Article
12.10.2021
184

Interview: Hardness alone is not decisive for wear protection.

Technology
Barrels

What are the parameters that actually matter when it comes to wear protection? An interview with Anna Rottstegge, Head of Research & Development at Reifenhäuser Reiloy.

Dr. Rottstegge, how did Reiloy come up with the idea of dealing with the specific topic of hardness in barrel wear protection?  

The main starting point of our research was some interesting contradictory feedback from our customers. First, they find that alloy hardness is specified as identical or very similar in the data sheets of all manufacturers. They therefore assume that this is the decisive factor when it comes to wear protection performance. On the downside, we find repeatedly in the field that the service life of our barrels is verifiably longer. This could not actually be the case if it depended only on the hardness of the alloy, which, as I said, is virtually the same for all suppliers. As a materials scientist, I have learned that wear protection does not only depend on hardness. That was my incentive to get to the bottom of the causes. So, we took a closer look at the microstructures in the Research and Development department and carried out our own wear tests.  

Where did you start analyzing the root causes? 

First of all, we analyzed nickel-based alloys. This is where all suppliers use high-hardness tungsten monocarbides. So, we started at this point and analyzed every lead thoroughly. We carried out intensive comparative analyses on a wide variety of components, took a very close look at alloy compositions, tested the microhardness of individual phases, evaluated surface coatings using optical analysis, and then carried out an optical analysis of size distribution. 

Dr. Anna Rottstegge received her doctorate at the Ruhr University Bochum in 2017 and has been Head of Research and Development at Reifenhäuser Reiloy Extrusion Systems since 2019.

That sounds like a lot of effort. Did the results justify the effort?  

Absolutely. Because we were able to prove conclusively that there are deviations in matrix composition and the volumetric content of carbides. We assumed that longer service life would go hand in hand with higher carbide density. But it soon turned out that the carbide density of only a few market competitors was lower than ours. In fact, the major difference of our nickel-based alloys compared to other manufacturers is carbide size distribution. Or to put it another way: The greater performance of our wear protection comes not only from the higher density of its carbides. In actual fact, it is their size that provides the additional advantage. We were even able to demonstrate that they are ten times greater than the carbides of all other nickel-based alloys on the market. But of course, there are also cases among competitors where lower carbide density, an unfavorable distribution, or a higher porosity have a negative impact on service life.  

Did that surprise you?  

Yes and no. It was already a conscious decision at Reifenhäuser Reiloy to select larger carbides in order to counteract specifically coarse abrasive wear in the barrel. The carbides themselves are not attacked by abrasive substances, but are simply in the way - the large ones more so than the small ones. Picture it like stones in a field that is being plowed: The plow simply pushes the smaller stones aside, but the larger stones present an obvious obstacle. In a similar way, larger carbides obstruct abrasive forces and offer significantly more resistance than smaller ones. In order to prove this scientifically, we procured or recreated alloys with smaller carbides and carried out wear tests on them, both in the lab and in actual use. Even we were totally surprised by the degree of positive evaluation compared with our own alloys. This confirms what we already learn in our materials science studies: That wear protection depends not only on macrohardness, but on the entire microstructure. 

Why don't other suppliers also use larger carbides in their nickel-base alloys?  

Perhaps because the entire manufacturing process required to achieve the desired surface quality and the required diameter tolerances is particularly difficult and costly when large carbides are used. We decided to use large carbides because of their utility value and we then optimized the manufacturing process accordingly. Here, we have the major advantage that we were able to match the surface treatment of the wear protection layer (honing) to the exact requirements needed since we develop and manufacture the tools ourselves. This is also a core process at Reifenhäuser Reiloy!  

So far, we have talked about nickel-based alloys. What about hardness as a comparative parameter of iron-based alloys?  

It’s a similar story here. First, we also asked ourselves why our iron-based alloys achieve verifiably longer barrel service life at the same hardness. And then, in precise microstructure analyses, EDX analyses of the chemical composition, and, above all, in microhardness measurements, we worked out that it is due to the almost ceramic layer that other alloys do not possess.  

Could you explain this in a little more detail?  

We are talking here about ceramic phases in combination with boron and carbon - mainly chromium-based - which are produced by the melt during the casting process. The objective of our concept is to generate a protective shield out of ceramic phases. These phases are still bonded in a metal matrix but occupy a large volume. In other words, the layer has a high hardness but is still sufficiently elastic. The entire process is geared to this. To produce the ceramic phases exactly the way we require, the composition, process temperatures, and process times must be precisely maintained. The process window is very narrow and must be exactly controlled. But we always get it right since we use the special Reiloy centrifugal casting process that we have continuously adapted and optimized over the years and we execute the entire process on specially developed machines.