The suitability of crusher wear liners includes correct liner profile design to produce the desired shape and production output. Once the liner design is established, the selection of wear liner material and reliability becomes the next most important consideration.
Manganese steel was traditionally produced within Australia for local requirements and there was a good understanding between the manufacturer and end user to ensure liner performance expectations were met. As local production continues to decline, it is important wear liner suppliers become the link between the manufacturer and end user to ensure the end user does not rely on the often costly trial and error method to obtain the best outcome.
Crushing Equipment, a division of Scaw Metals, has offices and stores in Perth, Melbourne and Brisbane, with a highly experienced team to represent the Scaw Metals Foundry Group in South Africa. The Scaw Metals Foundry group has been manufacturing manganese steel liners for over 50 years, supplying to Australia through original equipment manufacturers and direct to end users.
The purpose of this article is the general introduction of manganese steel and the variables that exist. Manganese steel crusher wear liners are a cast product and, as such, are subject to a number of variables often only controlled by the manufacturer. To ensure he will get the best and most consistent quality product that will perform in his environment without premature failure or damage to his crusher, the quarry manager should consider the following points and ask the relevant questions where applicable:
1. Understanding manganese steel.
2. Elements in manganese steel.
3. Manganese steel work hardening.
4. Manganese steel grades.
5. The ideal wear liner.
6. Operating and casting variables leading to liner failure.
UNDERSTANDING MANGANESE STEEL
Hadfield?s manganese steel was discovered in 1882. It was originally designed by Sir Richard Hadfield as a cast alloy of carbon and manganese that has a high toughness and ability to work harden. In the as-cast condition, the material contains a brittle network of grain boundary carbides (Figure 1) and is subject to cracking during the manufacturing process prior to heat treatment. It obtains its unique toughness properties following a water quench heat treatment process (Figure 2). In the heat-treated condition, the material has a stable single phase austenite structure at room temperature and is absent of any grain boundary carbide network (Figure 3). It is important to note that the manganese steel will revert back to the as-cast brittle structure if subjected to post-heat treatment thermal processes such as welding without appropriate procedures.
ELEMENTS IN MANGANESE STEEL
The primary elements in manganese steel are carbon, manganese and iron. Manganese above nine per cent creates a stable austenitic structure at room temperature. Carbon has a high solubility in austenite and creates a strong resistance to the movement of dislocations within the austenite grains, hence work hardening.
The rate of work hardening is directly related to the carbon content and the maximum carbon content that will remain in solution in austenite is related to the manganese content. This leads to the introduction of three grades currently available, nominally 12 per cent Mn, 18 per cent Mn and 23 per cent Mn. There is significant variation within each of the three grades including carbon variations and other element additions (eg Ni, Cr, Mo, Ti).
It is not the purpose of this report to detail these other than to highlight the importance of discussing the grades available with your supplier to ensure they are best for your application. As an example, the addition of chrome with a decrease in carbon may be beneficial to one application but detrimental to another. This particular grade is not usually supplied unless requested.
WORK HARDENING OF MANGANESE STEEL
The unique properties of austenitic manganese steel include the ability to work harden when deformed. This results in a hard wear layer forming in the deformed area backed up by the soft and ductile core. This, in effect, gives the material mutually exclusive properties of hardness (wear resistance) and ductility (impact strength). A microstructure of the hardened wear surface of the manganese steel wear liners showing slip planes inside the grains is illustrated in Figure 5.
Work hardening can only happen if the material is permanently deformed, therefore, in applications of pure or low-stress abrasion the desired property of higher hardness cannot be achieved, resulting in increased wear. The degree of work hardening is directly related to the degree of deformation. Liner wear can often be improved by a change in wear liner profile design without changing the material composition. Surface hardness testing can be carried out on worn liners to determine the wear surface hardness profile to assist with the selection of future liner design and material.
MANGANESE STEEL GRADES
Crushing Equipment has developed three grades of manganese steels with assistance from original equipment manufacturers and ongoing feedback from end users operating in a variety of crushing applications. There is evidence a liner can have up to double the wear life changing from a standard manganese grade to a higher C-Mn grade when crushing a low compressive strength and high silica content material. At the same time, using the high C-Mn grades for high compressive strength rock can lead to premature failure before the liner reaches its expected life. The three primary grades include:
a. Grade 845: Standard Hadfield Manganese Steel: 13 per cent Mn and 1.2 per cent C.
b. Grade 846: 18 per cent High Carbon Manganese Steel: 18 per cent Mn and 1.3 per cent C.
c. Grade 843: 23 per cent XAlloy: 23 per cent Mn and 1.35 per cent C.
All the above manganese steel grades are generally covered by the relevant specifications in standards AS2074 grade H1A, BS3100 Grade BW10, ASTM A128 Grade A and SABS 407 grade 1 containing carbon levels ranging from one per cent to 1.35 per cent and manganese from 11 per cent. Carbon is the main element that will determine the life of the wear liner. As the carbon level is increased, a corresponding increase in manganese content is made to accommodate carbon in a fully austenitic material that will retain sufficient fatigue strength for the application. The high C-Mn grades give the best results compared to standard manganese when crushing the lower compressive strength and high silica content materials.
THE IDEAL CRUSHER WEAR LINER
The ideal wear liner will give the maximum wear life for the crushing application at a consistent production output without damage to the crusher. Wear liner design is important for the ideal liner but once established will remain a constant. Wear liner material is not always given the consideration it requires and often does lead to premature failure, leaving the quarry manager wondering if it was an operator error or manufacturer/supplier error. The worn liner illustrated in Figure 6 is a good result. Although the liner is excessively worn (operator error) it has remained in one piece with no significant damage to the crusher, particularly the fitting faces. If a jaw crusher liner or a cone crusher liner breaks, the repair costs become extreme.
OPERATING AND CASTING VARIABLES LEADING TO LINER FAILURE
In the event of poor performance or crusher wear liner failure, it is important to establish the cause prior to repairing the crusher and replacing the liner. The cause may be difficult to establish and often the visible evidence is not necessarily the true cause of the failure. The primary causes of wear liner failure fall into two main categories:
1. Liner fitment. Correct installation and dimensional fitment of the crusher liner is important to ensure it is securely clamped to the fitting face. This usually includes retightening jaw liner bolts and cone liner clamping bolts following a short period after startup and a repeat as required. If the liner is allowed to flex during the crushing process liner failure can be quick due to the through work hardening of the liner (refer Figure 5). Cone crusher mantles that are not securely clamped, have poor fitment (particularly metal to metal fitment) and are restricted from self tightening during their crushing life will eventually crack and can result in significant damage to the crusher. The bolt or bucket tooth that enters the crusher chamber may dent the liner but should not always be deemed the cause of the liner failure.
2. Liner mechanical properties. The general mechanical properties for two common grades of Manganese steel are as follows:
The mechanical properties in Table 1 are typically for a 25mm section size and the properties of these grades will vary significantly with changes to section size. High levels of manganese will enable higher carbon contents to be used and it is the carbon that has the greatest effect on the mechanical properties. The more carbon that is dissolved in the austenitic microstructure, the greater the work hardening capacity of the material. If the carbon is not fully dissolved it occurs as grain boundary carbides and has a detrimental effect on the toughness of the material and wear properties. The effectiveness of the heat treatment process therefore has a significant effect on mechanical properties and this is, in turn, influenced by the liner section size, manganese carbon ratio, material grain size and presence of other elements either as deliberate additions or residual from each melt.
The variables outlined in categories 1 and 2 above are indicative of what can lead to liner failure and potential crusher damage. All the parameters included above can be readily identified and quantified following an investigation into a liner failure. Liner fitment may be difficult following the failure, however subsequent microscopic examination will determine the mode of failure and support either fatigue failure due to liner fitment or material mechanical failure. Where microscopic and chemistry testing may identify the problem as a material mechanical failure, this may have been poor material selection or a manufacturer?s error.
The investigation of wear liner poor performance and failure often requires the assistance from the supplier and individual liner traceability becomes important. Crushing Equipment Pty Ltd operates an accredited Quality Management System and each wear liner supplied from the Scaw Metals Foundry Group has a unique identification number traceable to all manufacturing stages including heat treatment, dimensional and chemistry reports.
There are many points to be considered when selecting and reviewing wear liners for the crushing plant to ensure you are getting the best result. The crusher?s performance remains a key issue when it comes to product consistency, output and cost per tonne. The quarry manager must use all available resources, internal and from suppliers, to achieve the best outcome.
Lew Dilkes is a qualified metallurgist with Crushing Equipment Pty Ltd.