Abrasion (or abrasive wear) is probably the most easily recognized form of wear. It is self-evident, for example, that hard particles will scratch softer surfaces, when they are forced against, and moved relative to, those surfaces. Not to be confused with solid particle erosion, which involves the striking of surfaces by gas-borne particles, abrasion is normally associated either with surfaces involved with the movement of packed particles (soil, sand, rocks, etc.), or hard particles trapped between machine surfaces. The former case is generally known as two-body abrasion or low stress abrasion; the latter is known as three-body abrasion or high stress abrasion. High stress abrasion is generally regarded as the more severe, because it can induce fracture of the abrasive particles, thereby ensuring the presence of sharp edges for the cutting action.
In the field of metallic materials, it has been found that alloys whose microstructures contain large volume fractions of hard precipitates (carbides, for example) provide the highest resistance to two-body (low stress) abrasion. Thus, the high-chromium irons, which contain large precipitates of chromium carbide, are favored for many earthmoving applications. For even higher resistance to low stress abrasion, mixtures of steel and carbide particles are available for co-deposition by welding. In this case, the steel melts in the welding arc, but the carbide particles are transferred intact from the welding consumable to the weld pool, where they become locked in place by the re-solidifying steel. These so- called composite materials normally contain tungsten carbide.
While a high volume fraction of carbides or intermetallics may be beneficial to low stress abrasion resistance, it is very detrimental to ductility. Consequently, it is common for weld overlays of the high chromium irons and tungsten carbide composites to crack during cooling, or upon impact in service. To cope with conditions requiring a modicum of ductility, therefore, alloys with moderate precipitate contents are available.
Cast cobalt alloys with different levels of low stress abrasion resistance are available, and, as with the high chromium irons, the higher the carbon content, the higher in general is the resistance to this form of wear (in the case of castings and weld overlays). In choosing cobalt alloys, however, the need for low stress abrasion resistance is often tempered by accompanying needs for corrosion resistance and crack-free overlays, both of which require the carbon content to be minimized.
To assess the low stress abrasion resistance of materials in the laboratory, the dry sand / rubber wheel test described in ASTM Standard G 65 is normally employed. The test procedure involves forcing a sample against a rotating, chlorobutyl rubber wheel (of diameter 229 mm), while feeding sand of a well-defined size and shape (rounded quartz grain sand, 212 to 300 µm diameter) to the wheel / sample interface at a specified rate. Data (relating to 2000 revolutions of the rubber wheel) for several wrought alloys are presented in the following chart, along with the corresponding value for STELLITE® 6 weld metal. These include 6B, which is compositionally similar to STELLITE® 6 alloy, but which exhibits much higher resistance to low stress abrasion by virtue of a more beneficial carbide structure. Other alloys include a tool steel (D-2), a carbon steel (1020), an austenitic stainless steel (316L), two low-carbon, cobalt-based materials (HAYNES® 25 and ULTIMET® alloys), and two nickel-based materials (625 and C-276 alloys). The difference in performance between Alloys 6 and 6B indicates the advantages of considering alternate product forms, in attempting to solve wear problems.