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The mechanism of sliding wear of lubricated flake graphite cast iron
Wear -
Elsevier Sequoia S.A.,
Lausanne - Printed in The Netherlands
THE MECHANISM OF SLIDING GRAPHITE CAST IRON
E.
WEAR
201
OF LUBRICATED
FLAKE
TAKEUCHI
The Tokyo Metropolitalz Industrial Research Institute (Japan) (Received
November
27, 1969)
SUMMARY
One aspect of the mechanism of sliding wear of lubricated cast iron is discussed. It was observed that when a lubricant, which is moderately oxidized, is applied to the sliding surface of cast iron, the lubricant is retained by the graphite particles on the surface of the cast iron and the oxygen in the lubricant moderately oxidizes the surface and improves its wear resistance. INTRODUCTION
In the study of the mechanism of wear of the surface of cast iron under conditions of lubricated sliding contact it is essential to determine the role of graphite in the iron in the lubricating action. Some authors consider that the dry sliding wear mechanism of cast iron can be explained in terms of the lubricating action of the graphite. Others hold the view that the elastic modulus and Brine11 hardness of the material are the dominant factors in the lubricated sliding wear mechanism of cast iron, and that the graphite plays only a minor rolel. LANES and others, however, point out the importance of graphite as a wear factor. Where large graphite flakes are randomly distributed, the cast iron has good wear resistance. The damage caused by adhesive or thermal shock wear on the sliding surface of cast iron has been studieda. It was observed that fracture started mainly at the tip of graphite particles, hence the higher the content of graphite the greater the probability of fracture and the lower the wear resistance. It is generally considered that the mechanism of wear under lubricated and non-lubricated conditions is similar but it is not clear how oxygen absorbed in the lubricant and the oil absorbed by the graphite4 influences the wear phenomena. The objective of the present research was to clarify this problem. OXIDATION
Oxidation
OF LUBRICATING
OIL AND ITS INFLUENCE
UPON WEAR
of lubricating oil
Lubricating oil absorbs oxygen from the air which gives rise to oxidation phenomenab. The state of oxidation of the lubricant may be known by direct determination of the amount of oxygen absorbede, examination of the physical and Wear, 15 (1970) 201-208
202
E. TAKEUCHI
chemical properties which change as the degree of oxidation changes7 or by examination of the oxides present*. In the present work, the progress of the oxidation reaction was followed by using the second method, that is by measuring the changes in the total acid content of the lubricating oil. Experiments were carried out with samples of a typical industrial lubricant having the viscocity of SAE # 20 Motor Oil. To facilitate oxidation, samples of the lubricant were heated to IOO’C and 150°C and then dry air was blown into them at the rate of IO l/min. This accelerated oxidation procedure was continued for 20011. During this process it was observed that the oxidation reaction proceeded extremely slowly for the first 25 h at IOO”C, then a sudden increase of the total acid value of the lubricants was observed. This was a characteristic oxidizing tendency of the lubricant under the treatment. Four samples of lubricant in different states of oxidation were examined. It was observed that, with the advance of oxidation, the viscosity of the lubricant increased while the surface tension of the oil film decreased indicating that the oil tends to lose its lubricating
effects with an extreme
advance
of oxidation.
Effect
of oxidation of lubricating oil on wear The effect of oxidation of the lubricating oil on wear was investigated samples of oil which had been given various oxidation treatments. A “pin type wear test machine was used 3. An oil refiner, Fig. I, was used to ensure conditions of lubrication during the test. The test temperature was ZO’C.
by using and ring” constant A control
23
_-_
Circulate
direction
of
lubricant
Fig. I. Construction of wear testing machine and circulation system of lubricant. I, Stator; 2, rotor; II, main shaft; 13, oil receiver; 14. isothermal bath; 15, thermostat; 16, lubricant; 17, heater; zoo W/h (automatic control), 300 W/h, 500 W/h; 18, glass filter; 19. gear pump; 20, glass wool filter; 21, cooling cylinder; 22, cooling water inlet: 23, cooling water outlet; 24, cooling bath. bt'ea?',15
(1970)
ZOI--208
SLIDING WEAR OF LUBRICATED FLAKE GRAPHITE CAST IRON
203
results of which are given in Fig. 2, was made with virgin oil. The results showed that when the oil temperature was 2o°C, a steady wear state was found at a sliding velocity of 3.40 m/set, and contact pressure of 40 kg/cm2. When the oil temperature rose to roo”C, the steady wear state was found at a sliding velocity of 3.40 m/set, and a contact pressure of IO kg/cmz. As shown in Fig. 2, when oils oxidized to various degrees were used, the steady wear state changed, for reasons stated later, and the wear loss tended to decrease. It appeared that when virgin lubricant was used, the frictional surface, even in the primary stage of wear, presented mainly a state of adhesion and this state persisted even in the continuous wear regime. But when the lubricants were oxidized for IOOh at IOO’C, in the primary wear range, evidence of oxidative wear was detected in parts of the sliding surface, which increased as the sliding length increased. In a sliding length of 200,000 m, evidence of the oxidative wear spread over the entire surface. Inspection of the worn sliding surface suggests that, because of oxidation caused by the oxygen containedin the lubricant, wear was less than that indicated by the steady wear state obtained with virgin lubricant. This accounts for the low wear loss per unit sliding length. Thus, when a moderately oxidized lubricant is used, the oxygen absorbed in the lubricant reacts with the metal of the sliding surface forming oxide test,
60 loo
100
Oxidized
200
140
oxidizeitime I
(hr)15G
temperature
(“C)
Fig. 2. Effect of oxidation of lubricant on wear (stator). Specimens: Stator, SAE 52100 Steel (85o’C oil quench, - 95°C subzero treatment, 180°C temp. Rotor, SAE 1055 Steel (goo’C oil quench, 570°C temp.). Wear condition: (A) contact pressure, 40 kg/ems; sliding velocity, 3.40 m/set; lubricant, SAE # 20 motor oil (Lub. No. z) ; oil temperature zo°C; (B) contact pressure, IO kg/cm2; sliding velocity, 3.40 m/set; lubricant, SAE # 20 motor oil; oil temperature, 100°C. Weav, 15 (1970) 201-208
204
E. TAKEUCHI
which facilitates the adhesion of the oil film to reduce the wear loss. With an excessively oxidized lubricant, however, because of deterioration in properties, thermal wear occurs with increasing wear loss, and a very unstable wear tendency. For the study of surface oxide, wear experiments were made at 100°C using a lubricant which had been oxidized at IOO’C for 100 h, then further oxidized at 150°C for 50 or IOO h, and the sliding surface of the stator was examined by electron diffraction. The results showed that the oxides were yFezOs and FesOs. On the sliding surface where thermal wear had occurred, a deposition of crystalline carbon, presumably formed by thermal decomposition of the lubricant, was detected. From these experimental findings it can be inferred that the sliding surface had reached a fairly high temperature, which suggests that when a steel test piece is used atmospheric nitrogen reacts with it to harden the sliding surface, as suggested by WELsP. In the present experiments, however, it was also found that the oxidation phenomenon has a beneficial influence in reducing wear. EFFECT
OF GRAPHITE
ON THE LUBRICATION
AND
SLIDING
WEAR
OF CAST IRON
Mean distance between graphite particles on the contact surface As it has been reported 10 that titanium containing cast iron with a eutectic graphite structure had excellent wear resistance under sliding conditions, an attempt was made to determine the effect of the spacings of graphite particles in cast iron on wear. Various kinds of cast iron with different graphite contents were used for the stator and cast iron and carbon steel SAE 1055 were used for the rotor, to provide various spacings of the graphite particles in the range from 0.01 to 0.08 mm. The graphite condition of the test pieces are given in Table I. TABLE
I
GRAPHITE
CONDITION
OF
SPECIMENS
.___
Graphite*
.SpeCiWWAS Kind
Mark
Stator
11 B
Rotor
c: s Y Z
* See American
Size
Shape
Type-A
No. 5
Type-A Type-n Type-A Type-X _
No. k-5 so. 7-8 No. 5-0 No. 6
Foundrymen’s
Society
Standard.
The mean space of the adjacent graphite particles on the contact surface of each test piece was measured by the following method. First the graphite structures of various cast iron test pieces were examined, and then the mean space value of the graphite particles was obtained by combining the data of the test pieces used in combination, superimposing the data of the rotor over those of the stator, as illustrated in Fig. 3. Figure 3(A) represents a superimposed graphite structure of a pair of test pieces seen against a background of section paper. Figure 3(B) shows how the mean space value was obtained. The graphite spaces across the reference points ao, al weav, rg
(1970) m--20X
SLIDING WEAR
OF LUBRICATED
FLAKE GRAPHITE
205
CAST IRON
b, were measured and the mean space value an, and bo, bl, ba,. . . b,+ calculated. Table II gives the mean space values obtained between the graphite particles on the contact surface of the test pieces; these fall within a range of 0.01-0.08 mm.
a2, . . . a,.+
Graphite deposited on surface of rotor. ,Graphite deposited on surface of I!!
(A)
bm-2
ststor.
“m
bm-1
(B)
00 bO
Fig. 3. Method of measuring graphite spacing on contact faces.
TABLE
II
RELATION BETWEEN
COMBINATION
Combination Mark
Stator
Rotor
II
A A B C
2 X Y Y
I* 13 ‘4
InfJzGence of the mean space surface upon wear resistance
OF
SPECIMENS
AND
MEAN
GRAPHITE
SPACE
VALUE
Mean graphite spacing (mm) 0.079 0.040 0.026 0.012
of graphite
particles
of cast iron deposited
on the contact
By varying the combination of the stator and rotor materials, the space between the graphite particles on the contact surface was controlled within the range o.or-0.08 mm, and wear tests were made to obtain the wear of the cast iron stator. Wear,
15
(1970)
201-208
206
E. TAKEUCHI
Maintaining the sliding velocity constant at 1.05 m/set and the sliding length at 10,000 m the contact pressure was increased from IO to 70 kg/cm2 by IO kg/cm2 increments. The pressure-wear curves of cast irons with different graphite spacings were compared. The results indicate that regardless of the differences in the graphite spacing similar wear characteristics exist. The various cast irons reach a stable wear state with maximum wear loss. When the absolute value of the wear rate obtained under wear conditions corresponding to the steady wear state was compared with the mean space value of the graphite particles on the contact surface, it was found that the wear loss tended to decrease almost linearly with the narrowing of the mean spacing between the graphite particles, suggesting that the closer-spaced graphite particles tend to preserve the continuity of the oil film, Fig. 4. Also, in the case of titanium containing eutectic graphite cast iron which, unlike the other cast irons, contained a small amount of ferrite, the relationship between the mean space value of graphite and the wear loss was similar to the pearlitic cast iron containing flake graphite. This is presumably attributable to the cell structure specific to this type of cast iron, which maintained the oil film.
Mean graphite
space value
(mm)
Fig. 4. Relation between steady wear state of cast iron (stator) and mean graphite space value on contact faces (* combination mark). Wear condition: Contact pressure: No. 11-13, 30 kg/cmz; No. 14, 50 kg/cmz; sliding velocity 1.05 m/set; sliding length ro,ooo m; lubricant 60 Spin.; oil temperature 2o’C.
These findings
show that when wear tests were carried
out on pearlitic
cast
iron with flake graphite or eutectic graphite combined with cast iron of the same structure or with steel to vary the mean graphite spacing, and the test conditions of load and speed were controlled to produce adhesive wear, the mean graphite space was found to have a pronounced effect on the protective power of the lubricating oil film and resultant wear. Effect of the mean graphite spacing on the contact surface of cast iron upon its wear characteristics Tests were carried out under the wear conditions of steady wear state deterWeav, 15 (1970)
201-208
SLIDING WEAR OF LUBRICATED FLAKE GRAPHITE CAST IRON
207
mined by earlier experiments for an extended sliding length of 100,000 m, and the wear characteristics of various types of cast iron were compared to determine the effect of the mean graphite spacing on the contact surface and to attempt to establish a relationship between the mean graphite spacing on the sliding surface and the wear loss per unit sliding length within the normal wear range. The results are plotted in Fig. 5. These confirm the earlier results that the smaller the mean spacing between adjacent graphite particles, the less the wear.
Mean
graphite
space value
6tm1)
Fig. 5 Relation between wearing tendency of cast iron (stator) in continuous wear range and mean graphite space on contact face (* combination mark). Wear condition: contact pressure: No. II--13,30 kg/ems, No. 14,50 kg/cmz; slidingvelocity 1.05 m/sec;lubricant 60 Spin.;oil temperature ZOYZ.
Summarizing, the graphite spacing on the contact surface was observed to affect the wear resistance of lubricated cast iron even in the range where adhesive wear predominates and regardless of whether in the initial or normal wear regimes. CONCLUSIONS
From the results of the experiments it may be concluded: (I) When lubricating oil is partially oxidized it supports the oxidation phenomenon of the sliding surface aiding the protective effect of the oil film and counteracting wear. If the lubricant is excessively oxidized, it tends to facilitate local thermal wear with an increase of wear rate. (2) The wear resistance of cast irons tested increased as the spacings between the graphite particles decreased in the primary and normal wear conditions as well as under conditions of adhesive wear. ACKNOWLEDGEMENT
The author wishes to express his indebtedness to Dr. C. Hisatsune, Prof. Emeritus at Nagoya University and to Prof. M. Tsuda, Kansai University for their guidance. Wear, 15 (1970) 201-208
E. TAKEUCHI REFERENCES I T. L. OBERLE, J. Metals, 3 (1951) 438. 62 (1940) 95. 2 P. S. LANE, Tvans. ASME, 3 4 5 6 7 8 g 10
E. TAKEUCHI. Wear, II (1968) 201. M. TSUDA, J. Japan Foundrymen’s Sot., 36 (1964) 154. G. H. FUCHS AND H. DIBMOND, Ind. Eng. Chem., 34 (1942) 927. R. W. DORNTE, Ind. Eng. Chem., 28 (1936) 26, 836, 1342. I (1958) 142; 2 (1959) 130; 3 (1960) 116, 389. T. YAMAJI, Bull. J.P.I., C. KROCER AND A. KALLER, Oel und Kohle, 19 (1943) 669. S. F. KAPFF, J. R. BOWMAN AND HOWY, J. Inst. Petvol., 31 (1945) 453 G. H. DENWON, Ivd. Eng. Chem., 36 (1944) 477. N. C. WELSH, J. Appl. Phys., 28 (1957) 960. Sot., 32 (1960) 635. E. TAKEUCHI, J. /apan Foundrymen’s
Weav,
r5 (1970)
201-208
Elsevier Sequoia S.A.,
Lausanne - Printed in The Netherlands
THE MECHANISM OF SLIDING GRAPHITE CAST IRON
E.
WEAR
201
OF LUBRICATED
FLAKE
TAKEUCHI
The Tokyo Metropolitalz Industrial Research Institute (Japan) (Received
November
27, 1969)
SUMMARY
One aspect of the mechanism of sliding wear of lubricated cast iron is discussed. It was observed that when a lubricant, which is moderately oxidized, is applied to the sliding surface of cast iron, the lubricant is retained by the graphite particles on the surface of the cast iron and the oxygen in the lubricant moderately oxidizes the surface and improves its wear resistance. INTRODUCTION
In the study of the mechanism of wear of the surface of cast iron under conditions of lubricated sliding contact it is essential to determine the role of graphite in the iron in the lubricating action. Some authors consider that the dry sliding wear mechanism of cast iron can be explained in terms of the lubricating action of the graphite. Others hold the view that the elastic modulus and Brine11 hardness of the material are the dominant factors in the lubricated sliding wear mechanism of cast iron, and that the graphite plays only a minor rolel. LANES and others, however, point out the importance of graphite as a wear factor. Where large graphite flakes are randomly distributed, the cast iron has good wear resistance. The damage caused by adhesive or thermal shock wear on the sliding surface of cast iron has been studieda. It was observed that fracture started mainly at the tip of graphite particles, hence the higher the content of graphite the greater the probability of fracture and the lower the wear resistance. It is generally considered that the mechanism of wear under lubricated and non-lubricated conditions is similar but it is not clear how oxygen absorbed in the lubricant and the oil absorbed by the graphite4 influences the wear phenomena. The objective of the present research was to clarify this problem. OXIDATION
Oxidation
OF LUBRICATING
OIL AND ITS INFLUENCE
UPON WEAR
of lubricating oil
Lubricating oil absorbs oxygen from the air which gives rise to oxidation phenomenab. The state of oxidation of the lubricant may be known by direct determination of the amount of oxygen absorbede, examination of the physical and Wear, 15 (1970) 201-208
202
E. TAKEUCHI
chemical properties which change as the degree of oxidation changes7 or by examination of the oxides present*. In the present work, the progress of the oxidation reaction was followed by using the second method, that is by measuring the changes in the total acid content of the lubricating oil. Experiments were carried out with samples of a typical industrial lubricant having the viscocity of SAE # 20 Motor Oil. To facilitate oxidation, samples of the lubricant were heated to IOO’C and 150°C and then dry air was blown into them at the rate of IO l/min. This accelerated oxidation procedure was continued for 20011. During this process it was observed that the oxidation reaction proceeded extremely slowly for the first 25 h at IOO”C, then a sudden increase of the total acid value of the lubricants was observed. This was a characteristic oxidizing tendency of the lubricant under the treatment. Four samples of lubricant in different states of oxidation were examined. It was observed that, with the advance of oxidation, the viscosity of the lubricant increased while the surface tension of the oil film decreased indicating that the oil tends to lose its lubricating
effects with an extreme
advance
of oxidation.
Effect
of oxidation of lubricating oil on wear The effect of oxidation of the lubricating oil on wear was investigated samples of oil which had been given various oxidation treatments. A “pin type wear test machine was used 3. An oil refiner, Fig. I, was used to ensure conditions of lubrication during the test. The test temperature was ZO’C.
by using and ring” constant A control
23
_-_
Circulate
direction
of
lubricant
Fig. I. Construction of wear testing machine and circulation system of lubricant. I, Stator; 2, rotor; II, main shaft; 13, oil receiver; 14. isothermal bath; 15, thermostat; 16, lubricant; 17, heater; zoo W/h (automatic control), 300 W/h, 500 W/h; 18, glass filter; 19. gear pump; 20, glass wool filter; 21, cooling cylinder; 22, cooling water inlet: 23, cooling water outlet; 24, cooling bath. bt'ea?',15
(1970)
ZOI--208
SLIDING WEAR OF LUBRICATED FLAKE GRAPHITE CAST IRON
203
results of which are given in Fig. 2, was made with virgin oil. The results showed that when the oil temperature was 2o°C, a steady wear state was found at a sliding velocity of 3.40 m/set, and contact pressure of 40 kg/cm2. When the oil temperature rose to roo”C, the steady wear state was found at a sliding velocity of 3.40 m/set, and a contact pressure of IO kg/cmz. As shown in Fig. 2, when oils oxidized to various degrees were used, the steady wear state changed, for reasons stated later, and the wear loss tended to decrease. It appeared that when virgin lubricant was used, the frictional surface, even in the primary stage of wear, presented mainly a state of adhesion and this state persisted even in the continuous wear regime. But when the lubricants were oxidized for IOOh at IOO’C, in the primary wear range, evidence of oxidative wear was detected in parts of the sliding surface, which increased as the sliding length increased. In a sliding length of 200,000 m, evidence of the oxidative wear spread over the entire surface. Inspection of the worn sliding surface suggests that, because of oxidation caused by the oxygen containedin the lubricant, wear was less than that indicated by the steady wear state obtained with virgin lubricant. This accounts for the low wear loss per unit sliding length. Thus, when a moderately oxidized lubricant is used, the oxygen absorbed in the lubricant reacts with the metal of the sliding surface forming oxide test,
60 loo
100
Oxidized
200
140
oxidizeitime I
(hr)15G
temperature
(“C)
Fig. 2. Effect of oxidation of lubricant on wear (stator). Specimens: Stator, SAE 52100 Steel (85o’C oil quench, - 95°C subzero treatment, 180°C temp. Rotor, SAE 1055 Steel (goo’C oil quench, 570°C temp.). Wear condition: (A) contact pressure, 40 kg/ems; sliding velocity, 3.40 m/set; lubricant, SAE # 20 motor oil (Lub. No. z) ; oil temperature zo°C; (B) contact pressure, IO kg/cm2; sliding velocity, 3.40 m/set; lubricant, SAE # 20 motor oil; oil temperature, 100°C. Weav, 15 (1970) 201-208
204
E. TAKEUCHI
which facilitates the adhesion of the oil film to reduce the wear loss. With an excessively oxidized lubricant, however, because of deterioration in properties, thermal wear occurs with increasing wear loss, and a very unstable wear tendency. For the study of surface oxide, wear experiments were made at 100°C using a lubricant which had been oxidized at IOO’C for 100 h, then further oxidized at 150°C for 50 or IOO h, and the sliding surface of the stator was examined by electron diffraction. The results showed that the oxides were yFezOs and FesOs. On the sliding surface where thermal wear had occurred, a deposition of crystalline carbon, presumably formed by thermal decomposition of the lubricant, was detected. From these experimental findings it can be inferred that the sliding surface had reached a fairly high temperature, which suggests that when a steel test piece is used atmospheric nitrogen reacts with it to harden the sliding surface, as suggested by WELsP. In the present experiments, however, it was also found that the oxidation phenomenon has a beneficial influence in reducing wear. EFFECT
OF GRAPHITE
ON THE LUBRICATION
AND
SLIDING
WEAR
OF CAST IRON
Mean distance between graphite particles on the contact surface As it has been reported 10 that titanium containing cast iron with a eutectic graphite structure had excellent wear resistance under sliding conditions, an attempt was made to determine the effect of the spacings of graphite particles in cast iron on wear. Various kinds of cast iron with different graphite contents were used for the stator and cast iron and carbon steel SAE 1055 were used for the rotor, to provide various spacings of the graphite particles in the range from 0.01 to 0.08 mm. The graphite condition of the test pieces are given in Table I. TABLE
I
GRAPHITE
CONDITION
OF
SPECIMENS
.___
Graphite*
.SpeCiWWAS Kind
Mark
Stator
11 B
Rotor
c: s Y Z
* See American
Size
Shape
Type-A
No. 5
Type-A Type-n Type-A Type-X _
No. k-5 so. 7-8 No. 5-0 No. 6
Foundrymen’s
Society
Standard.
The mean space of the adjacent graphite particles on the contact surface of each test piece was measured by the following method. First the graphite structures of various cast iron test pieces were examined, and then the mean space value of the graphite particles was obtained by combining the data of the test pieces used in combination, superimposing the data of the rotor over those of the stator, as illustrated in Fig. 3. Figure 3(A) represents a superimposed graphite structure of a pair of test pieces seen against a background of section paper. Figure 3(B) shows how the mean space value was obtained. The graphite spaces across the reference points ao, al weav, rg
(1970) m--20X
SLIDING WEAR
OF LUBRICATED
FLAKE GRAPHITE
205
CAST IRON
b, were measured and the mean space value an, and bo, bl, ba,. . . b,+ calculated. Table II gives the mean space values obtained between the graphite particles on the contact surface of the test pieces; these fall within a range of 0.01-0.08 mm.
a2, . . . a,.+
Graphite deposited on surface of rotor. ,Graphite deposited on surface of I!!
(A)
bm-2
ststor.
“m
bm-1
(B)
00 bO
Fig. 3. Method of measuring graphite spacing on contact faces.
TABLE
II
RELATION BETWEEN
COMBINATION
Combination Mark
Stator
Rotor
II
A A B C
2 X Y Y
I* 13 ‘4
InfJzGence of the mean space surface upon wear resistance
OF
SPECIMENS
AND
MEAN
GRAPHITE
SPACE
VALUE
Mean graphite spacing (mm) 0.079 0.040 0.026 0.012
of graphite
particles
of cast iron deposited
on the contact
By varying the combination of the stator and rotor materials, the space between the graphite particles on the contact surface was controlled within the range o.or-0.08 mm, and wear tests were made to obtain the wear of the cast iron stator. Wear,
15
(1970)
201-208
206
E. TAKEUCHI
Maintaining the sliding velocity constant at 1.05 m/set and the sliding length at 10,000 m the contact pressure was increased from IO to 70 kg/cm2 by IO kg/cm2 increments. The pressure-wear curves of cast irons with different graphite spacings were compared. The results indicate that regardless of the differences in the graphite spacing similar wear characteristics exist. The various cast irons reach a stable wear state with maximum wear loss. When the absolute value of the wear rate obtained under wear conditions corresponding to the steady wear state was compared with the mean space value of the graphite particles on the contact surface, it was found that the wear loss tended to decrease almost linearly with the narrowing of the mean spacing between the graphite particles, suggesting that the closer-spaced graphite particles tend to preserve the continuity of the oil film, Fig. 4. Also, in the case of titanium containing eutectic graphite cast iron which, unlike the other cast irons, contained a small amount of ferrite, the relationship between the mean space value of graphite and the wear loss was similar to the pearlitic cast iron containing flake graphite. This is presumably attributable to the cell structure specific to this type of cast iron, which maintained the oil film.
Mean graphite
space value
(mm)
Fig. 4. Relation between steady wear state of cast iron (stator) and mean graphite space value on contact faces (* combination mark). Wear condition: Contact pressure: No. 11-13, 30 kg/cmz; No. 14, 50 kg/cmz; sliding velocity 1.05 m/set; sliding length ro,ooo m; lubricant 60 Spin.; oil temperature 2o’C.
These findings
show that when wear tests were carried
out on pearlitic
cast
iron with flake graphite or eutectic graphite combined with cast iron of the same structure or with steel to vary the mean graphite spacing, and the test conditions of load and speed were controlled to produce adhesive wear, the mean graphite space was found to have a pronounced effect on the protective power of the lubricating oil film and resultant wear. Effect of the mean graphite spacing on the contact surface of cast iron upon its wear characteristics Tests were carried out under the wear conditions of steady wear state deterWeav, 15 (1970)
201-208
SLIDING WEAR OF LUBRICATED FLAKE GRAPHITE CAST IRON
207
mined by earlier experiments for an extended sliding length of 100,000 m, and the wear characteristics of various types of cast iron were compared to determine the effect of the mean graphite spacing on the contact surface and to attempt to establish a relationship between the mean graphite spacing on the sliding surface and the wear loss per unit sliding length within the normal wear range. The results are plotted in Fig. 5. These confirm the earlier results that the smaller the mean spacing between adjacent graphite particles, the less the wear.
Mean
graphite
space value
6tm1)
Fig. 5 Relation between wearing tendency of cast iron (stator) in continuous wear range and mean graphite space on contact face (* combination mark). Wear condition: contact pressure: No. II--13,30 kg/ems, No. 14,50 kg/cmz; slidingvelocity 1.05 m/sec;lubricant 60 Spin.;oil temperature ZOYZ.
Summarizing, the graphite spacing on the contact surface was observed to affect the wear resistance of lubricated cast iron even in the range where adhesive wear predominates and regardless of whether in the initial or normal wear regimes. CONCLUSIONS
From the results of the experiments it may be concluded: (I) When lubricating oil is partially oxidized it supports the oxidation phenomenon of the sliding surface aiding the protective effect of the oil film and counteracting wear. If the lubricant is excessively oxidized, it tends to facilitate local thermal wear with an increase of wear rate. (2) The wear resistance of cast irons tested increased as the spacings between the graphite particles decreased in the primary and normal wear conditions as well as under conditions of adhesive wear. ACKNOWLEDGEMENT
The author wishes to express his indebtedness to Dr. C. Hisatsune, Prof. Emeritus at Nagoya University and to Prof. M. Tsuda, Kansai University for their guidance. Wear, 15 (1970) 201-208
E. TAKEUCHI REFERENCES I T. L. OBERLE, J. Metals, 3 (1951) 438. 62 (1940) 95. 2 P. S. LANE, Tvans. ASME, 3 4 5 6 7 8 g 10
E. TAKEUCHI. Wear, II (1968) 201. M. TSUDA, J. Japan Foundrymen’s Sot., 36 (1964) 154. G. H. FUCHS AND H. DIBMOND, Ind. Eng. Chem., 34 (1942) 927. R. W. DORNTE, Ind. Eng. Chem., 28 (1936) 26, 836, 1342. I (1958) 142; 2 (1959) 130; 3 (1960) 116, 389. T. YAMAJI, Bull. J.P.I., C. KROCER AND A. KALLER, Oel und Kohle, 19 (1943) 669. S. F. KAPFF, J. R. BOWMAN AND HOWY, J. Inst. Petvol., 31 (1945) 453 G. H. DENWON, Ivd. Eng. Chem., 36 (1944) 477. N. C. WELSH, J. Appl. Phys., 28 (1957) 960. Sot., 32 (1960) 635. E. TAKEUCHI, J. /apan Foundrymen’s
Weav,
r5 (1970)
201-208