US 2933386 A
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2,933,386 METHOD OF SINTERING AND NITRIDI NG FERROUS BODIES V Leopold Pessel, Wyndmoor, ,Pa., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Application August 1,; 1956 Serial No.;601,353
3 Claims. (oi. 75-424 This invention relates generally .to methods for integrating compressed iron powder -articles while simultaneously rendering them rust-proof; More particularly, the invention relates to the simultaneous sintering and nitriding of compressed soft powdered iron bodies.
Soft iron powder bodies, even after sintering, are extremely susceptible to rusting, especially in a humid atmosphere. This may bepartly due to the porosity of such iron powder bodies which offers a relatively large area of surface in contact with theair. Previously attempts have been made to protect such sintered iron powder .bodies from oxidation by plating with another metal such as cadmium, zinc, chromium or nickel. Such attempts at plating, however, are not entirely satisfactory. The sintered iron body has a tendency to absorb the plating electrolyte which may later ooze out. This is not only unsightly and conducive to corrosion, but it may also atfect performance of the part where precise clearances are required. In addition, plating is a relatively expensive process.-
'It is, therefore, an object of this invention toprovide. an improved method for integrating a pressed powder body of substantially ferrous material while simultanea ously rendering the surface of the body rust-resistant.
Another object of the invention isto provide an improved method for integrating a pressed powder body of substantially ferrous material while simultaneously rendering the surface of the body rust-resistant without appreciably altering the magnetic properties ofthe body.
Still another object'of the invention is to provide an improved method for simultaneously sintering and nitriding a pressed powder body of ferrous material to integrate the powders while rendering the surface rustproof, without appreciably altering the magnetic properties of the body.
These and other objects and advantages of the invention are accomplished by sintering the pressed iron powder body while simultaneously nitriding the body during at least a part of the time it is being sintered.
The invention will be described in greater detail with particular reference to a cup-shaped magnetic pole piece such. as is useful for loudspeakers and the like. Ithas been found that such articles can be most economically manufactured by pressing soft powdered iron into the desired shape and dimensions and then integrating the pressed iron powder by sintering to produce a strong, continuous body. The invention, however, is applicable generally to the field of ferrous powder metallurgy.
It is known that the nitriding of iron produces an alloy or compound of iron and nitrogen which is highly rustresistant. In general, an iron body can be nitrided by heating the body to a temperature of about 9001000 F. in an atmosphere of undissociated ammonia (NH for about 40 hours. A complete description of the nitriding of iron is found on page 699 of the 1948 edition of the Metals Handbook, published by the American Society for Metals. -It is essential to the process that un dissociated ammonia be employedsince the iron will re- 2,933,386 Patented Apr. 19, 1960 and, though the process is reversible, the iron has such a great affinity for the monatomic nitrogen that it reactswith the nitrogen before it is converted to the di atomic state.
One of the nitrides thus formed is a hexagonal molecule I of the epsilon (E) phase of the iron' nitride alloy and as such is non-magnetic. However, it has been found that iron pole pieces can be nitrided according to the invention to the extent necessary to provide a protective rustproof nitride coating for the pole pieces without substantially altering the magnetic properties of the pole pieces. Furthermore, it has been found that nitriding and sintering the pressed iron powder pole pieces can be accomplished in one operation thus substantially reducing the cost of manufacture of such ferrous bodies. This is a departure from the teachings of the prior art relative to nitriding iron since the nitriding operationis thus carried out at sin'tering temperatures of the order of 2000" F. instead of the conventionally used 950 F. It might be expected that nitriding at temperatures of the order of 2000 F. would result in excessive nitride formation and thus deleteriously affect the magnetic properties and physical dimensions of the ferrous body so treated. Ithas been found, however, in the present invention, that although nitriding does produce expansion of the body (and by expansion is meant a permanent structure growth), the sintering process produces a contraction so as to compensate or balance out the expansion. The contraction is due to coalescence of the iron powder particles upon sintering. Thus an iron powder body may be nitnded simultaneously during the entire sintering operation. However, excessive nitriding (that is, more than is required for merely providing a ferrous body with a rust-proof coating) does affect the magnetic properties of the ferrous body. Where optimum magnetic properties are not required, the ferrous bodies may be simultaneously sintered and nitride'd during the entire 'sintei'ing operation according to the invention.
Example I Pressed iron powder bodies each having a cup-shape and Weighing about 1 oz. are placed in a furnace having an atmosphere of undissociated ammonia (NH and maintained at a temperature of 2100 F. for 15 minutes. Thereafter the body is cooled to room temperature in a reducing atmosphere such as hydrogen or nitrogen and hydrogen (dissociated ammonia, for example: N +3 H The pressed iron cups so treated have excellent rustresistant properties and their magnetic properties are virtually unaffected.
Example II Pressed iron powder bodies are placed, as in Example I, within a furnace having an atmosphere of dissociated ammonia (N -(6H and heated to a temperature of at least about 1800 F. Preferably the temperature is in the range of 2000 to 2100" F. v The bodies are kept at this temperature for about 15 minutes. Thereafter, while maintaining the temperature as stated, undissociated ammonia (NH is pumped into the furnace and maintained therein for about 5 minutes-at which time the sintering and desired nitriding of the body is completed. The furnace may then be turned-01f and the body allowed to cool to room temperature in an atmosphere of dissociated ammonia (N +3H or any other inert or nonoxidizing atmosphere which may be pumped into the furnace after ceasing to pump the undissociated ammonia (NH through the furnace. This processing technique also resulted in bodies having excellent resistance to rusting and magnetic properties suitable for using the cups in loudspeaker assemblies.
Example III A pressed iron powder body is placed in a crucible and drawn through a horizontal tubular furnace heated throughout its length at a temperature of about 2100 F. The first portion of the furnace contains an atmosphere of dissociated ammonia (N +3H H or another re- .ducing or inert atmosphere, and the rate of travel of the pressed iron body is adjusted so that the body will be maintained at this temperature and in this atmosphere for about 15 minutes. The next succeeding portion of the furnace contains an atmosphere of undissociated ammonia (NH the temperature of this portion is substantially the same as in the first portion of the furnace (2100 F.). The rate of travel with respect to the length of this zone is such that the body remains in this nitriding atmosphere for about 5 minutes during which time the sintering of the body is completed and the body is nitrided. The furnace terminates in an unheated portion, preferably cooled as by a water coil. This terminal portion of the furnace contains a non-oxidizing atmosphere such as H dissociated ammonia (N +3H Upon reaching room temperature the body is removed J from the furnace altogether and is finished.
It will be observed that the sequence of steps recited in Examples II and III calls first for sintering in a nonreactive, non-oxidizing atmosphere. The actual batch treated in the examples comprised 15 pounds of oneounce cups. For this batch the sintering time was about 15 minutes and during this period most of the sintering operation is completed. The sintering time will vary, however, with such factors as the furnace capacity, the size of the batch, the shape of the parts, the container carrying the parts,;,etc. Since the time and temperature of sintering depend upon many variables, it is not possible to specify exact sintering times which will be appropriate in all cases. The object of sintering is to give the iron powder body a density, ductility and toughness similar to that of solid iron or low carbon steel. Hence the time of sintering must be long enough to achieve these results. The temperatures necessary to accomplish this are generally above about 1800 F.
In the Examples II and III the next step after sintering is to complete the sintering operation and to simultaneously nitride the body. This step is accomplished by maintaining the sintering temperature and keeping the body in an atmosphere of undissociated ammonia. In the examples referred to, the nitriding step is accom: plished in about five minutes; but as in the case of sintering, the many factors already indicated make it impossible to specify the exact nitriding time which will apply in all cases. nitride formed is excessive (more than required for protection), the magnetic properties of the body are deleteriously affected since the nitride formed is non- As previously pointed out, if the p manufacturing efiiciency obtained by carrying out the sintering and nitriding operations in one continuous operation at the same temperature, is significant and highly desirable.
The non-reactive, non-oxidizing atmosphere need not be dissociated ammonia (N -MH if desired, hydrogen or nitrogen separately may be employed as well as other known reducing or inert atmospheres.
What is claimed is:
1. The method of processing a pressed powder body consisting essentially of iron comprising the steps of: partially sintering said body at a temperature of about 1800 F. to about 2100" F. in a non-oxidizing atmosphere; then, while continuing said sintering, nitriding the surface of said body in an atmosphere containing monatomic nitrogen at substantially the same temperature for a brief period.
2. The method of processing a pressed powder body consisting essentially of iron comprising the steps of: partially sintering said body at a temperature of about 2100" F. in a non-oxidizing atmosphere; then, while continuing said sintering, nitriding the surface of said body in an atmosphere of undissociated ammonia at substantially the same temperature for a few minutes.
3. The method of processing a pressed powder body consisting essentially of iron comprising the steps of: partially sintering said body at a temperature of at least 1800 F. in a non-oxidizing atmosphere; then, while continuing said sintering, nitriding the surface of said body in an atmosphere of undissociated ammonia at substantially the same temperature for a few minutes, and thereafter cooling said body in a non-oxidizing atmosphere.
References Cited in the file of this patent UNITED STATES PATENTS 1,864,567 Walter June 28, 1932 1,989,186 De Bats Jan. 29, 1935 2,051,454 Morris Aug. 18, 1936 2,333,573 Kalischer Nov. 2, 1943 2,666,724 Beller Jan. 19, 1954 FOREIGN PATENTS 1,107,756 France Aug. 10, 1955 OTHER REFERENCES Goetzel: Treatise on Powder Metallurgy, vol. I, pages 697, 698, published 1949.