed a wear behavior which was worse than for the other tests. This behavior could have resulted from a change in the wear mechanism, which induced a higher wear. Indeed, the friction curve showed a behavior totally different after 300 000 revolutions. Friction was more stable, but higher (m;0.6) than at the beginning of the test. There was no significant difference in the friction behavior among the different tests. The first part of the curve shows low friction, followed by an unstable part where the friction increases and decreases quickly, then followed by a more stable but higher friction. The mean estimated pressure was 5 Ny5.2 mm2 s0.96 Nymm2 (9.6 bar). A series of three scratches were performed on the substrate before and after heating at 2508C for 200 h. Only plastic deformation of the coating without any delaminations or important cracks was observed (Fig. 4). The general form of the microstructure of the two analyzed amorphous deposits is a two-phase solid with dendritic chromium borides in a matrix of iron–chro-
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Fig. 4. Scratch test on a CoqCrC coating.
mium or nickel–chromium (depending on the type of coating) which was predominantly amorphous (40– 50%). Fully amorphous structures are moderately stable (crystallizing under service conditions and not providing significant wear improvements). There was no significant difference in the friction behavior of the different tests for the amorphous A coating. The curve was very stable and the mean friction coefficient was approximately 0.5. The wear of the chip was not measurable ( just a few scratches). Some small cracks in the coating outside the track without any important delaminations and a high density of transverse cracks through the track were observed. The coating seems to be very brittle. There was no significant difference between the two tests of the metallic amorphous A coating, before and after 2508C, and between the two grinding directions. The scratch behavior of the coating before and after the test at 2508C seemed quite similar. The wear behavior of amorphous coating B is different from what was obtained for the other coating. The friction curves for the two longest tests showed several periods of high and stable friction followed by short periods of low friction. This high friction would explain the high wear rate, whereas friction was more stable and lower for the two other tests. There was a preferential wear of the chip in the sliding direction for the two longest tests and not measurable wear for the tests at 60 and 100 h. This was due to the fact that the wear mechanism was different along the tests. The mean estimated pressure for the longest test (200 and 160 h) was: 5 Ny1.5 mm2s3.3 Nymm2 (33 bars). The mean estimated pressure for the shorter tests (100 and 60 h) is: 5 Ny 8.41 mm2s0.6 Nymm2 (6 bars). The scratch test track of metallic glass B (Fig. 5) showed some small cracks in the coating outside the track without any important delaminations and a lot of transverse cracks through the
track. The wear behavior of amorphous coating A was much better than amorphous B coating, and the friction behavior was more stable. The information obtained with scratch tests showed that type A was more brittle than type B (radial cracks through the coating were numerous). 4. Conclusions Chromium plating is widely used as a confirmed solution to cope with the various operating needs including low wettability, high hardness, good wear resistance and low cost. The as-deposited Cr-hard plating has a hardness of approximately 1000 HV. The deposit is very brittle and the high degree of internal tensile residual stresses causes it to be microcracked spontaneously. The requirements for the highest possible standards in terms of working life, operational safety and low maintenance leave the possibility for some, localized, antiwear applications. The surface solutions for increasing wear resistance are normally not compatible with the high thermal conductivity required for the mold’s primary function. In some areas of the mold, the heat extraction capacity is in excess to what is desired or wear-prone areas are meaningless in the global heat extraction. Among these areas that can be improved, could be cited the lateral plates of slab and thin slab casters (also called end-plates), the bottom end of the mold (where the air-gap is already formed and heat transfer is limited to radiation) and guiding devices (grip plates or foot-rollers) at the exit of the mold. The reduction in porosity plus the thermal treatment associated with the chemical slurry treatment leads to fully dense coating chemically bonded to the substrate. Chemically hardened metal based coatings additionally show friction coefficients lower than those of electroplated
A. Sanz / Surface and Coatings Technology 146 – 147 (2001) 55–64
63
Fig. 5. Scratch test on an amorphous coating (amorphous B). (a) Scratch perpendicular to the roughness direction; and (b) scratch parallel to the roughness direction.
hard chromium. The WC–CoqWS materials have now been introduced in the above mentioned areas of the mold with an excess of heat extraction capacity and under severe wear conditions. Ceramic based chemically hardened coatings show better wear behavior than conventional hard-chromium layers with similar friction coefficients. Materials like the YSZqCH have a simpler processing schedule (single chemical slurryqannealing at the end) than the WC–CoqWS materials but cannot match the wear resistance of the latter. The high thermal resistance of both the YSZ and YSZqCH does not allow their use on the whole mold surface but just some selected areas. Some new electrodeposited Co alloys are harder than Ni deposits and are becoming increasingly interesting for casting purposes. Co rich plating leads to the formation of wear resistant surfaces able to promote the formation of adherent cobalt oxide in sufficient quantity to produce a glaze and to prevent metal-to-metal contact. Electrochemical solutions for depositing CoqCrC coatings are very sensitive to bath contamination by metals (Cu in particular). This family of coatings could be used on top of a nickel bond coat with proper masking of the areas not to be coated (a similar procedure to the one followed with the NiqCr electrolytic coatings in continuous casting molds). The amorphous coatings family is limited to application where the working temperature of the surface is kept below 6008C (in order to avoid crystallization). The electroplated amorphous coating candidate themselves as hard-Cr plating alternatives, but not for high-
speed caster where maximum peak temperature could be harmful for the coating. The working life of a mold depends on several different variables (steel grade, quality standards, mold fluxes, casting parameters, etc.), but coatings can give a significant impulse toward high wear resistance, chemical inertness and higher productivity through increased machine availability and through strand retention of nominal dimensions. Vitae Alejandro Sanz: Materials Science Engineering Degree at the Simon Bolivar University. Master Science ´ (DEA) at the Ecole Nationale Superieure de Chimie (Institut National Politechnique). Ph.D. at Ecole Nation´ ´ al Superieure de l’Aeronautique et de l’Espace. Current position executive manager of materials development for the Danieli & C group. References w1x A. Sanz, Revue de Metallurgie-CIT, ´ March (2000) 353. w2x D.A Breslin, A. Hetherrington, P.N. Walker, MPT Int. March (1999) 68. w3x K.C. Mills, A review of the ECSC-under research on mould fluxes, Commission of the European Communities: Technical Steel Research, EUR13177 1991. w4x S. Chandra, I.V. Samarasekera, J.K. Brimacombe, A. Bakshi, B.N. Walker, Ironmaking Steelmaking 23 (6) (1996) 512. w5x Y.M. Won, T.-J. Yeo, K.H. Oh, J.-K Park, J. Choi, C.H. Yim, ISIJ Int. 38 (1) (1998) 53. w6x G. Xu, J. Cui, X. Na, J. Iron Steel Res. Int. 5 (1) (1998) 13. w7x K.J. Sorimachi, S. Nabeshima, ISIJ Int. 84 (2) (1998) 103.
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