US3433685A - High magnetic permeability cast alloy - Google Patents

Description

United States Patent HIGH MAGNETIC PERMEABILITY CAST ALLOY Roy R. Albertzart, Jr., Saginaw, Mich., assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing. Filed May 20, 1966, Ser. No. 551,528 US. Cl. 14835 3 Claims Int. Cl. C22c 37/00 ABSTRACT OF THE DISCLOSURE A ferritic cast iron having a high magnetic permeability is disclosed. In a preferred form the composition contains 2.2-2.7% carbon, 1.22.0% silicon, 0.01-0.10% titanium and the balance substantially all iron, and has been subjected to a malleabilizing heat treatment so as to be substantially free of flake graphite and matrix pearlite.

This invention relates to a castable ferrous alloy having high magnetic permeability and more particularly to a cast ferrous alloy which is suitable for applications requiring a magnetic permeability in excess of 1600 gausses per oersted at a magnetizing force of 5 oersteds and 210 gausses per oersted at a magnetizing force of 70 oersteds.

Alternators have replaced generators in the ignition systems of most automobiles produced in the United States. Low-carbon steel stampings commonly have been used as rotor poles for such alternators. However, in applications requiring increased magnetic permeability, I have found that a cast iron rotor pole composition offers advantages over a steel stamping. I have also found that a castable ferrous alloy may be used advantageously to form other components requiring high magnetic permeability such as, for example, a magnetic clutch disk for air conditioner compressors.

The principal object of the present invention therefore is to provide a cast ferrous metal alloy having a high magnetic permeability which can be produced by conventional methods at a reasonable cost. This and other objects are attained with a cast iron of essentially ferritic microstructure comprising by weight 0.01-0.10% titanium, 2.2-2.7% carbon, -1.2-2.0% silicon and the balance substantially all iron. A cast malleable iron having the following composition has been found to be particularly useful in applications requiring high magnetic permeability: carbon 2.5- 2.7% by weight, silicon 1.2-2.0%, titanium 0.01-0.10%, manganese 0.300.60%, sulfur 0.15% maximum, phosphorus 0.05% maximum and the balance substantially all iron. In addition to the composition ranges specified, it is critical that the cast material have a microstructure comprised of ferrite and graphite, the graphite being present in the form of nodules or temper carbon. The presence of combined carbon as pearlite or of uncombined carbon as flake graphite, adversely affects the magnetic permeability of my composition. I have found that cast irons comprising by weight 2.2 to 2.7% carbon, 1.2 to 2.0% silicon, 0.01 to 0.10% titanium and the balance substantially all iron, and further having a ferritic microstructure which is substantially free of combined carbon as pearlite, or of uncombined carbon as flake graphite, have an unexpectedly high magnetic permeability of 1600 gausses per oersted or higher at a magnetic force of 5 oersteds and 210 gausses per oersted or higher at a magnetic force of 70 oersteds. This of course means that when the above specified cast iron is actually subjected to a magnetic force of 5 oersteds a magnetic flux density of at least 8000 gausses is obtained, or to a magnetic force of 70 oersteds a magnetic flux density of at least 14,700 gausses is obtained.

The respective concentrations of carbon and silicon 3,433,685 Patented Mar. 18, 1969 which are suitable for use in my ferrous composition are within the cast iron range and methods and equipment for melting and casting such compositions are well known. However, I have found that when titanium is incorporated into a ferritic cast iron of the above composition, the magnetic permeability is increased to the point at which the composition is useful in automobile alternators and the like. In accordance with my invention, titanium preferably is added in amounts comprising from 0.01-0.10% of the weight of the alloy as titanium metal, ferrotitanium, titanium-silicon alloy or in any other suitable combined form which does not introduce unwanted impurities.

While carbon, silicon, and titanium are the critical alloying constituents in my magnetically permeable cast iron composition, small amounts of other alloying elements may be added in order to more readily attain the desired microstructure of ferrite and temper carbon or nodular graphite. Such a microstructure in cast irons is commonly produced in the prior art in ferritic malleable iron and ferritic nodular iron. Malleable iron of course, is produced from a white cast iron by a suitable annealing cycle. Nodular iron on the other hand is produced from the molten alloy by the addition of inoculants, such as magnesium and cerium, which cause the uncombined carbon to separate as nodules or spherulites of graphite. Depending upon the base composition of the alloy, the amount of the inoculants, and the rate of cooling a subsequent anneal, may be required to convert residual pearlite to ferrite. It is known that minor amounts of alloying elements may be added to aid in the formation of a suitable ferritic malleable or ferritic nodular iron and they will be discussed below in more detail. While these alloying elements do not directly improve the magnetic permeability of ferrous alloys, it is apparent that they contribute to the practice of my invention by enhancing formation of graphitic temper carbon or nodules. A ferritic malleable iron of the above-defined composition with respect to carbon, silicon, and titanium is the preferred form of my cast magnetically permeable alloy.

In the production of ferritic malleable iron, it is desirable that carbon remain in combined form until after the molten alloy has been cast and has solidified as white cast iron. To this end it is known that bismuth and/ or tellurium may be added as mild carbide stabilizers to prevent mottle during casting. Bismuth and/ or telluriurn may effectively be 'added for this purpose in quantities from about 0.002- 0.2% by weight. To balance the carbide stabilizing effect of the bismuth and tellurium, it is also known that small amounts of boron may effectively be added. The boron does not materially affect iron carbides during casting but it does accelerate carbide decomposition during subsequent annealing. Boron is preferably addedin an amount equivalent to about 00001-0002 percent by weight of the alloy. Tellurium may be added as ferrotellurium or as tellurium metal. Bismuth may be added as bismuth metal or as bismnith alloyed with boron. Boron may be effectively added as 'ferroboron, as an alloy with bismuth, or as a borate as for example sodium borate.

It-is also known that the spherulitic graphite structure character of nodular iron is produced by the addition of one or more suitable elements to the molten cast iron. For example, magnesium, cerium, columbium, lithium, sodium, barium and other elements will produce the sphenulitic graphite structure. of these elements magnesium and cerium are commercially important and in many applications both magnesium and cerium are employed. Suflicient magnesium is added to obtain a residual amount of magnesium usually not in excess of 0.08%. As is well known in the art, a nodular iron microstructure containing essentially pearlite or essentially ferrite or mixtures thereof may be obtained as desired.

The production of both malleable iron and nodular iron is well known to metallurgists and either method may be used to produce my composition provided the final cast alloy is essentially free of matrix pearlite and of free graphite. A small amount of rim pearlite can be tolerated but is not preferred. Carbon either in the form of pearlite or of flake graphite adversely affects the magnetic permeability of the cast ferrous alloy particularly in the relatively unsaturated portion of the hysteresis curve.

As is known, it is very difficult to eliminate sulfur which is almost always present in small amounts in cast irons. Since sulfur stabilizes cementite and thus pearlite, it is preferable that its carbide stabilizing effect be neutralized by incorporating a small but effective amount of manganese. In accordance with my invention, it is preferred that the sulfur content be kept below a maximum of about 0.1 5- 020% sulfur. Accordingly, it is preferred to add about 0.30-0.60% manganese as for example ferromanganese which will assure a proper balance between the manganese and sulfur contents.

An example of a preferred embodiment will further describe the manner in which my invention may the practiced. An alternator rotor pole was cast suitable for use in automobile alternators such as is found on present model automobiles. The composition of the cast alloy comprised by weight, 2.34% carbon, 1.8% silicon, 0.37% manganese, 0.134% sulfur, 0.10% titanium and the balance substantially all iron. The melt was prepared in an induction furnace using scrap iron as the base metal. Titanium was added as 30% ferrotitanium and silicon was added as silicon metal. These additions were made to the melt in the induction furnace at approximately 2750 F. The tapping temperature was 2800 F. The rotor was cast in a green sand mold with an oil sand core. After the casting has solidified and cooled, it was removed from the sand mold and cleaned. It was then subjected to the following annealing cycle to produce an essentially ferritic malleable iron. The casting was heated in a suitable furnace over a period of three hours to 1550 F. In the next three and three quarter hours the temperature was slowly increased to 1725 F. at which temperature the casting was then maintained for a period of about eight and a half hours. The cast rotor was then cooled from 1725 F. to 1375 F. in about another one and a half hours. The casting was maintained at a temperature between about 1325 F. and 1375 F. for a period of an additional eight hours at which time the casting was air cooled to room temperature. Thus approximately twenty-four hours are required for the complete malleabilizing heat treatment. At the completion of the heat treatment the microstructure of the casting consisted only of ferrite and graphitic temper carbon. No comlbined carbon or flake graphite was apparent.

The cast rotor pole was then incorporated into an otherwise standard commercially available alternator and the following magnetic properties of the component were determined:

Flux density Magnetic force oersteds: gausses 8,800

From the tabulated data it is observed that the magnetic permeability of the above specimen is 1760 gausses per oersted at 5 oersteds and about 218 gausses per oersted at 68 oersteds. Other cast iron composiitons within the limits specified have all been found to have magnetic permeability values of, or in excess of, 1600 or 210 gausses per oersted when subjected to magnetic forces of .5 or 70 oersteds respectively. It will be apparent to one skilled in the art that an alternator rotor pole of my ferrous composition is entirely suitable for use in automotive alternators. Moreover, the pole may be manufactured by a casting process which is preferred to low-carbon steel stampings currently in use.

While my invention has been described in terms of a specific embodiment, it appears that other applications can be devised by those skilled in the art and therefore the scope of my invention is intended to be limited only by the following claims.

I claim:

1. A ferritic cast iron of high magnetic permeability having a matrix consisting essentially of ferrite and being free of flake graphite, said alloy consisting essentially by weight of about 0.010.10% titanium, 2.22.7% carbon, 1.22.0% silicon and the balance 1r0n.

2. A ferrous cast alloy of high magnetic permeability having a matrix consisting essentially of ferrite and being free of flake graphite, said alloy consisting essentially by Weight of about 0.01-0.l0% titanium, 2.22.7% carbon, 1.2-2.0% silicon, 0.30-0.60% manganese, sulfur not in excess of 0.15%, phosphorus not in excess of 0.05% and the balance iron.

3. A cast ferritie malleable iron characterized by a magnetic permeability in excess of 1600 gausses per oersted at a magnetic force of about 5 oersteds and 210 gausses per oersted at a magnetic force of about 70 oersteds, said ferritic malleable iron consisting essentially by weight of about 0.010.10% titanium, 2.2-2.7% carbon, 1.22.0% silicon, 0.30-0.60% manganese, 0.0020.02% of at least one element selected from the group consisting of bismuth and tellurium, 0.00010.002% boron, and the balance iron, said alloy being characterized by a microstructure which is essentially free of flake graphite and matrix pearlite and contains only a minor amount of rim pearlite.

References Cited UNITED STATES PATENTS 1,636,657 7/1927 Schwartz 148-138 X 1,707,753 4/1929 Boegehold 148-35 X 2,501,059 3/1950 Kluijtmans 14835 X 2,579,452 12/1951 Eckman et al. -123 2,901,386 8/1959 Saives 14835 X 3,189,492 6/1965 Laudenslager et al. 75123 X FOREIGN PATENTS 240,017 6/1960 Australia.

CHARLES N. LOVELL, Primary Examiner.

US. Cl. X.R.