Cylinder liner wear in low speed diesel engines

Cylinder liner wear in low speed diesel engines J. Nadelt and T.S. Eyre*

Wear in low speed diesel cylinder liners can vary over wide limits: generally up to 0.15 mm per thousand hours of operation on diameter is considered acceptable. In some cases wear of 2 or 3 mm per thousand hours may occur and would be considered most unsatisfactory. Most of the high wear cases were reported in the 1950's and could be associated with two changes which occurred: heavy fuels containing a high sulphur content began to be used and overcharged engines were coming into use. Since then a continual increase in the output per cylinder has taken place (Fig 1) with an inevitable increase in mechanical and thermal stresses. For many years high wear rates were attributed to corrosion and sulphur in the fuel was thought to be the primary cause: manufacturers even proposed the use of an austenitic iron to improve corrosion resistance. The introduction of vanadium (up to 0.25%) into the metallurgy of the cast liner brought about a considerable reduction in wear. More recently, high wear rates have been attributed to scuffing. Although scuffing is now generally accepted as the main reason for high wear rates, it was generally thought that scuffing depended more on the running conditions than on the cylinder liner material. A more likely explanation is that as the power outputs have increased it has become increasingly difficult to maintain an adequate oil f'dm between the piston and liner surfaces. Under conditions of marginal lubrication metal to metal contact occurs and the metallurgy of the surface then becomes important I . *Department of Metallurgy, Brunel University, Kingston Lane, Uxbridge, Middlesex UB8 3PH, UK tDaros, $433 01 Partille 1, Sweden /, /

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Before solutions to the problem of wear can be developed it is necessary to study the problem itself and outline the main characteristics of wear. It is, therefore, necessary to examine the topography and metallurgy of worn cylinder liners and to compare these with the available research literature to identify the basic mechanisms. It should then prove possible to establish a link between wear in service and wear mechanisms studied under laboratory conditions. If this is achieved, improved materials can be developed based primarily on laboratory wear machines. Scanning electron microscopy (SEM) has been used in this investigation for direct viewing of the wear topography combined with optical microscopy through wear surfaces to study the metallurgical changes leading to wear damage. Without exception grey cast iron is used for cylinder liner castings. These irons generally contain 'A' type graphite flakes in a peaditic matrix. Some manufacturers alloy to produce dispersed hard phases of phosphide or carbide, or in some cases both. Castings may contain up to 0.6% phosphorus and up to 0.3% vanadium. Some castings contain up to 0.8% copper, usually present to ensure freedom from ferrite. The precise microstructure is controlled by its carbon equivalent value, alloying additions, section thickness, and its precise cooling rate. The carbon equivalent value (CEV) is given by: CEV = % C +

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CEV is related to section thickness (Fig 2) and it will be observed that for a constant melt composition the precise microstructure will depend upon the section thickness because of its effect on the cooling rate. Different section thicknesses in the same casting may, therefore, have different microstructures and mechanical properties. Different size liners of the same composition will also have significantly different structures. Although the composi.tion and therefore the microstructure of the iron varies from

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Table 1 Composition of cylinder liner cast irons Element

C

Si

P

Cr

V

Cu

Composition, %

2,5-3,5

up to 1,5

up to 0.8

up to 0.3

up to 0.3

up to 0.8

one manufacturer to another, Table 1 gives an indication of the range to be expected. Although this investigation concentrates exclusively on wear of cylinder liners, the influence of the piston ring metallurgy must not be ignored. This is of particular importance now as a wider range of piston ringcoatings are already being used or are under investigation. The actual liners examined do not represent carefully controlled experiments but are drawn from a wide range of those that have been commented on from time to time by customers in the field. The evidence has been collected over a long time. The main objective, therefore, is to present evidence about the wear mechanisms that appear to be associated with a whole spectrum of behaviour as reported by engine operators.

Fig 3 Cylinder liner surface showing very low wear

Fig 4 Unworn cylinder liner

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Cylinder liner with low wear Wear after 60 000 h was 0.01 mm per thousand hours and was therefore satisfactory. The cast iron contained phosphorus, copper, vanadium and titanium. The wear surface is covered with parallel grooves (0.0010.004 mm/s wide) and evenly dispersed small pits (Fig 3). These parallel grooves, which are typical of abrasive wear, have probably been caused by the grinding action of ash, oxides etc, from the fuel or air. The graphite flakes can be clearly seen so that no significant plastic deformation has occurred. It would appear that an abrasive action has been responsible for polishing away the original machining which almost completely obscured the graphite flakes (Fig 4). Within the pitted regions shown in Fig 3 a plate like structure is evident (Fig 5) and this can be attributed to the selective chemical action of the sulphuric acid formed from the product of sulphur and condensation. The areas of phosphide eutectic can also be seen and it would appear that it has been attacked more extensively than the pearlitic matrix. Carbides are least affected either by the abrasive or chemical action and stand out in relief (Fig 6). In some areas the surface attack is rather patchy (Fig 7), the upper part being bright and reflective whilst the lower half is etched. This appearance may be attributed to differential corrosion caused by differences in wettability, or a difference in surface structure brought about by work hardening or phase transformation in the bright areas. Fig 8 shows a micro section through a liner, the edge being protected with a layer of electroplated copper. There is no deformation of the pearlite or graphite while there is deeper etching of the phosphide and carbide stands proud of the wear surface. Wear would appear to be caused by an abrasive action removing the original machined surface and gradually revealing the graphite flakes. There is no evidence of surface deformation or scuffing. Surface corrosion would appear to be responsible for producing an irregular surface

Fig 5 Etching of both pearlite and phosphide eutectic

topography consisting of shallow pits and protruding hard phases.

when wear progresses further to expose the phosphide. The presence of these hard particles (650-850 Hv) could then contribute to further wear by abrasion.

Liner with medium wear The liner shown in Fig 9 has worn at 0.7 mm per thousand hours. The iron contains vanadium and chromium but no phosphorus, titanium or copper. There are no light abrasion marks although there is evidence of some larger grooves (up to 0.01 mm) which are, however, well rounded.

Wear in different positions on the liner The examples discussed so far have not been located on any particular part of the liner surface. In another examination it proved possible to select samples from the top, middle and bottom dead centre position of a

There is evidence of corrosion within the grooves and generally over the whole surface. Corrosion is both more extensive and more well developed than that shown in Fig 3. The evidence is rather inconclusive, but it would appear that at some time heavy abrasive wear has occurred which has now changed to pred.ominantly corrosive wear.

Heavy wear Wear at the rate of 1 mm per thousand hours of operation has occurred. The liner contained copper, vanadium, titanium and phosphorus. Fig l0 shows that there is no evidence of the graphite structure or of surface etching. The surface consists of a plate-like flowed structure leading to the appearance of delamination which would be expected to produce metallic plate-like debris. There is some evidence of abrasion but this is not the predominant mechanism. The presence of transverse cracks in the flowed layers is evidence for the surface stresses exceeding the tensile strength of the metal. A number of features are observed from microsections taken from this liner. Fig 11 clearly demonstrates the plastic flow which has lead to the development of the flowed plate-like structure shown in Fig I0. Flow would appear to be concentrated around the graphite flake structure and it is possible to visualise how debris fractures from this surface. Fig 12(a) shows evidence of plastic deformation of the pearlite and immediately at the wear face the carbide plates have broken up into smaller particles. There is no evidence of structural changes which would indicate surface heating. Fig 12(b) shows how the phosphide eutectic has broken up immediately below the wear surface. This could contribute to hard particles falling out of the surface

Fig 7 Differential etching

Fig 8 Microstructure era worn layer

Fig 6 Carbides standing proud of etched pearlite

Fig 9 Cylinder liner with medium wear

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Fig IO Cylinder liner with very heavy wear

Fig II Microstntcture through a liner with heavy wear

Fig 12 Microstructure through cylinder liner with heavy wear

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Fig 13 Wear at different positions on the same liner (a) tc dead centre (b) mid-position (c) bottom dead centre

of a large number of liners. It is, therefore, possible to draw some general conclusions for which a good deal of qualitative evidence is available. Three wear mechanisms operate either separately or together: abrasion, corrosion and scuffing. Of these, scuffing appears to be particularly catastrophic, producing metallic plates of debris by a process of delamination. The surface characteristics are similar to those observed during running-in of piston rings, although in these liners there is no evidence of 'white layer' formation. There is also considerable similarity to 'severe' metallic wear which takes place above the mild/severe transition load, where no 'white layer' is normally observed. A plate like debris is, however, produced.

Fig 14 Micro-section through top dead centre wear area

liner which had exhibited a very low wear rate. At the top dead centre position wear was primarily by abrasion and the rubbing action revealed what appeared to be hard phases standing out in relief (Fig 13(a)). A differential wear process appeared to be operating, particles which ploughed through the pearlite were obstructed by the hard phase, 'climbed' over the top, and wear continued on the other side of the obstruction. At the mid position there is no evidence of abrasion but the microstructure of the pearlite and the phosphide hard phase are clearly evident (Fig 13(b)). At the bottom some of the original machining is still evident, graphite flakes are visible, abrasion has taken place but there is no evidence of either the differential wear referred to earlier, or of corrosion (Fig 13(c)). Micro examination showed that there was a relatively large volume of hard phases present (12%) and these stand proud of the wear surface at the top dead centre position (Fig 14).

Discussion The photographs which form the basis for this investigation are representative of a large number which have been collected over a period of years and cover the examination

The presence of hard phases would appear to increase the mild/severe transition load. In this respect the presence of both phosphorus and vanadium would appear to be advantageous. Where hard phases are present, both differential wear and corrosion processes occur and there is some evidence to suggest that the former is advantageous while the latter may contribute to increased corrosion rates. Phosphide is attacked by the corrosive media more rapidly than carbides. Both the type and extent of wear varies down the cylinder liner wall and to improve resistance to abrasive wear it may only be necessary to protect the cylinder liner for some distance just below the top dead centre position. An increase in corrosion resistance would be particularly significant over the middle of the liner. Abrasion appears to be responsible for the normal mechanical wear that occurs in the majority of liners. In liners with a long wear life corrosion is observed and this may contribute indirectly to increased abrasion. The combined action of abrasion and corrosion appears to produce a smooth surface with well defined graphite and a pitted and etched surface which may aid the retention of lubricant on the surface.

References 1

Gotothan D.W. A review of the causes of cylinder liner w e a r in marine diesel entries.Inst. Mar. Engrs. (1978) Vol 90, Ser A, Pt 3 of 7, 137-163

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