NewsInformation Center
The wear methods of high-alloy carbon steel used in mining tools
2019/07/12
Share:
Wear testing
General methods for wear testing have limitations and cannot always give a fair answer about the wear rate in a real application. The wear mechanisms are generally specific for each issue.While choosing the most appropriate test for a specific type of wear, there are a couple of things to consider. A test that is fast enough, controllable, and possible to continuously measure friction and wear, cost effective, repeatable and gives enough tests for statistically accuracy is preferred.
The most desirable is to find a test that is close to the reality, where the wear type and environment is the same for the test and the application. When finding a suitable and realistic test method, the wear rate on the specimen compares with the “real” worn material in macroscopic and microscopic scale.
Wear testing may be performed in different ways, depending on wear environment and wear type. Abrasive and adhesive wear tests can be separated into two types, open or closed. In a closed type the test piece is in contact with the same track on the other part. Thus, if the test piece does not meet the same contact surfaces more than once, the system is open.
As mentioned, wear is mostly measured by material loss and by microscopic studies of the surface to identify wear mechanisms.
Some of the commercial test methods for wear are the pin on disk-test, which tests adhesive wear, the dry/wet sand rubber wheel (RWAT) and the dry/wet sand steel wheel (SWAT) which tests three-body abrasion wear, all are ASTM standard test methods. It is possible to find specially constructed test machines for different wear mechanisms and applications for materials, one of those are a tumbling mill, a test for erosion abrasion wear.
Pin-on-Disk
The Pin-on-Disk seen in Figure 12, is described in the ASTM standard, G99, “Standard Test Method for Wear Testing with a Pin-On-Disk Apparatus”. The target specimen is placed in the end of a pin with direct contact to a disk, often made of a reference material. A pressure is placed upon the pin to maintain contact between the specimen and the disk, a motor makes the disk spin around. The wear is measured by material loss, which makes it important to measure the tested component thoroughly before and after the test. It is possible to test specimens in different shapes; spherical, flat and pin geometries. The parameters that can be controlled are the pressure on the pin, sliding speed and sliding distance. With advanced machines it is possible to control the temperature and atmosphere. The test can be performed with an opened or a closed system, in an open system the contact area between the pin and disc is never repeated whereas in a closed system the contact area is reused and repeatedly loaded.
The dry/wet sand/rubber wheel abrasion test
The dry sand rubber wheel abrasion test is the one covered in the ASTM standard G65,“Standard Test Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel Apparatus”.Dry particles, abrasives, are led into a space between the material test specimen and a spinning rubber wheel as shown in Figure 13. The wheel is a counterweight so that it maintains contact with the particles and the specimen. The specimen is weighed thoroughly before it is placed in the machine. The wear is measured by material loss and after a specific amount of particles has run through the equipment the test piece is weighted thoroughly again. One specimen is tested at the time. The behavior of the specimen in the rubber wheel abrasion test, RWAT, is not only depending on the properties of the specimen but also on the surrounding conditions. The condition includes of the abrasive particles (their type, size and shape), the nature of the wheel(hardness and stiffness) and the nature of the environment.
In the rubber wheel abrasion test the abrasives generally remain intact during the process,which produces soft abrasion.
The wet sand/rubber wheel test is described in the ASTM standard G105, “Standard Test Method for Conducing Wet Sand/Rubber Wheel Abrasion test”, where the abrasive material is an aqueous slurry besides from the abrasives; the test works similarly as the G65 standard.
The dry/wet sand/steel wheel abrasion test
The steel wheel abrasion test, works the same as the dry/wet sand rubber wheel. The difference is that the spinning wheel is made of steel which makes the stress from the particles on the material specimen becomes larger. The test is described in the ASTM standard B611, “Standard Test Method for Conducting wear Resistance of cemented Carbides”. According to the standard,the test is preferred when testing cemented carbides. The specimens’ behavior in the SWAT is depending on both the specimen itself and the surrounding conditions as in the RWAT. In the SWAT it is possible to use steel wheels made of different types of steel, it could be either a reference material or a wheel made of the same material as the specimen depending on the conditions required to emulating.
In the steel wheel abrasion test the abrasives generally fracture during the process, which produces hard abrasion.
Tumbling mill apparatus
The tumbling mill test seen in Figure 14 consists of a drum with the specimens in the lining. The custom made tumbler can be charged with both wet and dry abrasives. As the tumbling mill is rotating the abrasives continuously are in contact with the different specimens in the test. The machine simulates adhesive wear were the abrasives are in sliding contact with the specimens.
It is possible to simulate erosion wear if the machine is built with lifters inside which makes it possible to transfer the abrasives higher before dropping down. One test session includes several specimens and the controllable parameters are abrasives (type, size and shape) and the rotation speed. Since the wear rate is depending on the weight loss of the specimens they are being weighted before and after each test.
Particle erosion rig
At the ?ngstr?m laboratory in Uppsala a custom made particle erosion rig seen in Figure 15 is available. The rig is supplied with particles in to a spinning plate; and the particles are ejected against the specimens through peripheral holes. This causes erosion wear on the specimens. Up to 18 specimens at the time can be tested. Conditions that can be changed are particle size and type, velocity and impact angle.
Erosion Tests by Solid Particle Impingement Using Gas Jets
The standard test G76: “Standard Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jets”, permits erosion wear. A gas jet spreads particles against the specimen and causes erosion wear as seen in Figure 16. Conditions that can be changed are particle amount, particle size and type, impact angle, velocity. The small nozzle prevents bigger particles; this will also lead to wear of the nozzle.
Previous: The purpose and types of fatigue testing
N e x t : Discussion about relevant wear mechanisms
