Optimize your grinding parts
Maintenance News Customer Service
Every comminution process is accompanied by the phenomenon of wear. Wear is a loss of material from the surface of components. The area subject to the most intensive wear in Loesche mills is that of the grinding tools, i.e. grinding roller and grinding plate. The prevailing form of wear in the grinding roller – grinding plate system is abrasion, i.e.: • hard particles penetrate into the grinding tools, • whereby the depth of penetration is dependent on the hardness of the grinding body material; • in the case of multiphase materials (chilled castings – embedded carbides) local score grooving processes occur, resulting in a selective removal of material.
The choice of wear material which is to be used in vertical mills is determined not just by the abrasivity of the grinding stock. What is required is a consideration of all the economic factors, such as costs, plant availability, simultaneity of utilisation of all the wear parts – referred to in their entirety by the term “LIFE CYCLE COST”. Loesche offers the right material for every application. In practice three different material groups have gained acceptance: • Grinding parts made from chromium alloy cast iron • Hardfaced grinding parts • Grinding parts made from composite casting
The harder the particles to be ground, the greater the removal of material or wear. Increasing wear has a negative impact on the comminution effect of the grinding tools.
Typical wear pattern on a Loesche tyre
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Worn tyre
Worn tyre when installed together with grinding plate
Sign of wear at a grinding plate
Collar formation on a worn-out tyre
Grinding parts made from chromium alloy cast iron
100 µm
Structure of hardened Ni-Hard IV
Chromium alloy cast iron has proven itself as the standard material used for grinding parts in everyday applications. These materials are known for example by the trade names Ni-Hard IV and Cromodur. The carbon in the cast iron is present in chemically bonded form as iron carbide or chromium carbide. This produces a very hard structure which offers high wear resistance. The matrix is usually modified by heat treatment to martensite so as to offer the hard carbides a firm hold and thereby further increase wear resistance. In order to avoid embrittlement of the material and with it a risk of fracture, it is necessary for the components to be tempered by further heat treatment. Tempering leads to a loss of hardness and wear resistance. Due to this, tempering is only carried out at very low temperatures. Despite these low temperatures the impact resistance is appreciably improved. The manufacture of grinding parts containing high levels of chromium requires detailed knowledge of materials and plenty of experience. To achieve the desired material properties, it is essential to adhere exactly to the parameters of temperature and heating duration. Materials containing high levels of chromium have good to very good resistance to abrasion. The general hardness of 630 to 800 HV20 produces uniform, anticipated wear. Grinding parts made from this material are used in coal mills and in raw meal and cement mills. Although contrary statements are made in the literature, grinding parts made from chromium alloy cast iron can nowadays be armoured safely and economically. Thanks to hardfacing the useful life can be significantly increased compared with wearresistant castings. The down-times caused by hardfacing are unavoidable. Depending on the amount of material to be deposited, down-times ranging between a few days and more than a week may be anticipated. The number of repeatable weld deposits on a grinding part is dependent on the previous service life, the load and the condition of the previous deposits. The correct combination of hardening and tempering leads to a wear resistant material with sufficient impact resistance for many functions and which can also be regenerated.
Tyre made from Cromodur with worked seat
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Hardfaced grinding parts
Hardfacing refers to the deposition of a high-alloy material as surface protection on heavily stressed metallic components. The grinding tools are made from either a weldable cast steel or alternatively wearresistant iron castings. Hardfacing can be completed with a layer thickness of up to 80 mm. Welds are deposited with a filler wire in order to minimise the application of energy to the grinding parts. Structure of a hardfacing application
The welding materials contain high levels of chromium and carbon. Depending on the degree of wear resistance required, further carbide-forming materials, such as niobium, vanadium, etc. are used. A weld-deposited hard layer increases the wear resistance of the ductile cast steel. In this thermal process high-strength carbides are formed within a matrix, resulting in a highly wearresistant layer. For most tribological systems there are appropriate filler metals to suit the parent metal. Some of these material combinations are mechanically machinable, offer a high degree of safety against fracture, and provide a far greater useful life than grinding parts made from a wear-resistant casting.
Tyre during regeneration
Hardfacing can be carried out inside or outside the mill. Regular wear measurements provide information on the state of wear of the grinding parts. Evaluation of these measurements produces specific wear rates, makes it possible to provide a good assessment of the utilisation ratio and the remaining service life, and provides information on when to introduce necessary regeneration measures or when to acquire new grinding parts.
Completed weld-deposited tyre
Grinding parts made from composite casting In the case of composite materials two or more materials are structurally bonded in such a way that each component is stressed in that area in which it demonstrates the best properties. Grinding tools are made up of a so-called metal matrix composite (MMC). This involves shaped ceramic pieces being embedded in ductile cast iron. The shaped pieces themselves are sintered from non-metallic oxides together with chromium carbides. This combination makes the grinding tools particularly hard and wear-resistant with simultaneously high impact strength. The matrix of cast iron increases the mechanical machinability of the grinding tools. Then heat treatment is also required here in order to achieve the final hardness.
These composite materials provide a longer useful life than standard materials and also hardfacing. The material is already being used by various customers and has received an entirely positive evaluation. It must be noted that regeneration by hardfacing is not possible once the wear limit has been reached. The advantages at a glance: • Highly wear-resistant • Very high impact resistance • Simple machinability • No hardfacing necessary • Thus investment in a welding machine not necessary
Tyre made from composite material. The shaped ceramic pieces can be clearly made out.
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Wear of grinding parts has a negative effect on: • Mill throughput • Product quality (fineness, specific material surface and grain size distribution)
Improving wear protection results in: • Reduction of costs • Stabilisation of product quality and mill throughput • Increase in availability • Reduction of down-times • Prolongation of maintenance intervals
If you require further information or have a specific query, e-mail our Customer Service team at:
[email protected]
Wear rate of different wear materials as a function of the grinding stock
6
Wear rate [g/t]
5
4
3
2
1
0 Coal mill
Chromium alloy cast iron
Raw meal mill
Composite casting
Cement mill
Hardfaced
Loesche LM 46.2+2 C/S roller grinding mill, Kingston, Jamaica, 2006
Loesche GmbH Hansaallee 243 40549 Düsseldorf, Germany Tel. +49 - 211 - 53 53 - 0 Fax +49 - 211 - 53 53 - 500 E-Mail:
[email protected] www.loesche.com
