Wear mechanisms in hard-facing alloys Cobalt-based alloys, such as the Stellite series, find application in the hardfacing of machine components. However, the relation between the morphology of the facing material and the mechanism of abrasive wear has not been elucidated fully. The structure of the alloys consists of a mixed chromium carbide eutectic in a strong, ductile cobalt-based matrix.
the Metallurgy group of the Division. In addition, the Production Engineering group have an interest in the performance of cutting tools in metal machining and the Engineering Ceramics group in the performance of zirconia and zirconia-based cermets dies in metal forming operations, zirconia nozzles in paper pulp manufacture and molybdenum disilicide bonded boran carbide for such applications as shot blasting nozzles. In these projects, the main emphasis is on engineering aspects of production and usage rather than tribology.
There is no difference in the fundamental wear mechanism operative with the different abrasive media such as sandstone, silica powder and silicon carbide powder. The observed difference P. 114.ROBINSON in the rates of abrasion is attributable to abrasive particle size and the energy REFERENCES associated with the particles moving over the surface. Neither the surface of 1 Bailey S. O. and Perrott, C. M. 'The the abrasive particles nor the surface of behaviour of a multiple cutter array the workpiece show any evidence of on a tunnellingmachine', Proc. Symp Wear, Inst. Engs. Aust., Perth (1974) the formation of machining chips during abrasion. The abrasion of the sur2 Bailey,S. G, and Perrott, C. M. 'Wear processes exhibited by WC-Corotary face occurs by the indentation of the cutters in mining', Wear, 29, (1974) abrasive particle into the cobalt matrix, 117 followed by a ploughing action to form 3 Perrott, C. M. 'Design faults in cutters a groove which results in heavy deforfor raise boring' Division of Tribomation of the surrounding matrix. physics Report 30/ME 1974. Some of the material displaced from 4 Perrott, C. M. 'Rock drilling experithe groove forms a built-up edge along ments progressreport', Division of its length while the remainder is exTribophysics Report to Titan-Fagersta Ltd., (1973) truded through the carbide eutectic honeycombe to appear as protrusions 5 Perrott, C. M. and Robinson, P. M. 'Mechanisms of wear of tungsten on the surface away from the groove. carbide cobalt tools. I. Rock drilMaterial removal from the surface ling and cutting applications', J. Aust. occurs by the adhesion and smearing of Inst. Metals, to be published the protruding matrix onto the surface 6 Perrott, C. M. and Robinson, P. M. of subsequent abrasive grains which 'Mechanisms of wear of tungsten carpass across the metal surface. At low bide cobalt tools. 11. Metal cutting stress levels, the carbide phase in the structure interrupts the grooves formed by the abrasive particles but at higher stresses the carbide bridges deform plastically and groove formation becomes Abrasive wear in agricultural continuous. Deep etching experiments after abrasion indicate that the carbide bridges eventually fail by ductile fracIn a survey of the durability of agriculture after extensive plastic deformation. tural machinery I undertaken in 1966/7, abrasive wear was found to be responThe abrasion resistance of the alloy is sible for about 40% of the failures. associated, therefore, with the ability Damage occurs primarily to soil engagof both the matrix and the carbide ing equipment, but also to seals or maceutectic to deform plastically and abhine parts between which soil or other sorb the energy associated with the particles can penetrate, by abrasives enmoving abrasive particles rather than trained in fluids, and in the cutting, milwith the resistance of the hard carbide ling and processing of crop materials. to micromachining. The wear meIt has recently been estimated that the chanism is an example of adhesive wear cost of replacement parts for tillage imduring abrasion 16. plements is of the order of £8 million per annum in the UK, and the labour OTHER ACTIVITIES cost for replacing these worn parts is of the order of £1.5 million per annum. The work on materials tribology desIn addition, interruptions in work plans cribed above is part of the activity of
applications' Z Aust. Inst. Metals, to be published 7
Larsen-Basse, J., Perrott, C. M. and Robinson, P. M. 'Abrasive wear of
tungsten carbide cobalt composites. I. Rotary drilling tests', Mater. ScL Eng., 13, (1973) 83 8
Blombery, R. 1., Perrott, C. M. and Robinson, P. M. 'Abrasive wear of
tungsten carbide cobalt composites. 1I. Wear mechanisms, Mater. ScL Eng., 13, (1973) 93 Blombery, R. 1. and Perrott, C. M. 'Wear of tungsten carbide - cobalt composites during abrasion on Silicon carbide', J. Aust. Inst. Metals, to be published
'The classification of hard-facing consumables, draft Aust. standard', Australian Welding Research Association, Document P8-18 74
Blombery, R. I. and Perrott, C. M.
'How to use the AWRA system for the selection of hard-facing alloys', Australian Welding Research Association, Document P8 17 74 12
Perrott, C. M. and Robinson, P. M.
'Hard-facing of machine components, some factors affecting the validity of field trials, Australian Welding Research Association, to be published 13
Blombery, R. 1. and Perrott, C. M.
"The optimum service lives of wearing components', Proc. Sym. Wear, Inst. Eng. Aust., Perth (1974) 14
Blombery, R. I. and Perrott, C. M. 'Planning of field tests for breaker steels, Division of Tribophysics Report 6/ME 1973
Blombery, R. 1. and Perrott, C. M. 'Wear of sprayed tungsten carbide hardfacing deposits', Wear, 29, (1974) 95
Blombery, R. I. and Perrott, C. M.
"Adhesive Wear Processes Occurring during abrasion of stellite alloys', J. Aust. Inst. Metals, to be published
mach inery affect timeliness, particularly during seeding or harvesting; crop losses may occur as a result of worn components; and for all operations energy consumption increases with wear. Thus there is considerable incentive for research work on abrasive wear, and at the National Institute of Agricultural Engineering this is being conducted with the object of understanding the wear process and assessing the performance of a wide range of materials under two-body abrasive wear, It is hoped that research will eventually lead to improved wear-resistant materials, materials application and design.
TRIBOLOGY international February 1975
ABRASIVE WEAR B Y SOIL Work has been carried out over a number of years with the objective of ac-
counting for abrasive wear in terms of properties of the wearing materials and the soil. In field trials with soil cutting edges it has been found 2 that the wear characteristics of the soil depend upon the type, shape and size of the abrasive; the type, shape and texture of stones; the resistance of the soil to penetration by stones; and the extent of shattering of the abrasive or stone surface in contact with the cutting edge. In British soils, the abrasive is quartz or flint, with Vickers diamond pyramid hardness of about 11 000 MN/m 2, occurring in a wide range of particle sizes and the loading of these particles is caused by contact of an implement with stones which are forced through the soil.
groove and the material lying below the surface. Thus the principal variable in determining the abrasive wear resistance of such a material is its stress-strain behaviour. ASSESSMENT OF WEAR RESISTANT /VIA TER/A LS The determination of the wear resist-
ance of materials is a lengthy task, since misleading results can be obtained if studies are limited to the laboratory. Wear studies on abrasive papers show, for example, the importance of the abrasive grit size and applied load, but do not reproduce the exact conditions met in the field in respect of the wide size
range of abrasive particles, and of stone impacts and contacts. Recent work has been aimed as assessing the efficacy of hardfacings and surface diffusion treatments 7, since these may have a considerable future in improving the wear life of soil working equipment. Tests covered a wide range of materials and deposition methods, and attempts have been made to correlate the relative wear resistances with the microstructural properties of the coatings and variables in the wear environment. Hardfacings cannot be applied indiscriminately to wearing parts because of
The effect of some field variables on the abrasive wear of materials can be simulated in the laboratory using a testing method established by Khrushchov a, in which small cylindrical specimens are worn on abrasive papers (Fig 1). THE MECHANISM OF ABRASIVE WEAR
Over the last five years a study has been made of the damage to surfaces worn by abrasion and by a trepanning operation 4. During abrasion of ductile materials considerable plastic deformation occurs, and natural strains up to eight have been revealed by a specially manufactured composite material s (Fig 2). Where wear grooves are geometrically similar, the surface deformation zones lying below these grooves are also geometrically similar and the strain as a function of depth below the surface is proportional to both the abrasive grit size and to the square root of the load per unit area. At the surface, the strain is independent of the grit size and load, and the limiting strength of worn surfaces 6 is determined by the metallurgical structure of the material, the flow stress prior to wear, the strain hardening capacity of the material and, in some cases, the fracture properties of the material and abrasive, and the stress system developed during the wear process. Measurement of the microhardness and strain distributions below worn surfaces has provided data for an analysis of the energy balance during abrasive wear s . In ductile materials the work done in forming a wear groove is largely balanced by the energy expended in plastically deforming the material from the
The NIAE laboratory abrasion testing machine
Fig 2 Plastic deformation o f a copper- silver solder laminate material after abrasive wear
TRIBOLOGY international February 1975
their cost. Current work is designed to assess the optimum extent and pattern of coverage to provide economic wear life. THE EFFECT OF MICROSTRUCTURE ON ABRASIVE WEAR Work on a variety of materials has led to the conclusion that materials with high soil abrasive wear resistance must have a structure which includes a phase much harder than the abrasive in a matrix which approaches or exceeds the hardness of the abrasive. Both the hard phase and the matrix should ideally be capable of strain hardening under wear, but the initial condition may be achieved by mechanical and/or thermal processing. The scale o f the structure is important. There is a significant effect of relative size of the hard phase and wear debris when the abrasive is both very much harder than the hard phase 8,9 and when the abrasive is o f the same hardness or
softer than the hard phase ~°. When the hard phase is relatively small, the effect is merely to strengthen the material as a whole but when the hard phase is relatively large it may offer a powerful obstruction to penetrating abrasive particles. Work is planned to investigate the effect of hardness, structure, size, shape and dispersion o f the hard phase, and the effect of hardness and other mechanical properties o f the matrix.
E. F. GOBEL. Translatedand Edited by A. M. BRICHTA Newnes-Butterworths; 211 pp; £5.80 To many engineers whose principal interest and function in life is the pressing job of converting a neat conceptual design into a real-life piece o f machinery, nothing is more irratating than to consult a b o o k which purports to treat the problem in hand but which presents it in a form so generalized or obscure as to defy immediate recognition. While it may be flattering to realize that the remnants of one's intellectual curiosity are being appealed to, pressure of time or circumstance usually denies the full measure of gratification. Those who are prepared to admit to the occasional - or perhaps frequent - experience of such emotions will welcome A. M. Brichta's translation o f the third German edition o f Dr Gobel's b o o k on the design of rubber springs. It is clear, direct, prac-
114.A. MOORE 7
Richardson, IL C. D., Jones, M. P.,
and Attwood, D. G., 'A pilot survey of the durability of farm machinery', Proc Agric Engng Syrup, Division 2, paper 26, lnst Agric Engrs. (1967) Richardson, R. C. D., 'The wear of metal shares in agricultural soils,' Ph.D. thesis, London, (1969)
JBooh Rubber spring design
tical and well-aimed at the needs of the design engineer. For the most part, the subject matter is covered comprehensively and in a well balanced manner. The first twothirds o f the b o o k is devoted to basic conce F is, calculation methods and principles o f construction of different spring forms. The remainder is well stocked with specific examples from a wide field of typical applications, including such diverse areas as instrument mountings, bridge bearings and the use o f rubber as a pressure transmission medium in metal forming. While there is much to praise in the simplicity and directness of the treatment, in some respects the b o o k can be criticized for the failings of its virtues. Certain topics are not simple, yet there is a tendency for these to be dispatched with the same concise finality as is appropriately applied to the more obvious points. A particularly notable example occurs in the context o f dynamic stiffness. The fact that the stiffness of a rubber spring increases
Khrushchov, M. M. 'Resistance to wear by abrasion as related to hardness', lnst Mech Engrs Conf Lubric Wear, paper 46, pp 655-9 (1957) Richardson, R. C. D., 'The maximum hardness of strained surfaces, and the abrasive wear of metals and alloys', Wear 10 (1967) pp 353-82 Moore, M. A. 'The strain hardening and fracture of ferritic materials under abrasive wear', Ph.D. thesis, Newcastle upon Tyne, (1974) Moore, M. A., Richardson, R. C. D.
and Attwood, D. G. 'The limiting strength of worn metal surfaces, Metallurgical Trans, 3 (1972) pp 2485-91 Moore, M. A. 'The abrasive wear resistance of surface coatings' To be published in J Agric Engng Res. Richardson, R. C. D. 'The wear of metals by bard abrasives', Wear 10 (1967) pp 291-309 Moore, M. A. 'The relationship between the abrasive wear resistance, hardness and microstructure of ferritic materials'. Wear 28 (1974), pp 59-68 Richardson, R. C. D. 'The wear ofme~ tals by relatively soft abrasives', Wear I t (1968) pp 245-75
with the frequency of vibration is not only a point of great interest to the curious but also one of potential importance in the field of resiliently mounted high speed rotors. The b o o k whets the appetite by bringing the topic under a section head, but then proceeds to dismiss it in less than half a page with a recommendation to apply correction factors of 1.1 to 1.4 (depending on the rubber). Unfortunately, we are not enlightened as to why the phenomenon occurs or why a fixed correction factor is adequate or at what frequency it becomes necessary to employ it. It would, howeverl be somewhat churlish to over-emphasize such rare lapses, as it would to complain o f the occasional translation infelicity such as 'Synthetic rubbers are produced by chemical synthesis in the chemical plants of the chemical industry'. The overall impression is o f a comprehensive yet concise book which gets quickly to the point. It is packed with useful information, is clearly and copiously illustrated and is convenient for ready reference. The book has been attractively produced and, for a technical work in today's inflationary climate, is not unreasonably priced. R. E. L ECKENB Y National Centre o f Tribology Risley, UK
TRIBOLOGY international February 1975