I am often asked how hard the components I make are. The question is of course legitimate, as we have been conditioned over the decades to believe that hardness is the measure of all things. For a better understanding, today I would like to address the question of why the hardness of hard faced components does not alone determine the service life of a component and why a lower hardness can still lead to a longer service life.
By definition, hardness is what one material resists the penetration of another. The hardness test uses different shaped diamonds to penetrate the surface and measures the depth penetrated. Very hard metallic materials are measured using the Rockwell HRC test method, minerals are usually classified according to Mohs.
From tool making we know many different materials with final hardnesses between 45 and 65 HRC. The selection of the material and the type of heat treatment in combination define the material properties and determine the durability of the component in its very specific environment. The stresses are often a combination of heat, friction, chemical and impact stress.
My highly wear-resistant components with a hard faced surface usually consist of a cost-effective, soft base material – such as S235 – and a hard wear layer. The operating conditions here are always a combination of different types of wear, caused by friction, impacts, acids and heat.
The wear layer is applied using various welding processes and essentially consists of a matrix with embedded carbides.
To put it simply, carbides are created in the melting and solidification process (similar to the casting process) through the reaction of high-quality metals with carbon. This is how chromium, vanadium, niobium and tungsten carbides are formed. The hardness of the carbides ranges from 900 HV to 2900HV. Tungsten carbide is the leader here, with a hardness of 8.5-9.0 on the Mohs hardness scale, meaning it is almost as hard and wear-resistant as diamond.
Unfortunately, the harder the carbides are, the more expensive they are. We therefore use a combination of the relatively inexpensive chromium carbide together with higher quality and more expensive carbides made from vanadium, tungsten, niobium, etc.
The carbides are the actual wear partner. The harder and more densely packed they are in the material, the longer they will generally last.
The matrix of our materials is the carrier material that holds the carbides in place and prevents them from breaking out. If we imagine an exposed concrete slab as a comparison, the matrix is the concrete and the carbides are the pebbles. If the matrix is too hard, the carbides break out when impacted; if it is too soft, they are torn out.
Basically, we differentiate between hard and soft matrices. Examples as follows:
Boron generates a brittle-hard matrix which is very good at erosion and abrasion, but not so good at impacts.
Manganese, on the other hand, creates a tough, hard, easily deformable and work-hardening matrix, which is very good at impact stress.
Another function of a matrix is the temperature and corrosion resistance, which is created by the elements chromium and nickel.
Looking at everything together, we see that the correct conception of the matrix is extremely important. To do this, we need to know the operating conditions of the wear parts in order to be able to address them individually. This is the only way to ensure that they last as long as possible.
If we now carry out a hardness test on our wear layer using the above-mentioned Rockwell test method HRC, a carbide or the matrix can be hit during a test. If we hit a carbide with the diamond cone, the hardness cannot be measured because the cone does not penetrate. However, if we hit the matrix, the neighboring carbides prevent us from penetrating deeply and the scale shows quite different values. For this reason, we measure the hardness at different points and then average the value.
Conclusion
In summary, it can be said that the combination of different carbides together with a functional matrix determines how long the material can withstand stress, not the hardness alone. The trick for us is to develop and implement the optimal ratio between manufacturing costs and operator benefits of a coating.
The picture shows the HRC hardness test in our QS laboratory.
DURAPARTS and its partner companies have been providing individual wear protection solutions in German-speaking countries for many years.
