US2225968A - Decarburized white iron casting - Google Patents

Description

Patented Dec. 24, 1940 UNITED STATES PATENT OFFICE DECARBURIZED WHITE IRON CASTING No Drawing. Application June 12, 1939,

' Serial No. 278,724

2 Claims.

This invention relates to the formation of decarburized white iron castings.

The primary object of the present invention is to improve the practice in connection with the 5 formation of decarburized white iron castings.

There are certain advantages involved in casting articles of a cast iron" composition in place of casting the article from a "steel composition.

The high carbon content of the cast iron results in a composition which has a lower melting point than a steel .composition and one that is more fluid, especially at lower temperatures. The

greater fluidity of the cast iron permits casting in thin-walled sections and in castings of intricate' shape, if desired.

In accordance with the invention 1 provide a white iron casting of desired shape and of suitable composition with the carbon substantially all in combined form and decarburize the same at least to the point where massive carbides are removed whereby. a ductile, ferrous article or casting is produced. Ordinarily the decarburization is carried out to a point somewhat beyond that at which the massive carbides are removed. In some applications of the invention the casting is completely or substantially completely decarburized. It will be understood also that when the casting is of intricate shape and has walls of diiferent thicknesses that the decarburization may be carried out to the point where the thicker walls or sections will have at least the massive carbide removed and the thinner sections will be completely or substantially completely decarburized. Thus the article may have portions of different carbon content so long as the less readily decarburized portions are decarburized to the point where at least the massive carbides are removed.

The composition of the white iron is such that it has sufficient carbon to render the same rather fluid to permit casting in intricate shape. In addition, the white iron contains small amounts of copper and vanadium which produce an important effect on the decarburizing operation. By control of the conditions involved the white iron casting may be decarburized to produce an article free or substantially free of temper carbon and containing whatever combined carbon is desired. In some applications of the invention where the article may have walls or portions of rather thick section there may be a few widely scattered temper carbon spots at the central portion thereof, the surface and outer portions being free of temper carbon spots and containing whatever combined carbon is desired.

The decarburized article in accordance with the invention may be readily heated to a welding temperature or to a copper brazing temperature such as 2100 F. or higher without becoming mushy or undergoing permanent growth as would be the case in heating a malleable iron or cast iron article. No hard brittle constituent is formed when an article produced in accordance with the invention is welded to other parts, whereas, as is well-known in welding the ordinary "malleable iron having temper carbon, that is, the usual annealed white cast iron," a hard brittle constituent is formed which is detrimental and weakens the weld. The treated casting in accordance with the present invention may therefor be used to advantage in places where it is I desired to weld or braze the same to steel or to another casting of the same kind.

The decarburization is preferably carried out by subjecting the casting to the influence of moist hydrogen or other highly decarburizing gas at a 20 high temperature for a period of time suflicient to decarburize the casting to the extent desired. The temperature of the moist hydrogen or other decarburizing gas is preferably within the range 1600-2000 F. 25

The small amounts of copper and vanadium in the white iron have an important effect on the decarburizing operation. The addition of a small amount of copper in a white cast iron containing a small amount of vanadium greatly acceler- 30 ates the rate of decarburization. In this way the time and cost of decarburizing the same thickness of metal is reduced. The vanadium improves the physical properties of the final product by preventing the formation of any primary graphite in 35 the metal as cast and assists in the process of attaining a final structure free from temper car-- bon spots. In decarburizing a white cast iron containing a small amount of vanadium in accord-' ance with the invention the process of decarbu- 4o rization is believed to be by means of the reaction between the moisture in the hydrogen gas, for example, and the carbon in solid solution in the casting. As the carbon near the surface is removed, thus creating a difference in carbon 45 concentration between the surface layer and the center, and carbon in solution at a point away from the surface migrates toward the surface because of this diiference in carbon concentration, the iron becomes unsaturated with respect to car- 50 bon so that more of the undecomposed iron carbides dissolve in the iron and start migrating toward the surface. In this way is acquired a final structure which in thin sections of about sheet metal thickness is continuous and free from any It voids which would be formed it the iron carbide were to break down into iron and temper carbon.

' The outer portions of thicker sections are similar in structure to the thin walled portions but may contain in the inner central portion" thereof a few widely scattered temper carbon spots. The addition of a small amount of copper speeds up the decarburizing of the white iron containing vanadium.

The carbon content of the white iron casting is ordinarily kept quite high in order that the iron have a rather high degree of fluidity. In general, an approximate range of 25-43% carbon is suitable. Silicon and manganese may be varied as desired and, in general, are present in amounts equivalent to the usual commercial white iron suitable for annealing to form "malleable iron." For example, up to about 1.00% of manganese, or higher in special applications, if desired, and up to approximately 1.25% or more of silicon may be present in the white iron casting. Ordinary impurities as sulphur, phosphorus, etc. are usually kept low-as in ordinary commercial white cast irons. The composition is so chosen that the original casting will have the carbon all, or substantially all, in combined form.

The vanadium is present in small amount say about 25-55%, while about .50-l.50% of copper is suitable, with about 1.00% preferred at present.

The following is a specific example of a white iron composition that is satisfactory for use in carrying out the process: I

2.75% carbon, 1.25% silicon, .40% manganese, .40% vanadium, 1.00% copper; sulphur and phosphorus low, e. g. .15% sulphur and .05% phosphorus, and the balance iron.

In decarburizing a certain size casting of the specific analysis given for forty hours at 1900 F. in a moist hydrogen atmosphere the elimination of all massive cementite is to a depth of .185,

while with a similar casting under the same conditions but without copper the massive carbide is eliminated to only .150". It will be seen, therefore, that the copper has an important efl'ect on reducing the time necessary to decarburize the casting.

It will be understood that a decarburized article in accordance with the invention may be, if desired, subsequently carburized or otherwise heat treated entirely or in part in any way now known in the art in order to obtain physical propertie suitable for particular applications.

It will be understood that castings of many different shapes may be made by my process and that many changes not specifically enumerated may be made in the embodiment of my invention described herein without departing from the principles of my invention and therefore I do not intend to limit the patent granted for my invention except as necessitated by the prior art.

I claim:

1. The, method of making a ductile, ferrous metal article which comprises forming a white iron casting having substantially no free carbon and containing small amounts of copper and vanadium and decarburizing said casting by subjecting the same to the action of a decarburizing medium at a high temperature, said small amount 01' copper being about .50% to 1.50% and the small amount of vanadium about 25% to 55% of the white iron casting.

Y 2. The method of forming a ductile, ferrous metal article which comprises providing a white iron casting having substantially no free carbon and containing about 2.75% carbon, about 1.25%

silicon, about .40% manganese, about .40% vana-