Developments in the metallurgy of crusher wear parts are helping operators reduce the impact of wear on their operations
By Simon Walker, European Editor
From loading run-of-mine ore to handling concentrates for storage or shipment, managing wear is a constant battle. Mines that work soft, non-abrasive ores are few and far between, so for the vast majority of operations, keeping wear under control is a significant cost item in the production budget. Get it wrong, and the financial implications can be severe: hence the incentive to clad high-wear components with materials that can both withstand continuous abrasion and protect machines and materials-handling equipment made from standard steels.
And, while excavator buckets and truck beds bear the initial brunt of run-of-mine ore, crushers run a close second in terms of the tonnage of material handled and their persistent exposure to abrasion on the one hand, and the massive forces needed to break rock on the other. Abrasion, needless to say, results in wear, while the continuous crushing forces will test the mechanical integrity of any material exposed to them. For this reason, wear parts that are installed in crushers have a double function. Not only must they protect the machine itself, but they must be physically strong enough to survive in a brutally tough working environment.
E&MJ asked some of the world’s leading crusher manufacturers and wear-parts suppliers for their views on a number of key aspects relating to the design and production of wear parts for crushers. There is obviously a trade-off in place between durability and purchase cost, given that the production of wear parts involves highly sophisticated casting techniques using specialist alloys, so it is often the case that the most suitable wear-part materials for a particular operation can only be determined once ore has been running through the crusher at design production levels over time.
The Best Materials
The development in 1882 of austenitic manganese-rich steel (Hadfield steel), containing between 11% and 14% manganese and around 1.2% carbon, led to its use in high-wear applications. Major advantages for this material include its toughness and ductility, and the fact that continuous surface impacts result in work-hardening without any increase in brittleness. In consequence, Hadfield steels and their technological descendants provide both strength and abrasion resistance; qualities that are essential for wear parts that can withstand the rigors of the crushing process.
Chris Sydenham, technical director at CMS Cepcor said that premium manganese steel grades today have a higher manganese content, commonly in the 18%-24% range. In addition, he said, other alloying elements are often added, the most common being chromium and molybdenum, and to a lesser extent titanium, vanadium and nickel. These specific modifications to the metallurgical composition are targeted to achieve improved wear resistance and toughness.
As Steven Hanny, product line manager for mining crushing chambers at Sandvik confirmed, there are many types of manganese steel. The most common usually contain different alloying elements to enhance the toughness and hardness. More manganese is needed to bind with the other alloy materials, however; a higher manganese content does not necessarily mean that the wear parts will be better in a specific application. Working with alloys that are suited for different applications has been one of the significant improvements over the last 20 years, he added. By using different manganese alloys, materials can be customized to a customer’s applications, leading to increased productivity.
However, as Metso’s vice president for crusher wear solutions, Osmo Mäki-Uuro, pointed out, adding other alloying elements may enhance one characteristic but weaken others. The individual situation must be well understood before recommending alternatives, he said.
Not all of the research has been into alloying metals. “Recent advances have focused on higher carbon grades to improve wear performance, with other additions to maintain heavy-section properties at these higher carbon levels,” said John Dillon, vice-president for engineering and technical services at ESCO Corp.
“At ESCO, we patented an alloy that incorporates aluminum to increase the amount of carbon we can introduce in the alloy without sacrificing ductility. To my knowledge, that is the highest carbon grade in routine production. The introduction of higher carbon grades—starting about 20 years ago—has been the most significant improvement to the metallurgy,” Dillon said.
High-manganese steels are not without their challenges when it comes to manufacturing. Wear parts of all types are usually foundry cast, with one of the key requirements being to control the formation of carbides within the material as it is cooling. Heat treating, used to strengthen the material further, can also cause carbide precipitation, especially in thick sections.
Work-hardening Gives Longer Life
One of the key attributes of high-manganese steel, work hardening generates a tougher surface to the part, helping to protect it from wear. Of equal importance, said Hanny, is that the main material is very strong and tough and can withstand high forces without cracking. Manganese steel develops a low-friction polished surface that results in better crushing properties for the crushing chamber, while being tough with properties that prevent crack propagation and allow for longer life times.
Low-alloy steels and special high-chrome materials have expanded their application areas and today offer significant life improvements, said Mäki-Uuro. In addition, metal matrix composites—involving the addition of polymers and ceramics—are increasingly playing a bigger role in wear solutions, and have given some impressive results. Using these materials is also broadening the usage range for some crusher technologies, he went on, with their lower weight loss per ton when used in wear parts meaning that equipment can be economically used in more abrasive applications today than was the case in the past.
Other companies have successfully explored the potential of increasing the manganese content of the alloy, as Alan George, marketing communications manager for Columbia Steel, explained. Referring to the company’s Xtralloy 24% manganese steel for crusher wear parts, he noted that no other foundry can manufacture wear parts with this manganese content, with the company having been delivering 24% Mn crusher wear parts to the mining industry since 1988. The company reports that Xtralloy has significantly higher manganese and carbon contents than conventional Hadfield steel, claiming it has up to double the wear life compared with earlier materials, especially when handling tough, abrasive rock.
According to Hanny, casting thicker sections in high-manganese alloys can be difficult. Steel with a higher manganese content has lower heat transfer characteristics during quenching, which makes it more difficult to cast thick pieces. To compensate for the lower heat transfer, other alloying metals must be added, which can make the casting process both more complex and more expensive, he said.
“The fluidity of manganese steel is relatively good, making it possible to cast complex shapes,” said Sydenham. However, manganese steel achieves its inherent properties of good toughness and the ability to work-harden through heat treatment, which involves water quenching from its solutionizing temperature of above 1,000°C. This severe quenching action causes distortion issues to the part, especially those of complex shapes having differing section thickness across the part.”
“Process controls in melting and heat treatment have continued to evolve, with a resulting improvement in consistency from quality manufacturers,” said Dillon.
Changing Geology Brings New Challenges
Clearly, the economic life-span of crusher wear parts depends on more than just the materials from which they are made. Changing rock parameters, such as the silica content, will have a noticeable impact on the change-out cycle time, as will the tonnage of material being handled and the design of the crushing chamber itself.
A general trend is that today’s crushers draw more power and use higher crushing forces than before. Because of this, said Mäki-Uuro, the physical properties of the wear parts must meet the changed conditions, while aiming to minimize the wear rate in terms of the material weight loss (in grams per ton of output product).
There is also the question of changing conditions in the geology of orebodies now being worked compared with, say, 20–30 years ago. As Osmo Mäki-Uuro pointed out, there are significant differences between the iron ores produced in the U.S. (hard, abrasive taconite) and in Australia, where the ore is less abrasive. And, he said, mines being developed as greenfield projects today are often based on deposits that are hosted in less-abrasive rocks than where existing mines are being expanded.
Hanny agreed that there have been changes in the rock characteristics being handled today compared with 20 years ago. These change even within an individual mine, by depth and location, as a result of variations in the geological conditions and the rock/ore mineralogy. Usually mines encounter less-weathered rock as they get deeper, with the potential for less alteration and harder material. “What we can see is that mining companies are working lower-grade ores, and much more barren rock, which can be harder on account of their mineralogy,” Hanny said.
“One factor that is having an effect is that ore grades are declining, so more rock has to be processed to produce the same amount of mineral. Attention to crusher design and alloy selection is critical to limit the cost of the crushed stone,” said Dillon.
Crushers are usually key components within any mineral-processing system, so operators strive to minimize the downtime involved in changing out wear parts. In addition, each mine has to strike its own balance between the cost of buying replacement parts and the cost of lost production while fitting them. Cheaper parts may not translate into lower operating costs when looking at the bigger picture.
While all of the major suppliers of wear parts, both OEMs and independent after-market companies, guarantee compatibility of their components with the equipment they are intended for, that does not necessarily mean that exchanging old-for-new will be straightforward. Crushers by their very nature have a challenging lifestyle, which can translate into difficult change-out conditions as old liners wedge tightly into place.
According to Mäki-Uuro, Metso offers a wide range of options in its services portfolio, from change-out service to total life-cycle services programmed. Many customers, he added, carry change-out components such as spare head and shaft assemblies that allow for quick change-outs with the wear parts then being replaced in the workshop.
“The company regards top cone-crusher serviceability as a key customer benefit, and we are also developing different lifting tools for our parts that will make maintenance work faster and safer,” said Hanny. “Sandvik’s ASRi (Automatic Setting Regulation) systems keeps track of liner wear, which makes it easier to plan liner changes and minimize interruptions in production.”
“Whenever possible, ESCO adds cast-in-place lifting devices directly to its wear parts, which support safe handling procedures to hoist these large parts in and out of crusher frames,” said Dillon.
“Our engineers are constantly striving to improve the design of our products to improve safe handling and to facilitate ease of fitting,” said Sydenham. “An example of this would be our alloy steel concave segments, where we have recently provided a solution for a customer by reducing the number of segments in a set from 60 to 16. This not only reduces the number of parts being lifted, but also takes far less time to fit.”
Optimizing Wear-part Costs
The cost of replacing liners and other wear items in crushing plant on a regular basis is a function of the material being crushed, the quality of the wear parts being used and, most importantly, the way that the crushing chamber is set up. As Mäki-Uuro explained, the way the rock flows through the crushing chamber is critical, and depends on the chamber design. Each application needs to be tailored to the individual site conditions, depending on the input material characteristics and the type of product that is required: this may be vastly different for a mining operation or for aggregates production.
Clearly, then, there can be significant cost differences for replacing wear parts, not only in terms of the parts themselves but also in the labor needed. Liners that wear quickly will need replacing more frequently, and the crusher will be down more often as a result.
So, what should mines expect in terms of regular replacement costs, as a proportion of the initial cost of a crusher?
“Wear-part life is directly related to the material characteristics and mining operations. As a very general rule of thumb, one could estimate somewhere between one-third and one-half,” said Hanny.
“The consumption of wear parts is so dependent on the application of the machine and factors such as the compressive strength and work index,” said Sydenham. In a worst-case scenario, a mining company might spend roughly 50% of the capital cost of the base crusher per year on wear parts for that machine.”
“Part lifetimes have improved in recent years. Since higher carbon grades were developed, wear resistance has improved and, subsequently in most applications, the parts last longer. Better designs have optimized the placement of wear metal to improve production through the crusher while providing better wear life. Wear life should really be measured as tons of material produced rather than hours, so crushing performance is a critical area of focus,” said Dillon.
The clear message from all of the manufacturers is that each operation has to work out the best solution to crusher operation for their own individual circumstances, which may well change over time.
According to Hanny, one top priority within the mining industry is to reduce downtime due to liner changes. Even a short unscheduled maintenance stop in production can mean a big profit loss. Environmental impact is also increasing in importance with our customers, so we are working with materials, lifetimes, the production process and logistics so as to minimize our and our customers’ environmental impact.
Other materials are also playing a role. “Materials other than manganese steel have been introduced for crusher wear parts, especially in primary gyratory concave segments,” said Sydenham. Various grades of pearlitic and martensitic low steel and iron heat-treated to high hardness are gaining more popularity over manganese steel concave segments. Some manufacturers have also done trials on casting high-hardness alloy iron inserts into the manganese steel matrix material to enhance wear resistance.”
“In the future, I see more customized solutions at each crushing stage to enhance the total value of the operation, helping to optimize the product shape, amount of fines, system capacity, energy usage and wear costs for each individual customer,” said Mäki-Uuro.