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Considering Composites


If the last several years have taught us anything, it is that we must be able to operate lean, especially in these trying economic times. In addition to closely managing labor and inventory, it is equally important to carefully manage consumable purchases. It is not enough to look for the lowest piece price, instead our focus needs to be on the total cost of operations. For instance, part A may have a lower unit price, however part B and its superior wear life may afford the operator lower cost per operating hour and increased equipment up time.

This consideration is of particular importance to operators of both vertical and horizontal shaft impact crushers where a significant portion of the operating cost is the maintenance and acquisition of wear parts. Iron/ceramic composites last longer and can help reduce the number of part change outs which afford the operator with lower consumable costs, fewer maintenance hours for change outs and more productive operating hours.

The most common solution to impact crusher wear parts is high–chrome iron (CrMo WI), as it offers resistance to medium impact and sliding abrasion wear (Figure 1 wear factor chart). In this chart, high-chrome white iron is shown to have a significantly better wear factor than many abrasion resistant (AR) steels, manganese steels and to some extent, chrome carbide overlays. However when crushing hard rock, even wear-resistant high-chrome iron will eventually wear out.

It is possible to increase the wear resistance of high-chrome iron by using composite technology where one material is added to another to yield improved combined properties. However, the manufacturing of high-chrome iron/ceramic composites in production quantities is challenging. Spokane Industries has successfully created and manufactures Si-Tec, the integration of hard ceramic material into a metal matrix, yielding a wide range of long-life wear products. To preserve this strategic position, this successful implementer possesses several patents.

High-chrome iron has been used for impact crusher wear parts for decades. Impact crushers are capable of crushing a wide variety of rock ranging in hardness from limestone to granite. Crushing limestone with high-chrome iron wear parts will result in low-level wear, as the feed material is much softer on a relative basis than the iron.

However when crushing materials containing quartz or silicates, even high-chrome iron will tend to wear from the gouging and frequent impact of these extremely hard materials. At the other end of the extreme, using only hard ceramic material in an impact crusher application would likely result in fractured ceramic material; however when the ceramic is encapsulated in a tougher material such as high chrome iron, the resulting composite is able to withstand medium impact and high abrasion. Examples of successful Si-Tec applications are shoes and feed discs, and vertical shaft impact crushers, respectively.

Why Composites?
To understand why composites wear longer it is important to first understand why wear occurs. Wear most often occurs when the feed material is harder than the wear part material. In this case, the composite contains a constituent of high-chrome iron with very hard carbides (697 HV) as well as a ceramic constituent with a hardness of 1,900 HV.

The feed material may contain hard quartz and/or silicates with hardnesses ranging from 600-1,250 HV. As the hardness of the feed material exceeds that of the carbides in the iron, it slowly gouges and thus removes iron, until it meets the ceramic constituent. Because the ceramic is 1.5-2 times as hard as the abrasive feed material, it fortifies the iron and greatly impedes the ability of the quartz and silicates to gouge and remove iron. Over time the ceramic material will wear, but just as a shark replaces its teeth, Si-Tec composites expose fresh ceramic to once again fortify the high-chrome iron.

The combination of two durable constituents results in a composite, which is longer lasting than either of the composite constituents individually. This is proven with field data collected by an operator running vertical shaft impact (VSI) crushers outfitted with Si-Tec impeller shoes. In VSI crusher applications, ceramic composites resist wear from impact and sliding abrasion of rock as it is dropped through the feed chute on to a rotating table covered with a protective feed disc and is hurled centripetally by the impeller shoes to crash into surrounding wear parts.

Tests were run with both unfortified high-chrome iron and Si-Tec high-chrome iron/ceramic composite shoes. The data measured impeller shoe wear life in terms of hours of operation for a specific feed size and material.

As in many crusher processes, a jaw crusher was used as the primary crusher. Following the jaw, the output is screened to control feed size into the VSI. In this example, the feed is very hard and thus highly abrasive to high-chrome iron. Four separate batches were run using four unique impeller shoe sets.

The first batch was crushed using an unfortified high-chrome iron shoe. The next three batches were crushed using composite shoes each with a unique ceramic profile, Gen I, II or III (see Table 1). The differences between these three iterations are the locations of the ceramic constituent. The design iterations are the result of introducing an initial ceramic profile, studying the resulting wear pattern and then designing successive ceramic profiles to better fortify the iron and optimize wear life.

The additional hours of crushing are the result of a more uniform wear pattern with the Si-Tec composites than with the unfortified high-chrome iron shoes. The image below (Figure 2) shows an example of each of the iterations after crushing operations were completed. The unfortified shoe (shown on left) has a narrow wear pattern associated with focused erosion deep into the body of the shoe.

At this point, production was stopped so that the shoe could be replaced to avoid damage to the shoe brackets. In this very abrasive application, the unfortified shoe was removed after four hours of operation.

The three shoes to the right in the picture were fortified with successive iterations of Si-Tec ceramic composites. While all three have wider wear patterns than the first, the most impressive is the right-most shoe, Gen III. After 15 hours of crushing, this shoe has the widest and shallowest wear pattern, and as a result, the longest wear life.

The iron/ceramic composites wear longer, which translates to longer wear life and fewer change outs. Further, there is a commercial comparison based not simply on the cost per impeller shoe, but shoe cost per hour of operation. This analysis is shown in Figure 3 (page 62). When taking in to account the longer wear life and cost of the ceramic composite shoes, an analysis of the shoe cost per operation hour shows a significant advantage associated with the use of Si-Tec wear parts.

While results are likely to vary with feed type, feed size and machine configuration, in this application, the relative shoe cost per hour of operation of the Si-Tec Gen III is one-third that of the unfortified high-chrome iron.

Conclusion
Si-Tec Ceramic Composites are very effective at resisting wear in impact crusher applications where the wear components are exposed to medium impact loading and intense sliding abrasion. It is the addition of ceramic material to the high-chrome iron that fortifies the metal matrix against the abrasive silicates and quartz feed.

Although ceramic/metal composites often sell at a premium to the unfortified high chrome iron, the composites typically wear considerably longer. In this case, on a relative cost per hour of operation basis, the operating cost of Si-Tec ceramic composites is 28 percent of that of the unfortified shoes. Further, because the Gen III shoe of this test lasted more than three times as long as the unfortified, using it saves interruptions to production and downtime associated with the change out of two sets of impeller shoes. By reducing the frequency of lengthy shutdowns for shoe replacement, the operator realizes increased production hours and thus incrementally greater production volumes.

It is important to note, that the wear life for a given wear solution will vary in length with variances in feed (size and type), crusher (type, RPM and configuration) and moisture content. Thus, a trial of the same Gen III shoe may have different results with changes to any of the above parameters.

For these reasons, Si-Tec ceramic composites are an ideal wear part solution for use in medium-impact, high-intensity sliding abrasion applications. Ceramic composite wear parts reduce the cost of operation thus supporting higher profits for each operating hour. The savvy shopper will focus on cost of operations rather than simply lowest piece price when selecting wear parts.

Information for this report courtesy of Spokane Industries.