Weor,65(1980)131-133 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

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Letter to the Editor

Comments on “The unlubricated wear of cast irons”

Kawamoto and Okabayashi [l] have given a detailed account of their experiments on the unlubricated wear of cast irons in which they relate their results to two temperatures, the flash temperature and the mean temper- ature. In their definition the flash temperature is obtained by converting the highest instantaneous thermo-e.m.f. generated at the sliding interface into a temperature by using the appropriate e.m.f. uersus temperature data. They acknowledge that this flash temperature (which varies in their experiments between 200 and 1000 “C) is not the maximum asperity temperature since many individual asperities of differing temperatures and contact resistances are connected in parallel to provide the e.m.f. from which the flash temper- ature is derived. Their mean temperature is the temperature of the wearing surface as extrapolated from the temperature gradient along the pin assum- ing steady unidirectional heat flow and infinite pin length; the mean temper- atures were between 40 and 400 “C. Thus Kawamoto and Okabayashi infer that an asperity on the cast iron wearing surface is subjected to temperature fluctuations which can rise above the flash temperature when there is con- tact with the countersurface and which can then fall towards the mean tem- perature when there is no contact in the vicinity.

The purposes of this letter are to suggest that these results and conclu- sions are not satisfactorily explained by Kawamoto and Okabayashi and to suggest a model of severe wear which explains many of these experimental results in a way which is consistent with both the thermal conditions of the asperities and the metallurgical behaviour expected of cast iron.

While Kawamoto and Okabayashi give no details of metallographic examination of the worn pins they do report a “hardened white layer” ob- served on 15% of the ferritic iron surfaces when the flash temperature (mea- sured against constantan) was about 700 “C. This is evidence that 15% of the surface had been heated above 800 “C (the eutectoid temperature) in an experiment when the flash temperature was 700 “C. Since the area of con- tacting asperities at any instant is less than 1% of the total area, it is possible that at 400 OC, which is the flash temperature coinciding with the maximum wear rate, a significant fraction of the contacting asperities are at temper- atures above 800 “C!.

The mechanism of severe wear to which this argument points is the phase transformation mechanism [ 21 : when asperities are frictionally heated

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above the eutectoid temperature they transform to austenite with a carbon content which depends on temperature and time above the transformation temperature and on the distance carbon has to diffuse. The mean temper- ature is that to which this austenite is quenched when there is no local fric- tional heating, and if this is below the M, which is appropriate to the austenite composition there will be a cooling martensitic transformation. The proposal is that maximum severe wear rates coincide with repeated heat- ing and cooling transformations in the contacting asperities which disrupt the oxide film on which mild wear depends.

According to this model the transformation kinetics of Fe-C alloys control the wear rate, and the following are the changes in conditions which are expected to reduce the wear rate from its maximum value; they coincide with the changes in conditions expected to reduce the volume transforming: (1) a decrease in flash temperature so that lesser volumes are above the eutectoid isotherm; (2) an increase in flash temperature at constant mean temperature and M, so that those volumes transformed on the heating cycle take more time to cool and are therefore more likely to be reheated hy fric- tion before they reach M, ; (3) an increase in the carbon content of the austenite causing a decrease in M, and therefore, at constant mean and flash temperatures, a decrease in the volume transformed; (4) an increase in the mean temperature so that less martensite is formed on each cooling cycle at constant flash temperature and constant carbon content of the transforming region.

The evidence presented by Kawamoto and Okabayashi is consistent with this in the following ways.

(1) In all cases the wear rate falls sharply from its maximum when the flash temperature decreases.

(2) In each of their three irons the wear rate falls from its maximum as the flash temperature increases above about 700 “C. Other things being equal a higher flash temperature makes it less likely that the requisite time will be available to cool to M,.

(3) This fall in wear rate is much more rapid for pearlitic irons than for those containing ferrite. Austenite carbon content will be higher, other things being equal, when a pearlite transforms; therefore M, will be lower and consequently there will be less volume transforming at the same mean temperature. Clayton [ 31 has reported that the mean free path in ferrite is related to a severe wear rate in carbon steels; the reason may be the same.

(4) As the mean temperature increases to about 400 “C (above the M, for medium carbon austenites) the measured flash temperatures are always above 800 “C! so that the asperities are austenitic and in the absence of sur- face transformations the mild wear regime resumes.

I suggest that there is strong circumstantial evidence for the phase trans- formation mechanism of severe wear in the experiments of Kawamoto and Okabayashi and I regret that they did not discuss the possibility in their paper.

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1 M. Kawamoto and K. Okabayashi, Wear, 58 (1980) 59. 2 J. J. Stobo,J. Amt. Inst. Met., 18 (1973) 147. 3 P. Clayton, ht. Conf. on Wear of Materials, Dearborn, 1979, in Wear, 60 (1980) 75.

(Received July 2, 1980)

J. J. STOBO

Repco Research Pty. Ltd., P.O. Box 349, Dandenong,

3175 Victoria, Australia