US2049834A

US2049834A - Manufacture of hot galvanized articles

Info

Publication number: US2049834A
Application number: US53510A
Authority: US
Inventors: Flint C Elder
Original assignee: Individual
Current assignee: Individual
Priority date: 1935-12-09
Filing date: 1935-12-09
Publication date: 1936-08-04
Anticipated expiration: 1953-08-04

Description

Aug. 4, 193e. Y F. ELDR 2,049,834

MANUFACTURE OF HOT GALVANIZED ARTICLES Filed Dec. Y'9, 1955 ZVZZ; 9@

le? a@ ,1024 j.

Patented Aug. 4, 1936 UNITED STATES.

PATENT OFFICE MANUFACTURE OF HOT GALVANIZED ARTICLES 16 Clains.

This invention relates to the manufacture of coated metal articles, and while not limited thereto relates more particularly to the manufacture of hot galvanized or zinc coated ferrous metal articles; such as iron rods, wires, strips and sheets. The invention of the present application 'is a continuation in part of that shown and described in my copending application, Serial No. 692,684 led October '1, 1933.

Heretofore, in the coating of metal articles with dissimilar metals, and particularly in the galvanizing of ferrous articles, it has been the' practice to immerse the article to be coated in a hath of molten zinc for such a period of time as is necessary to alloy the coating metal to the superficial areas of the base metal. Upon Withdrawal from the coating bath, the coated article will be found to have, in general, three distinct metallic constituents in superimposed relation when viewed in cross-section. The innermost portion of the article will, of course, be the base metal. superimposed thereon will be a thin alloy layer, or layers', of the coating metal combined with the base metal.

In the case of a ferrous base metal and a zinc coating, this alloy layer, or layer's, will be ironzinc compounds represented, for the most part, by the chemical symbols, FeZna and FeZnv. superimposed atop this alloy layer will be a stratum of pure coating metal, or zinc, which will have adhered to the article upon its withdrawal from the bath, and which will have become frozen thereon upon being exposed to the air to constitute the outermost layer and surface of the coated article.

Where it has been deemed desirable. to increase the coating upon the base metal, according to prior art methods, the coated article has been returned to the same, or to a similar, coating bath for re-immersion therein. The temperatures of the secondary bath, and the length of time of immersion, have been such as to materially increase the total bulk of the coated articles. has clearly indicated that the total increase in weight of coating in every instance has been accompanied by a growth of the alloy layer, and that the outer layer of pure coating metal. is not increased appreciably over the initial coating.

It was found that the coatings on such articles, when coated in accordance with methods heretofore known, possess low ductile properties and brittleness, resulting in cracking and failure of the coating when subjected to the However, analysis of the final product exing and bending stresses occasioned by average usage. The cause for this undesirable brittleness has been definitely ascribed to the alloy strata of the articles coated as above described; particularly where one prolonged immersion or two or more immersions are made in an effort to increase the coating. Such increase of the time of immersion has always resulted in a material increase of the alloy strata without contributing to the thickness of the 1o outer layer of pure coating metal. i

It is apparent from the above discussion that it is highly desirable to acquire a coatingfupon a base metal composed almost entirely of the pure coating metal; such as zinc, wherein the alloy layer intermediate the base metal and the' pure coating metal is held to the irreducible minimum of that formed by the initial dip in the conventional way. The alloy layer formed from such an initial dip is not of suicient magnitude to impair the physical properties of the coating on the coated article. However, subse- -quent immersions in a molten coating bath in the conventional way do cause an increase in the alloy layer on the coated article to .thereby seriously impair the physical properties of the coating.

The principal object of this inventionis to provide a novel-method of. manufacturing hot galvanized or zinc coated metal articles, whereby a materially heavier coating will be formed on the articles than has heretofore been possible of production by hot galvanizing in accordance with prior art methods.

Another object of this invention is to provide a novel method of manufacturing hot galvanized or zinc coated metal articles whereby the zinc coating will have a materially greater thick- Vness of relatively purer zinc than has heretofore even been considered possible in hot galvanizing.

Still -another object is to produce coated articles having a heavy, pure coating of a metal thereon, which coating will be homogeneous, strong and possessed of good ductility and excellence of nish.

In the manufacture of hot galvanized or zinc coated articles the article to be galvanized is prepared for the galvanizing step by cleaning the surface of the article, fluxing the. article, then passing the so prepared article into and through a molten bath of high grade relatively pure zinc.

' The article passing from the bath of molten zinc has a'coating composed of an alloy of the 55 2 aoeaeaa base metal and the zinc next to the base metal and an outer layer of substantially pure zinc. 'Ihe amount of coating; that is, the thickness of coating on the metal article, has heretofore been regulated by the time ofimmersion of thearticle in the bath, either in one or more immersions.

In order to make a specic disclosure of this invention it will be described as applied to the 1o manufacture of hot galvanized or zinc coated iron or steel wire, it being understood that by a slight modification the same process may be applied to other metal articles in the form of strips, sheets, bars or shapes.

In the general method of galvanizing iron or steel wire the wire or rod is run successively through an acid cleaning bath, water washing bath, uxing bath, and then into a bath of molten high grade or substantially pure zinc y from which it emerges through a suitable wiper and then is wound upon a standard take-up block. The temperature of the molten zinc forming the bath through which the wire rod to be coated is passed is commonly carried at from 840 to 900 degrees Fahrenheit. The amount of coating applied to the wire or rod has heretofore been regulated solely by the time of immersion of the wire or rod in the zinc bath; that is, bythe length. of immersion and the speed at which the wire rod travels through the bath.

Increasing the length of time of immersion of the wire rod or other article in the molten zinc bath increases the amount of thickness of the coating obtained on the wire rod within well defined limits, since in actual operation it is recognized thatafter a predetermined time of immersion there is substantially no further increase of the amount of coating obtained. Ex-

perience has shown'that a #12 gage (.1055'f diameter iron or steel wire regardless of the increase in the length of time of immersion will not receive a. single coating greater than that equal to approximately 1.25 ounces of coating 4 5 per square foot of wire surface. If this same wire is re-galvanized in accordance with the usual practice, except for omitting the acid cleaning, the wire will be found to retain sub-. stantially the same weight of pure coating as resulted in the iirst galvanizing. In other words, experience has shown that doubling the total time of immersion or re-immersing galvanized wire, rods and the like causes no appreciable increase in the weight of pure galvanized coating.

' The present invention provides for increasing the weight of zinc or galvanized coating on wire, rods and the like far beyond that heretofore thought possible.

In the drawing: 7

Figure l shows a diagrammatic elevation of apparatus in carrying out the method of the invention.

Figure 2 shows a slightly modied form of apparatus in carrying out the invention.

Figure 3 shows a cross-section through a wire or rod coated in accordance with the invention.

In carrying out the method of this invention the wire,.rod'or other material A, as desired or terial exits from the furnace 2 it is passed continuously through an acid bath contained in a vat 3, th n through a washing vat il, containing 75 wash W ter, and then through the flux inq a o necessary, is annealed by passingr it continuously through the annealing furnace 2. As the ma .uxing tank 5, thence through a molten zinc bath contained in the zinc pan 6. The zinc pan 6 is provided with a burner l at its entrance end which maintains the zinc bath molten, and the portion of the bath closest to said 5 burner is of materially higher temperature than the portion of the bath farthest away from said burner. The material A to be coated enters the molten zinc or galvanizing bath at its hottest point, substantially above the burner l, and 10 travels through the bath at such a speed and for such a distance as to remain in the bath a suiiicient length of time to cause an alloying 'of the zinc and the metal to be coated, and

the accumulation of the maximum amount of 15 substantially pure zinc coating surrounding the so formed alloy layer. The wire, rod or other article to be coated exits from the molten zinc bath and travels continuously in the atmosphere for a sufficient length of time to cause the zinc 20 coating to freeze. After the coating is frozen or solidified the coated wire or rod is successively passed at intervals into the relatively cool end of the galvanizing or zinc bath to increase the Weight of coating. In the drawing the wire 2 or rod is shown as passing from the galvanizing or zinc bath in the pan 5 upwardly over a sheave or roll 8, then downwardly into the zinc bath and around a sheave or roll 9 immersed in the zinc bath, then upwardly and over a 30 sheave or roll l0 located materially above thel pan 6, and then downwardly through the molten zinc or galvanizing bath under a sheave or roll l2, thence outwardly over a suitable guide sheave Ml to a take-up reel l5. 35 If desired, it will be understood that even more successive passes may be made through the molten zinc bath to further increase thickness of the zinc coating. "Ihe successive passes of the coated metal through the molten zinc bath 40 are accomplished with great rapidity' and are l for relatively short intervals only, whereby additional substantially pure zinc is applied to the coating to materially increase its Weight and thickness and while the alloy portion of the 45 coating remains substantially unchanged. It is desirable that the successive passes of the coated article after the first coating is formed be made in a molten zinc bath of materially less temperature than the zinc bath into which the un- 50 coated base metal enters, although this is not t necessary. f

It will be understood that the nearer the melting point the molten bath can be maintained thel less critical the control over the amount of coat- 55 ing acquired in the second bath becomes. This is explained by the fact that the heat of such a bath is insuicient to cause even the superncial areas of the initial coating to melt, whereby the initial coat is preserved in its entirety. The 60 hotter the baths succeeding the initial bath are, the more rapid must be the immersions of the pre-coated base metal therein to prevent the melting off of the pre-coat, and, hence, the results become even more diiicult of predeter- G5 mination. However, it Will be understood that a heavy, substantially pure coating may be formed in a bath of even higher temperature thanpreceding baths, provided that the duration of immersion is proportionately diminished, and/or provided that the precoated base metal has been proportionally chilled before immersion.

In Figure 2 the modified apparatus for carrying out this invention is substantially the same as that described above, with the exception that v2,049,834 the galvanizing pan has Vbeen modified. The

same reference numerals will be applied to like parts of the apparatus of Figure 2. The galvanizing pan of Figure 2 is designatedl by the numeral 6. The pan 6a is divided into two chambers X and Z, by a partition wall 20 so as to prevent circulation of the molten zincV from the one chamber to the other. Material to be coated enters the chamber X of the pan 6a and passes through this chamber, then out over the guide roll or sheave 8a and down under a guide sheave or roll 9a in the chamber Z. From under the roll 9a the metal passes upwardly through the atmosphere and over a guide sheave or roll Ill, thence downwardly under a guide roll or sheave l2, and over a guide sheave or roll I4 to the usual take-up block, as described for 'the apparatus in Figure 1.

Experience has shown that in carrying out'thev method of this invention, withI apparatus similar to that described above, the relatively short immersions of the coated article in the molten bath of zinc having a less temperature than that normally maintained for galvanizing, causes a material increase after each immersion in the weight and thickness of the relatively pure zinc coating on the article. The zinc bath in which the successive short immersions are made is preferably maintained at a temperature as near to the melting point of zinc as is practical, and with successive immersions for length of time in the order of less than one second for #l2 gage wire, the relatively pure zinc coating can be increased to practically any desired amount.

In coating wire according to the present invention, I have placed almost 10 oz. of zinc per square foot of wire surface on a #12 gage iron wire, and have maintained the ferrous content of that coating to below 1% by weight. This is over four times the amount of zinc coating acquired by prior art methods on the same gage wire fabricated in accordance with the custom- 'ary practice on competitive articles.

As a specific example, #l2 gage steel wire has been coated, using a zinc or spelter temperature of 840 to 850 degrees Fahrenheit at the entering end of the galvanizing pan, and a length of immersion in the zinc bath to form the initial coating of approximately one minute. The resultant initial coating had a maximum weight of approximately 1.25 ounces wire surface. An additional immersion of the initial galvanized wire in the spelter bath having a temperature approximately at the melting point of the zinc for one second caused an increase in weight of the galvanizing or zinc coating of considerably more than five ounces per square foot of wire surface, and additional short immersions caused a similar increase in the relatively pure zinc coating, without appreciably increasing the alloy constituency.

While the exact theory on which this invention is based is not certain, it is a generally accepted fact that hot galvanized wire, rod or other article has at least three different layers in the coating, as illustrated in FigurcB of the drawing; that is, next to the base metal there is a high iron content or ferrous zinc alloy, designated in the drawing as 30, and on the outside of this iron or ferrous zinc alloy is the second alloy layer of lower iron content and more correctly termed a zinc iron or ferrous zinc alloy, which is designated in the drawing by the numeral 3l. zinc immersed or galvanized wire, rod or other per square foot of The third or surface layer ofthe article is composed of substantially pure zinc, as designated in the drawing by the numeral 32..

Heretofore, the three layers above described formed the complete coating. The two innermost alloy layers are relatively thin in comparison with the outer relatively pure zinc layer, and because of their iron or ferrous content the inner layers have a melting point considerably higher than the outer or pure zinc layer. The alloy layers are, therefore, in a. solid condition while the pure top layer is in a molten conditionas a galvanized wire or article emerges fromthe galvanizing bath. The generally accepted theory is that the amount of pure zinc in theouter layer is practically constant with a fixed bath temperature and speed, but the two alloy layers increase materially with time of immersion, the rate of increase becoming progressively slower as the alloy layers increase in thickness since the zinc has to diffuse through the solid alloy layers. The alloy layers are solid-because their melting points are higher than the bath temperature commonly carried in galvanizing.

The amount of the outer relatively pure layer of zinc galvanizng depends on the amount of liquid or molten zinc carried on the wire as it passes from the bath, and this amount of zinc depends on the fluidity of the bath, the lower the temperature within limits the more galvanizing or zinc will be carried out of the bath on the article frozen thereon.

In the present process the initial coating is allowed to freeze and then the so-coated article or wire is returned to the bath (either the same bath or a second one) and allowed to remain in the bath-for a length of time only sufficient to permit additional molten metal to freeze upon the initial coating without melting it and the article is then immediately withdrawn from the bath.

., Examination of the article upon withdrawal from the bath after the second dip has disclosed that no bond is 'formed between the Vfirst and second coatings. In the case of wire, the second coating resembles a zinc pipe shrunk upon the pre-coated wire or base metal. It has been deduced from this that the pre-coated base metal, when introduced into a bath of molten zinc, rapidly absorbs the heat from that portionof the zinc .bath immediately surrounding and near it, thereby actually causing. a mass of zinc to freeze upon it while still in the bath. Thereafter, the coated article is immediately withdrawn before it can become saturated with the heat of the bath; i. e., before its refrigerating effect is neutralized, whereby the zinc coating thus formed is retained in toto without melting off. Thus, it will'be clearly seen that if no bond is formed'between the first and subsequent coatings there can be no melting of the first coating. This is significant because of two reasons; first, the entire initial coating is preserved without loss. In this connection I have found that such heat required to melt even the supercial areas of the initial coating will cause that coating to run off entirely. Hence, even if bonding can be theoretically effected by this method, it is so extremely critical as to be uncontrollable as a practicable matter. Secondly, if the initial coating is not melted, nor heated to melting point, then obviously, as above noted, there can be no increase of the undesirable alloy constituency. Additional immersions may be made until the desired weight of coating has been built up 'iti on the article, and it will be found that the alloy layer of the initial coating will show no apparent increase from such successive short mi.; 1.1. ersio. The coated article or wire is preferably allowed to cool between the successive coating steps to a temperature appreciably below the melting point of zinc. f

Referring again to the drawing it will be noted that the layer of relatively pure zinc 32 formed by the initial coating is less than twice the thickness of the combined alloy layers 3@ and 3l, and that successive layers of relatively pure zinc, 33 and Sli, which result from successive short immersions as described heretofore, are substantially the same thickness or greater than that of the pure zinc layer of the` initial coating.

After an article has been coated in accordance with the method outlined herein, l provide that it shall be worked, either hot or cold, in any suitable manner, as by rolls, drawing dies, etc. This nal working not only serves to rene the grain structure of the coatings and to improve the finish yor luster thereof but also effects a complete coalescence between the several coatiings to make them into one, integral, homogeneous coating. All tendency of the coating to crack of flake od when flexed or distorted in use is thereafter precluded.

It will be seen from the above description thatvmy method applies to cases where it is desirable to place a substantially pure coating of one metal upon another without permitting the growth of an undesirable alloy stratum between the two. Obviously, then, my invention relates to the coating of any article thatA has an initial coating of the same metal already formed thereon, as well as to those uncoated articles -f'which includes rapidly dipping which receive the initial coating in accordance with the conventional practices recitedv herein. 'Ilherefore, my invention is equally applicable to articles pre-coated in any way, as by electrolytic deposition, spraying, ilowing, extrusion, etc. Hence, any of the conventional coating processes may be substituted for the initial dip stepdisclosed herein merely for purposes of illustration.

To recapitulate my invention, I` take an article to be coated in accordance herewith, which either hasan initial coating alloyed thereto or .upon'which I form such a coating in any suitable way. After the'initial coating has thoroughly set, I increase that coating' by rapidly dipping the article into a molten bath of the same metal, and instantly withdrawing it before the initial coatecan'be melted or otherwise dissipated. Successive rapid dips ofonly a second or less in duration may be made to still further increase the thickness of the coating any desired amount.

While I have shown and described certain specific embodiments of my invention it is to be understood that I do not wish to be limited exactly thereto, since various modifications may be made without departing from the scope of-my in ven'- tion, as deilnedvby the following claims.

I claim:

1. The method of forming 'an integral, homogeneous coating upon an article having a preformed coating of a given metal alloyed thereto said article in a molten bath of said given'metal to acquire a covering thereof without melting or otherwise affecting said preformed coating, permitting the second coating to set, and thereafter treating lwhich includes rapidly acides@ the coated article 'to eect coalescense between the coatings.

2. The method of forming an integral, homogeneous coating upon an article which includes allcying a solid coating of a given metal thereon, 5 subsequently immersing the initially coated article in a bath of said given metal and instantly withdrawing the same to acquire a secondary coating without melting the solid coating, and thereafter treating the coatings to effect coales- 10 cense therebetween.

3. The method of covering an article with a. relatively pure, heavy coating of Aa, given metal which includes bonding a coating of ysaid given metal to the article to be coated, and thereafter l5 rapidly dipping the coated article in a molten bath of said given metal to acquire a substantial covering thereof upon the initial coating without melting or otherwise affecting the integrity of the initial coating.

4. The method of covering an article with a relatively Dure, heavy coating of a given metal which includes bonding a coating of said given metal to the article to be coated, permitting the bonded coating to set, rapidly dipping the coated 25 article in a molten bath of said given metal to freeze an unbonded layer thereof upon said coated article, and thereafter treating the article so coated to integrate said bonded coating and unbonded layer into a. single homogeneous coating.

5. The method of increasing the coating upon a base metal having an initial coating of a given metal bonded thereto which includes rapidly dip# ping said pre-coated base metal in a molten bath of said given metal to acquire -a coating thereof 35 upon the initial coating without affecting the integrity of said inital coating. f

6. The method of increasing the coating upon a base metalhaving an initial coating of a given metal bonded thereto which includes rapidly dipping said pre-coated base metal into a molten bath of said given metal to form an unbonded additional coating thereon without affecting the integrityof said initial coating, and thereafter treating said coatings to integrate the same.

7. The method of continuously coating metallic articles with a given metal which includes bonding a coating of said given metal upon the articles to be coated'and thereafter rapidly dipping said articles lat least once in at least one molten bath of said given metal to form a plurality of coatings on said articles without affecting the ne'xt preceding coating, and thereafter treating said articles to completely integrate said coatingsand to improve the finish and physical properties thereof.

8. The method oi' galvanizing articles which includes rapidly dipping an article having'a preformed coating of zinc bonded thereto into a molten bath of zinc to acquire a substantial additional coating thereof upon the preformed coating without affecting the integrity of said preformed coatingf 9. The method of hot galvanizing articles dipping an article having a preformed coating of zinc bonded thereto at least once in at least one bath of molten zinc to acquire a substantial additional coating thereof without affecting the integrity of the preceding coating, and thereafter treating said article to integrate` all coatings, each coating in turn being permitted to completely set before being introduced to 'the next succeeding step oi the method.

10. As an 'article lof manufacture a metallic 75 article having an integral homogeneous coating formed thereon by the hot-dip method in excess of 2.5 oz. per square foot of surface and with an impurity content of less than 1% by weight.

11. As an article of manufacture a. metallic article having a hot-dipped coating thereon in excess of 2.5 oz. per square foot of surface, said coating being integral, homogeneous and bonded to said article.

12. As an article of manufacture a metal article having a hot-dipped zinc-coating bonded thereto in excess of 2.5 oz. per square foot of surface, said coating being substantially pure, integral and homogeneous.

13. The method of coating attenuated metal stock which includes passing said stock through a bath of molten metal and alloying a coating of the bath metal thereon, permitting said coating to solidify, and thereafter rapidly passing said coated stock through a bath of said molten metal to lacquire a relativelyheavy coating thereof acquire a relatively 'my of the latter.

upon saidl initial coating without affecting the integrity of the latter.

14. 'Ihe method of hot-galvanizing attenuated metal stock which includes passing said stock through a bath of molten zinc to alloy coating of zinc thereon, permitting said zinc coating to solidify, and thereafter rapidly passing said coated stock through a bath of molten zinc to heavy coating thereof. upon said initial coating without affecting the integ- 15. The invention recited by claim 13 which further includes treating the coated stock to integrate said coatings, and to improve the physical characteristics and finish thereof.

15 16. The invention recited by claim 14 which further includes treating the zinc-coated stock to integrate said coatings, and to improve the physical characteristics and finish thereof.

FLINT C.v EIDER.