- Email: [email protected]
A high temperature test rig for sliding and rolling wear
Wear, 115 (1987)
139
A HIGH TEMPERATURE WEAR* ASA
HAMMARSTEN
Uppsala University,
139
- 150
TEST RIG FOR SLIDING
AND ROLLING
and STURE HOGMARK Institute
of Technology,
Box 534, S-751 21 Uppsala (Sweden)
Summary New equipment for wear testing at elevated temperatures has been constructed. Test samples in the shape of blocks or rollers are pressed against a rotating disc (500 mm in diameter) which can be heated to 900 “C. The periphery velocity is continuously variable between 0 and 50 m s-l. When testing rollers, the angle between the disc and the roller axis can be varied from 0 to 5” to obtain a desired interfacial sliding at the contact surface. It is possible to load and unload the test specimen periodically and to record the frictional force. The ability of the test to reproduce the wear behaviour of guide rollers used in hot rod rolling mills was evaluated for two materials, tool steel (AISI D2) and Ferrodur, a sintered steel containing titanium carbides. Stationary guides of one grey cast iron grade were also included in this comparison. The specimens have been tested against a disc of structural steel (AISI 1045). For a particular set of test parameters it was found that the surface structure of the laboratory specimens very much conforms with that of the service case aimed at. Typical wear characteristics are microabrasion by oxide particles, thermomechanical surface cracking and adhesion of the hot material to the specimen surface. Parameters like contact pressure, rolling and sliding distance between the guide and rod material are extremely difficult to control in a hot rod rolling mill. Therefore this test equipment is expected to be of great value for the selection and development of new materials in mill guides and for similar applications in sliding or rolling contacts at elevated temperatures.
1. Introduction Wear of materials is often a problem in service environments because of fluctuating test conditions and high expenses. A great number of laboratory tests have therefore been developed. *Paper presented at the Nordic Symposium Technology,
LuleB, Sweden,
0043.1648/87/$3.50
on
Tribology,
LuleP
University
of
June 15 - 18,1986. @ Elsevier Sequoia/Printed
in The Netherlands
140
As soon as high ambient temperature is a determining factor in the wear process the number of available laboratory tests is reduced. A few t&s have been developed for different purposes. Fretting [l], sliding 12 .2 1 and abrasive [5] wear have been tested at high ambient temperature including high temperature contacting materials. This is accomplished by different types of oven heating. An important aspect of high temperature wear is the influence of the chemical environment which causes oxidation and other types of corrosion. Control and variation of such conditions are ?.hereforr desirable and have been applied by some authors [ 1,4, 51. Thermomechanical cycling often occurs in industrial apphcatmns hktl hot rolling. Therefore isolated tests of thermal fatigue have been commonly used [6 - 81. However, the results may be hard to apply to cases where thr thermal cycling is combined with mechanical deformation [91 Interesting work has been done on the wear of hot-working roiling rniir rolls using laboratory tests [ 10 - 141 and through tests in service [ 15 i 1 ri 1. The knowledge of the wear process of working rolls cannot be directly transferred to the wear of guide rollers and even less to the sliding wear conditions of stationary guides. Some service studies have been done on the wear of guide rollers [18] but, as the results of these studies are extremely uncertain owing to large unpredictable fluctuations in the contact condi. tions, a laboratory test is highly desirable. Zimmermann has developed an interesting test apparatus [19] which is similar to the one described in this report. The high temperature rig described in this paper was originally constructed to simulate the wear of guide materials used in hot rod rolling mills. Test specimens are forced to roll or slide against the periphery of a heated disc of rod material. A comparison is made between the rig test results and results from tests in service,
2. Test equipment 2.1. Performance The high temperature test rig is meant for testing rollers or blocks on the periphery of a rotating disc which can be heated up to 900 “C. The contact conditions can be either rolling or sliding or rolling with superimposed sliding. The normal force between specimen and disc is controlled by dead-weights. The contact can be continuous or periodical with eligible times for contact and separation respectively. The frictional force and the temperature of the disc are continuously recorded during each test. The material loss caused by the wear of the specimen is measured by weighing. Table 1 presents the performance data for the test rig. 2.2. Construction The high temperature rig is schematically presented in Fig. I The speed of the motor is arbitrarily regulated by frequency The drive is transmitted by belts to the disc shaft 1. The rotationai
control. speed of
141 TABLE
1
The performance
of the high temperature
Motor Frequency control (Hz) Disc dimensions (mm) Periphery velocity (m s-‘) Temperature (“C) Specimen: Roller dimensions (mm) Angle offset (deg) Stationary block dimensions Contact conditions
test rig
(mm)
1440 rev min-‘, 7.5 kW 0 - 100 500 x 12 0 - 50 20 - 900 35 - 45 X 24 o-+5 55 x 15 x 9 Normal force (N) 100 - 1500 Contact cycles (on/off) l-99/1-99 Water cooling of test piece
(s)
m SPEED
Fig. 1. Principal sketch of the high temperature test rig: 1, rotating disc; 2, specimen (exemplified by a roller); 3, load cell; 4, pneumatic cylinder; 5, dead-weights; 6, lever; 7, burner nozzle; 8, angle adjustment for roller.
the disc is recorded and displayed digitally. The recording of the rotational speed combined with the frictional force registration by the load cell 3 is used to count the effective number of revolutions when the specimen 2 makes contact with the disc. The test will be automatically interrupted after a preset number of revolutions. Two different types of specimen holders have been used for rollers and stationary blocks. A sketch of each holder is presented in Fig. 2. The construction of the roller holder 8 enables the roller axis to be inclined 0” - 5” with respect to the disc axis. This misalignment superimposes a sliding component at the roller-disc contact surface. The water cooling system and the bearing construction are identical to those used in conventional hot rod mill guide holders. The holder for the stationary blocks is
142
cooled by water. Water can also be directed to the contact region between specimen and disc. The holder arm pivots around an axis which in turn is attached to the load lever 6. The load lever pivots around a fixed axis. :I load &l 3 is attached under a fixed plate for the measurement of the frictional force between specimen and disc. Dead-weights 5 are attached to the load lever to obtain a desired normal force. The specimen can be brought into intermittent contact with the disc by a pneumatic cylinder 4 which lifts the load lever 6 up and down. ;1. pneu matic switch regulates the cylinder and is controlled by a timer which switches it on and off. Also, it is possible to preset the test duration. A mixture of liquefied petroleum gas and oxygen in a gas flame ‘i is used to heat the rotating disc. A pyrometer (IR radiation) is used tci c:ontrol the temperature of the disc. To prevent the disc shaft bearings from overheating, the centre of the shaft is provided with an axial channel for cooling water. The load celi is also water cooled through a copper pipe connected to the load cell support plate.
3. Experimental
details
3.1. Testing Testing was performed in a hot rod mil1 and in the laboratory test rig using similar test parameters. Since both rollers and stationary guides are frequently used in hot rod mills both types of guides were included. 3.1.1. Tests in service In Fig. 3(a) the design of a guide set-up is shown in front i>C a pan of rolls. The hot rod wire meets the stationary guide formed as a funnel and proceeds towards a pair of rollers which finally leads the rod wire into the entrance slot of the milling rolls Guide rollers were tested at a position of the mill plant where the rod has a speed of about 20 m s -’ and a temperature of 900 -. 1000 “C. Each roller pair was tested with interjacent weighing and surface examination in the scanning electron microscope. The number of hot rods passing the roller pair was recorded for each test. The average time for the passage of a single rod was estimated at 70 s. Stationary guide samples were selected from guides used under normai conditions and studied by scanning electron microscopy (SEM) and light optical microscopy. The exact history of the rod passages for these guides is unknown but is of little importance since the contact conditions vary randomly.
3.1.2. Tests in the high temperature After parameters
a series of pretests were chosen.
rig
in the laboratory
rig a suitable
set of test
(a) Fig. 2. Holders for (a) rollers holder B, off-axis angle scale specimen F, clamp G, cooling Fig. 3. (a) Set-up of stationary guide roller with a radial track.
(b)
and (b) stationary blocks. Roller with bearings A, turntable C, cooling water inlet D, specimen holder arm E, stationary water H, tap for directing water to the contact region J. and roller
guides
at the entrance
of a pair of rolls.
(b) A
For mechanical reasons the disc temperature could not be allowed to reach the service temperature. The oxides formed on the surfaces in contact play an important role in the wear process [lo]. However, a temperature of 700 “C was assumed to be adequate as the same type of iron oxides are formed on the test materials above 570 “C as at 900 - 1000 “C. In a service case, considerably fluctuating contact conditions prevail owing to, for example, the misalignment of the guides or variation in size and shape of the hot rod cross-section. While testing rollers the normal force was chosen to be 500 N which proved to give a wear rate of the same order of magnitude as that in service. The periphery speed of the service rollers will only coincide with that of the rod at a certain radius owing to the curvature of the roller track (see Fig. 3(b)). Inside and outside the radius of rolling contact a sliding component will be superimposed. This is an important factor to consider. The axis of the cylindrical specimen rollers used in the test rig was therefore misaligned with respect to the disc axis in order to reproduce these service conditions. At a misfit angle of 2” a relative sliding component will occur which corresponds to superimposed sliding 2 mm away from the circumferential lines of pure rolling contact of radially tracked guide rollers. A rolling speed of 20 m sP1 results in a sliding component of 0.7 m s-l. In the service case each rod passage involves acceleration of the stationary roller and an average rolling distance of 1400 m. To emphasize the
144
acceleration phase in the test equipment, the time of contact in each test, cycle was shortened. Furthermore, the time interval between t.wo consecutive contacts was prolonged to ensure that the roller came IO a complete stop. The on/off time cycle was set to 5 s/15 s corresponding to 100 XIIof rolling during each cycle. As in the service test the roller is eontmuously cooled with water. For stationary blocks a load of 100 N was chosen which gave a reasonable wear rate. The sliding velocity was fixed at 5 m s-’ and the t~imc of contact during each cycle was adjusted to give the same contact distance (100 m) as for the roller guides. Table 2 summarizes the test parameters for the rollers and stationary blocks. TABLE
2
Parameters
Rollers Blocks __.__
used in the laboratory
700 500 700 100 -. _~~..__. -._----~_-._.
20
test of rollers
0.7 i
_ _--.
and stationary
_.
blocks
S/l 5 2Oi5
.-. ..._-.-
l-I+ Yes, n
3.2. Materials Three conventional types of guide materials have been used in the tests. A high-alloyed tool steel (AISI D2) and a sintered steel, Ferrodur, were used for the rollers while a grey cast iron was used for the stationary samples, AISI D2 consists of about 15 vol.% chromium carbides in a martensitic matrix with a small amount of retained austenite. Ferrodur consists of titanium carbides in a martensitic matrix with a high chromium content. The carbide volume of Ferrodur is larger than t,hat of D2 and the carbide distribution is more even. The high content of titanium gives the material a low density which shortens the acceleration phase of the roller. Grey cast iron is a commonly used material in stationary guides because of the low cost and relatively good performance. The hot rod material in the service tests has been various grades oli mild steel and a structural steel, AISI 1045, was chosen as the disc material in the laboratory test. Chemical compositions and room temperature hardness of the test materials are given in Table 3. 4. Results 4.1. Guide rollers 4.1.1.
Material
loss
Figure 4 shows the results from field testing presented as wear us. number of passed rods. The general observations are a significantly lower wear
145 TABLE 3 Chemical
composition
(wt.%) and Vickers’
hardness
of test materials
Material
C
Si
Mn
MO
Cr
V
Cu
Fe
TiC
HV300
AISI D2 Ferrodur AISI 1045 Grey iron
1.55 0.65 0.45 3.0
0.30 0.3 2.5
0.25 0.6 -
0.80 3.0 -
12.0 14.0 -
0.80 -
0.8 -
84.3 48.5 98.6 94.5
33 -
710 1240 200 160
N
A
1800. 1
1600. 14001
1ZOC 1000
A
l x00 600 400
B
zoo
j 200
tl ,
.
, 400 lie. of
_
600
800
rods
100
1 0
Fig. 4. Wear of service-tested guide rollers D2 one pair; oa, Ferrodur one pair. Fig. 5. Wear of rollers rodur one roller.
us. number
us. the number
of sequences
of rods:
00, D2 one pair; AA,
in the test rig: 0, D2 one roller;
m, Fer-
rate for Ferrodur than for D2 rollers and a considerable scatter in the test results for the D2 rollers. In Fig. 5 the laboratory test results are given us. the number of contact sequences. In principle the relation between the wear rate of D2 and Ferrodur in the service test is reproduced. Comparing the results of Figs. 4 and 5, the number of cycles is the same as the number of rods but the rolling distance is 14 times longer in the service test. 4.1.2. Surface characteristics of service-tested rollers The wear surfaces of service-tested D2 and Ferrodur rollers showed a network of cracks (see Figs. 6(a) and 6(c)). The very characteristic crack network on Ferrodur develops early in the wear process. The individual cracks will coarsen gradually but the crack density is roughly constant. In D2 cracks occurred most frequently in connection with patches of adhered rod
(cl Fig. 6. Characteristic wear surfaces (SEM) of service-tested rollers of U2 ((a Ferrodur ((c) and (d)). The rolling direction is indicated by arrows.
1WILL( !)‘I] and
material, the presence of which was proved by an energy dispersive spectroscopy Cr KCYline scan (see Fig. 6(a)). The patches constitute a thin optically dark discontinuous film which was more frequent on D2 rollers than on Ferrodur (see Figs. 6(a) and 6(c)). At higher magnifications chromium and titanium carbides were revealed as protruding from the matrix of the D2 and Ferrodur rollers respectively (see Figs. 6(b) and 6(d)). 4.1.3. Surface characteristics of laboratory-tested rollers The characteristic crack pattern is reproduced in the laboratory test (see Figs. 7(a) and 7(c)). The crack formation is again somewhat less typical on D2 and will occasionally occur in connection with thin adhered layers on this material (see Fig. 7(a)). Protruding carbides are revealed in between the cracks (see Figs. 7(b) and 7(d)) and it is clearly seen that abrasive wear of the matrix material has occurred on the D2 roller. 4.2. Stationary
guides 4.2.1. Surface characteristics
of service-tested
In Fig. 8 two types of wear surface same sample are shown. These demonstrate
guides
structures originating from the abrasive (Fig. 8(a)) and adhesive
(b)
(c)
(d)
Fig. 7. Characteristic wear surfaces (SEM) of laboratory-tested rollers of D2 ((a) and (b)) and Ferrodur ((c) and (d)). The rolling direction is indicated by arrows.
(a)
(b)
Fig. 8. Characteristic wear surfaces (SEM) of service-tested stationary guides. (a) Domination of abrasive wear. (b) Adhesive wear and pitting. (The arrows show the direction of rod sliding.)
(Fig. 8(b)) wear respectively, rial removal. A cross-section layer with varying thickness
which are the dominant mechanisms of mateof the sample showed a white-etching surface up to 50 pm under both types of surfaces. The
148
pitting on the adhesively etching layer.
worn surface
seems to be connected
with the white-
4.2.2. Surface characteristics of laboratory-tested blocks In the laboratory tests the material loss was measured with and without water cooling of the contact region. It was evident that the water cooling increased the wear rate. Characteristic surface topographies from the two types of tests are seen in Fig. 9. The water-cooled specimen shows a surface where abrasive wear was dominant while adhesive wear was dominant for the surface where cooling was not applied.
Fig. 9. Characteristic wear surfaces (SEM) of laboratory-tested blocks. of water-cooled specimen. (b) Adhesive wear of specimen not subjected of the contact interface. (The arrows show the direction of disc sliding.)
(a) A~asivc- wear to water cooling
5. Discussion Wear of guide rollers for hot rolling occurs by a combination of finescale abrasion caused by oxide fragments from the hot rod surface and by thermal cracking. The relative importance of these mechanisms depends on the roller material. In this investigation, with rollers of D2 and Ferrodur, abrasion is the dominant mechanism of material removal. Thin patches of rod material frequently adhere to the roller surface and may thereby reduce the wear rate. However, very thick patches may cause damage to the surface of the rolled material. The laboratory test reproduced the wear mechanisms typical of guide rollers. Figure 4 implicitly demonstrates the problems of uncontrolled experi, mental parameters encountered in the field testing of guides. The two pairs of D2 rollers suffered about the same amount of wear though the number of rolled rods differs by a factor of seven. There is also a great amount of scatter between the rollers within each pair which is probably because of the misalignment of the guide box. Hence, being accurately controllable,
149
laboratory testing in the high temperature rig is believed to facilitate substantially the development and ranking of materials for roller guide applications. In this test equipment it is also possible to separate the contributions to the total wear from pure rolling, interfacial sliding and sliding during acceleration by proper variation of the test parameters. This has been done in an extensive investigation which also involves variations in disc velocity, temperature, contact pressure etc. The results will be published later. Also the simulation of stationary guide wear seems promising although the comparative field testing was limited to one material.
6. Conclusions (1) The dominant wear mechanism for guide rollers is abrasion by oxide fragments. (2) In the long run thermomechanical cracking can accelerate material removal if the roller material is too sensitive to this mechanism. (3) The laboratory test proved able to simulate both the abrasive and the thermomechanical wear mechanisms. (4) The laboratory test is therefore estimated to be of great value for the selection and development of guide materials.
Acknowledgments This work was financially ciation (Jernkontoret) and the Development. Personal thanks masters Association who have equipment.
supported by the Swedish Ironmasters AssoNordic Fund for Technology and Industrial are addressed to the members of the Ironcontributed to the development of the test
References 1 R. B. Waterhouse, Tribal. Int., 14 (1981) 203 - 207. 2 P. Crook and C. C. Li, in K. C. Ludema (ed.), Proc. Int. Conf on Wear of Materials, Reston, VA, April 11 - 14, 1983, American Society of Mechanical Engineers, New York, 1983, pp. 272 - 279. 3 A. F. Smith, Tribol. Znt., 18 (1985) 35 - 43. 4 J. L. Sullivan and N. W. Granville, Tribal. Znt., 17 (1984) 63 - 71. 5 S. Soemantri, A. C. McGee and I. Finnie, Wear, 104 (1985) 77 - 91. 6 E. Nes and P. Fartum, Stand. J. Metall., I2 (1983) 107 _ 111. 7 R. Sandstrom, A. Samuelsson, L.-E. Larsson and L. Lundberg, Stand. J. Metall., 12 (1983) 99 - 106. 8 L.-A. Norstrom, M. Svensson and N. Ghrberg, Met. Technol., 8 (10) (1981) 376 381. 9 J. M. Quets and R. C. Tucker, Jr., Thin Solid Films, 84 (1981) 107 118. 10 A. Magnee, C. Gaspard and M. Gabriel, CRM Rep., 57 (1980) 25 - 39.
150 11 V. R. Howes and M. P. Amor, Wear, 72 (1981) 105.120. 12 J. Kihara, K. Nakamura, K. Doya and M. Suenaga, Proc. 3rd Int. Colloq. on iubrication in Metal Working, Esdingen, January 1982, Esslingen Technische Akadamie. Esslingen, Vol. 1,Part B, 1982, 55.1 - 55.7. 13 S. R. Tittagala, P. R. Beeley and A. N. Bramley, Met. Techno/., 9 (1982) :!:il !A:;% 14 W. Schumacher, Mater. Eng., 101 (3) (1985) 23 - 25. 15 K. Edsmar, Iron Steel Eng., (April 1979) 80 88. 16 P. Funke, J. Holland and R. Kulbrok, Stahl. Eisen, 98 (1978) 403 409, 17 W. L. Roberts, Lubr. Eng., (November 1977) 575 - 580. 18 B. Jacob and H. Klaas, Neue Huette, 26 (1981) 333 - 336. 19 R. Zimmermann. W. Dahl and K. H. Mommertz, Stahl Eisen, IO3 (1983 j 685 St)
139
A HIGH TEMPERATURE WEAR* ASA
HAMMARSTEN
Uppsala University,
139
- 150
TEST RIG FOR SLIDING
AND ROLLING
and STURE HOGMARK Institute
of Technology,
Box 534, S-751 21 Uppsala (Sweden)
Summary New equipment for wear testing at elevated temperatures has been constructed. Test samples in the shape of blocks or rollers are pressed against a rotating disc (500 mm in diameter) which can be heated to 900 “C. The periphery velocity is continuously variable between 0 and 50 m s-l. When testing rollers, the angle between the disc and the roller axis can be varied from 0 to 5” to obtain a desired interfacial sliding at the contact surface. It is possible to load and unload the test specimen periodically and to record the frictional force. The ability of the test to reproduce the wear behaviour of guide rollers used in hot rod rolling mills was evaluated for two materials, tool steel (AISI D2) and Ferrodur, a sintered steel containing titanium carbides. Stationary guides of one grey cast iron grade were also included in this comparison. The specimens have been tested against a disc of structural steel (AISI 1045). For a particular set of test parameters it was found that the surface structure of the laboratory specimens very much conforms with that of the service case aimed at. Typical wear characteristics are microabrasion by oxide particles, thermomechanical surface cracking and adhesion of the hot material to the specimen surface. Parameters like contact pressure, rolling and sliding distance between the guide and rod material are extremely difficult to control in a hot rod rolling mill. Therefore this test equipment is expected to be of great value for the selection and development of new materials in mill guides and for similar applications in sliding or rolling contacts at elevated temperatures.
1. Introduction Wear of materials is often a problem in service environments because of fluctuating test conditions and high expenses. A great number of laboratory tests have therefore been developed. *Paper presented at the Nordic Symposium Technology,
LuleB, Sweden,
0043.1648/87/$3.50
on
Tribology,
LuleP
University
of
June 15 - 18,1986. @ Elsevier Sequoia/Printed
in The Netherlands
140
As soon as high ambient temperature is a determining factor in the wear process the number of available laboratory tests is reduced. A few t&s have been developed for different purposes. Fretting [l], sliding 12 .2 1 and abrasive [5] wear have been tested at high ambient temperature including high temperature contacting materials. This is accomplished by different types of oven heating. An important aspect of high temperature wear is the influence of the chemical environment which causes oxidation and other types of corrosion. Control and variation of such conditions are ?.hereforr desirable and have been applied by some authors [ 1,4, 51. Thermomechanical cycling often occurs in industrial apphcatmns hktl hot rolling. Therefore isolated tests of thermal fatigue have been commonly used [6 - 81. However, the results may be hard to apply to cases where thr thermal cycling is combined with mechanical deformation [91 Interesting work has been done on the wear of hot-working roiling rniir rolls using laboratory tests [ 10 - 141 and through tests in service [ 15 i 1 ri 1. The knowledge of the wear process of working rolls cannot be directly transferred to the wear of guide rollers and even less to the sliding wear conditions of stationary guides. Some service studies have been done on the wear of guide rollers [18] but, as the results of these studies are extremely uncertain owing to large unpredictable fluctuations in the contact condi. tions, a laboratory test is highly desirable. Zimmermann has developed an interesting test apparatus [19] which is similar to the one described in this report. The high temperature rig described in this paper was originally constructed to simulate the wear of guide materials used in hot rod rolling mills. Test specimens are forced to roll or slide against the periphery of a heated disc of rod material. A comparison is made between the rig test results and results from tests in service,
2. Test equipment 2.1. Performance The high temperature test rig is meant for testing rollers or blocks on the periphery of a rotating disc which can be heated up to 900 “C. The contact conditions can be either rolling or sliding or rolling with superimposed sliding. The normal force between specimen and disc is controlled by dead-weights. The contact can be continuous or periodical with eligible times for contact and separation respectively. The frictional force and the temperature of the disc are continuously recorded during each test. The material loss caused by the wear of the specimen is measured by weighing. Table 1 presents the performance data for the test rig. 2.2. Construction The high temperature rig is schematically presented in Fig. I The speed of the motor is arbitrarily regulated by frequency The drive is transmitted by belts to the disc shaft 1. The rotationai
control. speed of
141 TABLE
1
The performance
of the high temperature
Motor Frequency control (Hz) Disc dimensions (mm) Periphery velocity (m s-‘) Temperature (“C) Specimen: Roller dimensions (mm) Angle offset (deg) Stationary block dimensions Contact conditions
test rig
(mm)
1440 rev min-‘, 7.5 kW 0 - 100 500 x 12 0 - 50 20 - 900 35 - 45 X 24 o-+5 55 x 15 x 9 Normal force (N) 100 - 1500 Contact cycles (on/off) l-99/1-99 Water cooling of test piece
(s)
m SPEED
Fig. 1. Principal sketch of the high temperature test rig: 1, rotating disc; 2, specimen (exemplified by a roller); 3, load cell; 4, pneumatic cylinder; 5, dead-weights; 6, lever; 7, burner nozzle; 8, angle adjustment for roller.
the disc is recorded and displayed digitally. The recording of the rotational speed combined with the frictional force registration by the load cell 3 is used to count the effective number of revolutions when the specimen 2 makes contact with the disc. The test will be automatically interrupted after a preset number of revolutions. Two different types of specimen holders have been used for rollers and stationary blocks. A sketch of each holder is presented in Fig. 2. The construction of the roller holder 8 enables the roller axis to be inclined 0” - 5” with respect to the disc axis. This misalignment superimposes a sliding component at the roller-disc contact surface. The water cooling system and the bearing construction are identical to those used in conventional hot rod mill guide holders. The holder for the stationary blocks is
142
cooled by water. Water can also be directed to the contact region between specimen and disc. The holder arm pivots around an axis which in turn is attached to the load lever 6. The load lever pivots around a fixed axis. :I load &l 3 is attached under a fixed plate for the measurement of the frictional force between specimen and disc. Dead-weights 5 are attached to the load lever to obtain a desired normal force. The specimen can be brought into intermittent contact with the disc by a pneumatic cylinder 4 which lifts the load lever 6 up and down. ;1. pneu matic switch regulates the cylinder and is controlled by a timer which switches it on and off. Also, it is possible to preset the test duration. A mixture of liquefied petroleum gas and oxygen in a gas flame ‘i is used to heat the rotating disc. A pyrometer (IR radiation) is used tci c:ontrol the temperature of the disc. To prevent the disc shaft bearings from overheating, the centre of the shaft is provided with an axial channel for cooling water. The load celi is also water cooled through a copper pipe connected to the load cell support plate.
3. Experimental
details
3.1. Testing Testing was performed in a hot rod mil1 and in the laboratory test rig using similar test parameters. Since both rollers and stationary guides are frequently used in hot rod mills both types of guides were included. 3.1.1. Tests in service In Fig. 3(a) the design of a guide set-up is shown in front i>C a pan of rolls. The hot rod wire meets the stationary guide formed as a funnel and proceeds towards a pair of rollers which finally leads the rod wire into the entrance slot of the milling rolls Guide rollers were tested at a position of the mill plant where the rod has a speed of about 20 m s -’ and a temperature of 900 -. 1000 “C. Each roller pair was tested with interjacent weighing and surface examination in the scanning electron microscope. The number of hot rods passing the roller pair was recorded for each test. The average time for the passage of a single rod was estimated at 70 s. Stationary guide samples were selected from guides used under normai conditions and studied by scanning electron microscopy (SEM) and light optical microscopy. The exact history of the rod passages for these guides is unknown but is of little importance since the contact conditions vary randomly.
3.1.2. Tests in the high temperature After parameters
a series of pretests were chosen.
rig
in the laboratory
rig a suitable
set of test
(a) Fig. 2. Holders for (a) rollers holder B, off-axis angle scale specimen F, clamp G, cooling Fig. 3. (a) Set-up of stationary guide roller with a radial track.
(b)
and (b) stationary blocks. Roller with bearings A, turntable C, cooling water inlet D, specimen holder arm E, stationary water H, tap for directing water to the contact region J. and roller
guides
at the entrance
of a pair of rolls.
(b) A
For mechanical reasons the disc temperature could not be allowed to reach the service temperature. The oxides formed on the surfaces in contact play an important role in the wear process [lo]. However, a temperature of 700 “C was assumed to be adequate as the same type of iron oxides are formed on the test materials above 570 “C as at 900 - 1000 “C. In a service case, considerably fluctuating contact conditions prevail owing to, for example, the misalignment of the guides or variation in size and shape of the hot rod cross-section. While testing rollers the normal force was chosen to be 500 N which proved to give a wear rate of the same order of magnitude as that in service. The periphery speed of the service rollers will only coincide with that of the rod at a certain radius owing to the curvature of the roller track (see Fig. 3(b)). Inside and outside the radius of rolling contact a sliding component will be superimposed. This is an important factor to consider. The axis of the cylindrical specimen rollers used in the test rig was therefore misaligned with respect to the disc axis in order to reproduce these service conditions. At a misfit angle of 2” a relative sliding component will occur which corresponds to superimposed sliding 2 mm away from the circumferential lines of pure rolling contact of radially tracked guide rollers. A rolling speed of 20 m sP1 results in a sliding component of 0.7 m s-l. In the service case each rod passage involves acceleration of the stationary roller and an average rolling distance of 1400 m. To emphasize the
144
acceleration phase in the test equipment, the time of contact in each test, cycle was shortened. Furthermore, the time interval between t.wo consecutive contacts was prolonged to ensure that the roller came IO a complete stop. The on/off time cycle was set to 5 s/15 s corresponding to 100 XIIof rolling during each cycle. As in the service test the roller is eontmuously cooled with water. For stationary blocks a load of 100 N was chosen which gave a reasonable wear rate. The sliding velocity was fixed at 5 m s-’ and the t~imc of contact during each cycle was adjusted to give the same contact distance (100 m) as for the roller guides. Table 2 summarizes the test parameters for the rollers and stationary blocks. TABLE
2
Parameters
Rollers Blocks __.__
used in the laboratory
700 500 700 100 -. _~~..__. -._----~_-._.
20
test of rollers
0.7 i
_ _--.
and stationary
_.
blocks
S/l 5 2Oi5
.-. ..._-.-
l-I+ Yes, n
3.2. Materials Three conventional types of guide materials have been used in the tests. A high-alloyed tool steel (AISI D2) and a sintered steel, Ferrodur, were used for the rollers while a grey cast iron was used for the stationary samples, AISI D2 consists of about 15 vol.% chromium carbides in a martensitic matrix with a small amount of retained austenite. Ferrodur consists of titanium carbides in a martensitic matrix with a high chromium content. The carbide volume of Ferrodur is larger than t,hat of D2 and the carbide distribution is more even. The high content of titanium gives the material a low density which shortens the acceleration phase of the roller. Grey cast iron is a commonly used material in stationary guides because of the low cost and relatively good performance. The hot rod material in the service tests has been various grades oli mild steel and a structural steel, AISI 1045, was chosen as the disc material in the laboratory test. Chemical compositions and room temperature hardness of the test materials are given in Table 3. 4. Results 4.1. Guide rollers 4.1.1.
Material
loss
Figure 4 shows the results from field testing presented as wear us. number of passed rods. The general observations are a significantly lower wear
145 TABLE 3 Chemical
composition
(wt.%) and Vickers’
hardness
of test materials
Material
C
Si
Mn
MO
Cr
V
Cu
Fe
TiC
HV300
AISI D2 Ferrodur AISI 1045 Grey iron
1.55 0.65 0.45 3.0
0.30 0.3 2.5
0.25 0.6 -
0.80 3.0 -
12.0 14.0 -
0.80 -
0.8 -
84.3 48.5 98.6 94.5
33 -
710 1240 200 160
N
A
1800. 1
1600. 14001
1ZOC 1000
A
l x00 600 400
B
zoo
j 200
tl ,
.
, 400 lie. of
_
600
800
rods
100
1 0
Fig. 4. Wear of service-tested guide rollers D2 one pair; oa, Ferrodur one pair. Fig. 5. Wear of rollers rodur one roller.
us. number
us. the number
of sequences
of rods:
00, D2 one pair; AA,
in the test rig: 0, D2 one roller;
m, Fer-
rate for Ferrodur than for D2 rollers and a considerable scatter in the test results for the D2 rollers. In Fig. 5 the laboratory test results are given us. the number of contact sequences. In principle the relation between the wear rate of D2 and Ferrodur in the service test is reproduced. Comparing the results of Figs. 4 and 5, the number of cycles is the same as the number of rods but the rolling distance is 14 times longer in the service test. 4.1.2. Surface characteristics of service-tested rollers The wear surfaces of service-tested D2 and Ferrodur rollers showed a network of cracks (see Figs. 6(a) and 6(c)). The very characteristic crack network on Ferrodur develops early in the wear process. The individual cracks will coarsen gradually but the crack density is roughly constant. In D2 cracks occurred most frequently in connection with patches of adhered rod
(cl Fig. 6. Characteristic wear surfaces (SEM) of service-tested rollers of U2 ((a Ferrodur ((c) and (d)). The rolling direction is indicated by arrows.
1WILL( !)‘I] and
material, the presence of which was proved by an energy dispersive spectroscopy Cr KCYline scan (see Fig. 6(a)). The patches constitute a thin optically dark discontinuous film which was more frequent on D2 rollers than on Ferrodur (see Figs. 6(a) and 6(c)). At higher magnifications chromium and titanium carbides were revealed as protruding from the matrix of the D2 and Ferrodur rollers respectively (see Figs. 6(b) and 6(d)). 4.1.3. Surface characteristics of laboratory-tested rollers The characteristic crack pattern is reproduced in the laboratory test (see Figs. 7(a) and 7(c)). The crack formation is again somewhat less typical on D2 and will occasionally occur in connection with thin adhered layers on this material (see Fig. 7(a)). Protruding carbides are revealed in between the cracks (see Figs. 7(b) and 7(d)) and it is clearly seen that abrasive wear of the matrix material has occurred on the D2 roller. 4.2. Stationary
guides 4.2.1. Surface characteristics
of service-tested
In Fig. 8 two types of wear surface same sample are shown. These demonstrate
guides
structures originating from the abrasive (Fig. 8(a)) and adhesive
(b)
(c)
(d)
Fig. 7. Characteristic wear surfaces (SEM) of laboratory-tested rollers of D2 ((a) and (b)) and Ferrodur ((c) and (d)). The rolling direction is indicated by arrows.
(a)
(b)
Fig. 8. Characteristic wear surfaces (SEM) of service-tested stationary guides. (a) Domination of abrasive wear. (b) Adhesive wear and pitting. (The arrows show the direction of rod sliding.)
(Fig. 8(b)) wear respectively, rial removal. A cross-section layer with varying thickness
which are the dominant mechanisms of mateof the sample showed a white-etching surface up to 50 pm under both types of surfaces. The
148
pitting on the adhesively etching layer.
worn surface
seems to be connected
with the white-
4.2.2. Surface characteristics of laboratory-tested blocks In the laboratory tests the material loss was measured with and without water cooling of the contact region. It was evident that the water cooling increased the wear rate. Characteristic surface topographies from the two types of tests are seen in Fig. 9. The water-cooled specimen shows a surface where abrasive wear was dominant while adhesive wear was dominant for the surface where cooling was not applied.
Fig. 9. Characteristic wear surfaces (SEM) of laboratory-tested blocks. of water-cooled specimen. (b) Adhesive wear of specimen not subjected of the contact interface. (The arrows show the direction of disc sliding.)
(a) A~asivc- wear to water cooling
5. Discussion Wear of guide rollers for hot rolling occurs by a combination of finescale abrasion caused by oxide fragments from the hot rod surface and by thermal cracking. The relative importance of these mechanisms depends on the roller material. In this investigation, with rollers of D2 and Ferrodur, abrasion is the dominant mechanism of material removal. Thin patches of rod material frequently adhere to the roller surface and may thereby reduce the wear rate. However, very thick patches may cause damage to the surface of the rolled material. The laboratory test reproduced the wear mechanisms typical of guide rollers. Figure 4 implicitly demonstrates the problems of uncontrolled experi, mental parameters encountered in the field testing of guides. The two pairs of D2 rollers suffered about the same amount of wear though the number of rolled rods differs by a factor of seven. There is also a great amount of scatter between the rollers within each pair which is probably because of the misalignment of the guide box. Hence, being accurately controllable,
149
laboratory testing in the high temperature rig is believed to facilitate substantially the development and ranking of materials for roller guide applications. In this test equipment it is also possible to separate the contributions to the total wear from pure rolling, interfacial sliding and sliding during acceleration by proper variation of the test parameters. This has been done in an extensive investigation which also involves variations in disc velocity, temperature, contact pressure etc. The results will be published later. Also the simulation of stationary guide wear seems promising although the comparative field testing was limited to one material.
6. Conclusions (1) The dominant wear mechanism for guide rollers is abrasion by oxide fragments. (2) In the long run thermomechanical cracking can accelerate material removal if the roller material is too sensitive to this mechanism. (3) The laboratory test proved able to simulate both the abrasive and the thermomechanical wear mechanisms. (4) The laboratory test is therefore estimated to be of great value for the selection and development of guide materials.
Acknowledgments This work was financially ciation (Jernkontoret) and the Development. Personal thanks masters Association who have equipment.
supported by the Swedish Ironmasters AssoNordic Fund for Technology and Industrial are addressed to the members of the Ironcontributed to the development of the test
References 1 R. B. Waterhouse, Tribal. Int., 14 (1981) 203 - 207. 2 P. Crook and C. C. Li, in K. C. Ludema (ed.), Proc. Int. Conf on Wear of Materials, Reston, VA, April 11 - 14, 1983, American Society of Mechanical Engineers, New York, 1983, pp. 272 - 279. 3 A. F. Smith, Tribol. Znt., 18 (1985) 35 - 43. 4 J. L. Sullivan and N. W. Granville, Tribal. Znt., 17 (1984) 63 - 71. 5 S. Soemantri, A. C. McGee and I. Finnie, Wear, 104 (1985) 77 - 91. 6 E. Nes and P. Fartum, Stand. J. Metall., I2 (1983) 107 _ 111. 7 R. Sandstrom, A. Samuelsson, L.-E. Larsson and L. Lundberg, Stand. J. Metall., 12 (1983) 99 - 106. 8 L.-A. Norstrom, M. Svensson and N. Ghrberg, Met. Technol., 8 (10) (1981) 376 381. 9 J. M. Quets and R. C. Tucker, Jr., Thin Solid Films, 84 (1981) 107 118. 10 A. Magnee, C. Gaspard and M. Gabriel, CRM Rep., 57 (1980) 25 - 39.
150 11 V. R. Howes and M. P. Amor, Wear, 72 (1981) 105.120. 12 J. Kihara, K. Nakamura, K. Doya and M. Suenaga, Proc. 3rd Int. Colloq. on iubrication in Metal Working, Esdingen, January 1982, Esslingen Technische Akadamie. Esslingen, Vol. 1,Part B, 1982, 55.1 - 55.7. 13 S. R. Tittagala, P. R. Beeley and A. N. Bramley, Met. Techno/., 9 (1982) :!:il !A:;% 14 W. Schumacher, Mater. Eng., 101 (3) (1985) 23 - 25. 15 K. Edsmar, Iron Steel Eng., (April 1979) 80 88. 16 P. Funke, J. Holland and R. Kulbrok, Stahl. Eisen, 98 (1978) 403 409, 17 W. L. Roberts, Lubr. Eng., (November 1977) 575 - 580. 18 B. Jacob and H. Klaas, Neue Huette, 26 (1981) 333 - 336. 19 R. Zimmermann. W. Dahl and K. H. Mommertz, Stahl Eisen, IO3 (1983 j 685 St)