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Study of wear of as-cast and heat-treated spheroidal graphite cast iron under dry sliding conditions
WEAR ELSEVIER
Wear 188 (1995) 61-65
Study of wear of as-cast and heat-treated spheroidal graphite cast iron under dry sliding conditions M.A. Islam, A.S.M.A. Haseeb, A.S.W. Kurny Department of Metallurgical Engineering. Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh Received 9 June 1994; accepted 26 January 1995
Abstract Wear behaviour of as-cast and heat-treated spheroidal graphite (SG) cast iron has been studied under dry sliding conditions using a pinon-disc type apparatus. Wear tests were carried out at a linear sliding speed of 0.88 m s-‘, under a constant load of 1.5 kg. All tests were performed in ambient air at room temperature. Extent of wear damage and wear mechanisms were investigated by means of weight loss measurement, optical microscopy, microhardness measurement and X-ray diffractometry on wear debris. The wear rate measured after 9500 m
of sliding is found to be about three times higher in the as-cast sample than in the heat-treated material. In the case of the heat-treated material, abrasive wear is the main wear mechanism. A combination of adhesive wear, delamination and surface fatigue is believed to operate in the as-cast material. Keywords: Wear; Dry sliding; Spheroidal
graphite cast iron; Wear mechanism;
1. Introduction
Spheroidal graphite cast iron (SG iron) is a relatively new addition to the cast iron family. The essential feature of SG iron is that the graphitic carbon exists in the form of nodules, whereas in conventional grey cast iron, the graphitic carbon is in the form of flakes. The flaky morphology of graphite provides easy sites for stress concentration at the sharp edges of the flakes and this makes grey cast iron extremely brittle. On the other hand, the nodular morphology of the graphitic carbon in SG iron reduces the stress concentration effect and can even act as a crack arrester. SG iron thus possesses much improved ductility compared with grey cast iron, or even steel. SG iron has, therefore, increasingly been used in recent years to replace both cast iron and fabricated steel with a substantial cost advantage in manufacturing [ 11. The mechanical properties like hardness, strength, elongation, modulus, impact resistance etc. of SG iron are well documented [ 21. Strength and elongation of SG iron can be varied within a wide range by changing its matrix. Ferritic SG iron, which is softer, will give maximum elongation while pearlitic SG iron is stronger and harder with moderate elongation. The strength and hardness of the SG iron can be improved further through proper heat-treatment as in martensitic SG irons. Although adequate information about the properties of SG iron relevant to structural applications is 0043-1648/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SsO10043-1648(95)06605-5
As-cast and heat-treated
iron; Adhesive and abrasive weat
available, information on the wear resistance of this material is relatively scarce. This study was, therefore, undertaken to investigate the wear resistance of SG iron both in the as-cast and heat-treated conditions. Improved wear resistance of SG iron which is expected through heat-treatment will be of great value in gears, pinions, crankshafts and similar parts where wear resistance is of prime importance.
2. Experimental
details
Spheroidal graphite cast iron, both in the as-cast and heattreated conditions, was used in this study. Details of the composition, production method etc. of the SG iron used can be found elsewhere [ 31. The heat-treatment consisted of hardening from 900 “C, followed by tempering at 550 “C for 1 h. The as-cast SG iron has a pearlitic matrix (hardness: 382 KHN) with a ferrite ring around the graphite nodules, while the heat-treated SG iron has a matrix of tempered martensite (hardness: 580 KHN) . Wear tests were carried out in a pin-on-disc type apparatus (Fig. 1) under dry sliding conditions in the ambient air at room temperature. SG iron in the form of a cylindrical pin of 8 mm diameter and 6.5 mm length was used. Grey cast iron discs of 80 mm diameter and about 10 mm thickness were used as the counter body. During the tests, the SG iron pin was pressed against the disc under a constant load of 1.5 kg. The disc speed was 300 rev min- ‘,
M.A. Islam et al. /Wear
188 (1995) 6145
Load
APin
5
(Fixed) -.
x
Counter
body
7-E
-..$s-cast ..
3 ..
SG iron A
‘.A_______------
I’ “E
?ii+ Fig.
1.Schematic of the wear test apparatus
&
2..
2
,lJ’
I I’
Heat treated
SG iran 0
which gave a linear speed of 0.88 m s-’ at wear track. For each experiment a new pin and a new disc were used. Test durations ranged from l-3 h. Before the tests, both the pin and the disc were degreased, cleaned thoroughly in water and dried immediately in acetone. All tests were carried out in ambient air at room temperature. After testing, the worn surface of SG iron pins was examined by optical microscopy. These were then cleaned thoroughly in running water, dried in acetone and again examined. Pins were weighed before and after the tests to determine the weight loss due to wear. At least three tests were carried out for each set of conditions and the average weight loss is reported in this paper. To reveal any change in the subsurface region of the SG iron pin, microhardness measurements were carried out on cross-sections through the wear track, using a Knoop indenter. Identification of the wear debris collected during wear testing was made by X-ray diffraction, using Cu K-a radiation.
0
2,000
4.000
6,000
Sliding distance,
8.000
10,000
m
Fig. 2. Wear rate of SG iron pins as a function of sliding distance.
-1 .*
A
SG iron __,H~* P _-As-cast --__ -_ r* - ‘_&’ I/ /I // // i
;pG
iron,
_
3. Results and discussion The wear rates of both heat-treated and as-cast SG iron are shown as a function of sliding distance in Fig. 2. The wear rate of the as-cast sample initially reaches a high value, then decreases and falls to a steady value. In the case of the heattreated sample, the wear rate does not show a maximum. After the initial increase, the wear rate becomes steady. In this test geometry, as the test proceeds, pin wear changes the initial line contact to an area contact. Hence the contact pressure decreases continuously during the test. Such a change in contact pressure during wear testing is also encountered in other test geometries such as in the cross-cylinder test [4]. The wear data normalized with respect to instantaneous contact pressure is redrawn in Fig. 3. Both Figs. 2 and 3 are qualitatively similar although the difference in the wear rate between the as-cast and heat-treated samples is more pronounced when the nominal contact pressure change is taken into account. Fig. 4 shows the wear scars on the as-cast and heat-treated samples after sliding to 6333 m. Both micrographs were taken immediately after the test without any cleaning. Greyish debris covers much of the surfaces. Dark areas are graphite nodules. The graphite retains its nodular shape on the heattreated samples; while on the as-cast sample, the graphite is
0
2,000
4,000 Sliding
Fig. 3. Normalized distance.
6,000
distance,
8,000
ro,t
m
wear rate with respect to contact pressure
vs. sliding
elongated in the direction of sliding. This indicates considerable deformation around graphite nodules on the as-cast sample. It may be mentioned that the graphite nodules in the as-cast sample are surrounded by soft ferrite rings which are likely to get deformed. Sliding marks are observed on both samples. The as-cast sample sustains more damage due to wear. The scar on the as-cast sample also shows areas with a compact transfer layer, especially for longer test runs. Fig. 5 shows a typical example of a transfer layer. It is not continuous and occurs randomly on the scar. The wear scar on the heat-treated samples did not contain such a transfer layer. Fig. 6 shows wear scars after thorough cleaning in running water. The scar on the as-cast sample did not change much due to cleaning. Patches of debris, greyish in colour, are attached to the surface. Deformed areas are seen on the scar. The wear scar on the heat-treated sample is clean and smooth. Very fine ploughing/sliding marks are the main feature of this scar. The wear action has caused mainly polishing on this
M.A. Islam et a.! /Wear
188 (1995) 61-65
63
greater than that of the Fe304 line, possibly indicating a greater amount of Fe0 in the debris. It is clear that during the wear process, oxidation of iron had taken place. It is well known that oxidation also takes place during the wear of steels [ 5,6]. Although both Fe304 and Fe0 were identified, Fe203 could not be detected in the debris. This may be explained in
Fig. 5. Transfer layer formed on as-cast sample after 9500 m of sliding.
Fig. 4. Optical micrographs of wear scars after 6333 m of sliding. Micrographs were taken immediately after the tests without cleaning of (a) the as-cast sample and (b) the heat-treated sample. The sliding direction is represented by the horizontal.
sample. Fig. 7 shows the cross-section through wear scars on both samples, the top being the scar surface. The edge of the wear scar on the heat-treated sample is sharp and smooth whereas that on the as-cast sample is rough and irregular. Spalling of layers at the surface of the as-cast sample is seen (as indicated by the arrow). From the examination of the surface and cross-section of wear scars, the dominant wear mechanism can be suggested. For the heat-treated samples, abrasive wear is the main mechanism. Fine debris particles generated in the wear process are believed to cause the microcutting/ploughing on the surface. Abrasive action of this fine debris also smoothens the scar. On the other hand, several mechanisms are believed to operate in the case of the as-cast sample. The presence of the transfer layer suggests the occurrence of adhesive wear in the as-cast sample. Delamination and/or surface fatigue also occurs, which causes spalling of material as seen in Fig. 7(b) . Abrasive wear seems to play an insignificant role in this case. X-ray diffraction data obtained from the wear debris from the heat-treated sample during 9500 m of sliding is summarized in Table 1. The debris consists mainly of iron powder, Fe0 and Fe304 powder. The intensity of the Fe0 line is
Fig. 6. Wear scar after thorough cleaning in running water of (a) the as-cast sample and (b) the heat-treated sample. Both samples were tested to a sliding distance of 3166 m.
M.A. Islam et al. /Wear
188 (1995) 61-65
takes place with the generation of lamellar wear particles. At low loads and speeds, mild wear is encountered and the corresponding wear particles are very fine and oxidized. This has also been confirmed by others [9]. In the present study of cast irons, where a load of 1.5 kg and speed of 0.88 m ’ have been used, wear particles are very fine and oxidized and the wear is of the mild type. Sexton and Fischer [ 91 and Czichos [ lo] have found that the wear mechanism in steel depends upon its hardness. In martensitic steel with higher hardness, abrasive wear tends to take place. But for low hardness values, adhesive wear is encountered. The results of the present study also conforms to the above. For heat-treated SG iron with a martensitic structure, abrasive wear has been found to be the main mechanism while on the softer as-cast samples, adhesive wear has taken place. During wear tests, the properties in the surface region of the specimen may change and, thus, influence the wear process. To investigate whether such a change has taken place,
s-
“0°0 I
50 firn 0: Fig. 7. Optical micrographs showing cross-section through wear scar on (a) the as-cast sample and (b) the heat-treated samples. The sliding direction is perpendicular to the plane of the paper. Table 1 Summary of the X-ray data of the wear debris collected sliding of the heat-treated sample Peak position (“) 35.6 41.91 44.7 82.5
Possible phases (M) Fe304 (311) Fe0 (200) Fe (110) Fe (211)
_I
0
40
(a)
80
Distance
120
below wear
160
scar, grn
1,000
during 9500 m
Relative intensity weak strong strongest weak
terms of the free energy of formation of oxides. The standard free energy of formation of Fez03 is much less negative in the temperature range of O-2000 “C compared with those of Fe0 and Fe304 [ 71. The formation of Fe0 and Fe304 is, therefore, more likely than the formation of Fe,O,. Welsh [ 81 studied the wear behaviour of steels in the load range of 0.05-40 kg and speed range of 0.017-2.66 m s-l, observing that wear can be severe or mild depending upon the load and speed. At higher loads and speeds, severe wear
z
0
800
0
I Y
0
00
vi 600
2 S 5 I 400 S! .I! ZE 200
OL 0
( b)
-
B
0 u
,8Boo:o
bo -
50
Distance
0
0
0
0
08
u
-u
u
100
below wear
0
150
scar, ,um
Fig. 8. Variation of Knoop hardness below the wear scar as a function of depth, (a) the as-cast and b) the heat-treated specimens. Both specimens were tested up to 9500 m of sliding.
M.A. Islam et al. /Wear
microhardness tests were carried out on the cross-sections below the wear scar on the as-cast and heat-treated samples. Fig. 8 shows the Knoop hardness as a function of distance below the surface. A slight increase in hardness is detected near the surface of the as-cast sample, whereas the heattreated sample shows a decrease. Considerable increase in hardness near the surface during wear is reported by others [ 111. The increase in hardness of the as-cast sample in the present case is thought to be due to strain hardening of the pearlitic matrix at the surface region which predominates over any frictional heating effects. The decrease in hardness at the contact surface of the heat-treated sample has been caused by temperature rise due to friction. The martensitic matrix is believed to be tempered by the frictional heat during sliding.
4. Summary and conclusion In this study the wear behaviour of as-cast and heat-treated spheroidal graphite cast iron has been investigated. The difference between the as-cast and the heat-treated samples lies in their matrix structure. The martensitic matrix of the heattreated sample is harder than the pearlitic matrix of the ascast sample. The high hardness of the heat-treated samples results in lower wear rates as compared with the as-cast samples. Wear mechanism is also seen to be different. While heat-treated samples wear mainly by an abrasive mechanism, the as-cast samples undergo a combination of adhesive wear, delamination/surface fatigue. Under the present experimental conditions, wear is thought to be of the mild, oxidative type. The hardness near the surface of the heat-treated samples is found to decrease during testing because of tempering caused by frictional heating. But the as-cast sample shows a slight increase in hardness due to strain hardening effect. However, this hardness increase is not high enough to cause any subsequent decrease in wear rate in the as-cast sample.
References [l] A.A. Zavaras and H.D. Broady, US Government Research Announcement Index, PE-126647/X4& National Center for Manufacturing Science, 1990. [2] G.N.J. Gilbert, BCIRA J., 16 (4) (1968) 342.
188 (1995) 6165
65
I31 S.S.A. [41 [5] [6] [71 [ 81 [9] [ 101
[ 111
Mogni, M.Sc. Eng. Thesis, Bangladesh University of Engineering and Technology, Dhaka, 1992. Wear Testing with a Crossed-Cylinder Apparatus, G83, American Society for Testing and Materials, Philadelphia, PA, 1984. P.D. Goode, Nucl. Instrum. Meth. Phys. Res., 839 (1989) 521-530. B.M. Khusid, E.M. Khusid and B.B. Khina, Wear, 165 (1993) 109112. F.D. Richardson and J.H. Jeffes, J. Iron Steel ht., 160 (1948) 261. N.C. Welsh, Phil. Trans. Roy. Sot. (Lond.), Ser. A, 257 ( 1964) 31. M.D. Sexton and T.E. Fischer, Wear, 96 (1984) 17. H. Czichos, in N.R. Loonis (ed.), New Direction of Lubrication, Materials, Wear and Surface Interaction in Tribology in the 80’s, Noyes, NJ, 1989. J. Don, Ph.D. Thesis, Ohio State University, 1982.
Biographies M.A. Islam: obtained his B.Sc.Eng. (metallurgical) degree in 1992 and M.Sc.Eng. (metallurgical) degree in 1994 from the Bangladesh University of Engineering and Technology, Dhaka. After obtaining his B.Sc.Eng. degree, he joined the Department of Metallurgical Engineering, BUET, Dhaka as a lecturer in 1992. He is now working as an assistantprofessor in the same department. His present research interest is surface engineering and heat-treatment of new materials. A.S.M.A. Haseeb: received his M.Sc. Eng. (metallurgical) degree from the Bangladesh University of Engineering and Technology (BUET), Dhaka in 1986 and Ph.D. in metallurgy and materials engineering from the Catholic University of Leuven, Belgium in 1992. He is presently working as an assistant professor at the Department of Metallurgical Engineering, BUET, Dhaka. His research area includes wear, surface engineering and thin films. A.S.W. Kurny: was born in 1949. He obtained his B.Sc. Eng. (metallurgical) degree in 1970 and M.Sc. Eng. (metallurgical) degree from the Bangladesh University of Engineering and Technology, Dhaka in 1973. Then he joined the Department of Metallurgical Engineering, BUET, Dhaka as a lecturer in 1974. After obtaining his Ph.D. in metallurgy from the Indian Institute of Science, Bangalore in 1982, he has been working as a professor in the Department of Metallurgical Engineering, at BUET, Dhaka. His current areas of interest are surface engineering and minerals processing.
Wear 188 (1995) 61-65
Study of wear of as-cast and heat-treated spheroidal graphite cast iron under dry sliding conditions M.A. Islam, A.S.M.A. Haseeb, A.S.W. Kurny Department of Metallurgical Engineering. Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh Received 9 June 1994; accepted 26 January 1995
Abstract Wear behaviour of as-cast and heat-treated spheroidal graphite (SG) cast iron has been studied under dry sliding conditions using a pinon-disc type apparatus. Wear tests were carried out at a linear sliding speed of 0.88 m s-‘, under a constant load of 1.5 kg. All tests were performed in ambient air at room temperature. Extent of wear damage and wear mechanisms were investigated by means of weight loss measurement, optical microscopy, microhardness measurement and X-ray diffractometry on wear debris. The wear rate measured after 9500 m
of sliding is found to be about three times higher in the as-cast sample than in the heat-treated material. In the case of the heat-treated material, abrasive wear is the main wear mechanism. A combination of adhesive wear, delamination and surface fatigue is believed to operate in the as-cast material. Keywords: Wear; Dry sliding; Spheroidal
graphite cast iron; Wear mechanism;
1. Introduction
Spheroidal graphite cast iron (SG iron) is a relatively new addition to the cast iron family. The essential feature of SG iron is that the graphitic carbon exists in the form of nodules, whereas in conventional grey cast iron, the graphitic carbon is in the form of flakes. The flaky morphology of graphite provides easy sites for stress concentration at the sharp edges of the flakes and this makes grey cast iron extremely brittle. On the other hand, the nodular morphology of the graphitic carbon in SG iron reduces the stress concentration effect and can even act as a crack arrester. SG iron thus possesses much improved ductility compared with grey cast iron, or even steel. SG iron has, therefore, increasingly been used in recent years to replace both cast iron and fabricated steel with a substantial cost advantage in manufacturing [ 11. The mechanical properties like hardness, strength, elongation, modulus, impact resistance etc. of SG iron are well documented [ 21. Strength and elongation of SG iron can be varied within a wide range by changing its matrix. Ferritic SG iron, which is softer, will give maximum elongation while pearlitic SG iron is stronger and harder with moderate elongation. The strength and hardness of the SG iron can be improved further through proper heat-treatment as in martensitic SG irons. Although adequate information about the properties of SG iron relevant to structural applications is 0043-1648/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SsO10043-1648(95)06605-5
As-cast and heat-treated
iron; Adhesive and abrasive weat
available, information on the wear resistance of this material is relatively scarce. This study was, therefore, undertaken to investigate the wear resistance of SG iron both in the as-cast and heat-treated conditions. Improved wear resistance of SG iron which is expected through heat-treatment will be of great value in gears, pinions, crankshafts and similar parts where wear resistance is of prime importance.
2. Experimental
details
Spheroidal graphite cast iron, both in the as-cast and heattreated conditions, was used in this study. Details of the composition, production method etc. of the SG iron used can be found elsewhere [ 31. The heat-treatment consisted of hardening from 900 “C, followed by tempering at 550 “C for 1 h. The as-cast SG iron has a pearlitic matrix (hardness: 382 KHN) with a ferrite ring around the graphite nodules, while the heat-treated SG iron has a matrix of tempered martensite (hardness: 580 KHN) . Wear tests were carried out in a pin-on-disc type apparatus (Fig. 1) under dry sliding conditions in the ambient air at room temperature. SG iron in the form of a cylindrical pin of 8 mm diameter and 6.5 mm length was used. Grey cast iron discs of 80 mm diameter and about 10 mm thickness were used as the counter body. During the tests, the SG iron pin was pressed against the disc under a constant load of 1.5 kg. The disc speed was 300 rev min- ‘,
M.A. Islam et al. /Wear
188 (1995) 6145
Load
APin
5
(Fixed) -.
x
Counter
body
7-E
-..$s-cast ..
3 ..
SG iron A
‘.A_______------
I’ “E
?ii+ Fig.
1.Schematic of the wear test apparatus
&
2..
2
,lJ’
I I’
Heat treated
SG iran 0
which gave a linear speed of 0.88 m s-’ at wear track. For each experiment a new pin and a new disc were used. Test durations ranged from l-3 h. Before the tests, both the pin and the disc were degreased, cleaned thoroughly in water and dried immediately in acetone. All tests were carried out in ambient air at room temperature. After testing, the worn surface of SG iron pins was examined by optical microscopy. These were then cleaned thoroughly in running water, dried in acetone and again examined. Pins were weighed before and after the tests to determine the weight loss due to wear. At least three tests were carried out for each set of conditions and the average weight loss is reported in this paper. To reveal any change in the subsurface region of the SG iron pin, microhardness measurements were carried out on cross-sections through the wear track, using a Knoop indenter. Identification of the wear debris collected during wear testing was made by X-ray diffraction, using Cu K-a radiation.
0
2,000
4.000
6,000
Sliding distance,
8.000
10,000
m
Fig. 2. Wear rate of SG iron pins as a function of sliding distance.
-1 .*
A
SG iron __,H~* P _-As-cast --__ -_ r* - ‘_&’ I/ /I // // i
;pG
iron,
_
3. Results and discussion The wear rates of both heat-treated and as-cast SG iron are shown as a function of sliding distance in Fig. 2. The wear rate of the as-cast sample initially reaches a high value, then decreases and falls to a steady value. In the case of the heattreated sample, the wear rate does not show a maximum. After the initial increase, the wear rate becomes steady. In this test geometry, as the test proceeds, pin wear changes the initial line contact to an area contact. Hence the contact pressure decreases continuously during the test. Such a change in contact pressure during wear testing is also encountered in other test geometries such as in the cross-cylinder test [4]. The wear data normalized with respect to instantaneous contact pressure is redrawn in Fig. 3. Both Figs. 2 and 3 are qualitatively similar although the difference in the wear rate between the as-cast and heat-treated samples is more pronounced when the nominal contact pressure change is taken into account. Fig. 4 shows the wear scars on the as-cast and heat-treated samples after sliding to 6333 m. Both micrographs were taken immediately after the test without any cleaning. Greyish debris covers much of the surfaces. Dark areas are graphite nodules. The graphite retains its nodular shape on the heattreated samples; while on the as-cast sample, the graphite is
0
2,000
4,000 Sliding
Fig. 3. Normalized distance.
6,000
distance,
8,000
ro,t
m
wear rate with respect to contact pressure
vs. sliding
elongated in the direction of sliding. This indicates considerable deformation around graphite nodules on the as-cast sample. It may be mentioned that the graphite nodules in the as-cast sample are surrounded by soft ferrite rings which are likely to get deformed. Sliding marks are observed on both samples. The as-cast sample sustains more damage due to wear. The scar on the as-cast sample also shows areas with a compact transfer layer, especially for longer test runs. Fig. 5 shows a typical example of a transfer layer. It is not continuous and occurs randomly on the scar. The wear scar on the heat-treated samples did not contain such a transfer layer. Fig. 6 shows wear scars after thorough cleaning in running water. The scar on the as-cast sample did not change much due to cleaning. Patches of debris, greyish in colour, are attached to the surface. Deformed areas are seen on the scar. The wear scar on the heat-treated sample is clean and smooth. Very fine ploughing/sliding marks are the main feature of this scar. The wear action has caused mainly polishing on this
M.A. Islam et a.! /Wear
188 (1995) 61-65
63
greater than that of the Fe304 line, possibly indicating a greater amount of Fe0 in the debris. It is clear that during the wear process, oxidation of iron had taken place. It is well known that oxidation also takes place during the wear of steels [ 5,6]. Although both Fe304 and Fe0 were identified, Fe203 could not be detected in the debris. This may be explained in
Fig. 5. Transfer layer formed on as-cast sample after 9500 m of sliding.
Fig. 4. Optical micrographs of wear scars after 6333 m of sliding. Micrographs were taken immediately after the tests without cleaning of (a) the as-cast sample and (b) the heat-treated sample. The sliding direction is represented by the horizontal.
sample. Fig. 7 shows the cross-section through wear scars on both samples, the top being the scar surface. The edge of the wear scar on the heat-treated sample is sharp and smooth whereas that on the as-cast sample is rough and irregular. Spalling of layers at the surface of the as-cast sample is seen (as indicated by the arrow). From the examination of the surface and cross-section of wear scars, the dominant wear mechanism can be suggested. For the heat-treated samples, abrasive wear is the main mechanism. Fine debris particles generated in the wear process are believed to cause the microcutting/ploughing on the surface. Abrasive action of this fine debris also smoothens the scar. On the other hand, several mechanisms are believed to operate in the case of the as-cast sample. The presence of the transfer layer suggests the occurrence of adhesive wear in the as-cast sample. Delamination and/or surface fatigue also occurs, which causes spalling of material as seen in Fig. 7(b) . Abrasive wear seems to play an insignificant role in this case. X-ray diffraction data obtained from the wear debris from the heat-treated sample during 9500 m of sliding is summarized in Table 1. The debris consists mainly of iron powder, Fe0 and Fe304 powder. The intensity of the Fe0 line is
Fig. 6. Wear scar after thorough cleaning in running water of (a) the as-cast sample and (b) the heat-treated sample. Both samples were tested to a sliding distance of 3166 m.
M.A. Islam et al. /Wear
188 (1995) 61-65
takes place with the generation of lamellar wear particles. At low loads and speeds, mild wear is encountered and the corresponding wear particles are very fine and oxidized. This has also been confirmed by others [9]. In the present study of cast irons, where a load of 1.5 kg and speed of 0.88 m ’ have been used, wear particles are very fine and oxidized and the wear is of the mild type. Sexton and Fischer [ 91 and Czichos [ lo] have found that the wear mechanism in steel depends upon its hardness. In martensitic steel with higher hardness, abrasive wear tends to take place. But for low hardness values, adhesive wear is encountered. The results of the present study also conforms to the above. For heat-treated SG iron with a martensitic structure, abrasive wear has been found to be the main mechanism while on the softer as-cast samples, adhesive wear has taken place. During wear tests, the properties in the surface region of the specimen may change and, thus, influence the wear process. To investigate whether such a change has taken place,
s-
“0°0 I
50 firn 0: Fig. 7. Optical micrographs showing cross-section through wear scar on (a) the as-cast sample and (b) the heat-treated samples. The sliding direction is perpendicular to the plane of the paper. Table 1 Summary of the X-ray data of the wear debris collected sliding of the heat-treated sample Peak position (“) 35.6 41.91 44.7 82.5
Possible phases (M) Fe304 (311) Fe0 (200) Fe (110) Fe (211)
_I
0
40
(a)
80
Distance
120
below wear
160
scar, grn
1,000
during 9500 m
Relative intensity weak strong strongest weak
terms of the free energy of formation of oxides. The standard free energy of formation of Fez03 is much less negative in the temperature range of O-2000 “C compared with those of Fe0 and Fe304 [ 71. The formation of Fe0 and Fe304 is, therefore, more likely than the formation of Fe,O,. Welsh [ 81 studied the wear behaviour of steels in the load range of 0.05-40 kg and speed range of 0.017-2.66 m s-l, observing that wear can be severe or mild depending upon the load and speed. At higher loads and speeds, severe wear
z
0
800
0
I Y
0
00
vi 600
2 S 5 I 400 S! .I! ZE 200
OL 0
( b)
-
B
0 u
,8Boo:o
bo -
50
Distance
0
0
0
0
08
u
-u
u
100
below wear
0
150
scar, ,um
Fig. 8. Variation of Knoop hardness below the wear scar as a function of depth, (a) the as-cast and b) the heat-treated specimens. Both specimens were tested up to 9500 m of sliding.
M.A. Islam et al. /Wear
microhardness tests were carried out on the cross-sections below the wear scar on the as-cast and heat-treated samples. Fig. 8 shows the Knoop hardness as a function of distance below the surface. A slight increase in hardness is detected near the surface of the as-cast sample, whereas the heattreated sample shows a decrease. Considerable increase in hardness near the surface during wear is reported by others [ 111. The increase in hardness of the as-cast sample in the present case is thought to be due to strain hardening of the pearlitic matrix at the surface region which predominates over any frictional heating effects. The decrease in hardness at the contact surface of the heat-treated sample has been caused by temperature rise due to friction. The martensitic matrix is believed to be tempered by the frictional heat during sliding.
4. Summary and conclusion In this study the wear behaviour of as-cast and heat-treated spheroidal graphite cast iron has been investigated. The difference between the as-cast and the heat-treated samples lies in their matrix structure. The martensitic matrix of the heattreated sample is harder than the pearlitic matrix of the ascast sample. The high hardness of the heat-treated samples results in lower wear rates as compared with the as-cast samples. Wear mechanism is also seen to be different. While heat-treated samples wear mainly by an abrasive mechanism, the as-cast samples undergo a combination of adhesive wear, delamination/surface fatigue. Under the present experimental conditions, wear is thought to be of the mild, oxidative type. The hardness near the surface of the heat-treated samples is found to decrease during testing because of tempering caused by frictional heating. But the as-cast sample shows a slight increase in hardness due to strain hardening effect. However, this hardness increase is not high enough to cause any subsequent decrease in wear rate in the as-cast sample.
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Biographies M.A. Islam: obtained his B.Sc.Eng. (metallurgical) degree in 1992 and M.Sc.Eng. (metallurgical) degree in 1994 from the Bangladesh University of Engineering and Technology, Dhaka. After obtaining his B.Sc.Eng. degree, he joined the Department of Metallurgical Engineering, BUET, Dhaka as a lecturer in 1992. He is now working as an assistantprofessor in the same department. His present research interest is surface engineering and heat-treatment of new materials. A.S.M.A. Haseeb: received his M.Sc. Eng. (metallurgical) degree from the Bangladesh University of Engineering and Technology (BUET), Dhaka in 1986 and Ph.D. in metallurgy and materials engineering from the Catholic University of Leuven, Belgium in 1992. He is presently working as an assistant professor at the Department of Metallurgical Engineering, BUET, Dhaka. His research area includes wear, surface engineering and thin films. A.S.W. Kurny: was born in 1949. He obtained his B.Sc. Eng. (metallurgical) degree in 1970 and M.Sc. Eng. (metallurgical) degree from the Bangladesh University of Engineering and Technology, Dhaka in 1973. Then he joined the Department of Metallurgical Engineering, BUET, Dhaka as a lecturer in 1974. After obtaining his Ph.D. in metallurgy from the Indian Institute of Science, Bangalore in 1982, he has been working as a professor in the Department of Metallurgical Engineering, at BUET, Dhaka. His current areas of interest are surface engineering and minerals processing.