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8/17/2019 Wear Rate Loesche
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Optimize your grinding parts
Maintenance News
Customer Service
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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 proc-
esses occur, resulting in a selective removal of
material.
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.
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 considera-
tion 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
Optimize your grinding parts
Typical wear pattern on a Loesche tyre
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Sign of wear at a grinding plate Collar formation on a worn-out tyre
Worn tyre Worn tyre when installed together with grinding plate
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Grinding parts made from chromium
alloy cast iron
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 tem-
peratures. 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 wear-
resistant castings.
The down-times caused by hardfacing are unavoida-
ble. 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 de-
pendent on the previous service life, the load and the
condition of the previous deposits. The correct combi-
nation of hardening and tempering leads to a wear re-
sistant material with sufficient impact resistance for
many functions and which can also be regenerated.
Tyre made from Cromodur with worked seat
Structure of hardened Ni-Hard IV
100 µm
Optimize your grinding parts
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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 wear-
resistant 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.
The welding materials contain high levels of chromium
and carbon. Depending on the degree of wear resist-
ance required, further carbide-forming materials, such
as niobium, vanadium, etc. are used.
A weld-deposited hard layer increases the wear resist-
ance of the ductile cast steel.
In this thermal process high-strength carbides are
formed within a matrix, resulting in a highly wear-
resistant layer.
For most tribological systems there are appropriate
filler metals to suit the parent metal.
Some of these material combinations are mechani-
cally machinable, offer a high degree of safety against
fracture, and provide a far greater useful life than grind-
ing parts made from a wear-resistant casting.
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
Tyre during regeneration
Structure of a hardfacing application
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Grinding parts made from composite casting
In the case of composite materials two or more materi-
als are structurally bonded in such a way that each
component is stressed in that area in which it demon-
strates 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 themselvesare 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 custom-
ers and has received an entirely positive evaluation.
It must be noted that regeneration by hardfacing is notpossible 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.
Optimize your grinding parts
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Wear rate of different wear materials as a function of the grinding stock
W e a r r a t e [ g /
t ]
Coal mill Raw meal mill Cement mill
HardfacedChromium alloy cast iron Composite casting
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 specifi c query, e-mail our Customer Service team
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Loesche GmbH
Hansaallee 243
40549 Düsseldorf, Germany
Tel. +49 - 211 - 53 53 - 0Fax +49 - 211 - 53 53 - 500
E-Mail: [email protected]
www.loesche.com
Loesche LM 46.2+2 C/S roller grinding mill, Kingston, Jamaica, 2006