US2370225A - Malleable iron - Google Patents

Description

Patented Feb. 27, 1945 UNITED STATES PATENT orrics I 'mnfim mou Alfred L. Boe'gehold, Detroit, Mich., assignor to General Motors Corporation, Detroit, Mich a corporation of Delaware No Drawing. Application January 6, 1941, Serial N0. 373,305

18 Claims. (Cl. 148-3) This invention relates generally to a ferrous alloy and to a process of producing the same.

More particularly the invention relates to white irons suitable for making malleable iron castings and the manufacture thereof.

One object of the invention is to provide improvements in the practice involved in the production of malleable iron castings.

Another object of the invention is to reduce greatly the time required to anneal white iron castings to produce malleable iron castings.

It is also an object of this invention to provide improvements in compositions of white irons suitable for malleableizing whereby compositions containing higher amounts of carbon and/or silicon than standard compositions may be employed.

A further object of the invention i to provide compositions suitable for making white iron castings of large size that are to be annealed or malleableized and in which the tendency to form large temper carbon spots during annealing is overcome.

Still another object of the invention is to provide improvements in compositions suitable for malleable irons that require hardening throughout, especially those for use in making castings of heavy section. I

Other objects and advantages of the invention will become more apparent as the description proceeds.

In accordance with the invention a relatively small amount of bismuth is added to cast irons to form white cast irons'suitable for malleableizing, the composition in any case being such that without the presence of bismuth therein it would be grey or mottled in whole or in part, and the bismuth being present in amount sufficient to cause the solidified casting to have the fracture typical of white cast iron. The use of bismuth in this way permits the use of a larger amount of silicon or other graphitizer in white irons that areto be annealed or malleableized than that commonly used. The increased graphitizer greatly reduces the time required to anneal or malleableize the white iron casting. In standard compositions used for white cast irons for malleableizing the silicon content may not run much above 1.3% (depending on the size and cooling rate) without causing precipitation of carbon in flake form during solidification of the casting. The present invention permits the use of larger amounts of silicon or other graphitizer and greatly reduces the time required for malleableizing the casting. The present invention also permits, if desired, greater carbon contents than those commonly employed in ordinary white iron compositions that are to be malleableized.

I have found that the power of bismuth to inhibit the graphitizing influence of silicon or other graphitizers during solidification of the iron is much greater than its graphitizing inhibiting power at the annealing temperature. The practical significance of this property is that-increased speed. of annealing can be obtained by using over 1.3% silicon or its equivalent without causing grey or mottled fractures as a result of graphite deposition during solidification which would occur without the bismuth.

The amount of bismuth required in any particular application is relatively small. The amount in any case is preferably the minimum amount needed in order to cause the solidified casting to have a white fracture under the cooling rate of the casting under consideration. Thus for any solidification rate, as the silicon and/or carbon are increased the amount of bismuthwill be increased to compensate therefor and to cause a white iron casting to be formed. For most purposes .002 to .02% bismuth added to the iron is all that is required.

The amount of bismuth in the solidified white iron casting is somewhat less than th amount supplied to the iron. This is due to volatization, etc" of the bismuth.

The form in which the bismuth is added to the iron is immaterial to the invention. One convenient method is to add metallic bismuth to the molten iron coming from the cupola or electric furnace.

The following is an illustrative example of procedure in accordance with th invention. A composition containing 2.75% carbon, 1.65% silicon, .40% manganese, and the balance iron with small amounts of ordinary-impurities, such as phosphorus, sulphur, etc., was cast into a, one and onequarter inch round section and was grey throughout. With .005% of metallic bismuth added to the molten composition the resulting casting was clear white in the samesized section. After heating to 1700 F. and holding five hours, the primary iron carbide was "completely decomposed. This compares with a time of from about nine to ten hours usually required with similar sizes of sections using common compositions containing 1.25% silicon.

If in the above example it is desired that the manganese b increased over the-amount. commonly used, 1. e., about .40%, or if it is desired to add other elements to the composition which modify the physical properties of the annealed iron, it may be necessary to raise or lower the amount of silicon present depending on whether the addition agent is a carbide stabilizer or a graphitizer. Elements in addition to silicon which may be classed as graphitizers include aluminum, barium, calcium, copper, lithium, magnesium, nickel, potassium, sodium, titanium and zirconium while antimony, boron, cerium, chromium, manganese, molybdenum, selenium, sulphur, tellurium, tin, tungsten, vanadium, may be classed as carbide stabilizers. Thus if it is desired to obtain 7 higher strength by increasing the manganese content over that of the standard composition, it may be necessary to increase slightly the silicon content to counteract the carbide stabilizing influence of the increased manganese content. n the other hand if it is desired to add an element such as nickel or copper for increasing the tensile strength or the malleable iron it will be necessary to decrease the silicon content in order to prevent precipitation of graphite becaus these elements act as graphitizers in the same manner as silicon except that they are not as powerful.

Malleable iron castings oi. heavy section must have a reduced silicon content in order to have the proper structure, 1. e., freedom from mottling or grey fracture. The reduced silicon content results in a longer time required for malleableizlng. The use of a small amount of bismuth enables the use of a higher silicon content to obtain a white fracture throughout the large section. In annealing or malleableizing such large sections of the white iron containing bismuth the temper carbon spots normally tend to become large and widely scattered and thereby retard the carbide decomposition. I have found that the presence in the white cast irons containing bismuth of a small amount oi copper prevents the occurrence of large and widely separated temper carbon spots. An example of a typical composition for large size castings having a rate of solidification similar to a two inch ro'und bar is one containing 2.75% carbon, 1.50% silicon, .40% manganese, .50% copper, .006% bismuth, and the balance iron plus minor amounts of usual impurities such as phosphorus, sulphur, etc. In general the following is a range of composition for the large sized sections: ZOO-3.25% carbon, LOO-2.50% silicon, .25- 2.00% copper, .35-1.50% manganese, .002-.02% bismuth. In any particular case the several individual elements are varied within the ranges of percentages given so that the castng without the addition of bismuth is grey or mottled in whole or in part and the bismuth is present in amount suflicient to cause a white iron to be formed.

Another phase of the invention contemplates the addition of further alloying elements n order to increase the ability of the malleableized iron to be hardened by heat treatment, it being understood, of course, that without these additional elements the malleableized iron may be heat treated to improve its physicalproperties. Wher ordinary completely malleableizd or ordinary pearlitic malleable iron are hardened by heating and quenching, it has been found that the iron does not harden satisfactorily throughout in large' sections even with drastic quenching. I have discovered that the inclusion of a small amount of molybdenum in white cast irons containing bismuth and copper results in compositions that after malleablelzing may be readily hardened in heavy sect ons, either by air or oil quenching depending on the composition. A typical composition of white iron which will possess after malleabilization a martensitic structure in a two inch round bar after air cooling from a hardening temperature of 1600 F., contains 2.50% carbon, .75% manganese, 1.50% silicon, 1.00% copper, .25% molybdenum, 005% bismuth, balance iron plus minor amounts of ordinary impurities such 1 as phosphorus, sulphur, etc. A similar composition of the same section size containing .40, manganese instead of .'l5% manganese will upon air cooling from 1600 F. be partly martensitic and partly pearlitic. An oil quenching in place of cooling in air will result in a iully martensitic structure. An alloy similar to that above but containing 1.50% manganese and .50% molybdenum will, even after an annealing cycle which would completely anneal ordinary malleable iron in which the cooling rate from 1400 to 1300" F. is 10 an hour, will possess a martensitic structure having a hardness of 440 Brinell. This composition when heated to 1600 F. and air cooled becomes harder and has a hardness of over 500 Brinell. The alloys with medium manganese and molyb denum contents after the standard annealing cycle for producing ordinary malleabl iron possess, depending on the amounts of manganese and molybdenum, a rang of structures including all ferritic; pearlite and ferrite mixed; uniform pearlite, pearlite and martensite mixed; and at the upper end of the hardness range, uniform martensite, these structures having hardnesses ranging from 146 to 400 Brinell and upwards. 'In general, the alloys in accordance with the invention having good hardenability in heavy sections after malleableization are composed substantially as follows: 13.50% carbon, .50-2.50% silicon, .30-1.75% manganese, .25-2.00% copper, .l0-.'75% molybdenum, .002-.02% bismuth, and the balance iron plus ordinary amounts of usual impurities such as phosphorus, sulphur, etc. In any particular case the several individual elements are varied within the ranges of percentages given so that the casting without the addition of bismuth is grey or mottled in whole or in part and the bismuth is present in amount suflicient to cause a white iron to be formed.

The term white iron cast as used herein means a carbon containing cast iron free of appreciable amounts of carbon in flake form that characterizes grey iron castings. I do not intend the term to exclude annealable castings that may contain some nodular or temper carbon but contain substantially no flake carbon. White iron as used commercially for the manufacture of malleable iron may have a small percentage of the structure containing flake graphite without detriment to the properties of the annealed or malleableized product. These small areas are known as mottles and are similar to grey iron in structure. Castings having the structure just described and conta n g bismuth are within the scope of the invention.

It will be understood that various modifications and changes in the embodiments of my invention described herein may be made without departing from the principle and spirit of my invention and I do not intend to limit the patent granted thereon except as necessitated by the prior art.

I claim: I

1. An annealable white iron casting containing bismuth, the composition of the casting without bismuth being such that it would form a grey or mottled structure and the bismuth being present in small but eflective amount sufllcient to obstruct ,iormation of flake graphite during solidification of the casting.

2. An annealable white iron casting containing n: solidification of the casting.

bismuth. the composition of the casting without bismuth being such that it would form a grey or mottled casting and the bismuth being present in small but effective amount up to about .02% suflicient to obstruct formation of flake graphite dur- 3. An annealable white iron casting character ized by the rapidity with which it canbe annealed to form malleable iron, containing a graphitizer equivalent in effect to at least about 1.3% silicon and containing a small but effective amount of bismuth up to about .02%, the composition of the castin'gwithout bismuth being such that a grey or mottled casting would be formed and the hismuth being present in amount suflicient to ob-'- struct formation of flake graphite during solidifi- .l-.75% molybdenum, and a small but effective sufllcient to obstruct formation of flake graphite during solidification of the casting.

7. An annealable white iron casting composed approximately as follows: 2.75% carbon, 1.65% silicon, .40% manganese, .005% bismuth, balance iron plus small amounts of ordinary impurities.

.8. An annealable white iron especially adapted for sections of large size composed substantially as follows: 2.00-3.25% carbon, LOO-2.50% silicon, .25-2.00% copper, .35-1.50% manganese, .002- .02 bismuth and the balance substantially all iron, the composition in any case being such that without bismuth a grey or mottled iron would be formed and the bismuth being present in amount sumcient to obstruct formation of flake graphite during solidification of the casting.

0. An annealable white iron casting having a grey or mottled iron would be formed upon solidirate of solidification similar to a two inch round I bar composed approximately as follows: 2.75% carbon, 1.50% silicon, .40% manganese. .5096

copper, .006% bismuth, and the balance substan- 10. An annealable white iron casting oflarge section composed substantially 4 follows: 14.50% carbon, 504.50% silicon, 304.75% manganese, Bit-2.00% copper, .10-.'I5% molybdenum, .002-.02% bismuth and the balance substantially all iron, the composition'in any case bslngsuchthatwithoutbiamuthagre'yormottled present in amount suflicient to obstruct formation of flake graphite during solidification of the castins 1 1. The method of producing a white iron casting suitable for annealing or malleableizing which comprises pouring a molten cast iron composition containing a small amount of bismuth, the molten composition without bismuth being such that a flcation and the small amount of bismuth being present in amount sufliclent to obstruct formation of flake graphite during solidification ofthe castins.

12. The method of producing a malleable iron casting which co prises, forming a white iron casting of a cast ron composition containing a smallamount of bismuth; the composition without bismuth being such that a grey or mottled casting of a cast iron composition containing at least about 1.3% silicon and a small but eflective amount of bismuth up to about .02%; the composition without the bismuth being such that a grey or mottled iron would be formed and the bismuth being sufficient to obstruct formation of flake graphite during solidification of the casting, and thereafter annealing said casting until at least substantially all of the primary iron carbide is decomposed.

14. The method of producing a malleable iron casting which comprises, providing a molten cast iron composition that would normally form a grey or mottled iron casting, adding thereto a small but effective amount of bismuth up to about .02% sufllcient to obstruct formation of flake graphite during solidification of the casting. and thereafter annealing said white iron casting to decompose at least substantially all of the primary iron carbide.

15. In a process of making a malleable iron article from a cast iron containing a graphitizer equivalent in effect to at least about 1.3% silicon by casting the article in the form of white iron and annealing, the step which consists in addin to the molten cast iron about .002 to .02% of bismuth, the bismuth acting to retard separation of flake carbon during solidification but ineffective to prevent rapid separation of temper carbon .during annealing.

16. An article of cast ferrous metal having the "micro-structure of malleable iron and consisting of an alloy of iron containing a graphitizing agent equivalent to at least about 1.3% silicon, and a small but effective amount up to about .02% of bismuth. 4

17. A malleable iron article containing about 2.00-8.25% carbon, 1.3%-2.50% silicon, 304.76%

- manganese and .002-.02% bismuth.

18. The method of producing a white iron casting' suitable for annealing or malleableizing which to prevent substantial separation of flake carbon during solidification whereby is produced a white would arm the bhnuth use is iron casting.

ALFRED L. DOIGEHOLD.