US1588111A - Casting having silicon-alloy surface - Google Patents

Description

Patented Jun 1926.

4 UNITED STAT S CHARLES n-zraconsor WILMINGTON, DELAWARE, Assrclvo'n TO. I. no sour-1m ATE F CE.

' NEMOURS -& compan on WILMINGTON, DELAWARE, *A CORPORATION or? DELA- WARE.

No Drawing.

lhis invention relates to ferrous metal a ferrous metal casting, such as cast iron.

and cast steel, the surface portion of which composition of the casting metal are 'ob t with silicon-or ferrosilicon in a granular.

f condition in such a mannerthat the casting metal on flowing into the mold does-not isalloyed with silicon to such an extent as to' substantially increase the resistance of said surface portion to corrosion, and especially to the corrosive action of acids.

I have discovered that a casting, in which" a surface portion, -or layer, thereof is alloyed .with a comparatively large proportlonof 51116011, can, lie-produced by bring-mg the casting-metal into contact with'suitably sub-divided silicon --or ferro'sil-icon during the casting operation, provided certain conditions with respect to temperature and served. v v g 7 Generally speaking, my new method com prises coating those surfaces-of the mold or cores of the mold inwhich tlie'casting is made, correspondingwith-the surfaces of the casting'.on which the alloy'is required,

' wash the metallic. coating from the coated surfaces, but when poured into the mold at the proper temperature flows over and melt-s v or dissolves the metallicc'oating. The skinchill or'chillingeifect exerted by the mold surfaces prevents thesilicon from diffusing v throughout the body ofthe casting metal so that an alloy is formed 'on' the'surfaceOf the castingcontaininga highpercentage of dried so that it adheres firmly to the To prevent the coating from being was'hed from the mold surfaces the flow o f'the' casting metal, the silicon in a granular; or comminuted condition is; mixed with" a suitable binderinto a=plastic mass and applied to'the mold or core surfaces and then surfaces, forming a coating containing spaces filled with binder between the silicon granules. lVhen poured: into the mold, the

molten casting metal-enters and displaces the binder from said spaces and melts 01f dissolves "the metallic particles.

'CASLIJIlNG HAVING SILICON-ALLOY SURFAC Application filed April 30, 1921. Serial-No; 465.686.

It is important toapply the coating evenly binder. For thisreason-it is necessary to ficiently large to permit the proper quantity of the particular casting metal to'enter and melt ordissolve the silicon. When granulation has been carried too far and the coating metal contains a .c0n slderable proportlon 0f materials of an extreme degreeof fineness,"say,-from 140 to- 100- mesh and finer, the coating does not.

furnish voids large enough to permitthe casting metal in sufficient volume' to sur-' round the-particles-of the coating metal and .dissolve or melt-it. An example 'ofthe' ef-,' fect ofthe eoating material in too .finea state 'of' granulation may be cited in the case of ferrosilicon and. cast iron.-

VVhen themold was coatedv with ferrosili con showing thefollowingsizing te s t-,ma-

terial passing through 18 mesh retained on 80 'mesh screen 35% by wt;, material passingg. through 30 mesh retained on -60 mesh screen 37% by wt, material passing thr'ou'ghlGO mesh 28% by' wt.-the voids of the --coating were not large enough to allow sufficient iron to enter and surround-the particles of ferrosilicon, consequently the iron. running into the mold; dissolved only athin-slgin surface of th'e-coating'and the casting produced'hadonly' a very thinlayer-of ferroa silicon on its surface which was not'cont'inu-f ous as there were spots .where theiron had failed to dissolve any ferrosilicon. On the other hand when the mold-was coated with l flferr'osilicon" sized to between 118 and 30 mesh,

madeinto; a plastic mass with the same binder used with the 'fine'r;:ma-terial,"--the finished casting poured at the-same temperature, contained an alloy high injsilicon from its surface fto. -the depth of 'helow the'surface.

The new'method as. applied to lf'cast iron may. be advantageously practiced with sili' con or-feirosilicon particleshavinga size between 15-and 35 mesh, that is, all of'the particles passing through-a 15 mesh'screen Pure crystallized: silicon melts at 2588 F.

caught on..a mesh screen case-of cast iron, particles sized, for example, between 20 and mesh. In surfacing cast iron with silicon according to my invention it is necessary to raise the temperature of'the cast iron above the melting point of the silicon in orderto obtain a true alloy onthe surface ofthe 11011.

and feriosilicon at a somewhat lower temperature, depending on the percentage of silicon it contains. A mold coated with errosilicon containing about 81% silicon, was poured at the ordinary temperature at which iron castings are usually poured, 1. e., about 2600 F. or slightly above: The metal failed to take up more than a thin coating of silicon and this coating when examined under the microscope, showed that the greater part of the silicon was composed of particles embeddedin a matrix of iron, still retaining their identity, as marked bytheir crystalline structure. However, when the iron was raised to a temperature of 2700 to 2800 F.

and then poured, the silicon was taken into solution-in the iron and a true silicon alloy a formed on the surface of the casting. -Cast steel on the other hand readily takes silicon -into solution at a temperature below 2600 Ff There are also other conditions neces-- sary for the, successful formation of silicon alloys oncas't iron and cast steel that will be described hereinafter. p

The depth to which the alloy penetrates below-the surface of the casting is very largely dependent on the thickness of the coating of the mold or core surfaces-and'in a general-way exceptfor slight general diffusion, the depth of penetration corresponds approximately with the thickness of the coating on the mold. This varies with the thickness and nature of the casting. Molds for heavy castings of thick sections which cool .slowly may be coated to a depth of "and even greater, while molds for light castings of thin sections which cool rapid-1y may only be coated to a limited depth (about 3-" to in order that the casting metal may .have time to dissolve or melt the entire coating before the casting metal begins to solidify. If the coating of the mold is too thick the resulting casting will have a pitted uneven surface with a' large part of the coating metal not forming a true alloy butsimply embedded in a matrix of the casting metal.

/ It is obvious that alloys may be formedon a, the inside surface of hollow castings-as well as onthe outside of the casting and that these alloys may be of the same character or of entirely different character on the same casting, as for example, chromium on the outside surface of aniron or steel casting to resist oxidation at hightemperature and silicon on the inside surface'to resist "corrosion by acids.

Ferrous metal castings on accountof their strength and cheapness offer the largest commercial field for this process. -When alloyed withthe proper metal or alloy to render them resistant to corrosion, by liquids or gases they serve to replacethe brittle and comparatively expensive/high silicon alloys containing from 12 to 18% of silicon used as acid-resistant metals.

My invention may be explained in more detail as follows, reference first being had to the surfacing of cast iron with silicon. f

-The molds to be used areflthe ordinary green sand molds either blackened 0r untreated in which iron castings are usually made. The silicon made intov apaste with sodium silicate .solution is applied to the mold or core surfaces and dried; baking of the mold is not necessary, ordinary skin drying being'all that is required for cast iron. A coating thick,'made from sodium-sili- 'cate solution and ferrosilicon in suitable 'prjo- --port1ons, will contain about 1 lb. of metalli silicon per-square foot.

In forming alloys; high 'in silicon on the surface of cast iron for resistance toacid it was found that when the iron was poured at theiordinary temperature at which iron castings are usually poured, i. e., about 2500- 2600 F., the metal failed to take up more mg when examined under the microscope showed that the greater part-of the silicon grains were embedded a matrix of iron and still retained their identity as marked by their crystalline structureand general contoun' By the use of the-rmit introduced into the ladle of metal after it had been tapped from the cupola it was possible to raise'the temperature of the cast iron to be tween 2800 and 2900 (pyrometer measurements). When the. metal was poured into the moldssurfaced with silicon or fer'rosi'li- 'con at thistemperature the iron melted the silicon and took it into solution. The skin 'chill of the mold prevented any great amount of diffusion of the silicon in the iron sothat an alloy high in silicon was built up on the surface of the casting to a depth corresponding substantially with the thickness ofthe silicon coating on the mold surfaces,

i. e., about on small" castings and on larger castings.

I also found that if the silicon with which the moldwas coated was too "finely divided, as before pointed out in the general discussion. it left such-small voids between the particles in the coatingthat the free caustic than a thin coating of silicon and this coatcon coating and the casting came out of the,

mold with only a very thin layer of ferrosilicon on its surface which was not continuous as there were spots where the iron had completely failed to fix any perceptible amount of silicon. It was first thought that the failure of iron to take 11 finely divided silicon was due to the action of the free caustic in the silicate of soda solution. The

caustic in the solution was therefore neutralized with boric acid. However, when the silicon was too finely divided the same general effect was obtained with the use of neutralized sodium silicate and also with the use of Glutrin binder (a product used in foundries for making cores and containing no caustic soda) as with unneutralized sodium silicate, and while this action was accentuated by the presence of the caustic soda, the behavior of too finely divided silicon in the coatings showed that in order to obtain a uniform alloy high in silicon on the surface of the casting it is necessary to maintain a certain degree of porosity in the coating of the mold by using silicon particles of the proper size so that the iron can run into'the voids and surround the particles of the 'silicon or ferrosilicon while it is at a temperature sufliciently' high to melt or take the silicon into solution.

Silicon or. high ferrosilicon sized to from 18 110 30 mesh and made plastic and adhesive to 'the inold or core surfaces with 40 B.

silicate of soda solution mixed 1 to 1 with silicon or ferrosilicon, sections cut from the.

castings, poured at a sufiiciently high temperature to obtain actual melting or soluti on ofthe silicon in the iron, show a band of demarcation containing more or less graphite carbon segregated between the highsilicon area (which has been formed on the face of the castings) and the underlying cast iron area.

In large castings of thick sections re} quiring a considerable time to solidify or set, the separated graphite may even become disseminated through the high silicon and to a sufficient extent to introduce a certain degree Such castings while much more resistant to corrosion by acids than. ordinary cast iron are not entirely satisfactory "as absolutely acid roof castings, since slow seepage of acid 1s likely to occur through-the .porous of porosity in the high silicon alloy itself.

graphite, if present'in considerable quantity, and eventually reach the body metal'belowthe highsilicon area.

In small castings or castings with thin sections which cool rather quickly this effect of the silicon on theseparation and segregation. of graphite is less marked and in most cases will not induce porosity in the high silicon area of the casting."

vThe undue separation ofgraphite by the action of the} high silicon content of the melted'iron and'the ill effects noted above may beminimized to suchan extent 'asto be harmless by conducting the casting operation' under the proper" conditions. In cast-'- ings having thick sections in which the metal requires too long a time to freeze or set, the separation of graphite may be lessened by placing chills or pieces .of metal in the mold opposite these sectionsto conduct the heat away more rapidly and induce freezing or settling of the metal while it'still retains its raphite content evenly distributed.

arbon-free ferromanganese which has "the power of absorbing and holding a large amount of carbon in the combined'state may be added to the iron in the ladle to absorb and hold the carbon from separation as graphite nduced by. the action of the silicon.

distributed through the structure of the iron by adding suflicient low carbon steel scrap to the cupola and ladle to lower the carbon con-' tent of'the iron to prevent the separation of graphite. When steel scrap is added to .the-

ladle as well as to the. cupola it is usually necessary to use thermit in the ladle to help melt the steel and keep the iron sufficiently fluid to pour properly.

Decarbu'rization of the iron can be accomioo in: I

plished by'introducing the iron tapped from the cupola. into a small Bessemer converter and blowing it for a suflicient, time to partially burn out its carbon content. If decar-,

burlzed toa suflicient extent the metal will have substantially the same carbon content I as steel but, unlike steel, will still'contain most of the impurities existing in cast iron.- 1 Operating under the proper conditions .above indicated, satisfactory .high silicon alloys resistant to corrosion by various gases and liquids maybe formed 0 n the surface of cast iron.

1 The character-of the silicon alloy formed in the steel lowers the melting point of the witha body metal of gray cast iron is indicated bythe percentages of silicon, given in the following table, at'various distances from the face of the casting Location of sample. Per cent silicon.

. 2. Drillings 1 from face of cast-ing 2. 68

3. a 1%? a: a: 7 7: 2.56 4. Gray cast iron (analysis of test bar) 2.39.

corrosive action of the above-mentioned" acids.

Microscopic examination of sections cut 'fromthe casting on a line perpendicular to 4 the alloyed surface also shows the gradation of thehigh silicon alloy on the surface of the casting into the. cast iron body below.

The samples were polished in the usual manner and then etched for 15 seconds with a solution prepared by adding 4'cc. of nitric "acid, sp..gr. 1.42 to 96 cc. oft-5% grain alcohol. The high silicon area was/found to extend toa depth. of 1/ efan inch from the edge of the sample indicating that the silicon content to thisdepth was above 10%.

- fIn addition to being resistant to acids castings 'made according .;to my invention .operation however should be conducted at a much lower temperature than was used' -for siliconon cast iron and certainprecautions must be-observed before highsilic'on have all of the inherent strength of gray cast iron by virtue of the underlying body metal, whereas ferrosilicon containing as little as 14% of siliconcannot be machined,

is so weak and brittle as to narrowly limit the'size and shape ofcastings into which it can be formed, and has such high shrink- .age that castings thereof are frequently marred by checks and cracks. By my invention silicon alloys may be formed on,

and to a. considerable depth below, the surface on any casting that may be made of gray cast iron irrespective of size or shape.

In forming silicon alloys on 'caststeel, the preparation of molds, application of .the silicon or. ferrosilicon, and the use of rbinders is substantially the same as in the case of silicon on cast iron. The casting alloyscan be built cast steel. a

Silicon is muchmore soluble in steel than inrcast iron and when taken into solution up on the surface of metal to amarked degree 'so that while re mainingat the same temperature, steel to which silicon has been added becomes much more fluid and requires a longer period to freeze or set. This-well known effect of steel is taken advantage of in the open hearth steel process of making low carbon steel for casting purposes. The same increased fluidity is produced in the steel poured into a mold coated with silicon or ferrosilicon. The metal while remaining the same temperature is so much more flui that it requires a longer cooling period to reach its freezing or setting point, and un-' less precautions are taken to increase the rate of cooling, lower the temperature of the casting metal, or slow up the rate. of solution of the silicon in the steel, diffusion of the dissolved silicon will take place throughout the steel before the chilling effect of the ordinary mold'surfa ces can act to face of 'the casting next the molds. For example steel from a heat of the following analysis:

. Per cent. Carbon I24 Manganese .65

'- Silicon .289 Phosphorous .038 Sulphur"; .037

was poured into a mold forproducing a casting 2" in thickness. The mold, coated on the face to a depth -of 4 of an inch with 93% ferrosilicon sized to from 18 to 30 -mesh, was takenfrom the baking oven at a temperature of 650 F. and immediately closed and cast. The temperature of the steel as it was poured into. the mold was 2660 ll .;(i. e. about the usual temperature of pouringlow-carbon open hearth steel cast ngs). An analysis of the face of the casting corresponding withthecoated surface of the mold showed 1.75% silicon while the back ofthe casting showed 1.01% ,silicon. The silicon rapidly dissolved from the build up a high silicon'alloy'on the surcoated face of the mold and taken into solution in the incoming steel lowered the freezing point of the metal to such an extent that almost complete and uniform diffusion of the silicon took place throughout the whole body of the casting before the skin chill of the mold could solidify the metal.

entering the hot moldat 2660 F. and build up a high silicon alloynextthe mold su r- 1 faces.

' To successfully-produce high silicon alloysv on the surface. of lowcarbon' steel castings it is necessary to pour the casting at as low a temperature as it is possible to pour the metal and have it run to all parts of the mold in question, A temperature of 2600 or as much below that as the nature of the casting will permit usually gives goed re 70 silicon in lowering the melting point of sults, especially if the mold has been allowed to stand and cool to room temperature before the casting is poured. p

In castings with thick sections where the metal would naturally cool slowly it is. ad-

visable when the nature of the casting will permit'to set chills, (i. e. blocks of steel) in the mold opposite these sections to increase the rate of cooling induced'by the mold surfaces.

-For steel castings, especially with low.

carbon steel, the silicon or ferrosiliconwith which the molds are coated .should be reduped toa much finer state of division than is the case with cast iron in order to furnish smaller voids for the steel to enter and thus slow up the rate of solution of the silicon next the mold surfaces.

at a fairly low temperature.

in the steel. In the example pointed out above ferrosilicon sized to from 18 to 30 mesh gave quick solution and almost complete diffusion of the silicon throughout the whole area of the casting even when poured Silicon or ferrosilicon sized from 20 to 60' mesh and even finer offers smaller voids in the coating forthe steel to enter and slows up the rate of solution sufficiently to enable the chilling effect ofthe mold to make itself felt before dissemination of the silicon throughout the body of the steel has occurred, with the result that an alloyhigh in silicon is built up Green sand molds skin driedbut notbaked may also be used to increase the chilling action of the mold surfaces and prevent too rapid diffusion of the silicon in the steel. The,danger.0f porous castings which frequently occur-when ordinary steel castings are made in green sand molds is obviated in thiscase by the tight, close-grained structure of the high silicon alloy on the surface of the castings next to the mold surfaces, and the skin drying of the mold surfaces.

Certain types of steel castings, the body metal of which it is, desirable 'to make of soft easily machinable steel having a high silicon alloy on the surface, are frequentlyof such shape that they do not lendthemselves to the method of using chills in the mold and of such a character, on account of thickness of section,fetc., that the chilling effect of green sand molds alone does not sufliciently check the solution and diffusion of the silicon or ferrosilicon in the steel to cause the formation of a silicon alloy of the desired silicon content on the surface of the castings, With castings of thistype I with silicate of soda solution for surfacing the molds gives the desired results. On cast-l ings of greater thickness of. section, xas for example, two inches or more section, it is necessary to'add a greater proportion of charcoal to" the ferrosilicon. As muchas from 20 to 30'% has been found to glve desirable results.- Advantage is taken here of the well known fact that moltensteel will dissolve a considerable amount of carbon in the form of'charcoal. Thissame action takes place in the mold during the casting operation and the solution of the carbon in the steel slows up the solution and diffusion of the silicon, and causes the formation of a high silicon alloy on the surface of the casting. The penetration of the carbon in the steel is confined substantially'to a depth corresponding to the depth of the coating 2f ferrosilicon andcarbon 0n the mold. surace.

Operating 9 under the proper conditions and observing the precautions necessitated by the character of the casting in question, silicon alloys maybe formed on the surface and to a depth of A; of an inch and more below the surface of steel castings containing from 30 to 50% or more. of silicon at the skin surface and grading off to low silicon content where the high silicon alloy merges into the steel body metal of the casting.

Sections cut from such castings will show thesame bonding of the high silicon, area above and the cast steel below that is shown by chromium alloyed on cast steelf In the merging band and in the high silicon .area no graphite will show as it did in the case of silicon and cast iron. The high silicon area composed of silicon and ferrite' will show a close grained dense structureand the merging band will show traces-of this structure together 'With'the ferrite and pearlite of the underlying steel body metal of the casting.

Castings made by the formation of high silicon alloys on the surface of cast steel have the acid proof qualities of high ferrosilicon and the inherent strength of cast steel and can be made into shapes and sizes for resistance to acid not possible with ferrosilicon on account of its brittleness and lack of mechanical strength.

As has been indicated above, castings having the desired properties can be made starting either with pure silicon or with ferrosilicon. In practicing the invention on a commercial scale, pure silicon is eliminated as a starting material, at the present time because of its high' cost. The ferrosilicons which I prefer t6" use are those high in silicon, although the silicon content may vary within wide limits. As a rule a ferrosilicon having 75% or more of silicongives good the artto which this invention relates. various changes in details may-be made in the method above described without departing from the spirit-and scope of my invention.

I claim v 1. The method of making a ferrous metal casting having a surface strongly resistant to the corrosive action of acids, whichcomprises coating a surface of the mold in which the casting is to be formed with a mixture of' ferrosilicon particles having sizes between 15 and 40 mesh and a binder therefor, hardening the coating, and then introducing into said mold molten ferrous metal at a temperature at which substantially all ofv the particles of ferrosilicon will dissolve therein.

2. The method of making a ferrous metal casting having a surface strongly resistant to the corrosive action of acids, which comprises coating a surface of the mold in which the casting is to be formed with a mixture of ferrosilicon particles containing more than 7 5% of silicon and having sizes between about 15 and mesh and a neutralized so-' dium silicate solution, drying the coating,

and then introducing into said mold molten ferrous metal at a temperature'at which said ferrosilicon particles will dissolve therein,

, said temperature being low enough to allow solidification tooccur while. the dissolved silicon is still concentrated near the surface cast iron at a temperature of from' about 2700 to 2900 F., the size of said particles of the casting.

. 3. The process of forming a silicon alloy on the surface of cast iron which comprises coating a surface of the mold in which the casting is to be formed with a mixture con-- taining ferrosilicon particles and a liquid 'binderl therefor, hardening the coating, and then introducing into said mold molten cast iron at a temperature above 2650 F., the

' size of said particles bein such as to permit ready penetration there molten cast iron.

4. The process of forming a silicon alloy on the surface of cast iron which com rises coating a surface ofthe mold in whic the casting is to be formed with a mixture containing ferrosilicon particles and a hquid binder therefor, hardening the coating, and then introducing into said .mold molten etween by said being such as to permit ready penetration therebetween by said molten cast iron,

5. The process of forming a silicon alloy on the surface ofcast iron which comprises coating a surfaceof the mold in which the casting is to be formedwith a mixture containing ferrosilicon particles sized between about 15' and 35 mesh and a liquid binder therefor, hardening the coating, and then introducing into said mold molten cast iron" at a temperature of from about 2700 to. 2900 F.

" 6. The process of forming a silicon alloyon the surface of cast iron which com rises coating a surface of themold in whic the casting is to be'formed with a mixture containing ferrosilicon particles having more than 75% of silicon and sized between about 15 and .35 mesh'and a binder for said particles capable of holding them on the surface of the mold, hardening the coating, and then introducing into said mold molten cast iron at a temperature suflicient to enable said iron to penetrate between, and dissolve, substantially all of said particles.

7. The pr'ocess of forming a silicon alloy.

iron at a temperature of from about 2800 to 8. The process of making a steel casting having a silicon alloy surface,,which comprises coating a surface of the mold in which the casting'is to be" formed with a mixture containing metallic particles comprisingsilicon of sizes between about 20 and 60 mesh and a liquid binder therefor, hardening said coating, and then introducing into said mold molten steel at a temperature near its solidifying point to permit solidification thereof before a major portion of the silicon which is dissolved thereby has become dispersed throughout the body of the casting.

9. The process of making a steel" casting- 'ihaving a silicon alloy surface, whichcom prises coating a surface of the mold which the casting is to be formed with a ing said coating, and then introducing into said mold molten steel at a temperature of from about 2575 to 2650 F.

10. The process of making a steel casting having a silicon alloy surface, which comprises coating a surface of the mold in mixture contaimng finely divided ferrosilicon and a liquid binder therefor, harden-o which the casting is to be formed with a mixture containing comminuted ferrosilicon sized between about 20 and 60 mesh anda liquid binder therefor, hardenin said coating, and then introducing into sai mold molten steel at a temperature near its solidifying point to, permit solidification thereof before a major-portion of the silicon'which is dissolved thereb has become dispersed throughout the loo y of the casting.

-11. The process of maln'ng-a steel casting having a silicon alloy surface, which comprises coating a surface of the; mold in which the casting is to be formed with'a mixture containing comminuted ferrosllicon sized between about 20 and mesh and having more than silicon and a sodium silicate solution, hardening said coating, and then introducing into said mold molten steel at a temperature of from about 2575 to 2650 F.

12. The process of making a steel casting having an iron-silicon alloy surface in the form of a surface layer containing a large percentage of silicon, the body of the casting having a relatively small percentage of silicon, which comprises coating a surface of the mold in which the casting is to be formed with a mixture comprising carbon in a form readily soluble in molten steel, metallic particles comprising silicon, and a binder for said particles, allowin said coating to harden, and then introd ucing into said mold molten steel at a temperature at which the carbon will be effective to prevent the silicon in said coating from diffusing freely throughout the body ofsteel.

13. The process of making a steel casting having a silicon alloy surface, which comprises coating asurfa'ce of the mold in which the casting is to be formed with a mixture containing carbon in a form readily soluble in molten steel, ferrosilicon of sizes between about 20 and 60 mesh and a liquid binder therefor, hardening said coating, and then introducing into said mold molten steel at a temperature near its solidifying point to permit solidification thereof before a major portion of the silicon which is dissolved thereby has become dispersed throughout the body of the casting.

14. 'The process of making a steel casting having a silicon alloy surface, which comprises coating a surface of the mold in which the casting is to be formed with a mixture containing carbon in a form readily soluble in molten steel, finely divided ferrosilicon and a liquid binder therefor, hardening said coating, and then introducing into said mold'molten steel at a temperature of from about 2575 to 2650 F.

15. As a new article of manufacture, a ferrous metal casting having silicon alloyed therewith at the. surface, the silicon content decreasing from a value of more than 30% at the surface to a value of less than 10% at three-eighths of an inch from the surface.

16. As a new article of manufacture, a ferrous metal casting associated at its surof the remainder of the.casting being substantially less than 20%1 v 18. As a new article of manufacture, a cast iron casting having silicon alloyed therewith at the surface, the silicon content of a surface layer one-eighth of an inch thick being about 30%, and the silicon content of the remainder of the casting being substantially less than20%.

19. As a new article of manufacture, a steel casting associated at its surface with silicon, substantially all of said silicon being alloyed with the steel of the casting, the silicon content being more than 20% at the surface and decreasing toward the interior.

20. As a new article of manufacture, a steel casting associated at its surface with silicon. substantially all of said silicon being alloyed with the steel of the casting, the silicon content being more than 20% at the surface and decreasing toward the interior, the alloy surface of the casting being obtainable by limited diffusion of silicon into the cast steel while the latter is in a molten condition.

21. As a new article of manufacture, a steel casting having silicon alloyed therewith at the surface, the body metal of the casting being of soft easily machinable steel, most of said silicon being concentrated in'a surface layer of about one-quarter of an inch or less in thickness, and said surface layer having a substantially greater carbon content than has the body of the steel casting.

22. As a new article of manufacture, a steel casting having silicon alloyed therewith at the surface, the bodymetal of the casting being of soft easily machinable steel, most of said silicon being concentrated in a thinsurface layer, and said surface layer having a substantially greater carbon content than has the body of the steel casting. In testimony whereof I aflix my signature.-

CHARLES B. JACOBS.