US2488512A - Cast iron - Google Patents

Description

Patented Nov. 1 5,

CAST IRON Henton Morrogh, Handsworth, Birmingham,

England, assignor to British Cast Iron Research Association, Al'vechurch, Birmingham,

England No Drawing. Application April 28, 1948, Serial No. 23,859. In Great Britain May 21. 1947 Claims.

This invention relates to improved cast irons of the corrosion-resistant type containing more than 10% 'byweight of silicon.

For the purpose of making castings resistant to corrosion by strong chemical reagents, a special cast iron is frequently used which contains, as its most important constituent, a large proportion of silicon. The usual alloying elements, namely, manganese, carbon, phosphorus, sulphur, are always. present and other elements such as molybdenum, copper, nickel, chromium, titanium and aluminium may be present either fortuitously or by design. For the purpose of corrosion-resistance the important element in these alloys is silicon, which may be present in any amount between 10% and 20%, although it is usual to have between 13% and 16% of this element. Cast irons containing these high percentages of silicon are very hard and brittle.

The carbon content of such irons has to be much lower than is usual in normal cast irons, and will vary between 0.3% and 2.0% depending upon the silicon content, and will tend to be lower the higher the silicon content. Almost all of the carbon in high silicon cast irons cast in sand moulds exists as graphite. The amount and form of the graphite considerably influences the mechanical properties of the material. Silicon reduces the amount of carbon required to form the eutectic concentration, that is, the amount of carbonheld in solution in the liquid metal during the final'stages of solidification. For instance, for pure iron-silicon-carbon alloys the eutectic carbon concentration is about 1.0% with about 13.5% silicon and only about 0.6% with 16.5% silicon. It is a matter of considerable practical importance (and also of difficulty) to keep the carbon content below the eutectic value. The graphite in sand-cast high-silicon cast irons usually exists in the form of coarse flakes, and graphite in excess of the eutectic concentration in very coarse flakes. The presence of coarse flake graphite adversely influences the mechanical properties of the metal. Also high-silicon cast iron containing coarse graphite tends to result in unsound castings, due to the formation of porosity cavities.

It is an object of the present invention to provide a high-silicon cast iron in which coarse flake graphite is entirely eliminated. A further object is to provide a high-silicon cast iron in which the graphite is present in very finely-divided form or as a mixture of very finely-divided graphite and nodular graphite. A still further object is to provide a process by which corrosionresistant cast irons free from coarse flake graphite can be obtained from irons which are hypoeutectic, eutectic or hyper-eutectic in carbon. Other objects will be apparent from the description which follows.

These objects are achieved according to the invention by the introduction of not less than 0.02% by weight of cerium into the iron.

I have found that the addition of 0.02% to 1.0% by weight of cerium to a molten high-silicon cast iron immediately before pouring causes the eutectic carbon to separate on cooling in the form of very finely-divided undercooled graphite and any excess of carbon over the eutectic amount in the form of nodular graphite. The result is a high-silicon cast iron having exceptionally fine undercooled graphite structures free from coarse flake graphite. The product shows improved mechanical properties, including less tendency to brittleness, and. a marked freedom from porosity cavities.

In order that the cerium added shall exercise its full effect the iron must be low in sulphur. Cerium is a powerful desulphurising agent, and if much sulphur is present cerium will be lost as cerium sulphide, which floats on the metal with the slag. I prefer to use an iron containing not more than 0.2% y weight of sulphur, and the finished casting should contain not more than 0.02% by weight of sulphur.

The cerium may be added in any convenient form, either as pure metallic cerium, mischmetall, ferro-cerium, cerium carbide or other alloy of cerium. I prefer to avoid the use of cerium alloys containing magnesium since the latter appears to lessen the solubility of the cerium in the iron. The cerium may be applied in the form of a reducible cerium compound.

It will be seen that the addition of cerium in accordance with the invention makes it possible to obtain satisfactory castings from high-silicon irons with a much higher carbon content than has hitherto been deemed acceptable. A further advantage is the reduction or complete elimination of the blowholes which, unless special precautions are taken in the melting and casting, may appear in high-silicon castings.

The invention is illustrated by the following example (in which all percentages are by weight) A charge consisting of 18 lb. of pig-iron (8.94% total carbon, 2.78% silicon, 0.57% manganese, 0.017% sulphur, and 0.029% phosphorus, the remainder being iron and adventitious impurities), 31 lb. of ferrosilicon (46% silicon) and 51 lb. of mild steel scrap was melted in an indirect eleceans 3 tric arc furnace. 50 lb. oi this metal was run into a ladle and 2 oz. oi. cerium mischmetall was added before pouring the metal into a sand mould.

Analysis of a sample from the casting showed:

Microscopic examination of sections 01 the casting showed the graphite partly in the form 01 very ilnely divided particles of undercooled graphite and partly in the form of nodules or spherulites.

Having thus described my invention, .what I claim as new and desire to secure by Letters Patcut is:

1. A cast iron 01 the corrosion-resistant type containing more than by weight of silicon and not less than 0.02% by weight of cerium.

2. A cast iron 01 the corrosion-resistant type containing more than 10% by weight of silicon, not more than the eutectic proportion of carbon, and not less than 0.02% by weight of cerium, the cast iron being substantially free from coarse flake graphite.

3. A cast iron or the corrosion-resistant type containing more than 10% by weight of silicon and more than the eutectic proportion of carbon, characterised in that the carbon is present partly in the form of nodular graphite. the rest of the carbon being in the form of finely divided graphite.

4. A cast iron 01 the corrosion-resistant type containing more than 10% by weight of silicon and from 0.02% to 1.0% by weight of cerium.

5. A process for the production of corrosionresistant cast iron which comprises adding to a molten iron, containing more than 10% by weight oi silicon and not more than 0.2% by weight of sulphur, such an amount of a cerium-providing a substance as will yield a casting containing not less than 0.02 71 by weight 0! cerium.

6. A process for the production 0! corrosionresistant cast iron which comprises adding to a molten iron containing more than 10% by weight of silicon, not more than the eutectic proportion 01 carbon, and not more than 0.2% by weight of sulphur, such an amount of a cerium-providing substance as will yield a casting containing not less than 0.02% by weight of cerium.

'7. A process for the production of corrosionresistant cast iron which comprises adding to a molten iron. containing more than 10% by weight of silicon, more than the eutectic proportion 01 carbon, and not more than 0.2% by weight of sulphur, such an amount or a cerium-providing substance as will yield a casting containing not less than 0.02% by weight of cerium.

8. A process as claimed in claim 5 in which the amount 01' cerium in the casting does not exceed 1% by weight.

9. A process as claimed in claim 6 in which the amount 01' cerium in the casting does not exceed 1% by weight.

10. A process as claimed in claim 7 in which the amount of cerium in the casting does not exceed 1% by WBIEht.

BENTON MORROGH.

REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Number Country Date 127,981 Great Britain June 11, 1919 237,554 Great Britain Feb. 11, 1926 OTHER REFERENCES 40 Paper No. 875, presented at the 44th annual meeting of the Institute of British Foundrymen, June 17 to 20, 1947. Published by The Institute or British I'loundrymen, Manchester, England.

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