Wear, 85 (1983) 257 - 266 257

THE UNLUBRICATED WEAR, OF FLAKE GRAPHITE CAST IRON

PATRICK W. LEACH and DOUGLAS W. BORLAND

Department of Metallurgy, University of Melbourne, Parkville, Victoria 3052 (Australia)

(Received October 20,1982)

Summary

The wear behaviour of flake graphite cast iron was correlated with the microstructural parameters of graphite volume fraction and flake size using a pin-on-ring specimen configuration. Pin specimens of cast iron were pre- pared under carefully controlled melting and casting conditions to provide microstructures with variation in either carbon content or flake size but with the same type A graphite structure and pearlitic matrix.

Mild and severe modes of equilibrium wear were identified, the predom- inant effect of both microstructural parameters being in the severe wear regime. Decrease in flake size and increase in carbon content are detrimental to the wear behaviour resulting in a marked increase in the severe wear rate and a decrease in the mild-to-severe transition load.

1. Introduction

Flake graphite cast iron components are widely used in machinery and automotive applications where fully lubricated conditions cannot be con- tinuously maintained or where lubrication is undesirable. It is generally acknowledged that under such circumstances the microstructure and, in particular, the distribution of the graphite phase is an important factor in determining wear resistance.

Early investigators of the sliding wear of grey cast iron recognized two main trends relating to the effect of graphite structure on wear: (a) superior wear resistance of random type A graphite structures to that of microstruc- tures containing undercooled or type D graphite structures; (b) increasing wear resistance with increase in flake size irrespective of graphite form [ 1, 21.

These conclusions have been widely adopted as a guide to the selection of cast iron components for optimum wear resistance [3 - 51 although it is now apparent that these studies were seriously limited because of the narrow range of test conditions used in assessing wear.

Later studies by Eyre and coworkers [6 - 91, Takeuchi [lo, 111 and Kawamoto et al. [12] have characterized the unlubricated wear of cast iron

0043-1648/83/0000-0000/$03.00 0 Elsevier Sequoia/Printed in The Netherlands

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over a wider range of applied load and sliding speed conditions and have identified equilibrium regimes of mild oxidation wear and severe metallic wear. A transition from the mild wear mode to the severe wear mode was evident at a critical combination of load and sliding speed.

Eyre’s conclusions regarding the effect of the type of graphite on wear are consistent with those of earlier studies, with type A graphite structures having significantly lower wear rates than type D graphite structures in both mild and severe wear regimes [ 61.

However, there is still little known about the effect on equilibrium wear behaviour of altering basic features of the type A graphite structure. Such information is significant practically given the wide variation in carbon content and flake size encountered in the specification of grey cast iron.

Two series of flake graphite cast irons, with variation in either flake size or carbon content, have been used to establish the influence of these factors on the mild-to-severe transition and the equilibrium wear behaviour in both mild and severe regimes. These cast irons were prepared under laboratory conditions with strict control of the factors influencing variation in both matrix and graphite microstructures. It is believed that such close control of microstructure has not previously been achieved, even in the carefully con- ducted studies referred to above [ 6 - 121.

2. Experimental procedure

2.1. Wear testing machine The pin-on-ring configuration was chosen because it has the advantage

of maintaining constant contact conditions during a test and because the same configuration has been widely used by other investigators of the wear ofgreycastiron [6-111.

The machine used in all the wear tests is shown schematically in Fig. 1. This machine employed a motor with a continuously variable speed range to drive the central shaft on which the ring was mounted. For this series of wear experiments, the sliding speed at the cylindrical surface of the ring was maintained at 200 cm s-l.

During the operation of the machine, a load was directly applied to the cast iron pin specimen by the suspension of weights from the steel frame. Counterbalancing weights were used to offset the weight of the frame and attachments exactly. Vertical movement of the frame was guided by ball bushing bearings on two hardened steel shafts. Full details of the design and construction of the machine are given elsewhere [ 131.

A new pin and an unworn ring were used in each test, both specimens being thoroughly degreased with trichloroethylene before contact. Rings were machined from AISI 4340 steel, hardened and tempered to 500 HV 30 and finally surface ground on their outer cylindrical surfaces. After grinding, all rings were 76.2 mm in diameter and 28.6 mm wide with a centre-line average surface finish R, of 0.27 + 0.04 pm.

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Suspended load

Fig. 1. Design features of the pin-on-ring wear testing machine.

Changes in the length of the pin caused by wear were monitored with a linear voltage displacement transducer (LVDT) and transmitted to a chart recorder, thereby allowing an uninterrupted measure of the pin wear during a test. Talysurf measurements of the wear track showed that wear of the ring was negligible under all loading conditions used in these experiments.

2.2. Materials for pin specimens Cast iron pin specimens 76.2 mm long and 7.9 mm in diameter were

machined from the centres of cylindrical castings (diameter, 30 mm) which had been prepared under carefully controlled melting and solidification conditions. One series of castings consisted of seven separate hypoeutectic compositions ranging from 2.90% to 3.40% C, with the same microstructure of type A graphite and pearlite. In a second series the average flake size of the graphite was varied, while an approximately constant total carbon con- tent, between 3.32% and 3.38% C, was maintained.

Type A graphite is found to give way to other forms of flake growth as the number of effective nuclei decreases [ 141. An exclusively type A graphite structure was therefore obtained by treatment of each melt with “superseed” innoculant (a proprietary strontium-bearing innoculant from the Union Carbide Corporation) and by control of the factors which determine the number of active nuclei present in the molten iron (i.e. the superheating tem- perature, the holding time at that temperature and the timing of the innocu- lant additions). Additions of 0.15% Sn were made to each melt to ensure a fully pearlitic matrix structure. In the second series of castings, variations in flake size were obtained by adjusting the cooling rates of individual castings during solidification; the faster the cooling rate, the finer and more heavily branched the flakes within the austenite-graphite eutectic cells [ 14 - 161.

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Table 1 lists the compositions of the alloys in both series of cast irons; Table 2 gives the corresponding measurements of flake size. Typical micro- structures of cast irons with small, medium and large flake size are shown in Figs. 2(a) - 2(c). Table 1 also contains measurements of graphitic carbon con- tent, each value representing the average of four separate determinations made by using a combustion technique.

Microhardness measurements of the pearlitic matrix in the first series of cast irons showed an approximately constant matrix hardness of 440 HV 0.050. In the second series the pearlitic hardness could not be controlled so closely; it increased from 428 to 456 HV 0.050 as the flake size decreased.

Wear tests were also conducted on a steel (AISI 9260) with a similar silicon content to that of the cast irons. These specimens were isothermally transformed to produce a pearlitic matrix with a microhardness of 445 HV 0.050. The composition of this steel together with that of the AISI 4340 steel used for the ring specimens is given in Table 3.

TABLE 1

Chemical analysis (wt.%) of cast irons with varying carbon content (cast irons 1 - 7) and varying flake size (cast irons 7 - 10)

cast Total Graphitic Mn Si S P Sn iron

1 2.90 2.18 0.65 2.20 0.055 0.13 0.13 2 3.00 2.24 0.72 2.65 0.055 0.13 0.17 3 3.08 2.32 0.72 2.20 0.055 0.13 0.13 4 3.17 2.46 0.66 2.60 0.070 0.15 0.16 5 3.20 2.54 0.68 2.50 0.070 0.15 0.15 6 3.32 2.58 0.66 2.30 0.065 0.13 0.13 7 3.40 2.65 0.67 2.35 0.065 0.13 0.13 8 3.35 2.58 0.74 2.65 0.070 0.15 0.15 9 3.38 2.60 0.67 2.50 0.050 0.11 0.14

10 3.32 2.56 0.67 2.52 0.060 0.13 0.14

carbon carbon (wt.%) (wt.%)

(wt.%) (wt.%) (wt.%) (wt.%) (wt.%)

TABLE 2

Flake dimensions for the series of cast irons with varying flake size

Cast iron Maximum flake size range

(mm)

ASTM graphite size reference number

8 0.20 - 0.26 4 9 0.36 - 0.46 3

7 0.48 - 0.60 2, 3 10 0.72 - 1.00 2

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Fig. 2. Typicai microstructures of (0.20 - 0.26 mm); (b) medium flake mm).

TABLE 3

cast irons (etchant, 1% Nital): (a) small flake size size (0.48 - 0.60 mm); (c)large flake size (0.72 - 1.0

Compositions of the pin material and of the ring steel

Steel Et*%) Sk.%) pt.%) Twt.%) &.%) Et.%) ;A.%) &.%)

AISI 9260 silicon 0.63 1.93 0.80 0.22 0.22 0.01 - 0.16 steel (pin material)

AISI 4340 steel 0.42 0.26 0.70 - 0.03 1.85 0.17 0.93 (ring material)

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3. Results and discussion

Typical curves of weight loss uersus time are shown in Fig. 3. From each curve it was possible to identify an initial period of transient or running wear followed by an equilibrium wear state during which the wear rate was constant. Measurements of this equilibrium wear rate are plotted as a func- tion of load in Figs. 4 and 5 for cast irons with varying carbon content and varying flake size respectively.

WEIGHT LOSS

6.0 (d)

20 TIME (mtnutes)

Fig. 3. Weight loss vs. time curves for the 3.40% C and 2.90% C cast irons at 200 cm s-* with an applied load of 15 kgf.

It is evident from Figs. 4 and 5 that for each of the cast irons there is an abrupt transition in the wear rate which occurred at a critical load. Associ- ated with the transition in the wear rate was a change in the appearance of the worn pin specimens. At loads below the transition, the surfaces of the pins were relatively smooth with only minor surface grooves. In contrast, at loads above the transition, the surfaces were irregular and heavily damaged giving further indication that a basic change in wear behaviour had occurred. These features are characteristic of a mild-to-severe wear transition; it is well established that such a transition (termed the Ti transition) occurs with flake graphite cast iron [6 - 91. Consequently, the terms mild and severe will be used to describe the equilibrium wear below and above the transition load respectively.

It is also apparent from Figs. 4 and 5 that increasing flake size and decreasing carbon content have very similar effects on the overall pattern of wear rate as a function of load. Both factors result in a significant increase in the mild-to-severe transition load and, at loads above the transition, a decrease in severe wear rate. In the mild wear regime, the wear rate increased linearly with increase in load up to approximately 6 kgf. At loads above 6

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50- SEVERE WEAR RATE

(x 10m6 g/cm 1

40-

30-

20-

10t

MILD WEAR RATE ( x 10-8 g/cm )

i

5ve--k-u 8 10 12 14 16 I8 LOAD (kgf)

Fig. 4. The pattern of wear rate us. load for cast irons with varying carbon contents at 200 cm s-l (0, 3.4% C; 0, 3.35% C; A, 3.20% C; A, 3.17% C; +, 3.08% C!;O, 3.00% C; n , 2.90% C).

Fig. 5. The pattern of wear rate us. load for cast irons of varying flake size ranges at 200 cm s-l (A, 0.20 - 0.26 mm;4 0.36 - 0.46 mm;*, 0.48 - 0.60 mm;O, 0.72 - 1.00 mm).

kgf, the slope of the wear rate uersus load curves decreased for those speci- mens still in mild wear. In general, the wear rate in this regime was not affected by variation in carbon content or flake size; the only exception was that the wear rate for specimens with the largest flake size had a consistently lower wear rate. These trends in wear behaviour cannot be related either to variation in silicon content within the cast irons (see Table 1) or to differ- ences in microhardness of the pearlitic matrix.

It is apparent that increasing carbon content and decreasing flake size have a detrimental effect on wear behaviour, tending to decrease the load at the T, transition and to increase the severe wear rate. This effect of carbon content is consistent with the supposition that the role of graphite particles is to act as discontinuities in the material, providing sites for subsurface crack propagation. Obse~ations by Takeuchi [lo] of surface damage during severe wear suggest that subsurface crack formation associated with graphite

264

flakes may play an important part in the severe wear process of cast iron. Larger amounts of graphitic carbon would create greater opportunities for crack nucleation and propagation and would result in the release of more wear particles.

Extensive metallographic observations have been carried out on wear specimens during the severe wear regime. These observations will be presented and discussed in a subsequent paper.

In the mild wear regime, the wear rates of cast irons may be compared with equivalent results obtained for the AISI 9260 silicon steel (Fig. 6); this pin material represents the same matrix structure as the cast irons without the presence of graphite particles.

A commonly accepted role of graphite during the wear of cast iron has been that of a surface lubricant whereby the graphite in the microstructure tends to spread out over the sliding surfaces thus acting as a self-generating layer which reduces the extent of metal-to-metal contact. Graphite layers of this type are an identifiable feature of the mild wear of cast iron under lubricated conditions [ 171, during sliding in vacuum [ 181 and during sliding in air [19] .

A similar lubricating effect can account for the fact that the mild wear rate of cast irons is lower than that of the AISI 9260 silicon steel (Fig. 6). A reduced frictional interaction caused by the presence of graphite on the wear surface would tend to lower the interface temperature and thereby reduce the rate of production of surface oxide. However, the lack of any detectable difference in mild wear rate between cast irons with varying carbon content

Fig. 6. A comparison of wear rate us. load for flake graphite cast irons and AISI 9260 silicon steel (A, 3.17% C cast iron; r, 2.90% C cast iron; 0, AISI 9260 silicon steel).

Fig. 7. Closure of flakes at the mild wear surface of the 3.35% C cast iron (medium flake size; taper ratio, 3 :l; etchant, 1% Nital).

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(see Fig. 4) suggests some qualification to this view of graphite in the micro- structure as a lubricant. Either the amount of graphite in the smeared layer does not depend on the amount of graphite in the microstructure or the amount of graphite in the smeared layer has no influence on the rate of removal of particles during mild wear.

One factor likely to have a significant effect in restricting the availabil- ity of graphite at the wear surface is the extrusion of graphite in the micro- structure adjacent to the surface and the collapse of the cavity in a manner similar to that detected by Samuels and Craig [ 201 during severe abrasion of flake graphite cast iron. Takeuchi [lo] and Sugishita and Fujiyoshi [19] have found clear evidence of this process during mild wear of grey cast iron leading to the closure of flakes at the surface, and a further example is shown in Fig. 7 for the 3.35% C cast iron.

The result of this process is a restricted availability of graphite from flakes adjacent to the surface. If a sufficient proportion of flakes are sealed off at the surface following their collapse, then variation in carbon content should have only a minor effect on the amount of graphite immediately available to act as a surface lubricant.

4. Conclusions

(1) In the mild wear regime, variation in flake size and carbon content of flake graphite cast iron has virtually no effect on the wear rate. This result is attributed to the restricted availability of graphite caused by the closure of flakes at the wear surface.

(2) Increase in carbon content and decrease in flake size result in a significant increase in the severe wear rate and a decrease in the mild-to- severe transition load.

Acknowledgments

The authors wish to thank Repco Research Pty. Ltd. for financial assis- tance throughout this project and the State Electricity Commission of Vic- toria for the use of casting facilities.

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