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(11) | EP 0 032 282 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Process for manufacture of cast iron with vermicular graphite and cast iron so produced |
| (57) A process for the production of cast iron with vermicular graphite in which molten
grey cast iron is introduced in the mould cavity of a conventional pouring system
which pouring system additionally includes one or more intermediate chambers containing
a vermicularizing agent in an amount sufficient to convert the graphite at least in
part to vermicular graphite. A nodular graphite inhibitor may be added to the molten
grey cast iron before it is introduced in the mould cavity. By a particular selection
of process parameters the amount of vermicularizing agent in the resulting cast iron
may be closely controlled. The pouring system is known for the production of cast
iron with nodular graphite but not the the production of iron with vermicular graphite.
A pouring system is shown in the drawing and includes a downsprue (1), an intermediate
chamber (2) which contains the vermicularizing agent, a gate (3), a runner bar and
an ingate (5). |
[0001] This invention relates to a process for the manufacture of cast iron with vermicular graphite.
[0002] In U.K. Patent No. 1,278,265 a process for the production of nodular or spheroidal graphite cast iron castings in which untreated molten grey cast iron is introduced-into the mould cavity by way of a conventional pouring system which additionally includes one or more intermediate chambers containing a nodularising agent in an amount sufficient to convert the graphite to nodular or spheroidal form is described.
[0003] It has now surprisingly been found that the pouring system described in the said prior patent enables castings of cast iron with vermicular graphite to be produced, if desired without the use of a nodular graphite inhibitor as mentioned below.
[0004] Vermicular graphite is a name given to flake graphite which has become rounded, thickened and shortened compared with the normal elongated flakes commonly found in grey cast irons. This modified form of graphite is known by several other names including "compacted", "quasi-flake", "aggregate flake", "chunky", "stubby", "up-graded", "semi-nodular" "P-type" and "floccular" graphite.
[0005] Most cast irons have elongated flake graphite structures and such irons are comparatively weak and brittle, but have good thermal conductivity and resistance to thermal shock. It is known however, that it is possible to produce cast irons having a nodular or spheroidal, graphite structure and these are ductile and comparatively strong. However, in some circumstances these nodular or spheroidal graphite structures have lower thermal conductivity and sometimes also poorer resistance to thermal shock. Irons with vermicular graphite structures combine the high strength and ductility often associated with nodular graphite irons whilst retaining good thermal conductivity and resistance to thermal shock.
[0006] The manufacture of cast iron containing vermicular graphite has been well known for many years by alloying the iron with magnesium. Various investigators have devised systems in an attempt to control the magnesium content. A narrow range of magnesium content is required to produce a vermicular structure and close control of the process necessary to achieve this has been difficult. One proposed method of control involves the addition of an alloying agent, normally magnesium, and an inhibiting or suppressing agent. The major problem with such two-part systems, e.g. magnesium alloying agent and a nodular graphite inhibitor is that, in practice, it is difficult to make accurate additions at the required levels, e.g. 0.015%. One part addition agents are known, that is agents comprising a combination of a suppressing agent and magnesium. However, in practise, it is difficult to achieve the correct balance of the two components.
[0007] We have now found that cast iron with vermicular graphite can be produced in a controllable manner if the pouring system of the prior patent referred to above is utilized wherein a vermicularizing agent is contained in the intermediate chamber or chambers and optionally a nodular graphite inhibitor is added to the molten grey cast iron charge.
[0008] Although the pouring system of the prior patent provides a high degree of control over the casting porcess, it was not to be expected that this pouring system could be used to produce vermicular graphite containing cast iron in a reproducible manner, since the production of vermicular graphite represents a half way stage between flake and nodular graphite.
[0009] Accordingly, the present invention provides a process for the production of cast iron with vermicular graphite wherein molten grey cast iron optionally containing a nodular graphite inhibitor is introduced into the mould cavity by way of a conventional pouring system which additionally includes one or more intermediate chambers containing a vermicularising agent in an amount sufficient to convert the graphite at least in part to vermicular graphite under the process conditions employed.
[0010] By the term vermicularizing agent used herein we mean an agent which is in general an alloy but can be a metal and which leads to vermicular graphite, under appropriate process conditions. The formation of vermicular graphite depends to a certain extent on the process conditions, as is well known in the art, which process conditions are for example described in U.K. Patent Specification 1,427,445. In general the vermicularizing agent will be a magnesium-containing alloy. This alloy may contain a metal, such as, titanium which will inhibit the production of nodular graphite. This titanium may also be added as ferrotitanium to the metal in the ladle prior to pouring. It may also inherently be present in the iron which makes up the charge. The presence of this titanium enables one to be less strict in controlling the level of magnesium. In principle, vermicular graphite is obtained when the magnesium level in the finished metal is below 0.02 to 0.025% by weight based on the weight of the iron. In the presence however of a nodular- graphite inhibiting metal, in particular titanium, but also zirconium higher levels of magnesium may be tolerated. A particularly useful additive in the vermicularizing alloy is cerium.
[0011] The vermicularizing agent may therefore be magnesium in the form of a 5% magnesium ferrosilicon alloy containing cerium.
[0012] A suitable base iron for the grey iron charge, excluding carbon, is:-
Si: - 2.17% by weight
Al: - 0.01% by weight
Ti: - 0.013% by weight.
[0013] According to the present invention the size of the intermediate chamber, the pouring rate of the molten grey iron and the amount of magnesium retained in the cast metal may be determined according to the relationship defined in our U.K. Patent Specification Serial No. 1,511,246.
[0014] That is, the total area of the base or bases of the chamber or chambers are equal to a "factor" x (metal pouring rate) x (total concentration of the agent in the cast metal, expressed as a proportion of the weight of the cast metal).
[0015] The "factor" is the reciprocal of the constant "k" as defined in Specification No. 1,511,246 and is dependent on the physical form and chemical composition of the agent, preferably magnesium as described in detail in U.K. Patent Specification 1,511,246.
[0016] According to this embodiment of the invention, the required amount of magnesium can be calculated with great accuracy.
[0017] For example, for a magnesium alloy agent with a grading in the range of 0.5 to 4 mm., the following constants (k) have been determined
k (6% Mg alloy) = .0225 kg/cm2 (0.032 lb/in2) sec
k (9% Mg alloy) = .0337 kg/cm2 (0.048 lbJin2)sec:
[0018] We have found experimentally, using a magnesium alloy as the agent, that a product which is cast iron containing vermicular graphite can be obtained when the total concentration of the magnesium in the cast metal expressed as a proportion of the weight of the cast metal is between for example 0.01% and 0.02%.
[0019] Thus, using a 6% Mg alloy with a grading in the range of 0.5 to 4mm and applying the relationship:-
[0021] The structure obtained is generally not totally vermicular graphite but is about 80% to 90% vermicular.
[0022] We have found that the amount of magnesium required to produce vermicular graphite is dependent on the sulphur level of the metal. The tests just described were carried out with a grey iron containing 0.01 to 0.015% by weight of sulphur. Where the sulphur content is higher, the amount of magnesium required will increase and vice-versa.
[0023] The invention is further illustrated by the accompanying drawing which illustrates one embodiment of a mould system used in the process according to the invention.
[0024] In this embodiment, molten metal containing a nodular graphite inhibitor enters the mould system by way of a pouring bush (not shown) down a downsprue 1 and into a reaction chamber 2 which contains the vermicularizing agent. A gate 3 regulates the outflow of metal from the chamber thereby ensuring a sufficient residence time for the metal in the chamber for reaction with the vermicularizing agent therein. From the gate 3 the treated molten metal flows via a runner 4 and ingate e.g. 5, to the remainder of the system comprising a riser and the mould cavity proper (not shown). Thus, as molten iron containing a nodular graphite inhibitor passes into the casting mould it contacts the vermicularizing additive in the intermediate reaction chamber 2 and the graphite in the iron is converted into vermicular form.
[0025] The present invention is further illustrated by the following Example which was carried out using a mould system as described above.
Example
[0026] A base metal containing a total carbon content of 3.4%, silicon`2.4%, sulphur 0.01% and titanium 0.015%, the balance being iron, was poured to obtain a total pound weight of casting, together with riser and runner bar, of 22 kgs (50lbs). The disc casting size was 50.8 cms (20 inches) x 2.54 cms (1.0 inch) and the pouring time was ten seconds.
[0027] The intermediate chamber contained a vermicularising agent consisting of an alloy comprising 5% magnesium, 0.3 to 0.5% calcium, 0.2% cerium, 45-50% silicon with the balance being iron.
1. A process for the production of cast iron with graphite by casting molten grey
cast iron into a mould cavity by way of a conventional pouring system which additionally
includes one or more intermediate chambers characterized in that the intermediate
chamber or chambers contain a vermicularizing agent in an amount sufficient to convert
the graphite at least in part, to vermicular graphite under the process conditions
employed.
2. A process as claimed in claim 1, characterised in that a nodular graphite inhibitor
is added to the molten grey cast iron prior to its introduction into the mould cavity.
3. A process as claimed in claim 1 or 2, characterised in that the amount of vermicularizing
agent in the resulting cast iron is controlled by selection of process parameters
according to the relationship:- area of the base or bases of the intermediate chamber
or chambers = (factor) x (metal pouring rate) x (total concentration of the vermicularizing
agent in the cast metal, expressed as a proportion of the weight of the cast metal).
4. A process as claimed in any of claims 1 to 3, characterised in that the vermicularizing
agent is magnesium or an alloy thereof.
5. A process as claimed in claim 4, characterised in that the process is carried out
such that the total concentration of the magnesium in the cast metal product expressed
as a proportion of the weight of the cast metal is 0.01 to 0.02%.
6. A process as claimed in any of claims 1 to 5, characterised in that the molten
grey iron contains titanium and/or zirconium.
7. A process as claimed in any of claims 1 to 6, characterised in that the cast metal
product contains from 80 to 90% vermicular graphite.
8. Vermicular graphite-containing cast iron when made by a process as claimed in any
of claims 1 to 7.