Elsevier Sequoia S.A., Lansanne - binted
RUN-XN AND GLAZE FORMATION
in the ~~~~rlands
CAST IRON SURFACES
IX. S. MONTGOMERY* IngersoEl-Rmd
;V.,I. 08540 fU.S.A .)
June IO, r9G9)
The chief processes involved in run-in of cast iron are smooth~g of the surface and formation of a surface coating derived largely from the graphite in the metal structure and iron oxide (Fe&). Much of the smoothing can be attributed to this coating rather than to a wearing off or “squashing” of asperities. It may be almost I. mil thick and its exact composition varies from point to point along the surface, In the case of a specimen run-in with a non-additive oil under laboratory conditions, the carbon content ranged from 30 to 100% and the remainder could be accounted for by I?esOa, although on an engine cylinder bore it contains significant amounts of other components. This “glaze” d~ubtl~s~y imparts a resistance to scuff~g during moment~y contacts between the mating smacks but, ~si~ly more ~rn~~~t, by covering surface irregularities it produces a smooth surface which allows hiiher loads to be carried by a fully hydrodynamic lubricant film. INTRODUCTION
The run-in process can involve a number of different kinds of changes in the metal surface. (I) It is generally agreed that improved conformity between mating surfaces results, although this is seldom explicitly stated. (a) It is also gener~y agreed that mating surfaces become smoother because of fearing or ~~squash~g’~ of surface asperitiesl. (3) In the case of surfaces ~ub~~t~ with oils contend cb~ically active additives, films are formed by the reaction of these materials with the metals. In other cases such as in unlubricated sliding, formation of oxide films occur during run-inal and it has been suggested that the generation of protective surface films during sliding comprises an essential part of the running-in process of machinerys. (4) Physical changes in the structure of the metal as a result of sliding or rolling have been demonstrated in certain cases [email protected]
it is doubtful that these are necessarily associated with run-in, Run-in and formation of glaze on cast iron cylinder bores has been little studied despite its great practical importance. The excellent wearing qualities of cast iron are g~~~~ally att~buted to a film of graphite on the surface derived from the graphitepre*
l?resen~address: Magss Research Center, Watervliet
Wat~rvli~t~ N.Y. [email protected]
sentinitsst~dure-es~‘~, andGRoszEK 1~x2 has been st~~dyingahs~r~t~~nof ail on graphite because he is convinced that it is rtf importance in the lubrication of cast iron, Despite this, there has been no work published where the thickness, composition, or importance of this postulated film has been investigated. EXAMINATION
OF: RUN-TN CAST IRON SURFACES
Glazed and run-in surfaces of pistons and cylinder bores were found to be considerably smoother, as expected, than were the unworn surfaces. This is in keeping with all the experience that running-in produces a smoother surface if it is not already SmO#th. Changes in structure of the iron were ~~v~~t~g~tedby means of rn~ta~~gr~~~~~c studies on a weal-glazed internal combustion engine cylinder bore. The surface of the bore beyond the ring travel was unworn and so was representative of the initial metal surface while the surface within the ring travel was run-in andglazed. A section through the unworn portion of the bore (Fig. I) showed a slight distortion of microstructure to a depth of just aver 0.0001 in.; no similar distortion could be detected in the glazed portion af the bore.
Fig. I. Section through the UnWornportion of a cast iron natural gas engine cylinder liner etched with nital, Note the thin skin of distorted metal.
The thin skin of distorted microstructure on the unworn bore was verified by microhardness measurements. One-to-ten taper sections were made in both the glazed and the unworn portions and nlicr~~bardnes~rn~~~ement~ made at depths of o,ouox, o.soo~g o.~~~~ o.o~o~~~U,OOO~~~ and o.oq in. Twelve measurements were made at each depth owing ta the extreme ~~ar~ab~~ity of the vaiues, and the graphite flakes in the structwre were avoided. ~~~~t~~~n the accuracy nf the nleasurement~, the Kmq.~
1 mil ,_
hardness at all depths on both specimens, with the exception of the 0.0001 in. depth on the unworn bore, were identical at about 335 K.H.N. The O.OOOI in. depth on the unworn bore was slightly harder at about 377 K.H.N. Evidently, the skin of slightly distorted microstructure left by the honing process had been worn off during run-in. It cannot be concluded, however, that removal of distorted metal is an essential part of the run-in process. Presence of a coating on run-in cast iron surfaces was investigated by examining sections through glazed surfaces of internal combustion engine pistons and cylinder bores and through the cylinder bore of a reciprocating air compressor all of which had been protected by nickel electroplates before sectioning. A relatively thick coating was found to cover all the glazed surfaces: it had apparently been derived chiefly from the graphite in the cast iron structure (f;igs. z and 3). No appreciable coating, however, was found on the unglazed surfaces. The thicknesses of the coatings on the engine piston and cylinder bore ranged from 0.0006 to 0.0008 in,, while the maximum thickness on the air compressor cylinder bore amounted to only about o.noorg in. A graphitic coating apparently is characteristic of glazed cast iron surfaces, but its thickness varies from case to case. LARORATOKY
A cast iron test block was run-in, mated with a carburized steel surface using a Dow Corning LFW-I variable speed testing machine operated in the oscillating mode. This mode was chosen because it corresponded with the action of a piston in a cylinder and, moreover, because it should result in more rapid run-in owing to loss of hydra)dynamic lubrication at the end of each stroke. The length of the track on the carburized steel ring was T. I. in and the speed of reciprocation r..4 c/set. The lubricant was a commercial additive-free, purified white minera oil of IIO cSt. kinematic viscosity at IOO'F and it was held at 180°F during the experiment. A load of 30 lb. was used which was the lowest which could be obtained. The initial mean bearing stress was about 24,000 p.s.i., but as the block wore it rapidly became much lower. The contact had increased from a line to an area 0.0535 in. wide after 430,ooo cycles; this corresponds to a mean bearing stress of only 2,24o p.s.i.
Fig.4. The change in surface roughness run-in surface. The scale is in mils.
o of a cast: iron caused by run-in.
(a). Original surface;
During the experiment, the coefficient of friction decreased from 0.21 to 0.12, and the contact area became considerably smoother (Fig. 4). The surface was protected with a nickel electroplate and a section examined metallographically. That part of the surface which had been sliding on the steel ring was covered with a graphitic layer 0.0003 in. thick (Fig. 5) while the surface which had not contacted the steel Wear, 14 (1969) w-105
Fig. 5. Cast iron which had been sliding on the steel in the experiment showing the generated phitic layer.
Fig. 6. Cast iron which had not contacted graphitic layer.
the steel in the experiment
showing the absence of the
We&y, =4 (MN
ring had no such coating (Pig. 0). The UJatjng was contiguous with the graphite in the iron structure.
and visually identical
As an initial experiment, glaze was scraped from the surface of a run-in cylin der bore and compared with scrapings from a location where there had been no wear and no glaze formation, The glazed surface was much more resistant to scraping than was the unglazed surface, although the unglazed surface had been previously found to be slightly harder. This apparent higher hardness of the glazed surface is well known to mechanics and must be attributed to the surface coating. Debyc-Scherrer X-ray diffraction film patterns of glaze showed only the presence of iron oxide (Fe&) and iron itself. The same materials were found in the scrapings from the unglazed surface, but there was very much less iron oxide. Iron oxide, then, is an important constituent of glaze. This is not unexpected since it is the product of the mild wear of iron and steels. The iron found in the scrapings is probably almost entirely attributable to fragments of the substrate iron dislodged by the scraping process. It is surprising that graphite was not identified in the glaze because on microscopic examination it appears to be the important component. Graphite, however, is difficult to determine with this technique. The investigation was continued using an electron probe X-ray nlicroanalyscr. With tlris instrument, the cnnrpositions of the coatings could be studied on metallographic sections at different locations along the surfaces. Ilnfortunately, the specific. form of the element cannot be distinguished; graphite and carbon-containing oil al)pear alike under tire probe as do iron oxide and small particles of iron metal. However, the presence of large iron fragtnents can be detected from the electron scan photographs. The compositions
of the surface coatings
at several locations
on the specimen from the laboratory run-in experiment and on a section of a glazed engine cylinder bore. The iron and carbon contents were different for these specimens and also varied from point to point along their surfaces. The carbon contents of the coatings directly above graphite flakes in the surface structures were appreciably higher than they were for the coatings as a whole. In the case of the coating on the laboratory test specimen, it ranged from 30 to roo”/b. The carbon was doubtless in the form of graphite with some absorbed oil because little, if any, thermal decomposition of the lubricating oil to carbon-containing residue would be expected under the conditions of the experiment. The remainder of the coating on the laboratory test spccimen can be arcounted Ear by iron oxide (I;e&). Glaze, then, on this specimen consisted of only carbon and iron oxide, The carbon content of the coating on the glazed engine cylinder bore ranged from 40 to 7oC;:i1and the Fe& content ranged from IO to 3oCT<).This leads to the significant observation that, while glaze can be a mixture of only carbon and iron oxide, on an authentic engine bore it contained 30-40 o/oof other components. The scan photographs corroborate this observation. This could possibly be partially accounted for by the presence of voids in the coating hut voids are certainly not conspicuous in the micrographs. Furthermore, there are some dense particles of foreign niaterial embedded in the coating, only some of which arc iron metal.
.&m-in on gray cast iron involves a number of processeS;. First, it is certain, although not from this work, that conformity of the mating surfaces improves during run-in if perfect conformity was not achieved in the manufacturing process. Furthermore, the mating surfa.ces become smoother during run-in. On the other hand, a hardening or distortion of the metal surface structure is not. necessarily involved and a thin surface skin of disturbed metal left by the manufacturiI1~ process may he w,vorn off, The most sjgnif~cant finding is that glaze produced on the surface during run-in is an appreciably thick coating derived PdrgeXyfrom the graphite in the east iron structure. It may be almost I mil thick and its exact composition varies from point to point along the surface. In the case of a specimen run-in with a non-additive oil under laboratory conditions, the carbon content ranged from 30 to IOO$;, and the remainder could be accounted for by Fe& On an engine cylinder bore, hnwvver, glaze curtained significant anl~unt~
&.ring steel undergoing cyclic stressing, J. Baszc E:lzc., 88 (1~66) 555. 8 J. j. ULJSH, Iv’. L. CRURE), AND G. H. KOI3lxSON, Microstructural and rcsiduai. stress changes in hardened steel due to rolling contact, 7’raws. Anz. Snc. ,“dr?tals, 5.# (1961) 390. 9 J, W LAAIJELLE, Wear resistance of cast-iron components, in Hwzdhook of Msckanmzi II’Puu, C. LIPSON AND L. V. COLWBLL, Eds., I;niv. of PIichigan Fress, Ann Arbor, 1961, p, 378. IO H. T. ANGUS AND A. Pk. I,A~B, Destruction of cast iron surfaces under conditions of dry sliding wear, PYOC. Conf. ~_~4~~~&~~~f~~z awd Wrer, ~~~i~~~~,r957. Inst. Mech. Eqrs., ~I_ondon), “957. p, 789, I 1 A. J. GROSZEK, Preferential absorption of compounds with long mcthylene chains on cast iron, graphite, boron nitride, and molybdenum disulfide, TYUPS. .?w. Svc. Lubvacat?vn Eqvs. I ‘) (1966) 67. ra A. J, GROSZEK, personal communication, I ~08.