US2485061A - Method of protecting metallic surfaces - Google Patents

Description

- EXAMINER as 6 54 M CROSS REFERENCE Patented Oct. 18, 1949 METHOD OF PROTECTING METALLIC SURFACE Peter J. Nebe, Gary, Ind., assignor to Carnegie- Illinois Steel Corporation, a corporation of New Jersey No Drawing. Application March 8, 1946,

Serial No. 653,174

1 Claim. 1

This invention relates to an improved annealing box, and more particularly to inner covers used in connection with hood type annealing furnaces, and still more particularly to a coating composition for such covers.

Annealing boxes formerly used in steel and other metal manufacturing industries usually were in the form of containers employed for the express purpose of preventing direct contact between oxidizing gases of the heating means and the stock being annealed; Two principal types of design were commonly employed: one represented by an upwardly opening box requiring a special lid for closure thereof; and the other substantially resembling a hood which was lowered over the stock to be annealed and rested on a base commonly through the medium of a sand seal or its equivalent.

The above boxes were used principally in connection with stationary annealing furnaces equipped with separate grates for fuel consumption in which the loaded boxes were placed and removed intermittently. Wide temperature variations encountered in this practice, together with the deleterious effect oftlhe highly oxidizing atmosphere of the open ila e upon the boxes, caused almost universal adoption to use of heavily ribbed and massive cast iron boxes to provide the necessary strength to resist high temperatures and oxidation or scaling.

Contemporary development of furnace design brought with it a radical departure from the above described method of annealing. The basic difference between the old and new practice consisted in making the annealing charge stationary and moving the furnace proper over it. In this manner it became possible to support enormous loads on a heavy permanent base, protect them from the harmful effects of furnace atmosphere with a comparatively lightweight cover or box subjected to no loads, and place the movable furnace over the whole assemblage.

- Seeking further to improve annealing practices the so-called radiant tube type heater has become almost universally adopted as a means of furnishing the desired annealing temperatures and has largely supplanted the older type of open flame heat. However, even with the adoption or the radiant tube heaters, which in a sense eliminated to a certain degree the harmful eflects previously encountered from open flame heating, it was found necessary to still use inner covers to maintain a sealed atmosphere around the annealing charge for the purpose of introducing deoxidizing gas therein.

z As a result, the changed function of the an nealing cover introduced pronounced changes in design thereof. The new design primarily tended to lighten the cover structure to the utmost in order to secure more rapid transfer of heat to the charge. As a result, today the accepted practice is to fabricate covers from welded together steel sheets which are corrugated for stiifness. Gratifying results are attained by use of the new lighter covers as reflected in increased tonnage annealed and more rapid annealing cycles.

As mentioned previously, adoption of radiant tube type heaters lessened materially the scal-' ing of inner covers, but scaling still occurs and is quite detrimental to the life of the new lightweight covers. Resort has been made to innumerable changes in design including among others the substitution of alloy heat resisting steel portions, and also covers constructed entirely of heat resisting alloys. The extremely light weight of the covers however generally causes them to become deformed when exposed to intensely concentrated heat which requires that they be frequently straightened. All 01- these facts are detrimental to the life of the covers as reflected in increased operating costs and unsatisfactory performance.

The present invention embodies the discovery that oxidation resistance of inner covers constructed from thin, lightweight ordinary carbon steel sheets is greatly increased by applying to their outside surfaces, i. e., those in contact with heat radiating from the radiant tubes, a mixture composed of graphite and water glass having a definite composition, and applied in a specific manner. When covers so treated are subjected to heat of the annealing process, the applied mixture assumes jet black color and a glaze-like flnish apparently entirely impervious to oxidizing gases, thus materially prolonging their useful life.

Practical experience demonstrates the need for maintaining a certain minimum concentration of graphite for any selected combination of sodium silicate and water, depending to some extent upon operating conditions of the annealing furnace. For average temperatures ranging between 1400 F. and 1600 F. in a radiant tube type furnace for instance, a coating mixture containing 18 to 25 pounds of powdered graglgtg, flve gallons of commercial sodium silica an five gallons of water, applied to the outer surface of the covers meets the requirements. Lower graphite concentrations lead to an undesirable conducive to a uniform and ncontinuous layer,

but preferably it is applied by meansof spraye ing. It has been found that coatings which are too thin do not offer suflicient'protection, while excessively heavy coatings will peel off and tend to interfere with evenly distributed and efficient heat transfer. sprayed preferably at.the rate of substantially one gallon of prepared mixture to .each' one thousand square feet of cover surface.-

Obscrvations leading in part to the presently developed stage orthe present invention indicate furthermore that better cover. protectionv is obtained when the mixture is sprayed on a warm coversurface within --a temperature range -of 4009 Ffito 700 F's-which is representative of the delredtemperature; On thisaccount the use of 'a brush for applying the mixture causes unevennistribution and as a result full benefitis not derived and ultimate cover life will not. be leeln'edm. The process of dipping covers in *the prepared mixture is obviously associated with pronounced mechanical difliculties and is unat- I tr'active iroma cost-standpoint.

Accumulated .data support thenresult that a much better coating-adherence and more effective prctectionis obtained when the graphite bearing flLmis-sprayed on a sli htly oxidized surface in 140 preference to! clean metaL In this-respect it is pm en'ed to treat. covers which have first undergone n weathering period during: which :a @thin oxide-film has formed as" a natural'result 'of being. exposed to the elements;

Iris-reducingthe invention to practice there prefer-ably rare mixed together II thoroughly v20 pounds of powdered graphite and dive gallons of water and then while the graphite. is still in suspension, there: isladded five gallon's'of comrpereial sodium silicate. exemplified :by that chauingm specific gravity of 40- B.- and; a relatively low alkalinityi '(siOzzNaz ra 0 approximately 3.25) While'stfll maintained in an agitated-state filnuidzure described-isapplied with .a spray-gun atithe'rate of oneogallonperone thousand square feet"of surface -.to' an :inner coverwheated to .a

temperaturewffrmn 400? F. to: woo After water contained i-nazthe-mixture has evaporated,

thesprayed inner cover is placedover thecharge no andlannnaled with it, and as a result .thesprayed on mixture is transformed into a jet black-glazelike, tightlytadhering coating, and the :so treated In this respect the covers are cover possesses the inherent qualities of long life and performance of the present invention.

To take fullest advantage of the benefits of the present invention it is preferable to coat inner covers by the method as described immediately afterlhelr initial trip .to .theannealing furnace asthispbviouslydprolongs their life for a greater period of time. Also, in this connection it is more feasible to secure'the desired temperature of 400 F. to 700 F. in the covers prior to the spraying operation, as residual heat remaining after the annealing operation is quite sufficientv to obtain the ultimate required for spraying.

Plant records have definitely demonstrated that inner covers treated according to the present invention deteriorate at an extremely retarded rate under conventional annealing conditions and can be efiiciently utilized for at least twice the length of time as compared to conventional untreated covers, provided the coating is not mechanically injured by careless handling.

I claim:

A method of protecting plain carbon steelinner covers of annealing boxes from oxidation-when such covers are heated to annealing temperatures of 1-i00 to 1030 F. I comprising weathering l-a cover and thereby forming a thin iron oxide film on its surface, heating the cover to an annealing temperature, cooling the cover to a temperature of 400 to 700 F., spraying on the cove'r surface while at this latter temperature a coating composition consisting .of' a suspension-of powdered graphite in an aqueous solution of so.- dium silicate, the proportions .beingc201to :25 pounds of graphite for each five gallons of sodium silicate and each five gallons ofwater, the composition being applied-approximately" in the amount of one gallon per 1000 square" feet of cover surface, and re-heating the cover to' an annealing temperature.

PETER J.- NEBEJ;

REFERENCES CITED I The following references are ofrecord in the me of this patent:

UNITED s'rarns. PATENTS Date II Number Name 1,467,398 Schumacher'; Sept. 11,1923 1,550,629 Paulus'; Aug. 18,1925 2,016,139 Eddison Oct;"1,"1935 2,047,087 Szymanowitz" July-7,1936 2,157,155 Work'et a1; May 9,1939 2,209,304 Alder July 19% FOREIGN PATENTS I Number I Country Date 284,219 Great Britain June 24, 1929 OTHER REFERENCES 4