US1632704A - Casting having chromium alloy surface - Google Patents

Description

Patented June 14, 1927.

UNITED STATES PATENT OFFICE.

CHARLES B. JACOBS, 0F WILHINGTON, DELAWARE, ASSIGNOR, EY EESNE ASSIGN- MENTS, TO ELEGTRO METALLURGICAE COMPANY, A CORPORATION- OF WEST VIB- GINIA...

No Drawing.

steel for various purposes has led to'ademand for containers which would withstand oxidation of furnace gases at high temperatures. To meet this need certain alloys composed of nickel, chromium and iron, have been devised. These were put on the market under the name of nichrome, pyroloy, nichroloy, hardite, etc. These alloys are all very similar and usually contain from 1U-15% chromium, 40-50% nickel, and the balance iron with the usual small constituents of manganese, silicon, carbon, etc., found in iron and steel. Alloys of the above composition are for the most part made by melting up various constituents in the proper proportions in electric furnaces and casting the melt in the shape of heat treating boxes, 1

retorts, muflies, etc. Alloys of this nature are expensive and have been somewhat unreliable on account of the lack of control in' the manufacture.

My experience in the construction of cyanide retorts, designed to with stand the action of oxygen-containinggases at a temperature of 200023OQ F., has led me to believe that such alloys are unsatisfactory on account of the variation in the product obtainable on the market and the high cost of the alloys. One sample of one of the abovenamed alloys, for example, would resist oxidation almost completely at the above temperature, while the next sample ofsupposedly the same composition would break down and oxidize badly in a very short time,

Knowing that metallic chromium was resistant to oxidation .to an extreme degree even at very high temperatures, it occurred to me that if wrought iron or steel, both of stronger than the nickel-chromium alloys of chromium.

casrme mvnie onnomum ALLOY soar-non.

Application filedAprfl 22, 1921. Serial Ilia. 463,565.

and otherwisebetter suited to the construction of retorts, could be surfaced with chromium, it would render the steel or iron sufficiently resistant at high. temperatures to serve my purpose. The first attempt to accomplish this was by electrolytic deposition of chromium, but this procedure, whilev working very well on small surfaces, was found impracticable of application to large retorts.

Upon further investigation I have discovered that a casting having a surface por- .tion, or layer high in chromium can be produced by bringing the casting metal into contact with chromium during the casting operation in a manner which I shall describe hereinafter in detail. By my new procedure, it is possible to produce a chromium alloycoating on a steel,casting the thickness of which coating may vary from or less to as desired. This coating in all cases remains on the surface of the casting as a distinct separate layer.

It is not impaired in quality by diifusion into the steel casting, even after long serviceat high temperatures. The coating is firmly fused to the casting itself and cannot be scaled or chipped off, even with the use of an air chisel.

Generally speaking, my. new method comprises coating those surfaces of the mold or scores of the mold in which the casting is made, corresponding with the surfaces of the casting on which the alloy is'required, with such a manner that the casting metal on flowing into the mold permeates and melts the chromium 'in a granular condition in or dissolves the metallic coating. The skinchill or chilling effect exerted by the mold surfaces prevents the chromium from diffusing throughout the body of the casting metal so that an alloy is formed on the surface of the casting containing a high percentage To prevent the coating from being washed from the mold surfaces by the flow of. the casting metal, the chromium .in a granular .or comminuted condition ismixed with a suitable binder into a plastic mass and applied'to the mold or COISUIfaCGS and then dried sothat it adheres firmly-to the Surfaces, forming a more or less porous coating containing spaces between chromiumgram.

l x H v v i ules into which the casting metal enters and displaces the binder and melts or dissolves the metallic particles.

It is important to"'apply the chromiumcontaining coating evenly all over the mold or the core surfaces, as the case may be, and to have the chromium particles of such size or in such physical condition that spaces between the particles are formed, into which spaces the cast metal can force its way, displacing any flux orbinder contained therein "and surrounding the particles, and either melt them or take them into solution during the time the casting metal remains in a molten condition in the 111016.. For this reason it is necessary to size the coating material so that the voids between the particles will be sufliciently large to permit the proper quantity of the particular casting metal to enter the voids and melt or dissolve the chromium.

The new method may be advantageously practiced with chromium particles having a size between 18 and 30 mesh, that is, all of the particles passing through an 18 mesh screen and substantially all of the particles being caught on a 30 mesh screen.

Steel on account of its freedom from the relatively large amount of impurities which go to make up cast iron takes chromium into solution more readily than does cast iron, and offers less difliculty in the formation of alloys on its surface, and on account of the greater purity of the metal the alloy is usually of a superior quality.

For a casting designed for resistance to I oxidation at high temperature chromium offers the best metal with which to form the alloy on the casting. Iron and chromium alloys containing as little as 12% ofchromium are quite resistant to oxidation at temperatures up to 1400-1600 F. When, however, the temperature reaches 1800 to 2000 F. or above, it requires a considerably larger proportion of chromium on the surface of the alloy to render it resistant to oxidation;

-A very thin film of pure chromium on the surface of iron or steel serves to render it resistant to oxidation at extremely high tempertures. Even so thin a film of chromium as may be deposited electrolytically on iron 'or steel renders it remarkably resistant to oxidation at high temperatures, providing the coating or skin is non-porous and con-' tinuous.

My invention may be illustrated in detail by the following example:

A mold for steel casting is prepared in mold while in the green state, i, e. before baking it (as is usual with molds for steel castings) is coated 'o the proper depth, for example, to 1/8 of an inch, with comminuted or granulated chromium mixed with-ordinary silic-ee a! soda solution. Sufficient causes the chromium to adhere to the surface of the sand composing the mold and after linking the mold to a temperature of about GOO-700 F. (i. e. the usual temperature of baking molds for steel castings) the metallic chromium is cemented to the surface of the mold so firmly that it can be rubbed or brushed 'with the hand without dislodging it. After the mold has been so prepared the casting of the metal is carried out in the usual manner. A pouring temperature of approximately 2650-2900 F. is usually found to be suitable for castings, the molds of which are coated with chromium to the depth of as much as 1/4". If the chromium is comminuted to too fine a state of division, for instance, finer than 50 mesh, and the molten steel has a temperature of about 2G50-2700 C., the voids between the metallic particles in the coating are too small to permit a sulficient quantity of the steel to enter them and remain molten long enough to dissolve and take the particles of chromium into solution in the steel. When such is the case onlya very thin alloy of chromium will be produced on the surface of the casting which under the microscope will reveal the fact that a part at least of the chromium grains are merely embedded in a matrix of steel and have not lost their identity as shown by their original crystalline structure and traces of original contour. Also the casting is apt'to show spots in which littleor no chromium has been taken up by the steel.

Silicate of soda gives the best results as a binder although other binders such as glutrin (a commercial product obtained by concentrating sulphite waste liquor), molasses or similar tacky substances may he used for causing the chromium to adhere to the. surface of the mold, but irrespective of the particular binder used. only a sufficient quantity-to just accomplish firm adherence of the coating to the mold should be used. An excess of binder destroys porosity of the coating and is difficult to displace by the casting metal.

It will be noted in the above example that although the temperature at which the metal is poured in forming a chromium alloy on cast steel may be somewhat below the IOU mam

I haye in practice made castings with steel on whlch high chromium alloy penetrating to a de th 0 was obtained on the surface 0 the casting ,in a retort weighing 276 lbs.-, and to the depth of A" in a retort .weighing 10670 lbs. 4

As an example of the resistance to oxidation at high temperatures of such castings in actual service may be cited the results obtained under the following conditions with the small retort mentioned above. The

retort was put. into service and continually I gases in contact with outside of retort,

21122200 F., duration of the heating 29 days, weight of retort at end of test 278 lbs., weight of retort before test 276 lbs., i pcrease in weight by oxidation 2 lbs. or 2%.

At the same time this test was made a solid cylinder of steel 2" in diameter and 30" long poured from the same ladle of metal used inpouring the retort but unprotected by chromium was placed in the furnace with the retort and left there during the entire test. The unprotected cylinder was oxidized to a depth of from A to 1" and very badly distorted and swollen be-' yond its original diameter. It contained only a core of unoxidized metal in the center.

' Sections out from the retort and samples for analysis taken by machine cuttings and drillin throw considerable light on what -took p ace during the 'casting operation.

Since the steel was poured into the mold at a temperature (2650 F.) below the melting point (2741 F.). of the 98 to 99%) chromium with which the mold was surfaced, the melted steel coming in contact with the chromium dissolved some of. the

at the mold surfaces, took place so ra idly that as a consequence an alloy hg in chromium, containing 39.56% chromium in the first 1/100 of an inch next to the mold surface and grading off to 9.45% at a depth of Ayof an inch below the surface, was

built up on the surface of the casting. That this is a true alloy and not a mere coating or overlay of melted chromium fused into the surface of the steel is shown by the chromium content at different depths in the section of the table:

' v I Per cent Location of sample. chromium 1. Skin to de th 01' 1/100" (machine cuttings); 39. 56 2. Additio 1 100" I machine cuttings) 3&5]. 3. Skin todept of 1 in! machine cuttings).-. 3 .08 4. Skin to de th oil rillingzfi 9. 45 5. 1/2-from of casting (drill g)--. 0. 66 6. 1" from face of caetingfldriilingi, 0.37

The steelwhich was used for the casting on which the above tests were made contained 0.00% of chromium. The analysis of. the heat from which this casting was pouredis as follows: I

\ Per cent. Carbon 0.28 Manganese 0.65 Silicon 0.282 Phosphorus 0.039 Sulphur- 0.041

, Cast steel retorts with a high chromium I alloy on the surface similar in composition to that justshown above have proved more resistant to oxidation at high temperatures, under the same conditions,' than ironchromium-nickel alloys containing from 50 to of nickel and 10 to 20% of chromium.

In addition to greater resistance to oxidation, cast steel "with a high chromium alloy on the surface possesses the strength and soundness inherent in steel castings.

Although cast iron is less desirable than steel for apparatus which is to be used at high temperatures, it may be expedient in some cases to use cast iron to form the body of the casting which is to be provided with a chromium alloy surface. It becomes necessary in this case to modify the procedure described above in connection with steel to the extent that the cast iron should be at a higher temperature when, poured than is necessary with steel, since the impurities existing in cast iron which total about 8 to 10% tend to prevent the chromium from going into solution in the cast iron, unless the latter be raised to a. high temperature, say slightly above 2741" F., the melting point of chromium. This offers no serious difliculty since by the use'of thermit the iron in the ladle may be easily raised to a temperature of 2800 to 2900 F. before pouring it into the molds.

- Instead of substantially pure chromium as the substance to be alloyed with the cast iron or steel, I may use a ferro-chrome containing a high percentage of chromium. The pure chromium is preferred, however, on account of the higher chromium content obtainable on the face of the casting.

An iron or steel casting obtainable by my new method and having as an integral part thereof an iron-chromium alloy surface concasting given in the following I taining more than 20% 'of chromium while the major portion of the casting contains less than 15% of chromium, is, as far as I know, a new article of manufacture.

Various changes in minor details may be made in the method above described without departing from the spirit and scope of my invention I claim 1. The method of making a ferrous metal casting having a chromium alloy surface layer that is strongly resistant to oxidation, which comprises coating 21 surface-in the mold in which the casting is to be formed to a depth of from" about one-eighth to onequarter of an inch with amixture composed of metallic particles comprising chromium and a binder for said particles, hardening the resulting coating, and then introducing into said mold-molten ferrous metal at a pouring temperature of from about 2650 to 2900 F., the size and disposition of said metallic particles in the coating being such as to permit ready penetration therebetween by the molten metal and solution of said particles by said molten metal.

2. The method of making a ferrous metal casting having a chromium alloy surface layer that is strongly resistant to oxidation at high temperatures, which comprises coating a surface in the mold in which the casting is to be formed to a depth of from about one-eighth to one-quarter of an inch with a mixture comprising a sodium-silicate solution and substantially pure chromium comminuted to a size between 18 and 30 mesh, baking the coated mold, and then introducing into the baked mold molten ferrous metal at a pouring temperature of from about 2650 to 2900 F. y

3; As a new article of manufacture, a fer- .rous metal casting alloyed at its surface with chromium, the latter forming a distinct layer in which the chromium content decreases from the surface toward the interior of the ferrous metal and is'sufliciently high at the surface to render the casting resistant to oxidation at temperatures up to at least 1800 F.

4. As a new article of manufacture, a ferrous metal casting alloyed at its surface with chromium, the surface layer consisting essentially of chromium and having a chromium content substantially greater than that of the interior of the ferrous metal, the percentage of chromium at the surface being sutficient to render the casting resistant to oxidation at temperatures up to at least 1800 F.

5. As a new article of manufacture, a steel casting alloyed at its surface with chromium, the surface layer for a depth of at least an eighth of an inch containing more than 25% chromium, the chromium content decreasing rapidly from said surface layer to the interior of the casting.

6. As a new article of manufacture, a steel casting alloyed at its surface with chromium, the alloyed portion forming a distinct layer having a. thickness of 1/16 to 1/4 of an inch and consisting essentially of chromium at the surface, and in which the chromium content is more than 25% at a depth of 1/16 of an inch and decreases progressively to less than 15% at a depth of 1/8 of an inch. 7

at an eighth of an inch below the surface, 30

less than 15%.

8. As a new article of manufacture, a steel casting containing chromium in the form of an alloy, the chromium content of a surface layer 1/100 of an inch in thickness being greater than 38% and at an eighth of an inch below the surface, less than 15%.

' 9. As a new article of manufacture, a steel casting containing chromium in the form of an alloy, thebody of the casting containing on an average less than 10% of chromium,

' and a surface layer 1/50 of an inch in thickness containing more than 35% of chromium,

the chromium content decreasing rapidly 9 from thesurface to the interior.

In testimony whereof I afiix my signature.

CHARLES B. JACOBS.