US1707753A - Malleable iron alloy - Google Patents

Description

Patented Apr. 2, 1929.

UNITED STATES PATENT OFFICE.'

ALFRED L. BOEGEHOLD, OF DETROIT, MICHIGAN, ASSIGNOR T0 GENERAL MOTORS RE- SEARCH CORPORATION, OF DETROIT, MICHIGAN, A CORPORATION OF DELAWARE.

MALLEABLE IRON ALLOY.

No Drawing.

It is an object of this invention to produce malleable iron castings of any size and shape by (1) pouring a molten mixture of iron, carbon and a graphitizing agent, (that is an agent capable of decomposing combined iron andcarbon into graphite and iron) to which has been added an alloying ingredient having the property of preventing the formation of flake carbon during the period of solidification and cooling, and (2) thereafter heat treating the casting until it becomes malleable.

The process of making malleable iron castings according to the heretofore usual practice consists in pouring into an ordinary green sand mold mo ten iron having a silicon content low enough to prevent the formation of carbon in flake form in the mold, then removing the casting and annealing it by subjecting it to heat treatment continuing for a period of about 110 hours. The heat treatment consists in first, raising the temperature of the casting and its environment to about 1600 to 1700 F., requiring about 30 hours, second, maintaining the temperature of about 1600 to 1700 F., for about 40 hours; third, slowly cooling'the casting to just below the lower critical temperature (which is approximately 1340 F.) and holding this temperature for about 35 hours; and, fourth, cooling to a temperature low enough to permit handling, which requires about 5 hours,-'

a total period of about 110 hours, which cannot be very much reduced.

Iron commonly used in the customaryprocess of making malleable castings has a silicon content of from .90% to 1.10%; it cannot run much above 1.30% in the described process of the prior art, in which the carbon content is usually 2.40 to 2.60%, without dangerof precipitating carbon in flake form, whlch would destroy the property of malleability. It is generally understood that the quantity of carbon plus silicon in .the mix employed in practicing the ordinary process must not substantially exceed 3.7 5%, if iron having the white fracture characteristic of castings ca Application filed March 14, 1927. Serial No. 175,435.

In view of the necessarily highcost of malleable iron production by the described stan'dard process, due to the long period of heat treatment necessary, I have sought to develop a process of making iron products, cast in molds, that may be annealed in a much shorter time; that may approximate or equal the standard malleable iron product in tensile strength, malleability and ductility, and equal or exceed it in machineability.

In Patent No. 1,591,598, issued to Harry M. Williams and myself on July 6th, 1926, is described and claimed a process for the production of malleable castings in which the desired shortening of the annealing time is accomplished by the employment of larger proportions of temper carbon forming or graphitizing agent, such as silicon, thancould be employed in said customary process; and, to prevent precipitation of the carbon in flake form during casting, owing to the presence of the added quantity of this ingredient, the iron is cast in chill molds, preferably permanent metallic molds, so that it will cool as rapidly as possible. Satisfactory malleables can be produced by this process. 1

However, the permanent mold process is not applicable to all shapes and sizes of castings; wherefore I have sought to develop a process of more general application, useful for the production of malleable castings of a wide range of shape and size.

It has also been proposed heretofore to shorten the annealing time of malleables by employing a lower percentage of carbon in the mix than has been used in the aforesaid usual process, this lower carbon percentage permitting an increase in the percentage of the graphitizing agent, such as silicon, with a consequent shortening 6f the annealing time. I have found that where this process (which may be here termed the low carbon process) is empl yed there is a limit to the sum total perce tage of silicon and carbon that canbe included in the mix, which total this process I have been governed by the following limiting conditions:

(1) Precipitation of flake graphite during solidification of the casting must be avoided; (2) A suflicient quantity of graphitizing agent to hasten annealing, as compared with the period of time required by said customary process, mut be present;

(3) The casting metal must have a low melting point and high fluidity in order to economize in time and fuel when melting, prolong the life of the refractories in the -melting receptacle and permit the metal to fill the interstices of the mold;

(4) The process must be adapted to produce castings in ordinary sand molds, although preferably also equally well adapted to the use of permanent metallic molds;

(5) The process must produce castings showing a white fracture independently of the thickness of the casting.

I have found that it is possible to satisfy all of the foregoing named conditions to a greater or less extent by adding to a suitable casting composition of iron, carbon and silicon or the like a suitable quantity of an ingredient having the property of obstructing the formation of flake graphite, which may be one of several metals capable of alloying with iron.

According to the invention that is the subject of this application I employ a composition of molten metal to be cast consisting (except for impurities commonly present in cast iron) of iron, carbon, and a graphitizing agent, preferably silicon, wherein the proportion of graphitizing agent is higher than can be used in the aforesaid customary process and wherein the total proportion of carbon and silicon may be considerably higher; and also an additional metallic ingredient that has the aforesaid property of preventing or obstructing the precipitation of graphite in flake form by the action of the graphitizing agent. The percentage of carbon should preferably be high enough to impart a low melting point and high degree of fluidity to the casting metal; the percentage of silicon or equivalent graphitizing agent can be made high enough to enable the casting to be anneale'd'in a short eriod oftime; by reason of the presence of t c said additional metalcon or the like present in the casting metal can be prevented during solidification and cooling of the casting and, in addition, a casting that will show a white fracture can be made independent of shape or size of cross section. Castings poured from molten mixtures as described can be rendered malleable by a relatively short heat treating or annealmg.

Among many substances tested it has been found that any one of the metals molybdenum, vanadium, nickel or copper, when introduced in suitable quantity in a cast iron mixture containing a proper proportion of silicon and carbon, has proved eflicacious in preventing formation of flake graphite during solidification and subsequent cooling of castings poured from the mixture. Of the several elements that prevent or retard formation of flake graphite one may have also an efi'ect of retarding the action of silicon in breaking down iron carbide to form temper carbon during annealing or heat treatment of the cast-ing, while anothermay assist this action. The characteristic action of the element selected with respect to the decomposition of iron carbide may affect the proportions of silicon or the like required in the mix.

It has also been ascertained that the addition'to the mix or casting metal of a small proportion of metal having the said property of preventing or obstructing formation of flake graphite makes it possible to use a large proportion of the graphitizing or carbon precipitating agent without that formation of flake graphite which would occur if said metalhad not been added.

In making white iron castings by sand mold processes, using a mix consisting of iron, silicon and carbon the sum of silicon and carbon cannot substantiallv exceed 3.7 5 of the mix, as heretofore stated. But, in an iron-silicon-carbon mixture containing'in addition .50% molybdenum, it is possible to include 2.59% carbon and 1.80% silicon (carbon plus silicon=4.39%) without producing a castingshowing a grey fracture; a molybdenumcontent of .50% with 2.48% carbon and 2.30% silicon (carbon plus silicon= 4.78%) also gives a casting without grey fracture. The addition of the 50% of molybdenum increases the amount ofsilicon that can be employed without imparting the characteristics of grey iron to the casting. Iron casting showing the best physical properties thus far obtained by this process have been poured from an iron mix containing 2.27%

carbon, 1.50% siligon (carbon plus silicon= 3.77%) and .50% molybdenum. Tests of malleable castings made from this alloy showed a tensile strength of 71800 pounds per sq. inch, and an elongation of 10.5%. Molybdenum may be introduced in the form of ferromolybdenum or molyte (a compound of molybdenum and calcium silicide).

Experiments with nickel as an alloying ingredient have proven that an iron. mix containing 2.11% carbon and 1.77% silicon (carbonplus silicon=3.88%) and a nickel content of 1.33% will afford a casting of white iron in spite of the fact that nickel itself is considered a graphitizing agent. The effect of nickel as a graphitizing agent here is more apparent at the annealing temperatures than at the higher temperatures in the mold during solidification. Annealed castings made from the described nickel-containing mix showed upon test a tensile strength of 62000 pounds per square inch and an elongation of 4.5%.

By introducing 25% vanadium into a mixture of iron casting-metal it is possible to use 2.26% of carbon and 1.81% of silicon (carbon plus silicon=4.07%), and still obtain a casting having a white fracture. Higher percentages of vanadium although giving a white iron casting appear to slow the annealing process. An annealed casting made of iron with 205% carbon and 1.81% silicon (carbon plus silicon=3.86%) and less than 25% vanadium has shown a tensile strength of 65000 pounds per square inch and an elongation of 10%. The introduction of copper into the casting metal permits, as does nickel, a much higher carbon plus silicon content than could be used to produce a casting giving a white fracture in the absence of copper. Using .50% copper, 2.30% carbon and 1.97% silicon, (carbon plus silicon=4.27%) castings have been obtained the fractures of which are white. However, annealed castings made from copper-containing mixtures do not possess as high tensile stren th and elongation as castings containing the other alloying ingredients mentioned.

The time required for annealing castings made by the process of this application is very much less than that required by the standard process of making malleable iron.'

A small number of castings containing a'lloying metals mentioned above were heat treated 19 hours in an open furnace, not packed in boxes, and were tested for physical properties with the results stated. In these tests 1 hours were required to heat the castings to annealing temperature, about lfi hours to anneal (i. e. to precipitate the combined carbon in the form of temper carbon) while maintaining the annealing temperature of about 1650 F., and about 2% hours to cool the castings to 1150 F., when they were removed from the heating apparatus.

'als for obstructing formation of flake graphite is poured into a 'mold, the rate of cooling in the mold is not a highly important factor in producing a casting having a white fracture, as it is in processes involving casting in permanent or other chill molds.

Whereas, also, castings made of ordinary iron of proper analysis to produce small malleablizable casting inprior known processes might show a white fracture in a castingof one inch section, a two inch section might show a grey fracture. But a casting made from one of the alloy compositions according to this invention of analysis proper to show a white fracture in a inch or 1 inch casting will show a white fracture also if the casting be of 2 inch or larger section. It will be obvious that this property is of very great advantage in the art of making malleables.

There are graphitizing or temper carbon;

forming elements other than silicon; for example, nickel as already mentioned, copper, titanium, zirconium and aluminum. Aluminum for instance may be substituted for silicon with substantially similar effects with respect to precipitation of carbon in temper carbon form and obstruction of formation of flake carbon. As, however, the presence of aluminum confers certain undesirable properties upon the metal in molten condition, which make it undesirable for casting, it is less suitable than silicon for use as a graphitizing agent. Where the graphitizing agent is also a metal that obstructs formation of flake graphite it may be possible to do without a separate graphitizing agent, or to use a smaller proportion of a separate graphitizing agent. But the quantity of alloying metal to be used under these circumstances is likely to be so large as to confer undesirable properties upon the casting.

The substances usually present as impuritiesv in mixtures used for makingmalleable iron castings are sulphur, phosph orus and manganese." In the casting mixture utilized in the'process of this application the quantity of sulphur can be as high as in any form of commercial malleable now known, but according to my experience it should be kept preferably below .085%. Phosphorous should pref-- erably be below 20%, and manganese below .50%. A manganese content above .50% renthan 2.00%. It should not be so high as to weaken the texture of the annealed casting, and I consider that more than 2.75% carbon is too high.

The silicon content should be high enough to form tempercarbon during a short annealing period but not high enough to form flake ders annealing more difficult. A mixture con- I graphite during solidification. These conditions place the low limit of silicon at about 1.30% and the high limit at about 2.50%.

. It should be apprehended that theproportion of either the silicon or carboningredient that may be used successfully in this process depends to some extent on the proportion of the other. In general, if the percentage of silicon is near the higher limit, t e percentage of carbon ought to be lower than if the silicon content were nearer the lower limit, and vice versa. My experience indicates that the sum total of silicon and carbon in the casting composition should not exceed 4.80%, more or less. In any eventthe relative proportions of carbon and silicon within the ranges mentioned should not be such as to impart the characteristics of grey iron to the casting; the castings should have a white fracture.

The limits of the percentage of metal used" as an obstructor of flake graphite formation are theoretically quite wide. Ordinarily as little as may be required to produce the desired result should be used. Otherwise the cost (particularly when using molybdenum or vanadium) would probably be prohibitive; also, other characteristics that are undesirable might be imparted to the alloy.

. Too much vanadium, for example, tends to prevent silicon from precipitating the carbon during the annealing operation, while W much copper tends to form globules of copper in the iron during solidification in the mold,- that is, a too high copper content will not wholly alloy with the iron. The permissible percentage of copper depends to an extent on the percentages of silicon and carbon present. The graphitizing properties of nickel and copper constitute another limiting factor to the quantity of nickel or copper that can be used, which must be less than'that which causes formation of flake graphite.

A principal feature of this process is the use of a relatively larger proportion of silicon, or similar grapliitizing agent, than in the customary process, together with an element alloyable with iron that obstructs formation of flake graphite. Although the total content of carbon plus silicon is generally larger than in the usual practice it is not necessarily so, but the silicon content should be larger; and this permits the use of snflicient carbon to provide a low melting point casting composition even though the total of carbon and silicon be considerably larger than in standard practice.

The metal to be cast may be melted in a suitable melting furnace such as an air furnace, electric furnace, or cupola used in connection with an electric furnace.

The castings may be annealed at the usual annealing temperature (around 1600 F.)

and may be heated gradually although rapidly to this temperature and after maintaining it quire about 50 hours, more or less, because of the time necessarily consumed in heating all parts to the annealing temperature and permitting cooling to a temperature at which the castings can be handled. -The time during which the annealing temperature should be maintained would be about 15 or 16 hours, the remainder of the 50 hours being required to bring the castings up to annealing temperature and thereafter to cool them.

In using the term white iron casting I mean a carbon containing iron free of carbon in flake form that characterizes ordinary grey iron castings. I do not intend the term white iron to exclude annealable iron castmgs that may contain some nodular or temper carbon but contain substantially no flake carbon.

Annealed castings produced by the process of this application show the same microstructure that characterizes standard malleable castings made in the heretofore customary manner, and appear in all respects to be adapted for use wherever malleable castings are needed.

While in order to comply with the statutes, I have set forth the principle of my invention and have given several analyses of casting metals for use in practicing it, including that one which I have thus far found to be most suitable, it will be understood that the principle may be applied by the skilled metallurgist with other analyses and that I do not intend to be limited to examples given except as indicated in the appended claims.

What I claim is 1. An annealable white iron casting free .from flake graphite comprising iron, carbon, a graphitizing agent capable of promoting formation of temper carbon during annealing, and a suitable proportion of a metallic ingredient having the property of obstructing formation of flake graphite during solidi- 'plus carbon not substantially over 4.80%.

4. An annealable white iron casting as degraphite during solidification of the casting fined in claim 1, comprising from 1.30% to is molybdenum. I 2.50% of silicon and from 2% to 2.75% of 7. An annealable white iron casting free 15 carbon. from flake graphite consisting of iron, car

5 5. An annealable white iron casting as debon and silicon, and a small proportion of fined in claim 1, in which the metallic ingredimolybdenum alloyed with the 11011. ent capable of obstructing the formation of 8. An annealable'white iron casting as deflake graphite during solidification of the fined in claim? containing iron, about 2.75% 20 casting is totally allo ed with the iron. carbon, about 1.80% silicon and about .50%

0 I 6. An annealable w iteiron casting as demolybdenum.

fined in claim 1, in which the metallic 1n redi- In testimony whereof I aflix my signature. entcapable of obstructing formation 0 flake v ALFRED L. BOEGEHOLD.