US2067896A - Surface hardened cast iron articles of manufacture - Google Patents

Description

' ly successful.

Patented Jan. 19, 1937 SURFACE HARDENED CAST IRON ARTICLES OF MANUFACTURE Marie Louis Andr Babinet, Boulogne-sur-Seine, France, asslgnor, by mesne'assignments, to The Nitricastlron Corporation, a. corporation of Delaware I No Drawing. Application July 14, 1931, Serial No. 550,758

16 Claims.

My invention relates to the surface hardening of articles made of cast iron alloys, the hardening being effected by the action of nitrogen-containing compounds, for example, ammonia.

It has heretofore been proposed to harden by nitriding a cast iron containing a small proportion of an alloying addition, such asone or more of the elements aluminum, silicon, manganese, chromium, nickel, cobalt, vanadium, molybdenum, tungsten, titanium and zirconium. However, while the nitride hardening eifect.obtained by introducing certain of the elements above mentioned into cast iron is always very much superior to that of an ordinary .cast iron, such hardening effect and the other resulting physical properties produced by the nitriding treatment vary considerably, depending upon the proportion of the allowing addition and the particular alloying element or elements comprising such addition. Consequently, cast iron alloys of these general types vary considerably in their suitability as regards their application for various industrial uses. Moreover, it is found when such castiron alloys are prepared in the ordinary man-ner,-that is, from materials melted in a' cupola and cast under ordinary conditi'ons,that the results obtained by nitriding are not uniform- Oftentimes the castings produced lack structural homogeneity, tend to be porous and will show segregations, particularly in those mentioned, there is a tendency for uneven absorption of nitrogen during the nitriding treatment with resultant unevenness of surface hardness and depth of case. Such defects'also give rise to a brittle, case and to a rough surface after nitriding, and, as a consequence, the nitrided product is often. found unsuited for industrial uses. While in some cases, as where the defects are confined to the surfaceportions of the castings, it is possible to machine away the defective portions before nitriding, this adds to the cost of manufacture and, besides, is not feasible in situations where thin castings, such as thin tubular articles, are being produced.

In order to insure good results in nitriding it is necessary that at least thesurface portions'of the castings to be treated shall have imparted to them a uniform physical structure, either through control of the casting process or through subsequent heat treatment or otherwise, and best restituent of the cast iron is distributed therein in very finely divided form.

It is an object of the present invention to prepare castings destined to be hardened by nitriding and having the desirable physical character- 'istics above mentioned. Further objects of the invention relate to the production of improved alloy cast iron compositions that are particularly suitable for nitriding and to the production therefrom of nitrided articles of manufacture.

I have found that castings that are particularly suitable for nitriding may be prepared from alloyed cast iron having a relatively'low carbon content, say, from about 2% to 3.5%, and which have been cast under conditions insuring rapid cooling, followed, if necessary, by appropriate heat treatment designed to produce precipitation of at least a large part of the carbon in the form of finely divided free carbon. The casting operation may advantageously be carried out by the centrifugal method of casting,-i. e., pouring the cast iron into a mold which is rotating at high speed about its axis. force, a dense homogeneous product is produced. Furthermore, castings produced by the centrifugal process have the advantage that the portion of the carbon that is present as elemental carbon, whether present in the metal as cast, or developed by suitable heat treatment, is uniformly distributed throughout the metal in finely, divided form.

The proportion of graphitic carbon that is found in the metal as cast will; as is known, be influenced by the carbon and silicon contents, and also to some extent by the proportions and kinds of alloying additions. Whenthe casting operation is carried out with the use of chilled molds, the cast metal will ordinarily contain a large proportion, if not all, of the carbon in the combined form. In such case, the castings should further be prepared for nitriding by subjecting them to suitable heat treatment to convert all or a large part of. the combined carbon into the form of finely divided temper carbon.

Another method of preparing castings having a uniform physical structure and characterized by even distribution of the graphitic carbon constituent in very finely divided form comprises superheating the charge of molten metal and then casting either directly from this stage or after cooling to the normal casting temperature. Superheating also brings about uniform-dissemination of alloying elements that might otherwise tend to formsegregatioris in the metal. Instead of superheating the charge, the same results may Due to the centrifugal e be accomplished by prolonged heating at a temperature above the melting. point and then.

should not in general be below about 1.4% orhigher than about 4.0% and preferably should be between about 1.5 and 3%. With compositions that are not characterized by a high degree of graphitization in theas cast condition or do not respond easily to an annealing or malleableizing heat treatment when the silicon content is maintained within the ranges above mentioned, it is possible to promote separation of carbon in elemental form without further increase of silicon by adding other elements that tend to promote graphitization or to render the cast product readily malleableizable when subjected to suitable heat treatment, e. g., aluminum, nickel and. tita nium. By substituting one or more of such elements for part of the silicon, it is possible to produce satisfactory alloys for the purpose in hand containing less than 1% of silicon. I have found that cast iron alloys that are particularly suitable for nitriding purposes, that respond well to the casting methods hereinbefore mentioned and that are characterized by particularly good general physical properties, such as strength, elastic limits, etc. may be prepared with proportions of carbon and silicon within the ranges above mentioned and in addition containing at least. 0.5%

and preferably between 0.5 and 3.5% of an alloy-.

ing element or a combination of alloying elements tending to promote nitride hardness, such, for example, as chromium, molybdenum, vanadium, tungsten and titanium.

In situations where maximum hardness is desired the cast iron may contain up to 2% of aluminum, which notably increases the hardness obtained by nitriding.

In order to improve the general physical characteristics of the alloy or to impart to it special properties for certain mechanical applications, further alloying additions may be made such as one or more of the elements nickel, copper, cobalt, manganese and zirconium.

The following is an example of a cast iron composition that is particularly suitable for nitriding and which may be used to form castings of thin section by employing one of the methods of procedure above outlined:

Percent Carbon about 2.5 Silicon d0 3 Manganese ldo 0.5 Aluminum do i 1 Chromium do 1.5

A heat treatment that has been found advantageous, not only in treating the particular composition above mentioned but otheralloys falling within the broad range hereinbefore disclosed, consists in annealing at a suitable temperature (for example about 900 C.), the duration of the annealing treatment depending upon the particular composition involved and the readiness with which the combined carbon is converted into the elemental form in the annealing treatment. In order to still further improve the structure of the metal and to impart to the casting the ability to acquire the maximum nitride hardness, it is recommended that after machining the casting be reheated to a temperature which may differ from the annealing temperature (for example about 875 C.) and then quenched in a suitable medium, for example an oil bath or a salt bath heated to about 200 C. The casting should then be tempered by re-heating to a suitable temperature (for example of about NO-750C.) and thereby brought into a uniform sorbitic state.

The casting in this state is then subjected to the nitriding treatment.

Some alloys falling within the range above mentioned may be brought to the sorbitic state by omitting the preliminary annealing treatment and, instead, merely quenching, say, from a temperature of around 950 C., and then re-heating to a lower temperature. ing necessary to eifect the nitriding will suifice, in lieu of the re-heating step,'for effecting such further change in the internal structure of the metal as would otherwise be effected in such reheating step.

It will be understood that the temperatures above mentioned are cited merely byway of example and that the optimum heating temperatures for annealing, for heating, for quenching and for tempering will vary somewhat according to the particular composition of the cast iron.

I claim:

, 1. Process of preparing nitride hardened cast iron articlesof manufacture, which comprises forming molten cast iron containing one-or more alloying elements adapted to render the cast iron susceptible of being surface hardened by nitriding, casting and heat treating the alloyed cast iron to develop in the cast product a physical structure characterized by uniform dissemination of at least a'large part of the carbon content in finely divided form as elemental carbon in a sorbitic matrix, and finally surface hardening such product by nitriding.

2. Process of preparing' t ed ca iron articles of manufacture, which comprises forming molten cast iron having a low carbon content and containing one or more alloying elements adapted to render the cast iron susceptible of being surface hardened by nitriding, casting the metal under such conditions as to insure, rapid cooling of the cast product, heat treating the cast product to produce uniform dissemination of at least a large part of the carbon content in finely divided form as elemental carbon-in a sorbitic matrix, and finally surface hardening such prod uct bynitriding.

In some cases the heat- 3. Process of preparing nitride hardened cast Q iron articles of manufacture, which comprises forming molten cast iron containing one or more alloying elements adapted" to render the cast iron susceptible of being surface hardened by nitriding, casting the metal while applying high pressure to the metal as it is cast and during the period of solidification to produce a dense product, heat treating the cast product to produce uniform dissemination of at least a large part of the carbon content in finely dividedform as elemental carbon in a sorbitic matrix, and finally surface hardening such product by nitriding.

4. Process 'of preparing nitride hardened cast iron articles. of manufacture, which comprises forming molten cast iron containing one or more alloying elements adapted to render the cast iron susceptible of being surface hardened by nitriding, centrifugally casting the metal, heat treating the cast product to produce uniform dissemination of at least a large part of the carbon content in finely divided form as elemental carbon in a sorbiticmatrix, and finally surface hardening such product by nitriding.

5. Process of preparin'gnitride hardened cast iron articles of manufacture, which comprises forming molten cast iron containing one or more alloying elements adapted to render the cast iron susceptible of being surface hardened by nitriding, heating the molten iron to bring about uniform dissemination therein of said alloying elements, then casting the metal, heat treating the cast product to produce uniform dissemination of at least a large part of the carbon content in finely divided form as elemental carbon in a sorbitic matrix, and finally surface hardening such product by nitriding.

6. Process of preparing nitride hardened cast iron articles of manufacture, which comprises forming molten cast iron containing one or more alloying elements adapted to render the cast iron susceptible of being surface hardened by nitriding, superheating the molten alloyed cast iron to bring about uniform dissemination therein of said alloying elements, casting and heat treating the alloyed cast iron to develop in the cast product a physical structure characterized by uniform dissemination of at least a large part of the carbon content in finely divided form as elemental carbon in a sorbitic matrix, and finally surface hardening such product by n triding.

7. Process of preparing nitride hardened cast iron articles of manufacture. which comprises forming molten cast iron containing one or more alloying elements adapted to render the cast iron susceptible of being surface hardened by nitriding, and before casting the molten cast iron holding it at a temperature above the fusion point for a sufiicient time to insure uniform dissemination therein of said alloying elements, casting and heat treating the alloyed cast iron to develop in the cast product a physical structure characterized by uniform dissemination of at least a large part of the carbon content in finely divided form as elemental carbon in a sorbitic matrix, and finally surface hardening such product by nitriding.

8. Process of preparing nitride hardened cast iron articles of manufacture, which comprises forming molten cast iron containing one or more alloying elements adapted to render the cast iron susceptible of being surface hardened by nitriding, forming the molten metal into'castings,

then subjecting such castings to an annealing heat treatment to develop therein a physical structure characterized by uniform dissemination of at least a large'part of the carbon content in finely divided form as elemental carbon, further iron articles of manufacture, which comprises forming molten cast iron containing less than 3.5

of carbon and in addition one or more alloying elements adapted to render the cast iron susceptible of being surface hardened by nitriding and centrifugally casting the metal under conditions insuring rapid cooling of the cast product,

annealing the cast produ t at about 900 C. to render it machinable, reheating the annealed product to a temperature between the lower thermal critical point and about 875 0., and quenching from such temperature. then reheating to about 700-750 C. to bring the cast product into a sorbitic state, and finally surface hardening such product by nitriding.

11. Process of preparing nitride hardened cast iron articles of manufacture, which comprises forming molten cast iron containing from 2 to 3.5% of carbon, from 1.4 to 4% of silicon. and

from 0.5% to 3.5% of an element selected from the group consisting of chromium, molybdenum. vanadium,.tungsten and titanium, heating the alloyed cast iron to bring about uniform dissemination therein of the alloying elements, casting and heat treating the alloyed cast iron to develop in the cast product a physical structure characterized by uniform dissemination of at least a parge part (of the carbon content in finely divided form as elemental carbon in a sorbitic matrix, and finally surface hardening such product by nitriding.

12. Process of preparing nitride hardened cast iron articles of manufacture, which comprise forming molten cast iron containing from 2 to 3.5% of carbon, from 1.4 to 4% of silicon, an effective and substantial amount but not exceeding about 2% of aluminum, and from 0.5 to 3.5% of an element selected from the group consisting of chromium, molybdenum, vanadium, tungsten and titanium, heating the alloyed cast iron to bring about uniform dissemination therein of the alloying elements, casting and heat treating the alloyed cast iron to develop in the cast product a physical structure characterized by uniform dissemination of at least a large part of the carbon content in finely divided form as elemental carbon in a sorbitic matrix, and finally surface hardening such product bynitriding.

13. An article hardened in its surface portions by nitriding and consisting of a cast iron alloy containing from 2 to 3.5% of-carbon, 1.4 to 4% of silicon and 0.5 to 3.5% of an element selected from the group consisting of chromium, molybdenum, vanadium, tungsten and titanium, and characterized by a physical structure and properties such as are produced when a cast iron alloy of like composition is cast and heat treated to develop in the cast product a physical structure characterized by dissemination of at least a large part of the carbon content in finely divided form .as elemental carbon in a sorbitic matrix, and

finally surface hardened by nltriding.

14. An article hardened in its surface portions by nitriding and consisting of a cast iron alloy containing from 2 to 3.5% of carbon 1.4 to 4% of silicon and 0.5 to 3.5% of an element selected 5 from the group consisting of chromium, molybdenum, vanadium, tungsten and titanium, and characterized by a physical structure and properties such as are produced when a cast iron alloy of like composition is cast centrifugally, thereby 1 producing a dense, fine-grained cast product, and

such cast product is heat treated to develop therein a physical structure characterized by a uniform dissemination of at least a large part of the carbon content in finely divided form, and is 15 finally surface hardened by nitriding.

15. An article hardened in its surface portions by nitriding and consisting of a cast iron alloy containing from 2 to 3.5% of carbon, 1.4 to 4% of silicon and 0.5 to 3.5% of an element selected 20 from the group consisting of chromium, molyb- -denum, vanadium, tungsten and titanium, and characterized by a'physical structure and properties such as are produced when a-casting of a cast iron alloy of like composition is subjected to an 25 annealing heat treatment followed by reheating to a temperature close to but short of the annealing temperature, then quenched from such reheating temperature,- then reheated to a still lower temperature suflicient to produce tempering, thereby developing in the casting a physical structure characterized by uniform dissemination of at least a large part of the carbon content in finely divided form, and finally surface hardened by nitriding.

' 16. An article hardened in its surface portions by nitriding and consisting of a cast iron alloy carboncontent in finely divided form and is finally surface hardenedby nitriding.

MARIE LOUIS ANDRE BABINET.