US2227217A - Production of malleable iron - Google Patents

Description

Patented Dec. 31, 1940 UNITED STATES PRODUCTION OF MALLEABLE IRDN Walter Crafts, Niagara Falls, N. Y., assignmto Electro Metallurgical Company, a corporation of West Virginia No Drawing. Application May 9, 1940, Serial No. 334,167

3 Claims. (.01. 148-213) This invention relates to the production of malleable cast iron.

Malleable cast iron is prepared by a process comprising two main steps. In the first of these 5 steps white cast iron, an extremely hard and brittle material in which the carbon is largely in the form of combined(carbide) carbon is produced; in the second, the white cast iron is converted to malleable cast iron. The conversion of white cast iron to malleable iron involves transforming the combined carbon of white iron to graphitic carbon in the form of nodules (tem-' per carbon) by holding the iron at certain elevated malleabilizing temperatures.

Depending in part on the composition of the iron being treated, the time required to convert white cast iron to malleable cast iron may be as long as three to six days. The treatment usually includes the steps of heating the iron to a temperature above the upper critical temperature, holding it at such temperature, slowly cooling it to about thelower critical temperature, holding it at this temperature for a time, and finally slowly cooling it to room temperature. The

length of time required for this treatment, and the expense involved in maintaining such high temperatures over long periods of time, are obviously disadvantages of the long annealing treatment. Moreover, the malleabilizing anneal does not always yield consistent results on difierent castings.

' Efiorts have been made for many years to provide a shorter annealing treatment; By carefully balancing the composition of white iron and by adding small proportions of alloying elements, it has been found possible to produce a material which may be graphitized in a much shorter time than ordinary white iron. For instance, the silicon content may be raised beyond that custom- 40 arily present inwhite iron, and copper added, as well as small proportions of elements which promote the formation of carbides, such as chromium. A material of such composition may be graphitized by the so-called short cycle anneal,"

about 80 hours on the average. In some cases, by the use of the short cycle anneal malleable iron'may be produced from white iron in as short a time as 15 hours. Extremely careful coirtrol of temperature must be maintained in the short cycle anneal.

' e short cycle anneal, while a definite advance in the malleabilizing art, has definite limitations in applicability. For instance, it may be used" successfully only with white iron of carefully balanced composition, and the long time anneal must still be utilized for most types of iron. Again, on occasion a white iron of normal composition given the short cycle annealing treatment will not be completely malleabilized.

-It is an important object of'this invention to 5 provide a method of treating white iron to enhance its responsiveness to a malleabilizing anneal. Another object is a method of treating white iron whereby its rate of graphitization during a malleabilizing heat treatment is in- 10 creased, Another object is a novel anduseful heat treatment for white cast iron. Another object is a method of treating white iron castings whereby complete malleabilization may be cons stently attained in a short time. 'Still another 15 object is to counteract the inhibiting eiiects of carbide-forming elements on the malleabilization 'of white cast iron.

These objectsare achieved by the invention, which is based on the discovery that the respon- 20 siveness of white cast iron to a malleabilizing anneal may be considerably enhanced by subjecting the white iron to a low temperature heat treatment prior to the annealing step. One result of this low temperature pretreatment is that the 25 rate of graphitization of the pretreated white iron is materially increased. The invention accordingly comprises a method of heat treating white cast iron to increase its graphitization rate when annealed. 1 30 In accordance with the invention, white cast iron which is to be transformed to malleable iron is first subjected to a heat treatment which com= prises heating the white cast iron at a temperature within the range of C. to 400 C., and 35 preferably between about C. and 300 C., for a period of time dependent on the composition of the iron and on the temperature 01' the treatment. The treating time necessary is shorter if a temperature near the upper limit of this 40 range is employed than if a temperature near the lower limit of the range is employed. The heating period is preferably at least ten hours and in some cases may be as long as two hundred hours, but no minimum time limit for all mate- 45 rials can be specified, the optimum time being determinable empirically for the particular composition of iron to be treated. The heat treatment may be carried out at a particular temperature within the given range or 'by slowly heating to or cooling the material through the range.

After a white cast iron article has been pretreated in the manner described, it may be annealed directly without cooling, or it may be allowed to cool before being annealed. An advan- 65 15 hour cycle=heating to 955 C. in 1 hour, holding 4 hours, cooling 'lographic tests to have tage of annealing directly after the pretreatment is that the heat in the article is not lost. Oi course,the annealing treatment necessary to malleabilize a white iron treated in accordance with the invention is dependent not only on the composition of the iron but also on the pretreatment given it, but in all cases the annealing treatment is considerably shorter than if the material had not been pretreated.

Certain experiments have been made in. which the effects of the heat treatment of the invention have been determined. In a typical in-- vestigation tests were made on cast irons having the following compositions and which contained no graphite visible by microscopic examination:

Iron A Iron B Percent Total carbon Combined carbon. Grsphitic carbon Silicon Remainder Remainder the following table in which T. C. means total carbon, C. C. means combined carbon, and F. C. means free carbon (graphite).

Graph- Annealite ing cycle' Percent T. C. 0.

Per- Percent cent C. F. C

Iron Pretreatment nuclei per sq. mm.

None None None None to 788 C. in 1 hour, cooling to 650 C. in 9 hours.

30 hour cycle=heating to 955 C. in 9 hours, holding 10 hours, cooling to 788 C. in 2 hours, cooling to 650 C. in 9 hours.

It will be apparent from consideration of the tabulated data that the heat treatment of the invention has a pronounced beneficial eflect on the material to which it is applied. Thus, the combined carbon content of those specimens which had been pretreated in accordance. with the invention was shown by chemical and metalbeen substantially converted to temper carbon, form of nodules or, nuclei. Those specimens which had not been pretreated before annealing were shown to have been only slightly affected that is, graphite in the by the annealing treatment. The facts that the treatment of the invention does not of itself transform the combined carbon of white iron to temper carbon and that the time required for the treatment is dependent on the composition of the material treated and the temperature selected for the treatment are also brought out by these tests.

While no structural change of any kind is apparent in white cast iron which has been subjected to the heat treatment of the invention, the responsiveness of so treated white cast iron to a malleabilizing anneal is enhanced. For example, there is a tendency for more nuclei to be formed on subjecting a pretreated white cast iron to a graphitizing anneal than are formed when a white cast iron which has not been so treated is annealed. Also complete malleabilization appears to be more consistently approached if white iron is treated after the manner of the invention, and the inhibiting effects of carbide-forming elements on malleabilization are efiectively counteracted. It is possible that these beneficial efi'ects of the treatment of the invention are due at least in part to the elimination of hydrogen from the iron.

For some purposes, an incompletely malleabilized material is desired. Such a material is usually obtained by control of the cooling rate and composition of the iron. In other applications a malleable iron having a decarburized rim is desired. This is obtained by packing the white iron in an oxidizing material such as scale and malleabilizing the iron while it is packed in such 'material. The process of the invention is beneficial in both of these applications as well as those in which complete malleabilization without material decarburization is sought, since one effect of the process is to shorten the time required for malleabilizatiom While specific examples of time and temperature of the heat treatment of the invention have .been given, such examples merely illustrate its principles and the invention is not limited to or by these examples. the invention is not limited to use with iron of any particular composition, and therein lies its great advantage.

I claim:

1. Method of preparing white cast iron for a malleabilizing anneal which comprises heating the iron with the range of C. to 400 C. for a time sufficient to enhance its responsiveness to the malleabilizing anneal.

2. Method of converting white cast iron to malleable iron which comprises preheating the white iron at a temperature within the range of 100 C. to 400 C. and thereafter heating it at malleabilizing temperatures until substantially all of the combined carbon has been converted to temper carbon.

3. Method of converting white cast iron to malleable iron which comprises heating the white iron at a temperature within the range of C. to 300 C. for a time sufficient to enhance its responsiveness to a malleabilizing anneal and thereafter heating it at malleabilizing temperatures until substantially all of the combined carbon has been converted to temper carbon.

I WALTER CRAFTS.

Moreover, the treatment of CERTIFICATE OF CORRECTION., Patent No. 2,227,217. December 51, 191m.

I WALTER CRAFTS.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, sec- 0nd column, line 52, claim 1, for the word "with" read "withinand that the said Letters Patent should be read with this correction therein that the samemay conform to the record of the case in the Patent Office,

Signed and sealed this 28th day of January, A. Do

Henry Van Arsdale, (Seal) Acting Commissioner of Patent so