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Discussion about relevant wear mechanisms
2019/07/12
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Based on how the steel from Ovako is being exposed to wear, test methods which achieve proper experiments will be discussed.
The thread
The thread is primarily exposed to adhesive wear. When grinding the metal surfaces against each other, the surface will be rubbed and irregular. At this point two-body abrasion is the present wear mechanism and occasionally loose particles will be present and the mechanism in the thread turns to three-body abrasion wear.
The drill bits
The drill bits are exposed to a severe environment with presence of water, loose abrasive particles, percussions and vibrations. The assumption is that the drill bits are being exposed to adhesive, two-body and three-body abrasive wear and/or erosive wear. The adhesive wear when rocks and water, the abrasives, wash over and erosive when the abrasives impact the component. The threaded button bits are also in contact with another surface, mixed with particles which cause three-body abrasion wear. The presence of water will cause a corrosive and lubricating effect. Though, based on the studies in both wet and dry environment the corroding effect is negligible compared to the rate at which a material is being removed.
This means the corroding effect can be ignored.
It can be assumed that all parts of the drill, both the drill thread and the drill bit are often being exposed to hard impacts, which also will affect the wear on the material.
Abrasive wear can be assumed to be the most commonly occurring mechanism on the current applications, since there are a lot of dirt and irregular surfaces. Adhesive wear are present but probably in a minor scale. This means a test method performing abrasive wear is more relevant than a test method performing adhesive wear. The adhesive wear will still be kept in mind though.
The pin-on-disk apparatus
A benefit with pin-on-disk is mainly the fact that it is a well-known machine that customers easily can relate to. Since it is an ASTM standard the test is good as a reference test when comparing different materials. The international standards have an advantage as they ideally can be reproduced in any other tribology laboratory. Disadvantages with pin-on-disk is the cost,it is not easy to build yourself and must be purchased. In some pin-on-disk machines spherical specimens are required for the test, this is expensive and complicated to make. Some pin-ondisks can test rods with a plane top which are much easier to make. The pin-on-disk machine does not test presence of water. Since water has a lubricating effect that will change the wear properties of a material, this might lead to substantial error in the result and the wear rate might be misleading. The wear on the drill bits is not at all similar to the wear that is being performed in a pin-on-disk apparatus. The wear in the drill threads, where two similar surfaces are in sliding contact with each other, is very similar though.
The sand rubber/steel wheel
The difference between G65, dry sand/rubber wheel and G105, wet sand/rubber wheel is the abrasive material. Dry particles or an aqueous slurry is used. These two tests are appropriate for softer steel, although the RWAT is still not a perfect copy of the reality. The dry/wet sand rubber wheel only results in abrasion wear. It does not perform adhesive wear. The required specimens are relatively large, recommended size being 76,2 mm long by 25,4 mm wide by 12,7 mm thick.
This could be a drawback with respect to the cost combined with the fact that only one test can be done at the time and that the standard involves rather large amount of abrasives. Although a benefit with the dry/wet rubber wheel is that the tests can be done both wet and dry. It is also possible to vary type, size and shape of abrasives. This is an advantage because of the varying conditions the steel from Ovako will be exposed to.
The positioning of the specimen has been discussed by Hutchings who designed an apparatus where the specimen is placed horizontally over a rubber wheel with the same dimensions specified in the ASTM standard G65. The load was attached directly above the specimen and the advantage over the ASTM design is the control of the sand feed. The amount of abrasives that actually abrades the specimen can be accurately determined by weighting the abrasives that are falling off the rubber wheel before contact with the specimen.
The dry/wet steel wheel abrasion test, following ASTM standard B611, is very similar to the RWAT. The SWAT includes a wheel of steel, using this test enables to receive higher forces. This results in higher stresses from the abrasives on the material specimen and may in some cases be more similar to what happens in the mining applications.
In some practical abrasive wear problems the system is of metal-on-metal nature and in those cases the sand rubber wheel test is not appropriate when it cannot recreate the environment which the real component is exposed to. Then the SWAT is the most appropriate test method. For Ovako this could be an interesting test method for the drill threads. The application is of metal-metal nature and it could be assumed that the adhesion wear in the drill thread will result in an abrasive wear.
The test could also be used for testing the material used in the drill bits, depending on the hardness of the steel. Harder steel is preferred as a component in the steel wheel abrasion test while testing softer steel specimens, like the drill bits, it is preferred to use the RWAT test method because there’s no metal to metal contact in the application. The rubber wheel abrasion test produces soft abrasion as mentioned before and the steel wheel abrasion test generally results in hard abrasion, where the abrasive particles tend to fracture during the process. When performing a steel wheel abrasion test the wheel is convertible, this means that it is possible to use either the same material as the specimens or a suitable reference material.
The tumbling mill apparatus
The tumbling mill apparatus is custom made, which means that Ovako would have to design and build one by themselves. It is preferable because it is possible to run several specimens at a time,which is an advantageous in comparison between the specimens when they were tested in the same environment, which will make it time effective which leads to being cost effective. A disadvantage is that there only is a small similarity between the test and the real conditions in which the drill threads or the drill bits are situated. The test could be used as a method for testing the steel on the drill bits were erosive wear is present.
The particle erosion rig
The particle erosion rig is quite similar to the tumbling mill. It has the same benefits because it is possible to run several specimens at the time and it is cost effective because it is possible to build. Abrasives used in the erosion rig are not limited like the ones in the standard test which makes it possible to choose a cheaper kind if needed, which makes it possible to experiment with different kinds of abrasives.
The erosion test by solid particle impingement
An erosion test by solid particle impingement using gas jets on the other hand has its limits in both the abrasives and the fact that only one specimen at the time can be tested. It has its benefits of being a standardized test method which makes it direct comparable to the same tests in another laboratory elsewhere.
Similarities
The micro-cutting mechanisms in two-body/ three body abrasive wear and erosive abrasive wear have turned out to be similar. A brittle target leads to micro-cutting/micro-cracking and a ductile target leads to micro-cutting/micro-ploughing. This is still not an indication that the sample would react similar to both erosive abrasive wear and two-body/three-body abrasive wear since the impact angle and applied force will be different. Since a general test method is to be found and the impact angle and applied force for a general application is impossible to submit, because there are different impact angles and applied forces for different applications,erosive abrasive wear and two-body/three-body abrasive wear will be set equal when suggesting a proper test method. This means the tumbling mill apparatus, the erosion rig and the solid particle impingement using gas jets also could be appropriate for testing two- and three-body abrasion wear.
Other discussed test methods
At all metal laboratories a grinding machine could be found. Grinding is the easiest way of testing two-body abrasion wear and has a large advantage with its availability. This means the test is cheap and easy to use. Those are the biggest benefits of using a grinding machine as a method for testing wear. A disadvantage about wear testing metals in a grinder is that no scientific articles could be found about this method which means it will be hard motivating while discussing with potential customers because of their unawareness. The fact that the grinding machine is the easiest way of testing wear is also a disadvantage; the wear mechanism achieved in the grinder may not at all be similar to the real situation and might lead to untrue signals of how the material would be treated for higher wear performance.
To put it into a general perspective, wear is mass or volume loss of material. If a general test method is required, it should perhaps be kept that simple. This could simply be put into a machine that grinds off material. The volume of material loss should be weighed and then put in to Archard's equation, so that a wear rate could be received.
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