Specialized metallurgical processes – compositions for use therei – Processes – Free metal or alloy reductant contains magnesium
Patent
1981-08-05
1984-02-07
Rosenberg, Peter D.
Specialized metallurgical processes, compositions for use therei
Processes
Free metal or alloy reductant contains magnesium
C22C 3308
Patent
active
044301235
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the production of vermicular graphite cast iron.
The term vermicular graphite cast iron is used to denote cast iron in which flake graphite as been modified to a rounded, shorter form compared with the graphite in normal grey cast iron. This modified form of graphite is also known by other names, including "quasi-flake" and "compacted".
Vermicular graphite cast iron may be produced by treating molten iron with magnesium in conjunction with titanium and one or more rare earth metals. Usually the magnesium is added as a 5% magnesium ferrosilicon containing cerium and titanium is added as ferrotitanium or titanium metal.
However it can be difficult to produce the correct graphite structure when making separate additions of the magnesium, titanium and rare earth metal, and an iron containing excessive titanium or an iron which has a nodular graphite structure due to the presence of insufficient titanium for the quantity of magnesium present can easily result.
These difficulties can be overcome by using special alloys containing magnesium, titanium and rare earth metals, and British Pat. No. 1,427,445 describes the production and use of such alloys.
British Pat. No. 1,515,201 describes a modified alloy of the type disclosed in No. 1,427,445 which in addition contains calcium. The presence of the calcium gives an alloy which, for a given added quantity, produces a vermicular graphite structure over a wider range of initial sulphur contents in the iron compared with an alloy containing no calcium. In thin section castings (less than 5 mm) treatment with magnesium and titanium gives unacceptable quantities of nodules and insufficient compacted graphite when the iron is well inoculated.
It has also been proposed to produce vermicular graphite iron by adding to molten iron one or more rare earth metals, for example cerium or mischmetall, which is a mixture of cerium and other rare earth metals. A process using rare earth metals is described in British Pat. No. 1,268,706. However as is stated in that patent when using rare earth metals alone it is necessary first to desulphurise the molten iron to an abnormally low level or to use a large quantity of rare earth metal in order to obtain a fully vermicular graphite structure. Further, the use of rare earth metals alone is confined to the treatment of hypereutectic irons.
It has now been found that vermicular graphite iron may be produced from molten irons having a wide range of sulphur contents without the need for a preliminary desulphurisation treatment, by the simultaneous addition of a rare earth metal and calcium, providing the additions of rare earth metal and calcium are kept within certain parameters.
According to the invention there is provided a process for treating molten iron containing carbon and sulphur to produce a cast iron having a vermicular graphite structure comprising adding to the molten iron simultaneously up to 0.3% by weight of the iron of one or more rare earth metals and in excess of 0.2% by weight of the iron of calcium the quantity of rare earth metal being within the range of 2 to 8 times the sulphur content of the molten iron.
Preferably the iron contains less than 0.05% by weight sulphur before treatment otherwise excessive dross may be formed in the iron during the treatment process.
If the ratio of the rare earth metal added to the sulphur content of the metal before treatment exceeds 8:1 the graphite is present in the cast iron mainly as spheroids or nodules, and there is also a tendency for carbides to be produced even though the form of the graphite may be good. When the ratio of rare earth metal to sulphur is very high, for example of the order of 18:1, a fully white iron is produced.
When the calcium addition is about 0.2% by weight or below the formation of flake graphite is promoted. Normally the amount of calcium added will not exceed about 0.7% by weight.
In general for a particular sulphur content the lower the quantity of calcium which is added the higher the quantity of rare earth metal added, and v
REFERENCES:
patent: 2889222 (1959-06-01), Kurzinski
patent: 3415642 (1968-12-01), Matsumoto
Foseco International Limited
Rosenberg Peter D.
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