US2971753A

US2971753A - Process and an oven for the baking of enamels on articles of ferrous metal

Info

Publication number: US2971753A
Application number: US519603A
Authority: US
Inventors: Hermans Fernand Victo Francois
Original assignee: Individual
Current assignee: Individual
Priority date: 1954-07-08
Filing date: 1955-07-01
Publication date: 1961-02-14
Anticipated expiration: 1978-02-14
Also published as: DE1017877B; GB801741A; FR1134010A; BE530224A; NL97227C

Description

Feb. 14, 1961 F. V. F. HERMANS 2,9 1,753 PROCESS AND AN OVEN FOR THE BAKING OF ENAMELS ON ARTICLES OF FERROUS METAL Filed July 1, 1955 2 Sheets-Sheet 1 2 v 40 J 6 9 41 2 5 I Q 9 42 I 5 F. V. F. HEghgANS ARTICLES OF FERRO Feb. 14, 1961 PROCESS AND AN OVEN FOR TH AKING OF ENAMEL US METAL Filed July 1 1955 2 Sheets-'Sheet2 United States Pate fit O F PROCESS AND AN OVEN FOR THE BAKING OF ENAMELS N ARTICLES 0F FERROUS METAL Fernand Victor Francois Hermans, 16 Rue Papeukasteel, Uccle, Belgium The present invention relates to a process for stoving enamels on articles of ferrous metal lying in a gaseous atmosphere containing combustion gases. 7 It is known that articles of ferrous metal which are raised to a temperature of several hundred degrees in the presence of air are oxidised rather rapidly if they are, at the same time, in contact with water vapour. This is the reason why it has been proposed to enamel articles of sheet-iron or sheet-steel or cast-iron articles either in electric ovens or in ovens heated by gas, oil or coal but containing a mufile in which the articles to be enamelled are protected from the combustion gases which heat the muflie externally and which contain water vapour.

Heating in electric ovens is more expensive than heating muffle ovens by gas. However, the latter ovens have the drawback of being of a relatively expensive construction, of necessitating periodic renewal of the muflies and of having a relatively high calorific inertia. All these ovens also have the drawback that the articles being enamelled are heated by radiation from bodies, the temperature of which is necessarily higher than the temperature required for baking the enamel, so that, if the articles being enamelled are accidentally left in the oven for too long a time, defects, such as blisters or changes of colours, are sometimes produced in the enamel. In addition these ovens do not allow thick articles and thin articles to be enamelled simultaneously, because the thin articles become superheated at the moment at which the other articles only reach the melting point of the enamel.

There are also ovens for enamelling articles of ferrous metal, in which these articles are heated by radiation from a base-plate, below which combustion gases pass before entering the oven and quietly lick its side walls up to openings made in its roof in order to enable them to escape through a chimney. In these ovens, the combustion gases do not come into direct contact with the articles being enamelled, notwithstanding the fact that there is no Wall separating them from the combustion gases which lick the side walls of the oven. Moreover, the gaseous atmosphere, which is in direct contact with the ferrous articles being enamelled, undergoes hardly any movement because it is not influenced by the fiow of the combustion gases along the side walls. These ovens have the drawback that their base-plate is heated much more strongly than their side walls and that, consequently, the articles being enamelled, which are heated by radiation, are appreciably hotter on the side opposite the base-plate than elsewhere. The result of this is that the enamelling that is produced there is very often irregular, although the defects due to oxidation of the metal are generally less marked there owing to the smaller period of heating which is due to the employment of a very hot base-plate.

The object of the present invention is a process for enamelling ferrous articles, whereby any trace of oxidation of the metal is obviated almost with certainty.

2,971,753 Patented Feb. 1.4, 196;

For this purpose, in the process according to the iiivention the combustion gases are made to travel, on contact with the articles being enamelled, at a minimum speed of one metre per second, preferably at about four metres per second.

Tests have shown that, when such a speed of the combustion gases was used on contact with the articles being heated up to the melting point of the enamel, no oxidation of the sheet or cast metal carrying the enamel to be vitrified was produced, notwithstanding the water vapour contained in these gases and licking the articles.

A similar elfect is not obtained in ovens for drying articles being enamelled, in which drying gases at a relatively low temperature pass, at a low speed, in contact with the articles being dried which remain in the oven often for two or three hours. In these drying ovens, the drying gases, which sometimes contain a small quantity of combustion gases, are at a temperature which generally does not reach 70 C. because the workmen should be able to enter the oven. The object of the low speed of travel of the drying gases is only to prevent the gases from becoming saturated with water and, consequently, from becoming incapable of removing the large quantity of water which accompanies the enamel when the latter is applied to the articles to be enamelled.

If the articles coated with enamel were dried for a few hours in an atmosphere saturated with moisture, they would, even after the enamel is baked in a mufiie, have the usual defects of imperfectly dried articles.

In contradistinction, when the articles have been prefectly dried, for example by a known method, the enamel can be baked, even at 800 C., by passing, in contact therewith, combustion gases which contain a large quantity of water, on condition that the speed of those gases is, at least, one meter per second, preferably about four metres per second.

This result, which appears to be in contradiction of the hitherto accepted ideas concerning the harmfulness of water vapour contained in the combustion gases, can be explained by the fact that, at such a speed, the combustion gases remove the thin layer of water vapour which, during the heating of the article being enamelled, is formed from the enamel covering the article being enamelled and remains adhering to the metal. The result of the removal of this layer of water vapour, which covers the articles, is also, in the case in which the combustion gases contain sulphur compounds, to reduce the corrosive action of these compounds on the enamel.

According to the invention, the speed of the combustion gases on contact with the articles being enamelled is such that, even with the water vapour present in these gases, the moisture, which adheres to the metal and which is formed as the result of the heating of the enamel to several hundred degrees, is carried off by these gases. The water vapour contained in the latter can retard the removal of this moisture only slightly and not sufficiently for it to be possible for the metal to oxidise on contact with air before the enamel melts.

The speed of forced travel of the combustion gases on contact with the articles being enamelled,'which is produced in the process according to the invention, is much greater than that of the slight current of air which is, at times, admitted into mufiie ovens by the effect of natural draught, with a view to reducing the oxidation of the metal. In the latter case, the slow renewal of the atmosphere of the mufiie has only the effect of allowing an easier removal of the moisture derived from the water of impregnation or from the water of constitution of the enamel owing to a lower content of water vapour in the air in contact with the articles of ferrous metal being enamelled in the mufiie.

A forced travel of air on contact with glass articles being 1 glass.

enamelled in a tunnel oven has already been proposed in he preheating ZAO'IIG Of these aTtiCls'. In that C353, 1101' air comes from the cooling zone of the oven and is sent over the articles in the preheating zones so as to lead away the solvents and the enamel vapours which are evolved and'to prevent them settling on'colderarticles inthe course off-being heated. It has never been stated whether the speed of this hot air was, on Contact with the article's,

sufficient to remove moisture that may be formed.

It is to be noted, in additiomtha't, in the case of glass ar 'ti'cle's being enemalled, one need not fear the oxidation of the articles in the presence 6f water vapour arid'that,

es" equently, even if moisture is formed during the preheating and is led away by the air at the same time as the solvents and the enamel vapours, the effect of this rernbval of the moisture is notto prevent oxidation.

It is the same in the part of these ovens where the baking of the enamel is carried out, without a muffie, in {the presence ofcombustion gases. in this part, not only isthere no oxidation to be prevented but no special device hasbeen proposed for sending the combustion gases over the articles being enamelled. Moreover, the speed of these gases is normally much lower than that of the gases used for heating sheet-iron or cast-iron articles being enamelled. As a matter of fact, the enamels for enamel- ,Iing these articles have, because they have a coefiicient of expansion that is nearly that of iron or cast iron, a melting point which is much higher than that of the enamels which can be applied to glass. The enamels generally employed for the treatment of ferrious articles are enamels with a titanium base, the melting point of which isabout 7 56 C. for those thatare to be applied to sheet ironand about 850 C. for those that are to be applied to cast iron. Previously, the melting point of the enamels which were available for this purpose was still higher. In the enamelling of glass, the enamels used should have a melting point that is lower than the softening point of the That is the reason why only enamels, the melting point of which generally does not exceed 550 C., are

employed. The result of this is that, in the enamelling of glass, the temperature of the heating gases is considerably lower than that of the gases for heating articles of ,ferrous metal and, consequently, the volume and'the speed ofthese gases are considerably less.

It is to be noted that'the drying of articles of ferrous metals with the aid of air, which is displaced at 'a minimum speed of one metre per second, has already been carried out with a view of reducing the oxidation of these articles during their drying. However, in that case, much lower temperatures were to be employed than those to which the articles being enamelled are raised in an enamelling oven. At these temperatures and'in the absence of enamel, the'moisture,'derived from the water of constitution of the enamel, has never been removed from these articles.

The travel of the combustion gases on contact with the ferrous articles being enamelled'may obviously be produced in various ways.

One of these ways consists in employing, in the oven, jets of gas leaving the burnerswhich are used for the heating. For this purpose, it is sufiicient to'direct the burners so that the atmosphere in the ovenis set in motion on contact with the articles. The speed of the jets may easily be obtained by acting on theoutlet section of the burners and on the supply pressure.

Another object ofthe invention is an oven for carrying 'outthe process accordingtothe invention.

Inone embodiment of the invention, an oven for the discontinuous enamelling of articles comprises gas bumers which open laterallyand are arranged in'sucha manner'that the jets of combustion-gases which leave-them meet halfwayacross the width.

In the case of a'tunnel oven having a-p'rehe'atingzone in which the articles already enamelled travel towardsthe outlet; giving; up-a part oi'their heat to the artic-lesbeing 16 of the oven.

4 i enamelled which are advancing towards a baking zone, this oven" comprises at least one fan, which ensures a transverse travel of the gaseous atmosphere on contact with the articles in the preheating zone, and gas burners which ensure the transverse passage of the gaseous atmosphere on contact with the articles in the baking zone.

Other features and details of the invention will appear in the course of the description of the drawings accompanying' the present specification, which represent diagrammatically and-only by way of example three constructions of an oven according to the invention.

Fig. 1 represents diagrammatically a vertical crosssection through a plane represented by the line 1-1 in Fig. 2 of an oven according to the invention for the intermittent baking of enamels.

Fig. 2 is a vertical longitudinal sectioncorresponding to the staggered line IIII of Fig. 1.

Fig. 3 represents diagrammatically, in plan, two'par-ts of a U-shap'ed tunnel oven according to the invention, in which the articles being enai'nelled are carried during their displacement in the oven.

- Figures 4 and 5 are, on a larger scale, cross-section'sof "the oven shown in Figure 3 which are denoted respectively by the lines IV-'1V and V-V of Figure 3.

Figures 6 and 7 are cross-sections, which correspond respectively to those of Figures 4 and 5, in a tunnel oven according to the invention, in which the articles are suspended during their displacement in the oven.

In these difierent figures, the same reference numerals denote'identical elements.

The discontinuous oven represented in Figures 1 and 2 comprise gas burners 2 which are fed withgas-'and-air through pipes denoted respectively by 3 and '4. These burners open laterally into the oven near the base-plate and send upwards, along the side walls 5, the jets of combustion gases formed at the outlets. These jets meet one another at about the middle of the width of the oven. The cur-rents of combustion gases are compelled to descend in the middle part of the oven towards the articles being enamelled which are carried by a grate 6.

The change in the orientation of the gaseous currents which meetone another is facilitated by the presence of "a deflector '7 at halfway across the width of the oven along its roof '8.

A part of the gases which lick the articles being en'a'nielled is led away by the jets leaving the bhrr'ier's, owing'tothefa'ct that openings 9 are made in'two lateral supports 10 and 11 which carry the grate 6. The latter is also supported by a middle wall 12 which, at the'saine time, assists in the division of'the gaseous current;

The oven' represented also comprises'two flues lii' which are separated from the baking chamber by walls-14.

However the latter leave a passagebetween the baking chamber and the fines 13 at one of their ends. This passage is formed by means of orifices 15 near the-door V The lines 13 also communicate with conduits 17 which are connected to the chimney (not shown) and with the suction side of a fan 18. The

latter ensures a forced travel of the gaseous atmosphere on contact with the articles being enamelled in the longitudinaldirection of the oven, that is'to-say in the direction perpendicular to the'plane determined by the jets issuing from the burners. The fan 18 therefore also ensures ap'artial recirculation of the combustion gases coming into contact-With the articles being enamelled. The longitudinal current of combustion gases,which is produced by the fan 18, isdis'tributed both below and above the grate '6.

Tests have' shown'that, when the speed of the combustion gases, on contact with'theferrous'articlesbeing enamelled; re'aehes a minimum-value of one-metre per second, appreciable oxidation of these'articles no longer ture which is formed from the layer of enamel between the instant at which the articles begin to be heated in the oven and that at which the enamel is melted. Preferably, the speed of the combustion gases on contact with the articles being enamelled is about four metres per second.

It can be seen that, in the process for baking enamels according to the invention, the movement of the combustion gases on contact with the articles being enamelled may be produced solely with the aid of gas jets issuing from the burners but, in certain cases, the arrangement of the articles in the oven with a view to filling it as much as possible is such that transverse currents cannot be obtained conveniently. In that case, it is useful to employ a fan for producing the movement of the gaseous atmosphere in contact with the articles in the direction which is suitable for all the faces of the latter to be capable of being licked by the gases at a minimum speed of one metre per second.

It is to be noted that the employment of burners, which produce a transverse travel, and of a fan, which ensures a longitudinal travel, is particularly advantageous, because all the faces of the ferrous articles being enamelled can thus be licked by the gaseous atmosphere at a speed which is sufiicient for removing the water vapour which is formed from the enamel during the heating of the articles. If it is feared that this speed will be too great when the enamel has melted, it can easily be reduced by stopping the fan.

The tunnel oven represented by Figures 3 to 5 comprises, in the known manner, a preheating zone 19, in which the articles 20 already enamelled travel towards the outlet and give up a part of their heat to the articles 21 being enamelled which are advancing towards a baking zone 22.

In the preheating zone 19, at least one fan 23 has been provided, which ensures a transverse travel of the gaseous atmosphere on contact with the

articles

20 and 21. The speed of travel of this atmosphere is, at least, one metre per second in order that the moisture formed by these articles during their heating should be removed immediately by the moving atmosphere.

In the baking zone 22, the articles being enamelled are licked by the combustion gases which, on their contacting these articles, travel at a speed which is also at least one metre per second, owing to the arrangement of the burners 2.

In the oven represented by Figures 3 to 5, the

articles

20 and 21 are carried and are displaced in preheating and baking zones which are wider than their height. In this case, the burners 2 are arranged laterally about halfway up. In addition, the burners which are situated on one side of the oven are so oriented that the jets which issue therefrom are directed upwards whilst the burners which are situated on the other side of the oven are so oriented that the jets which issue therefrom are directed downwards. This arrangement of the burners assists the travel of the gaseous atmosphere, on contact with the articles being enamelled, at the speed required for re moving the moisture which is formed from the enamel during the heating of these articles.

The oven represented in Figures 6 and 7 is also a tunnel oven, but the articles being enamelled are suspended therein. height than the Width because of the dimensions of the suspended articles being enamelled. In this case, a fan 23 also ensures a forced travel of the air in the preheating zone 19 on contact with the articles 21 being enamelled. In the baking zone 22, some of the burners 2 are arranged on one side near the base-plate, so that their jets are directed upwards, and the others are arranged on the other side near the roof so that their jets are directed downwards.

It is obvious that the invention is not exclusively limited to the embodiments represented and that many modifications may be made in the shape, arrangement and constitution of certain of the elements employed on carrying out the invention, it these modifications are not in contradiction with the subject matter of each of the following claims.

What I claim is:

1. A process for removing combined water from porcelain enamels and for vitrifying porcelain enameling material on articles of ferrous metal by fusion of the enameling material in an oven heated by combustion gases, comprising introducing said combustion gases into said oven in contact with the enamel-coated articles, circulating the combustion gases at an effective speed by applying a positive pressure to said gases during contact thereof with the enameling material being melted, removing from said oven the combustion gases as they are being circulated, and removing from said oven the combined water contained in the enameling material during the heating of the latter preceding their fusion as the circulated gases are removed from said oven, thereby preventing injurious oxidation of the metal by combined water originally contained in the enameling material.

2. A process according to claim 1 wherein the speed of said gases is at least one meter per second.

3. A process according to claim 1 wherein the speed of said gases is at least four meters per second.

References Cited in the file of this patent UNITED STATES PATENTS 1,819,772 Darrah Aug. 18, 1931 2,010,295 Drefiein Aug. 6, 1935 2,181,928 Vaughman Dec. 5, 1939 2,308,902 Weller Jan. 19, 1943 2,434,491 Elder et al Jan. 13, 1948 2,480,374 Larman Aug. 30, 1949 2,492,682 Carpenter Dec. 27, 1949 2,517,024 Prescott et a1 Aug. 1, 1950 2,676,007 Davis Apr. 20, 1954 2,676,008 Munker Apr. 20, 1954 In this oven, the baking zone is of greater