During the past 10 years, high-pressure roller presses have been employed at an increasing rate in the comminution of clinker. Due to this popularity, a need was seen to increase the service life of the roller surfaces.

As a result of the complexity of the surface stresses involved, various wear-protection concepts are available. In all cases, a tough base material is provided with a wear-resistant facing or lining, which can be applied to both solid and composite rollers. Surfacing options include chill castings, hardfacings, and stud linings combined with different types of rollers.

With a view toward cost efficiency and mechanical stability of the surface, the German firm Maschinenfabrik Koppern developed Hexadur, an abrasion-resistant material serving as wear protection for grinding rollers. The wear resistance of this protective lining results from a particular material combination and geometric pattern on the grinding rollers.

Previously, grinding rollers were constructed with a welded, 15-mm-thick, hard-facing lining as wear protection. Despite technological advancements in welding and the filler metals used for build-up, no satisfactory results could be obtained and generally service life was only a few thousand operating hours at many plants.

Hexadur grinding rollers, working in a Norwegian cement mill operated by Norcem, are said to have reached 16,000 hours lined with a 10-mm-thick, wear-protection system without a surface repair. During this period, the plant produced about 1.3 million tons of clinker.

In the metallurgical manufacturing of the Hezadur lining, a steel powder compacted at high temperature and pressure is used as the basic product. This process is said to yield a strong and pore-free material.

The Hexadur surface is coated with a wear-resistant layer applied to the tires or segments mounted to the rollers. This surface layer consists of two materials and is applied to a strong base material (greater than 1,000 MPa). The surface is structured so that a highly wear-resistant material (Component A) covering a large area is surrounded by interstitial material (Component B) of deliberately selected lower wear resistance (Photo 1).

The selection of two different materials is linked with process technology considerations as it is linked to feed intake performance of the rollers. >From the operations of hardfaced rollers, it is known that a profiling of the roller surface contributes to improving the feed intake and throughput characteristics. In the case of the Hexadur surface, these intake conditions are said to be positively influenced by open interstices.

Because of the lower wear resistance of Component B, the interstice material will be washed out after a relatively short period of roller operation. In the process, these spaces will be filled with pressed feed material serving as a kind of autogenous wear protection of the joints if a particular width-to-depth relation of the interstice is reached. The filled interstices are rougher than the normal roller surface and, as a result, the gripping behavior of the entire roller improves. Since the areas covered with Components A or B can be varied in size, these surfaces can be tailored to the specific feed size to be processed.

After commissioning at the Norcem clinker grinding plant, the capabilities of this new development soon came up for review. Previously, all installed welded rollers had to be replaced after 10 to 16 weeks (2,000 to 2,500 hours) of operation (Photo 2). On the newly installed roller that operated in conjunction with a conventional counter-roller, a wear rate of about 1 mm was noticed after 2,200 hours-the point at which the conventional roller had to be replaced. After about 7,200 hours of grinding clinker, wear was at less than 5 mm. At this point, conventionally welded roller had been replaced twice. Even foreign bodies occasionally passing through the press did not harm the Hexadur surface to any degree, according to its manufacturer.

The wear figures determined on the new roller during 15,000 operating hours are shown as a reduction in diameter in Figure 1. For comparison, a cumulative characteristic has been plotted indicating the diameter reductions encountered during this operating period with seven rollers with welded-on hardfacing.