Alloys or metallic compositions – Ferrous – Rare earth metal containing
Reexamination Certificate
1998-08-29
2001-01-23
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Rare earth metal containing
C420S578000, C420S029000, C420S030000
Reexamination Certificate
active
06177045
ABSTRACT:
This invention relates to a composition for inoculating grey iron and more particularly to a composition for the inoculation of a grey iron having a low sulphur content.
Inoculation is a process for controlling the solidification behaviour of the austenite/graphite eutectic and suppressing the formation of the austenite/carbide eutectic in grey cast irons. The inoculation treatment ensures that the cast iron has a fully grey structure, provided it is done just prior to casting of the iron, and produces benefits such as improved mechanical properties and machineability. A variety of inoculants have been used and many of those are based on ferrosilicon alloys. Other commonly used inoculants are alloys or mixtures of such elements as calcium, silicon, graphite, barium, strontium, aluminum, zirconium, cerium, magnesium, manganese and titanium.
Most inoculants, although effective in inoculating molten irons having a sulphur content of above 0.04% by weight, are unsatisfactory as inoculants for low sulphur irons having a sulphur content of 0.04% by weight or below.
In order to improve the response of low sulphur irons to inoculation, it has been proposed to add iron sulphide to the molten iron in order to increase the sulphur content. However, this procedure is only partially effective and can produce undesirable side effects.
GB-A-2093071 describes a method for inoculating molten iron involving the use of a source of sulphur and a reactant which forms a sulphide therewith which sulphide is capable of acting to provide nuclei in the form of graphite from the molten iron. The source of sulphur may be sulphur itself or a sulphide mineral such as chalcocite, bornite, chalcopyrite, stannite, iron sulphide or covellite. The sulphide forming reactant may be calcium silicide, calcium carbide, a cerium or strontium alloy, a rare earth and/or magnesium.
It has now been found that a ferrosilicon based composition containing rare earths and strontium can be used effectively as an inoculant for low sulphur iron, without the need to increase the sulphur content of the iron during the inoculation treatment, if the amount of each element is controlled within a certain range and the content of any calcium and/or aluminum which is present does not exceed a certain amount.
According to the invention, there is provided a composition for inoculating molten grey iron comprising by weight:
Rare earth
1.0-4.0%
Strontium
0.5-1.5%
Calcium
1.5% maximum
Aluminium
2.0% maximum
Silicon
40.0-80.0%
Iron
balance
Preferably the composition comprises by weight:
Rare earth
1.5-2.5%
Strontium
0.7-1.0%
Calcium
0.5% maximum
Aluminium
0.5% maximum
Silicon
70.0-75.0%
Iron
balance
The rare earth may be cerium, mischmetall containing nominally 50% by weight cerium and 50% by weight other rare earths or a mixture of cerium and other rare earths.
The inoculant composition is most preferably free of aluminum and calcium but if these elements are present the amounts should not exceed the limits indicated. Aluminum is, in general, considered to be a harmful constituent in inoculant compositions, and calcium has an adverse reaction with strontium and affects its performance.
The inoculant composition may be a particulate mixture of ferrosilicon and the other constituents of the composition but it is preferably a ferrosilicon based alloy containing the other constituents.
The inoculant can be made in any conventional manner with conventional raw materials. Generally, a molten bath of ferrosilicon is formed to which a strontium metal or strontium silicide is added along with a rare earth.
Preferably, a submerged arc furnace is used to produce a molten bath of ferrosilicon. The calcium content of this bath is conventionally adjusted to drop the calcium content to below the 0.35% level. To this is added strontium metal or strontium silicide and a rare earth. The additions of the strontium metal or strontium suicide and rare earth to the melt are accomplished in any conventional manner. The melt is then cast and solidified in a conventional manner.
The solid inoculant is then crushed in a conventional manner to facilitate its addition to the cast iron melt. The size of the crushed inoculant will be determined by the method of inoculation, for example, inoculant crushed for use in ladle inoculation is larger than the inoculant crushed for use in mould inoculation. Acceptable results for ladle inoculation are found when the solid inoculant is crushed to a size of about 1 cm down.
An alternative way to make the inoculant is to layer into a reaction vessel a charge of silicon and iron or ferrosilicon, strontium metal or strontium silicide and rare earth and then melt the charge to form a molten bath. The molten bath is then solidified and crushed as described above.
When the inoculant is made from a base alloy of ferrosilicon, the silicon content of the inoculant is about 40 to 80% and the remaining per cent or balance after taking into account all other specified elements is iron.
Calcium will normally be present in the quartz, ferrosilicon and other additives such that the calcium content of the molten alloy will generally be greater than about 0.5%. Consequently, the calcium content of the alloy will have to be adjusted down so that the inoculant will have a calcium content within the specified range. This adjustment is done in a conventional manner.
The aluminum in the final alloy is also introduced into the alloy as an impurity in the various additives. If desired, it can also be added from any other conventional source of aluminum or aluminum can be refined out of the alloy using conventional techniques.
The exact chemical form or structure of the strontium in the inoculant is not precisely known. It is believed that the strontium is present in the inoculant in the form of strontium silicicle (SrSi
2
) when the inoculant is made from a molten bath of the various constituents. However, it is believed that any metallic crystallographic form of the strontium is acceptable in the inoculant.
Strontium metal is not easily extracted from its principal ores, Strontianite, strontium carbonate, (SrCO
3
) and Celesite, strontium sulphate (SrSO
4
). However, the inoculant may be produced with either strontium metal or strontium ore depending upon the economics of the entire production process.
U.S. Pat. No. 3333954 discloses a convenient method for making a silicon bearing inoculant containing acceptable forms of strontium wherein the source of strontium is strontium carbonate or strontium sulphate.
The carbonate and sulphate are added to a molten bath of ferrosilicon. The addition of the sulphate is accomplished by the further addition of a flux. A carbonate of an alkali metal, sodium hydroxide and borax are disclosed as appropriate fluxes. The method of the 3333954 patent encompasses adding a strontium-rich material to a molten ferrosilicon low in calcium and aluminum contaminates at a sufficient temperature and for a sufficient period of time to cause the desired amount of strontium to enter the ferrosilicon. U.S. Pat. No. 3,333,954 is incorporated herein by reference and discloses a suitable way to prepare a silicon-bearing inoculant containing strontium to which a rare earth can be added to form the inoculant of the present invention. The addition of the rare earth is preferably done after the addition of the strontium, however, the sequence of the addition is not critical so long as the inoculant has the proper amounts of reactive elements. The addition of the rare earth is accomplished in any conventional manner.
The rare earth can come from any conventional source, for example, individual pure rare earth metals, mischmetall, rare earth of cerium silicide and, under appropriate reducing conditions, rare earth ores such as bastnasite or manazite.
There are the normal amount of trace elements or residual impurities in the finished inoculant. It is preferred that the amount of residual impurities be kept low in the inoculant.
It is preferred that the inoculant be formed from a molten mixture of the different constituents as described
Butler David
Ecob Christopher
White Douglas
Elkem ASA
Yee Deborah
LandOfFree
Composition and method for inoculating low sulphur grey iron does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Composition and method for inoculating low sulphur grey iron, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Composition and method for inoculating low sulphur grey iron will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2509550