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TEST METHODS FOR ABRASION/EROSION AND COMBINED STRESSES
2018/11/28
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Abrasive wear is a widely dominant wear mechanism especially in a lot of industrial applications. According to a “classic” definition by SAE [4] abrasive wear concerns the removal of material from a surface by mechanical action of abrasive (hard) particles in contact with the surface. Arbitrary classifications of abrasive wear are based on observed conditions.
Gouging Abrasion: The result of this type of abrasive wear is the removal of large particles from a metal surface. Worn surfaces show heavy gouges.
High Stress Grinding Abrasion: This type of abrasive wear occurs during the progressive fragmentation or grinding of the abrasive which was initially of small size and takes place on the surfaces employed to grind the abrasive. The wear is believed to be caused by concentrated compressive stress at the point of abrasive contact and to result from plastic flowing and fatiguing of ductile constituents and cracking of hard constituents of the metal surface. The use of the words “high stress” in this classification is intended to imply that the crushing strength of the abrasive is exceeded.
Low Stress Scratching Abrasion or Erosion: The result of this type of abrasive wear is scratching of the metal surface, and the scratches are usually minute. The stress imposed on the abrasive particle does not exceed the crushing strength of the abrasive.
Abrasive counterparts or particles are grooving the functional surfaces of machine components or parts, like tools, guidances, and raceways, under various tribological interactions. Numerous basic operations to process raw materials, among them crushing, classifying or conveying, are typical for mining, steel and many other industries, and unavoidably related to abrasion and different damaging effects due to abrasive particles, like erosion, peening-like processes and also impacting.
Core components of converting plants such as crushers are exposed to heavy wear and require efficient surface protection measures in order to avoid costly downtimes and to reduce costs for expensive spare parts [5]. Both wear resistance against abrasion and/or impact or the ability to withstand other complex mechanical actions are often required to maintain the material’s structure and shape of machine components and to extend the lifetime of machinery equipment efficiently.
CONDITIONS FOR SPECIFIC TEST PROCEDURES:
Different wear mechanisms and the resulting wear amount show a major influence on both the affected materials and the abrasive matter, e.g. depending on the kinematic and kinetic properties of abrasive particles,which is not surprising from a tribological point of view.
Though the systems’ approach should govern the considerations of wear behaviour a prevailing attitude can be observed in order to characterise the applicability of materials for certain conditions simply from material oriented tests. This is especially true for the design and selection of hardfacing materials like iron-based alloys that are to protect machinery equipment. The selection of the most effective wear protection solution, especially in case of combined wear, is either related to longterm practical experiences, in situ tests or to applying alloys according to their hardness or the content of specific hard phases such as tungsten carbide. Simplified tests are feasible in terms of economic restrictions and the need for statistically relevant results in a reasonable time frame. Thus a number of test methods have been introduced, more or less helpful for qualifying application oriented material properties with regard to tribological behaviour.
Yet it is unavoidable to study into detail the material behaviour and the material structure being aware of the specific stresses and wearing conditions that obviously vary according to the concerned application each.
In spite of expectations that might arise from practical engineering it is not seriously possible to characterize the tribological performance of materials simply from a single test. In order to evaluate the wear resistance under different fields of operational demands, it is necessary to make use of several test methods that not only provide abrasion but also other types of tribological stresses. This means that also the combination of stress variants have to be considered,which can be e.g. combination of abrasion and impacts,or additional stressing through high temperature. Another special condition could be due to corrosive effects which may occur through different media(liquids, gases).
Of course, different levels of tribological stresses that vary have to be considered in the scope of this paper from “mild abrasive wear” to heavy wearing conditions due to large abrasives and/or high local stresses due to specific contact forces or impacting.
Rotary platform double-head abrader:
This type of setup is primarily used for tests under mild abrasion conditions and commonly known as the TABER? Rotary Platform Abraser [7]. This “abrader” was already developed in the 1930’s in order to provide accelerated wear testing as it has been used for research and development, quality and process control, and generally for material evaluation. Several test procedures have been introduced into industrial,national, and international standards, e.g. [8-10]. The Taber Abraser generates a combination of rolling and rubbing to cause wear to the tested material or surface, respectively, being in contact with bonded abrasive particles .
Abrasives are applied in wearing rollers (abrading wheels) of different composition (hard particles and binder). Test specimens disks are spun on a turntable and are abraded by a pair of abrading wheels for a specified number of cycles under a specified load. The test method specifies that the change in haze of the test specimen be determined as a measure of abrasion resistance. It is more common, however, to see abrasion resistance reported as the change in mass of the test specimen or change in mass per number of cycles. Mass change is due to material loss from abrasion.
Thus wear is normally quantified as cumulative mass loss of the plate mating against the wheels, or as a“Taber Wear Index” (mass loss relating to 1000 cycles) typically after a test run of several 1000 cycles, with a typical test load (wheel load) of 10.2 N (corresponding to a mass of 1000 g).
Such type of abrasion tester is a versatile tool with many regular test options as to type of wheel compositions. But it also offers the opportunity to create specific combinations of test samples, e.g. to invert the function of the samples, this means to place the abrasives into the rotating base plate which then produces wear of the counteracting wheels. Depending on the abradant type and test specimen, the surface of the abrasive component may change (becomes clogged) due to the transfer of material from the test specimen to the abrading wheel, and thus must be cleaned at frequent intervals or replaced. The test conditions are comparatively mild due to the given level of load. This may be the reason why this type of test – in spite of its awareness level – is mainly used for testing of coatings and different types of surface finishing.
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