Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
1999-01-19
2001-04-24
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S540000
Reexamination Certificate
active
06221184
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the production of high-carbon cast steels which are more particularly intended for the manufacture of wearing parts, especially grinding media such as balls.
BACKGROUND OF THE INVENTION
In the mining industry, it is necessary to release potentially valuable minerals from their rock gangue for the purpose of concentrating them and extracting them.
In order to achieve this release, the ore must be crushed and finely ground.
In the grinding step alone, it may be estimated that 750,000 to 1 million tonnes of grinding media, in the form of spherical balls or cylpebs (frustoconical or cylindrical pebbles), are consumed annually in the world.
In grinding media, the following materials are mainly encountered:
1) low-alloy martensitic steels (0.7 to 1% carbon and alloy elements less than 1%) shaped by rolling or forging and then heat-treated in order to obtain a surface hardness of 60-65 RC;
2) chromium-alloy martensitic cast iron (1.7 to 3.5% carbon and 9 to 30% chromium) shaped by casting and heat-treated in order to obtain a hardness of 60 to 68 RC throughout the cross-section;
3) low-alloy pearlitic white cast irons (3 to 4.2% carbon and alloy elements less than 2%) not treated and having a hardness of 45 to 55 RC, obtained by casting.
Each of the current solutions has drawbacks which are specific to each of them:
for forged martensitic steels, the capital costs for forging or rolling machines, the heat-treatment plants and the energy consumptions are high;
as regards chromium-alloy cast irons, there are additional costs related to the alloying elements (mainly chromium) and to the heat treatments;
finally, for low-alloy pearlitic white cast irons, the manufacturing costs are generally quite low but the performance characteristics in terms of wear resistance are markedly inferior to the previous solutions. In addition, only grinding media of a size less than 60 mm are generally produced industrially.
More particularly, in the case of ores where the gangues are highly abrasive (for example: gold ore, copper ore, etc.), the current solutions are not entirely satisfactory for the users, since the contribution of the products and materials subjected to wear (balls and linings) remains large in the production costs of these potentially valuable metals.
SUMMARY OF THE INVENTION
The object of the invention is to provide a process for the production of cast steels having improved properties and most especially to remedy the drawbacks and shortcomings of the solutions in the prior art for wearing parts (in particular the grinding media), the composition, the shaping by casting and the post-casting cooling conditions of which make it possible to obtain a wear resistance (especially under very abrasive conditions) which is comparable to that of forged martensitic steels and chromium martensitic cast irons, but with a markedly lower cost, and is markedly superior to pearlitic cast irons for a comparable cost.
Other objects and advantages of the invention will appear to those skilled in the art on reading the following description of the characteristic elements of the invention and of particular embodiments thereof.
In the process of the invention, high-carbon steels are used having a composition expressed in % by weight of:
carbon
0.6 to 2%
manganese
0.5 to 6%
chromium
1 to 6%
silicon
0.4 to 1.5%
the balance being iron, with the usual impurity contents, in that they have non-equilibrium structures obtained directly after solidification.
Depending on the chemical composition and the cooling conditions, the structures of these steels may consist of:
a non-equilibrium structure of fine pearlite, containing between 1 and 1.5% by weight of carbon with a hardness lying between 47 and 54 RC;
a high carbon austenitic structure with a hardness lying between 15 and 30 RC;
a high carbon martensitic structure with a hardness lying between 60 and 65 RC.
Particularly preferably, the carbon contents are:
between 1.3 and 1.7% as regards the steels consisting of fine pearlite;
between 1 and 1.6% as regards the steels consisting of austenite;
between 0.6 and 1% as regards the steels consisting of martensite.
According to the invention, steels of the indicated composition are subjected, after casting and complete solidification, to a cooling from a temperature of at least 900° C. at a cooling rate lying between 7.5 and 1.0° C./sec down to 500° C. and a cooling rate lying between 2° C. and 0.4° C./sec from 500° C. to room temperature.
As regards the non-equilibrium pearlitic structures, specific compositions have proved to be particularly useful for the manufacture of grinding media, in particular balls having a diameter of 100-125 mm, wherein the alloy composition of the steel is:
carbon
of the order of 1.3 to 1.7%
manganese
of the order of 3 to 4%
chromium
of the order of 3 to 3.5%
silicon
of the order of 0.4 to 1%
and for the manufacture of grinding media, in particular balls having a diameter of 30-90 mm, wherein the alloy composition of the steel is:
carbon
of the order of 1.3 to 1.7%
manganese
of the order of 0.3 to 2.5%
chromium
of the order of 1.5 to 3%
silicon
of the order of 0.4 to 1%.
As regards the non-equilibrium austenitic structures, specific compositions have proved to be particularly useful for the manufacture of grinding media, in particular balls having a diameter of 100-125 mm, wherein the alloy composition of the steel is:
carbon
of the order of 1 to 1.6%
manganese
of the order of 4.4 to 5%
chromium
of the order of 3.5 to 4%
silicon
of the order of 0.4 to 1%
and for the manufacture of grinding media, in particular balls having a diameter of 25-90 mm, wherein the alloy composition of the steel is:
carbon
of the order of 1 to 1.6%
manganese
of the order of 2.6 to 4.1%
chromium
of the order of 2.5 to 3.5%
silicon
of the order of 0.4 to 1%.
As regards the non-equilibrium martensitic structures, specific compositions have proved to be particularly useful for the manufacture of grinding media, in particular balls having a diameter of 60-125 mm, wherein the alloy composition of the steel is:
carbon
of the order of 0.6 to 1%
manganese
of the order of 1.1 to 1.3%
chromium
of the order of 3 to 3.5%
silicon
of the order of 0.4 to 1%
and for the manufacture of grinding media, in particular balls having a diameter of 30-60 mm, wherein the alloy composition of the steel is:
carbon
of the order of 0.6 to 1%
manganese
of the order of 1.3 to 1.6%
chromium
of the order of 2.5 to 3%
silicon
of the order of 0.4 to 1%.
These various alloys were evaluated using the same procedure and each proved to be particularly useful depending on the levels and types of stress that are encountered in grinding in the mining industry.
The casting operation causes the shaping of the wearing pieces, and more particularly the grinding media, directly and it can be performed using any of the conventional casting techniques known in founding (especially die casting).
The non-equilibrium structures are obtained by extraction (knock-out) of the still hot casting from the casting mould and by adapting the chemical composition to the mass of the casting and to the rate of cooling (natural or preferably accelerated cooling) which follows extraction from the mould.
REFERENCES:
patent: 5855701 (1999-01-01), Bonnevie
patent: 2456137 (1976-08-01), None
patent: 0120748A1 (1984-10-01), None
patent: WO95/28506 (1995-10-01), None
Jacobson Price Holman & Stern PLLC
Magotteaux International S.A.
Yee Deborah
LandOfFree
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