On the importance of work hardening in the design of wear-resistant materials

HARDNESS DENSITY STRAIN PLASTIC
7
DISTANCE BELOW SURFACE
Fig. 1. Schematic illustration of the deformed surface layer of an abraded or eroded material. The curve on the right-hand side indicates the change in microhardness, dislocation density or strain with depth below the worn surface.
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Wear, 91 (1ห้องสมุดไป่ตู้83) 201 - 207
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ON THE IMPORTANCE OF WORK HARDENING WEAR-RESISTANT MATERIALS
A. BALL
IN THE DESIGN
OF
Department of Metallurgy and Materials Science, University of Cape Town, Me&es Building, University Private Bag, Rondebosch 7700 (South Africa) (Received March 16, 1983)
203
STRESS
MEAN
LEVEL
OF
---------
APPLIED
STRESS
FREQUENCY OF IMPOSED OF ABRASIVE
DISTRIBUTION OR STRESS LEVELS STRIKES
SPECTRUM
STRAIN
(No’s
of strikes
at a given stress
2. Rationale Material is lost from a metal surface by abrasive and erosive wear when a critical fracture strain condition is experienced. A worn surface can be considered to have reached this critical strain. This strain may be achieved locally after a single abrasive strike or as a cumulative effect of many strikes.
1. Introduction The objective of much of the research concerned with mechanical properties is the “design” of a material which will provide the optimum resistance to plastic deformation and fracture. “Materials design” consists of choosing the composition and heat treatment of material which will produce a microstructure with good resistance to the imposed deformations. This philosophy has proved successful, for example, in the development of aluminium alloys with high yield strength and adequate toughness for use in the aircraft industry. For abrasive and erosive wear obviously plastic deformation and fracture occur but the wear resistance of a microstructure cannot as yet be assigned to any one parameter such as yield strength or hardness which can be conveniently measured in the laboratory. The difficulties have been discussed in numerous papers by many workers. Recent works include those by Bauschke et al. [ 11, Foley and Levy [2] and Soderberg et al. [3]. This problem is addressed in this short paper and the important characteristics which may provide a basis for the design of material with wear-resistant microstructures are considered.
level)
Fig. 2. The hypothetical stress-strain curves for the wear of three classes of materials superimposed upon the frequency distribution of abrasive or erosive strikes of a given stress magnitude.
Summary By considering abrasive and erosive wear as a consequence of a series of impacting strikes within a given stress range, it is possible to demonstrate the advantage of materials with high work-hardening rates.
concentrated and microfracture of the surface layers will result. This type of material has no ability to blunt such an event and brittle spalling of the surface will take place. Materials with stress-strain curves of type II (Fig. 2) are soft and have high ductility. Although these materials show high strains prior to fracture, these strains are produced at low stress levels. The majority of abrasive strikes will create stresses which will immediately cause shear of the surface beyond the critical strain for fracture and materials will be lost by abrasion after one or more strikes. Clearly materials of types I and II do not have potential as abrasionresistant materials. The areas under their stress-strain curves are small and the toughness or work to fracture with respect to abrasion is low. On this basis a material with a stress-strain curve of type III has an advantage. The first abrasive strike at the mean applied stress level causes a limited amount of plastic strain and work hardening. However, if further strain is to be imparted into the surface a strike of higher stress is required. In practice this is probable, but the frequency of strikes of higher stress levels becomes
0043-1648/83/$3.00 @ Elsevier Sequoia/Printed in The Netherlands
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Analogies can be made with conventional stress-strain curves, but it must bc remembered that the strain is imparted at a very high rate and conditions of triaxiality exist when a surface is scratched by a tool or abrasive particle. Also, the deformation is dynamic in the sense that, as material is lost at the attainment of the critical strain, material well below the surface is at the point of plastically yielding. The situation is depicted in Fig. 1. If one wishes to design an alloy to resist abrasive wear, then attention must be given to providing a microstructure which ideally never accumulates the critical fracture strain under the imposed abrasive stress conditions or at least has a long lifetime before the critical strain is accumulated. Three possible approaches to achieving this can be considered (Fig. 2). Any item of machinery will experience a spectrum or distribution of abrasive stresses (as shown in the figure), but a mean level o, is considered initially for simplicity. Materials with a stress-strain curve of type 1 (it should be remembered that these stress-strain curves are hypothetical and refer to the high shear rate conditions and triaxiality pertinent to abrasion) should provide ideal resistance to abrasion since the applied stress never causes plastic yielding no matter how many times it is applied. However, materials with this type of stress-strain behaviour are very susceptible to sudden fracture. Any inhomogeneity in the structure will cause the applied stress to be