The Relative Wear Method is a tool by which we can assess wear in practical situations. By starting with the relative hardness value (Hr), the choice of material is facilitated and the service life in an application can be estimated. The relative wear method takes into consideration only the hardness of the materials involved. This simplification makes it easier to use the method.
In the laboratory and in field trials, on which the method is based, the parameters, such as force, velocity and geometry, vary from one test to the next. This means that the absolute wear varies for one and the same steel. The relative difference in wear between steels with different degrees of hardness varies less with the above parameters.
The model cannot predict the absolute wear. However, the degree of wear for a steel with a certain hardness can be related to how much a reference steel (such as mild steel or Hardox 400) wears under similar conditions. As long as the abrasive material is similar and has the same movement pattern, the relative difference will be the same, irrespective of the level of absolute wear.
SSAB has developed WearCalc 2.0, a software-based tool that enables customers to choose steel that is both optimal for both the job at hand and for the kind of rock being handled. The programme calculates the relative service life of abrasion-resistant spare parts that continuously come into contact with either sliding or falling rock, such as liner plates on dump trucks, shovel buckets and chutes in conveyor systems.
There are quantitative models for sliding and impact wear, eg Hardox 450 should last 2.1 times longer than steel ‘X’.
TWO TYPES OF SURFACE DAMAGE
Damage to the surface of the wear plate occurs during sliding wear. This can consist of either a cutting type wear (the abrasive material cut chips out of the wear plate) when the hardness of the rock is significantly harder than the wear material or a plastic deformation (which can resemble ploughing) when the hardness of the wear material is closer in hardness to the rock.
When the hardness of the wear plate or the abrasive material is changed so that the surface damage switches from cutting to plastic deformation type wear, there will be a large change in relative wear.
When we analyse a wear curve with a typical granite composition in Wear Calc 2.0, the type of wear transitions from a cutting type wear beginning at about a 400 Brinell wear plate. When you use a hardness of wear plate above 400 you move into the transition zone changing the type of wear.
Small changes in hardness can have great effect in the service life. Using granite in sliding wear as an example, changing from a Hardox 400 to a Hardox 450 we would expect a 70 per cent increase in service life.
When we run basalt, we get a different result. The transition period runs from 300 to 550 Brinell. In order to achieve the best service life you need to move as far up the transition curve as possible.
Comparing to Hardox 400, moving to a Hardox 450 achieves a 30 per cent increase. But if you move to a Hardox 550, you may expect a 480 per cent increase in wear life.
SELECTING THE IDEAL MATERIAL
Selecting the correct material is a balance between the cost of manufacture of the wear part and the expected service life. Higher hardness materials generally cost more to purchase, process and are less flexible in the areas that they can be used. However, when you understand the possible benefits, the extra costs initially may be warranted due to a lower total cost of ownership.
Glen Hooper is the Regional Technical Manager ? Pacific for SSAB.
About wear ? a guide from SSAB Oxelosund. www.ssab.com/brands/hardox/WearCalc 2.0.