US3115421A

US3115421A - Hot dip coating

Info

Publication number: US3115421A
Application number: US84712A
Authority: US
Inventors: Harvey W Seymour
Original assignee: American Chain and Cable Co Inc
Current assignee: FKI Industries Inc
Priority date: 1961-01-24
Filing date: 1961-01-24
Publication date: 1963-12-24
Anticipated expiration: 1980-12-24

Description

Dec. 24, 1963 H. w. SEYMOUR HOT DIP COATING 4 Sheets-Sheet 1 Filed Jan. 24, 1961 To Take Up FIG. 2

INVENTOR Harvey W. Seymour ZM] r f/ ATTORNEYS Dec. 24, 1963 H. w. SEYMOUR 3,115,421

HOT DIP CoATING Filed Jan. 24, 1961 4 sheets-sneetz /f ll INVENTOR Harvey W. Seymour ATTORNEYS Dec. 24, 1963 H. w. SEYMOUR 3,115,421

HOT DIP COATING Filed Jan. 24, 1961 4 Sheets-Sheet 3 FIC-3.7

FIG. 5

To Take Up l 1* y INVENTOR Harvey W. Seymour ATTORNEYS Dec; 24, 1963 H. w. SEYMOUR HOT DIP COATING 4 Sheets-Sheet 4 Filed Jan. 24, 1961 lll uw O ..4 wf,... w n .Vnnunm e. mw F @m.. w .da

lNVENTOR Harvey W. Seymour United States Patent ghan dr Cable Company, Inc., a corporation of New Filed Jan. 24, 1961, Ser. No. 84,712 Claims. (Si. 117-114) This invention relates to applying metallic coatings to base metals by the hot dip method, that is, by immersing the article to be coated in a molten bath of the coating metal and then withdrawing it from such bath. The invention is particularly directed to the invention of an irnproved method for maintaining the bath of coating metal in the molten condition and as free as possible from undesirable contaminants by keeping the bath in a quiescent condition and free of contact with metallic furnace and heating element components. An important object of the invention is to provide a method which can be used eiciently and economically for maintaining a bath of molten aluminum in good condition for hot dip coating of steel and other ferrous metals with aluminum. This application is a continuation-impart of my co-pending application entitled Hot Dip Coating, Serial No. 673,019, led Iuly'l9, 1957, now abandoned.

The :attainment of these results is achieved in accordance with the invention by establishing and maintaining the molten bath of coating metal in a crucible composed of refractory material which is inert to attack by the coating metal, and supplying the heat necessary for maintaining the bath in the molten condition Wholly by conduction or radiation through a thin-walled refractory heating element immersed in the molten bath or radiation from a heating element removed lfrom the bath. In this manner, the molten bath of coating metal is kept out of contact with metallic and other furnace components which might react with it or otherwise contaminate it. Moreover, the 'bath is thereby maintained in as quiescent condition tas possible, with the result that impurities which are adventitiously introduced and which form a separate phase of different density than the molten coating metal can separate from the main body of the bath under the influence of gravity and without interference by substanstantial convection currents within the bath.

In most hot dip coating operations the molten bath of coating metal is contained in a metallic pot, usually of steel or iron, and the metal in the pot is heated by combustion of a fuel beneath the pot. The hot combustion gases impinge directly on the bottom surface of the pot, and heat is transferred tot the molten coating metal by conduction through the wall off the pot. Such apparatus has the virtues of simplicity and low initial cost, and is very satisfactory for use in hot dip coating with low melting point metals which `are not particularly reactive at the coating temperature, such as lead and tin. Such apparatus also has Ibeen extensively (but less economically) used for coating ferrous metals with Zinc (galvanizing), though zinc (attacks the iron of the pot and consequently the life of cast iron direct-heated galvanizing pots is relatively short. For coating steel with aluminum, direct-heated cast iron pots are quite unsuitable, because the melting temperature of aluminum is considerably higher than that of zinc, and aluminum reacts with iron to form an intermetallic iron-aluminum compound much more rapidly at taluminizing temperatures than does Zinc at ordinary galvanizing temperatures.

Because of the mpracticability of using direct-heated iron pots for containing a bath of molten aluminum, the molten metal for hot dip aluminizing operations has heretofore most commonly been contained in a crucible of refractory material and has been heated therein by induction heating equipment. The refractory Crucible is made 3,115,421 Patented Dec. 24, 1963 ICC of material which is substantially inert with respect to molten aluminum, and the induction heating method avoids bringing the aluminum into contact with any metallic components which might be attacked by it or which might contaminate it. It has been found in actual experience, however, that use of induction heating of the aluminum bath in hot dip aluminizing operations is subject to many disadvantages. Induction heating by its very nature induces powerful convection currents within the metal bath, keeping it well agitated. Indeed, the design of induction heated aluminizing pot furnaces has generally taken advantage of this fact by locating the induction heating coils about channels provided in or near the furnace walls and through which the molten metal circulates from and to the main body of the bath.

Vigorous circulation of the molten aluminum in hot dip aluminizing is objectionable for a number of reasons. In the rst place, some air-oxidation of the molten metal in the pot is unavoidable, and the rapid circulation of the metal keeps the oxide mixed with the coating metal, rather than allowing it to be skimmed off as a dross. As a result, wires, sheets, or other articles coated by passing them through the agitated aluminum receive a coating contaminated with solid inclusions of aluminum oxide. Furthermore, contamination of the bath with iron is an unavoidable consequence of immersing ferrous articles to be coated in the bath. Iron dissolved from such articles forms in iron-aluminum compound which tends to segregate as a separate phase from the molten aluminum. Although it is appreciably heavier than the molten aluminum, it is unable to separate from the aluminum by settlement when the aluminum bath is being rapidly circulated by induction heating means. Induction heated furnaces, therefore, result in the ultimate build up of an objectionably high concentration of iron in the aluminum coating formed on articles passed through a bath of aluminum maintained molten in such a furnace.

The present invention attains the advantages of heating the coating metal under conditions which maintain the bath substantially quiescent (as in a direct-fired holding pot), while at the same time providing the advantages incident to confining the metal in a refractory Crucible containing no components which are attacked by the molten bath metal or tend to contaminate it. The method of the invention involves introducing the coating metal into a melting Crucible of non-metallic refractory material, heating the metal in the Crucible to above its melting temperature, passing the ferrous article to beneath the surface of the molten metal and withdrawing it therefrom with an adherent coating of said metal, and maintaining the metal at such temperature while dip coating the ferrous article by supplying heat thereto substantially entirely by at least one of conduction and radiation from heating means free of metallic components in direct contact with the metal. Thereby contamination of the bath of molten metal by metallic furnace and heating element components is avoided. Also, the method involves maintaining the bath sufficiently quiescent whereby contaminants introduced otherwise and which differ in density from the bath metal are enabled to separate from the main body of the bath metal under the influence of gravity and do not enter the coating on the article.

In one embodiment of the new method, the step of maintaining the metal at melting temperature while dip coating the ferrous article is carried out by supplying heat thereto wholly by conduction through a non-metallic refractory heating element maintained in direct contact with the metal. Alternatively, this step may be carried out by supplying heat to the molten metal substantially entirely by radiation from a heating element spaced from said molten metal, In either case, the bath is kept free of contaminants from metallic furnace and heating element components and can be maintained quiescent so that contaminants otherwise introduced can settle out.

The invention is particularly applicable to establishing and maintaining a molten aluminum bath for hot dip coating of ferrous metals with aluminum. It may however be used with advantage in other types of coating or treating operations, such as in galvanizing operations or in the high temperature patenting of steel wires in molten lead. The invention is particularly adapted for the continuous coating or treatment of articles of indefinite length, such as wires and sheets, by continuously advancing such articles through the molten metal of the bath.

Advantageous embodiments of the invention are described below with reference to the accompanying drawings, in which FIG. 1 is an elevation of a furnace in which a molten bath of aluminum is maintained, together with associated apparatus for the continuous aluminizing of one or more Steel wires;

FIG. 2 is a plan of the furnace shown in FIG. l;

FIG. 3 is a section taken substantially along the line 3 3 of FIG. 2;

FIG. I4 is an enlarged :cross section through the upper end of a heating tube of the character shown in FIGS. l to 3;

FIG. 5 is an elevation of a modified form of furnace and heating means for maintaining a bath of molten aluminum, together with associated apparatus for continuous aluminizing of steel wires;

FIG. 6 is a plan of the furnace and heating means shown in FIG. 5;

FIG. 7 is la sectional elevation on an enlarged scale of the heating means shown in FIGS. 5 and 6 g FIG. 8 is a plan of a radiant heating type furnace adapted to carry out the new method; and

FIG. 9 is a section taken substantially along the line 9-9 of the apparatus of FIG. 8.

The apparatus sho-wn in FIGS. 1 to 4 comprises a pot type furnace 10 for containing a bath of molten aluminum. The furnace comprises a steel shell 11 reinforced by ribs 12 and lined with several layers 13, 14 and 15 of heat insulating and refractory material. At least the innermost refractory layer 15 is of a material which is substantially unaffected by contact with molten aluminum at temperaturen of l200 F. to 1400o F., such, for example, as graphite or an aluminum oxide refractory. The refractory forms a crucible in which a body of molten aluminum 16 can be established and maintained.

A cover plate 17 overlies the furnace and the upper surface of the refractories. It is formed with a central opening above the central crucible in which the molten aluminum bath is received, in order to give access to the bath.

Heat required for maintaining the metal of the bath in the molten condition is supplied to it by conduction through the walls of refractory heating tubes 18. These heating tubes are immersed in direct contact with the metal of the bath, and advantageously extend from above the cover plate 17, through openings formed therein, to the bottom of the crucible in which the molten metal is contained. The bottoms 19 of the heating tubes are closed and, in the apparatus shown in FIGS. l to 3, are supported on the refractory bottom of the furnace crucible. The crucible itself is shown as being of T-shaped outline, and the heating tubes are positioned in recesses formed where the cross-bar of the T extends beyond its central leg.

The upper end of each refractory tube 18 is received in a socket fitting 20, which in turn is secured to the furnace cover plate 17 by studs 21 and is urged downwardly toward the cover plate by compression springs 22. The heating tubes 18 thus are held immersed in the molten metal in the furnace crucible despite their buoyancy therein, while the springs 22 accommodate motion due to thermal expansion of the heating tubes.

A branch pipe fitting 23 is secured to the upper end of the socket 20. A combustion tube 24, which may be composed of a refractory metallic alloy, extends through the straight run of the branch fitting 23 and through the heating tube 18 to near the closed bottom 19 thereof. The combustion tube is held centered in the heating tube by spacers 25 welded to it at its lower end, and it is supported in place by being welded or otherwise fastened to an apertured plug 26 which is screw-threaded into the upper end of the branch fitting 23.

A T fitting 27 is secured to the upper end of the combustion tube 24, above the branch fitting 23. Air under pressure from a compressed air source is admitted to the combustion tube through a pipe 28 connected to the side leg of this T, and a gas inlet pipe 29 extends into the combustion tube through the straight run of the T. The lower end of the gas inlet pipe terminates at a burner nozzle 3b, where fuel gas admitted through the gas inlet pipe mixes with air admitted through the air pipe 28.

The construction of the burner nozzle 3i) is best shown in FIG. 4. It comprises a metallic shell 31 which at its upper end is screw-threaded to the gas inlet pipe 29. The shell is configured to define an internal Venturi section 32 just below the point of its connection to the gas inlet pipe. Openinvs 33 adjacent the throat of the Venturi permit air to flow into the burner nozzle from the annular space between it and the combustion tube 24. The mixture of air and gas thus formed below the throat of the Venturi is ignited in the lower section of the nozzle, which advantageously is lined with a refractory composition 34 to protect the nozzle shell and to define the gas exit form of the nozzle.

Hot combustion gases flow downwardly from the nozzle through the interior of the combustion tube 24, out its lower end, and upwardly through the annular space 35 between the refractory heating tube and combustion tube. Thereby the interior surface of the refractory heating tube 18 is heated to a high temperature, and heat flows by conduction through the Wall of the heating tube into the bath of molten metal in which the heating tube is immersed. The combustion gases then fiow out through the branch of the fitting Z3, and through an exhaust pipe 36, from which they are directed by a hood 37 into a flue 33 for carrying them to the open atmosphere.

The apparatus shown in FIGS. 5, 6 and 7 differs from that described above only in details of construction of the furnace vessel and the heating tube assemblies. It comprises a pot type furnace 40 defining a crucible for containing a bath of molten aluminum 41. This furnace, like that described above, comprises a steel shell 42 lined with refractory material 43 which may be laid down in several layers. At least the innermost layer of refractory is a material which is inert with respect to molten aluminum. A cover plate 44 overlies the marginal portions of the upper surface of the furnace.

The molten aluminum of the bath 41 is heated by conduction through the walls of immersion heating elements 45. These heating elements comprise tubes 46 of refractory material which are immersed in direct contact with the metal of the bath. The tubes 46 are secured to head assemblies 47 which in turn are supported on brackets 47 projecting over the furnace crucible from the cover plate 44. The tubes 46 are closed at their lower ends, and are made of refractory material of good heat conductivity which is resistant to attack by molten aluminum. Each tube may with advantage be of oval or elliptical cross section, and is divided internally by a longitudinally extending partition 48 which preferably spans its minor axis. The partition 4S extends from the upper end of the tube to a point a short distance above the closed bottom, and divides the tube interiorly into a combustion chamber 49 and a iiue chamber 50.

An air inlet pipe 51 extends into the upper end portion of the combustion passage 49. Combustion air from a blower or other compressed air source is supplied to this pipe through the side leg of a T fitting 52. A gas inlet pipe 53 extends through the straight run of the T and through the air inlet pipe into the upper end portion of the combustion passage 49 0f the heating tube. Fuel gas supplied through the gas inlet tube 53 mixes with air delivered through the air inlet pipe 5l, and the resulting combustible mixture is ignited at the discharge end of the gas inlet pipe. (Electrically energized ignition means not shown may be provided for igniting the mixture initially or in the event the Harrie is inadvertently extinguished.) The hot combustion gases flow downwardly through the combustion passage 49, around the lower end of the partition 4.8, and up through the fue passage S0, thereby heating the interior surface of the heating element tubes 46. The combustion gases pass out from the liuc passage at its open upper end. A deecting hood 54 is provided to protect the air and gas inlet pipes from the emerging still hot combustion gases; and a collecting hood, similar to that shown in FIG. 3, may also be provided for directing these gases into an exhaust ilue by which they are carried into the open atmosphere.

The two forms of furnace described above are shown in FIGS. 1 and 5, respectively, in association with means for applying a coating of aluminum to one or more steel wires w. The wire is passed continuously over a guiding sheave s, down through the molten metal bath in the furnace crucible to a sinker roll r, thence vertically upwards to a head sheave h, and thence to a coiler or other take-up mechanism. The wire is suitably prepared for receiving an aluminum coating prior to passage over the guiding sheave s, and the aluminum coating which is formed thereon in consequence of its immersion in the bath of molten aluminum is cooled to below its freezing point in the course of its vertical upward movement to the head sheave h. The manner in which the wire is handled preparatory to, during and after application of the aluminum coating is conventional, and is not a part of this invention.

All parts of the furnace crucible and immersion heating tubes which come in Contact with the molten aluminum coating bath are of non-metallic refractory material. The refractory of which the furnace crucible is formed may be selected primary for its ability to resist attack by molten aluminum at the temperature of the bath. Thus it is not a source of contamination of the bath, and can be constructed so as to require a minimum of maintenance over a long useful service life. The heating tubes likewise should be made of a refractory material which is not subject to attack by molten aluminum at the bath temperature; and in addition they should be composed of a refractory which possesses good heat conductivity. Various materials meet these requirements, but the refractory comprising silicon carbide bonded into a unitary structure by silicon nitride is particularly satisfactory. It is substantially completely inert to attack by molten aluminum, and it is not affected by atmospheric oxidation at the temperatures to which it is exposed. Moreover, it is mechanically strong and is a fairly good conductor of heat. Heating tubes of silicon nitride bonded silicon carbide thus form effective heating elements for the bath of molten aluminum and do not constitute a source of contamination of the bath .or of serious maintenance problems or expense.

The foregoing description of FIGS. l through 7 concerns :the embodiment of the new method of the invention wherein the batlh is heated by conduction from a refractory heating element directly to the molten bath. ln

FIGS. 8 and 9, further furnace and heating means is At least the inner refractory layer 64 is resistant to attack by molten aluminum at operating temperatures of from l200 IF. to 1400" F. and graphite or an aluminum oxide refractory is suitable for this purpose. The marginal portion of the furnace 60 is defined by an edge plate 67 completely surrounding its sides. The width of the edge plate 67 is such that it overh'es :the walls of the furnace and leaves unobstructed virtually the entire surface of the bath of aluminum 66.

Spanning the edge plate 67 and covering all of the surface of the bath of salu-minum 66 but for a relatively narrow end portion thereof (FIG. 8) is a hood structure 69 of composite thickness. As shown in lFIG. 9, the hood structure comprises a metal frame 70 and an inwardly extending ilange portion 71 at its bottom edge. Over the outside of the frame 70 is a plurality of refractory elements 72 fitted between ribs extending from the frame. On the inside `surface of tihe frame 70,

several layers

73, 74 and 75 of refractory material `are mounted in place and supported principally by the fange 71 of' the frame. These refractory layers, and particularly the innermost layer 75, are also resistant to temperatures up to about `1400 fF.

The entire hood structure `69 constitutes a radiant heating element spaced in opposed relation to the greater part of the surface of the bath of aluminum 66. In addition, the hood defines `a Acombustion chamber ove-r the bath. At one end of the hood structure, suitable burners 77 eX- tend inwardly into the chamber to supply a combustible air-fuel mixture therewithin. At the opposite end of the hood, la iiue 78 communicates with the interior of the chamber and provides an exi-t for the burned combustion gases.

Adjacent the hood structure 69, means for applying a coating of aluminum to one or more steel Wires l80 are located. A wire Sii may 'be passed over a guide roller 8l (FIG. 8) adjacent the furnace, down through nhe molten metal bath 66 to `a sinker roll 82, and thence generally upwardly to a suitable coiler or take-up mechanism. The sinker roll 82 is supported by a suitable frame 813. As in the previous embodiments, the wire is prepared for receiving its aluminum coating prior to immersion in the bath of molten metal, `and las it passes upwlardly from the bath the adherent coating of aluminum is cooled to below its freezing point.

-In carrying out the method of the invention by the means shown in FIGS. 8 and 9, the bath of molten aluminum 66 is maintained at the proper temperature by heat supplied thereto from the burning fuel Within the hood structure 69. By far the greatest amount of the heat given olf by the burning fuel is absorbed by the

layers

73, 74 and 75 of refractory material on the inside of the hood frame 7 ti. Asa result, these layers become quite hot and radiate heat downwardly to the opposed surface of the bath y66. Of course, some of the heat from the fuel is transferred directly to 4the bath 66 by conduction but its amount is insignificant relative to the amount of heat radiated from the hood structure 69. tI-t often proves advanfnageous to cover the surface of `the bath of molten aluminum 66 with a floating layer of refractory material 84 possessing considerably higher heat emissivity than the molten aluminum surface in order to prevent a substantial amount of `the radiant heat from being reflected from the bath. Various refractories may be used for the layer 34 so long as they are buoyant land inert with respect to molten aluminum at a temperature up to about l4010 F. and also possess the necessary higher heat emissivity.

With the heating element spaced from the bath of molten aluminum, contamination o-f `the bath by metallic components of the heating means is rendered impossible. Only non-metallic refractory material is in direct contact with the bath and it can be sufficiently inert to attack by the molten aluminum to prevent any contamination of the bath from the furnace structure.

lIn each lof the foregoing embodiments, it is of course impossible to prevent contamination of the molten aluminum bath by atmospheric oxidation and by dissolution of iron from the wire or other felrous metal being coated. Atmospheric oxidation of the hot meta-l results in formation of aluminum oxide. This material is of lower density than metallic aluminum, and tends to float to the surface of the bath.y An important advantage of the invention is that the baths in all of the previous-ly described structures are maintained substantially quiescent, for heating all lof Ithe baths by conduction or radiation does not set up very strong convection or other currents, As a result, such oxidation of the aluminum as inadvertently occurs results merely in ,the formation of Ian oxide dross which collects at the surface of the metal and can easily be kept 4clear of the lvsn're being coated and can be skimmed H whenever an Iappreciable amount has accumulated.

lIron dissolved from the article being coated tends to form a segregated intermetaliic compound of iron and aluminum having a melting temperature above that of the aluminum bath itself, The iron in excess of the solubility in aluminum tends to collect fin a solid phase, the density of which is somewhat greater than that of the molten metal. Here again, the relatively quiescent cond-ition in which the bath is maintained by conduction or radiation in accordance with this invention is an advantage `of major importance, for it permits the segregated iron-aluminum compound to settle to the bottom of the cnucible, whence it can from time to time be removed. Thus, despite the continual accumulation of iron in the bath from the article being coated, the content of iron in the aluminum deposited as a coating on the article is kept from building up to an objectionable extent.

The method of the invention is particularly advantageous for use in coating ferrous articles with aluminum because of the corrosive effect of molten aluminum or metallic melting pots, and because of the susceptibility of a molten aluminum coating bath to objectionable contamination by iron and aluminum oxide whenever it is subjected to forces promoting vigorous bath circulation. The invention, however, is not limited in its applicability to aluminum coating operations. It can, for example, be applied with advantage to galvanizing operations. While an iron melting pot is more resistant to molten zinc at conventional galvanizing temperatures than it is to aluminum at aluminizing temperatures, it is not inert to Such attack, and the replacement of cast iron melting pots is an important element of maintenance cost in hot dip galvanizing operations. Such costs can be minimized, and improvements in the purity of the zinc coating can be achieved, by maintaining the zinc coating bath in the molten condition by the method of this invention. Again, the invention has an important field of applicability in high temperature patenting of steel wire by immersion in molten lead maintained at a temperature far above its melting point. At and near its melting point, lead does not seriously attack steel or iron, and it is ordinarily melted in steel or iron pots. At a temperature of 1500 F., however, lead exerts a marked corrosive effect on such melting pots. The cost of frequent replacement of the pots, and the contamination of the lead in consequence of its corrosive attack on the pot, are major factors in limiting the usefulness of high temperature lead patenting operations. By use of the method of this invention for maintaining a bath of molten lead at an elevated temperature, these elements of expense can be substantially eliminated from such patenting procedures, thus making them more generally attractive and useful.

I claim:

l. The method of hot dip coating a ferrous article with a coating metal for iron and iron alloys which comprises introducing said coating metal into a melting crucible of non-metallic refractory material, heating said metal in said crucible to above its melting temperature, passing the ferrous article to beneath the surface of the molten metal and withdrawing it therefrom with an adherent coating of said metal, maintaining said metal at said temperature while dip coating the ferrous article by supplying heat thereto substantially entirely by at least one of conduction and radiation from heating means free of metallic components in direct contact with said metal, whereby contamination of said molten metal is avoided, and maintaining said bath substantially quiescent whereby contaminants introduced otherwise and which diifer in density from the bath metal are enabled to separate from the main body of the bath metal under the influence of gravity and do not enter the coating on the article.

2. The method of hot dip coating a ferrous article with a coating metal for iron and iron alloys which comprises introducing said coating metal into a melting crucible of non-metallic refractory material, heating said metal in said crucible to above its melting temperature, passing the ferrous article to beneath the surface of the molten metal and withdrawing it therefrom with an adherent coating of said metal, maintaining said metal at said temperature while dip coating the ferrous article by supplying heat thereto substantially entirely by one of (a) conduction through a non-metallic refractory heating element maintained in direct contact with said metal and (b) radiation from a heating element spaced from said molten metal, whereby contamination of Said molten metal by metallic furnace and heating element components is avoided, and maintaining said bath substantially quiescent whereby contaminants introduced otherwise and which differ in density from the bath metal are enabled to separate from the main body of the bath metal under the influence of gravity and do not enter the coating on the article.

3. The method of hot dip coating a ferrous article with aluminum which comprises forming a molten bath of aluminum in a crucible of non-metallic refractory material which -is inert with respect to aluminum, passing the ferrous article to beneath the surface of the molten aluminum and withdrawing it therefrom with an adherent coating of said aluminum, supplying heat to maintain said bath at above its melting temperature during the course of dip coating said ferrous article wholly by combusting a fuel and passing the hot gases in heat-transfer proximity to non-metallic refractory heating means, transferring said heat from said heating means to said bath substantially entirely by one of (a) conduction through said non-metallic refractory heating means maintained in direct contact with said metal and (b) radiation from said heating means spaced from said molten metal, whereby contamination of said molten aluminum by metallic furnace and heating element components is avoided, and maintaining said bath substantially quiescent whereby contaminants introduced otherwise and which diifer in density from the bath metal are enabled to sparate from the main body of the bath metal under the influence of gravity and do not enter the coating on the article.

4. The method of hot dip coating a ferrous article with a coating metal for iron and iron alloys which comprises introducing said coating metal into a melting crucible of non-metallic refractory material, heating said metal in said crucible to above its melting temperature, passing the ferrous article to beneath the surface of the molten metal and withdrawing it therefrom with an adherent coating of said metal, maintaining said metal at such temperature while dip coating the ferrous article by supplying heat thereto wholly by conduction through a nonmetallic refractory heating element maintained in direct contact with said metal, whereby contamination of said bath of molten metal by metallic furnace and heating element components is avoided, and maintaining said bath substantially quiescent whereby contaminants introduced otherwise and which differ in density from the bath metal are enabled to separate from the main body of the bath metal under the inuence of gravity and do not enter the coating on the article.

5. The method of hot clip coating a ferrous article with a coating metal for iron and iron alloys which comprises introducing a molten bath of said coating metal into a melting crucible of non-metallic refractory material, passing the ferrous article to beneath the surface of the molten metal and withdrawing it therefrom with an adherent coating of said metal, immersing in said coating metal a thin-Walled non-metallic refractory combustion tube of good heat conductivity, supplying heat required to maintain said metal at above its melting temperature during the course of dip coating ferrous metal therewith wholly' by introducing and burning fuel in said combustion tube, whereby contamination of said molten coating metal by metallic furnace and heating element components is avoided, and maintaining said bath substantially quiescent whereby contaminants introduced from other sources and which differ in density from the bath metal are enabled to separate from the main body of the bath metal under the influence of gravity and do not enter the coating on the article.

6. The method of hot dip coating ferrous articles with aluminum which comprises establishing a molten bath of aluminum in a Crucible of non-metallic refractory material inert with respect to molten aluminum, passing the articles to beneath the surface of the molten metal and withdrawing them therefrom with an adherent coating of said metal, maintaining the bath of aluminum at above its melting temperature during the course of dip coating ferrous metal therewith by supplying heat thereto wholly by conduction through a non-metallic refractory heating element inert with respect to molten aluminum which is maintained in direct contact with said molten bath, whereby contamination of the aluminum by metallic furnace and heating element components is avoided, and maintaining said bath substantially quiescent whereby aluminum oxide and segregated ironaaluminum alloys are enabled to separate from the main body of the bath under the influence of gravity and do not enter the coating on the articles.

'7. The method of hot dip coating ferrous articles with aluminum which comprises forming a molten bath of aluminum in a Crucible of non-metallic refractory material which is inert with respect to aluminum, passing the ferrous articles to beneath the surface of the molten metal and withdrawing them therfrom with an adherent coating of said metal, immersing a thin-walled combustion tube of silicon carbide in said bath of molten aluminum, supplying heat to maintain said bath at above its melting temperature during the course of dip coating ferrous articles therewith wholly by combusting a fuel and passing the hot combustion gases through said combustion tube, whereby contamination of said bath of molten aluminum by metallic furnace and heating element components is avoided, and maintaining said bath substantially quiescent whereby contaminants adventitiously introduced which differ in density from aluminum and segregate therefrom are enabled to separate from the main body of the bath metal under the influence of gravity and do not enter the coating on the article.

8. In a method for applying a coating of a low melting point coating metal to a ferrous metal article which comprises irnmersing the article in a molten bath of the coating metal, the improvement which comprises containing said bath in a non-metallic refractory holding vessel, supplying the heat required to maintain said bath in the molten condition solely by conduction through a non-metallic refractory heating element maintained in direct Contact with the molten metal, whereby said molten metal is maintained free from contamination by the heating element, and maintaining said bath substantially quiescent whereby adventitious contaminants differing in density from said coating metal and which segregate therefrom are enabled to separate from the main body of the molten 10 metal bath under the influence of gravity and do not enter the coating on the article.

9. In a method for applying a coating of aluminum to a ferrous metal article which comprises immersing the article in a molten bath of aluminum, the improvement which comprises containing said bath in a holding vessel of non-metallic refractory material which is inert with respect to aluminum, supplying the heat required to maintain said bath in the molten condition wholly by conduction from hot combustion gases through a non-metallic refractory combustion tube maintained in direct contact with the molten aluminum, whereby said molten aluminum is maintained free from contamination by the heating element, and maintaining said bath substantially quiescent whereby adventitious contaminants of aluminum oxide and iron-aluminum alloys which differ in density from the molten aluminum metal and segregate therefrom are enabled to separate from the main body of the bath under the inuence of gravity and do not enter the coating on the article.

l0. A method of hot dip coating a ferrous article with a coating metal for iron and iron alloys which comprises introducing said coating metal into a melting crucible of non-metallic refractory material, heating said metal in said crucible to above its melting temperature, passing the ferrous article to beneath the surface of the molten metal and withdrawing it therefrom with an adherent coating of said metal, maintaining said metal at such temperature while dip coating the ferrous article by supplying heat thereto substantially entirely by radiation from a heating element spaced from said molten metal, whereby contamination of said molten metal is avoided, and maintaining said bath substantially quiescent whereby contaminants introduced otherwise and which differ in density from the bath metal are enabled to separate from the main body of the bath metal under the influence of gravity and do not enter the coating on the article.

ll. The method of hot dip coating a ferrous article with a coating metal for iron and iron alloys which comprises introducing a molten bath of said coating metal into a melting Crucible of non-metallic refractory material, passing the ferrous article to beneath the surface of the molten metal and withdrawing it therefrom with an adherent coating of said metal, supplying heat required to maintain said metal at above its melting temperature during the course of dip coating ferrous metal therewith wholly by introducing and burning fuel between the sur face of said molten bath and a radiant heating element in opposed spaced relationship with said surface, whereby contamination of said molten coating metal by metallic furnace and heating element components is avoided, and maintaining said bath substantially quiescent whereby contaminants introduced from other sources and which differ in density from the bath metal are enabled to separate from the main body of the bath metal under the influence of gravity and do not enter the coating on the article.

l2. The method of hot dip coating ferrous articles with aluminum which comprises establishing a molten bath of aluminum in a Crucible of non-metallic refractory material inert with respect to molten aluminum, passing the articles to beneath the surface of the molten metal and withdrawing them therefrom with an adherent coating of said metal, maintaining the bath of alumnium at above its melting temperature during the course of dip coating ferrous metal therewith by suplying heat thereto substantially entirely by radiation from a heating element spaced from said molten bath, whereby contamination of the aluminum by metallic furnace and heating element components is avoided, and maintaining said bath substantially quiescent whereby aluminum oxide and segregated iron-aluminum alloys are enabled to separate from the main body of the bath under the influence of gravity and do not enter the coating on the articles.

13. The method of hot dip coating ferrous articles with aluminum 'which comprises forming a molten bath of aluminum in a crucible of non-metallic refractory material which is inert with respect to aluminum, covering the surface of said bath with non-metallic refractory material which is buoyant and inert with respect to aluminum and which possesses substantially higher heat emissivity than the surface of said molten aluminum, passing the ferrous articles to beneath the surface of the molten metal and withdrawing them therefrom with an adherent coating of said metal, supplying heat to maintain said bath at above `its melting temperature during the course of dip coating ferrous articles therewith substantially entirely by combusting fuel and passing the hot combustion gases in heat-transfer proximity to a non-metallic refractory radiant heating element spaced in opposed relationship with said bath, whereby contamination of said bath of molten aluminum by metallic furnace and heating element l'components is avoided, and maintaining said bath substantially quiescent whereby contaminants adventitiously introduced which differ in density from aluminum and segregate therefrom are enabled to separate from .the main body of the lbath metal under the inuence of gravity yand do not enter the coating `on the article.

14. In a method for applying a coating of a low melting point coating metal to a ferrous metal article which comprises immersing the article in a molten bath of the coating metal, the improvement which comprises containing the bath in a non-metallic refractory holding vessel, supplying the heat required to maintain the bath in the molten condition substantially entirely by radiation from a radiant heating element maintained in opposed spaced relationship with the surface of said molten bath, where* by said molten metal is maintained `free from contamination by the heating element, and maintaining said bath substantially quiescent whereby adventitious contami- 12 nants differing in density from said coating metal and which segregate therefrom are enabled 'to separate from the main body of the molten metal under the influence of gravity and do not enter the coating on the article.

15. In a method efor applying a coating of aluminum to a ferrous metal article which comprises immersing the article in a molten bath of aluminum, the improvement which comprises containing said bath in a holding vessel of nonmetalli'c refractory material which is inert with respect to aluminum, covering the surface of said bath with a non-metallic refractory material which is buoyant and inert with respect to aluminum and which possesses substantially higher heat emissivity than the surface of said molten aluminum, supplying the heat required to maintain said bath in the molten condition substantially entirely by radiation from hot, combustion gases passed in heat-transfer proximity to a non-metallic refractory radiant heating element in opposed spaced relationship with the surface of said molten bath, whereby said molten aluminum is maintained free from contamination by the heating element, and maintaining said bath substantially quiescent whereby adventitious contaminants of aluminum oxide and iron-aluminum alloys which differ in density ifrom the molten aluminum metal and segregate therefrom are enabled `to separate from the main body yof the `bath under the influence of gravity and do not enter the coating on the article.

References Cited in the ile of this patent UNITED STATES PATENTS 1,719,512 Krembs July 29, 1929 2,768,075 Sterental Oct. 23, 1956 2,894,856 Schwendemann et al July 14, 1959 2,958,520 Fritz Nov. l, 1960

Claims

1. THE METHOD OF HOT DIP COATING A FERROUS ARTICLE WITH A COATING METAL FOR IRON AND IRON ALLOYS WHICH COMPRISES INTRODUCING SAID COATING METAL INTO A MELTING CRUCIBLE OF NON-METALLIC REFRACTORY MATERIAL, HEAT SAID METAL IN SAID CRUCIBLE TO ABOVE ITS MELTING TEMPERATURE, PASSING THE FERROUS ARTICLE TO BENEATH THE SURFACE OF THE MOLTEN METAL AND WITHDRAWING IT THEREFROM WITH AN ADHERENT COATING OF SAID METAL, MAINTAINING SAID METAL AT SAID TEMPERATURE WHILE DIP COATING THE FERROUS ARTICLE BY SUPPLYING HEAT THERETO SUBSTANTIALLY ENTIRELY BY AT LEAST ONE OF CONDUCTION AND RADIATION FROM HEAT MEANS FREE OF METALLIC COMPONENTS IN DIRECT CONTACT WITH SAID METAL, WHEREBY CONTAMINATION OF SAID MOLTEN METAL IS AVOIDED, AND MAINTAINING SAID BATH SUBSTANTIALLY QUIESCENT WHEREBY CONTAMINANTS INTRODUCED OTHERWISE AND WHICH DIFFER IN DENSITY FROM THE BATH METAL ARE ENABLED TO SEPARATE FROM THE MAIN BODY OF THE BATH METAL UNDER THE INFLUENCE OF GRAVITY AND DO NOT ENTER THE COATING ON THE ARTCLE.