US2650888A

US2650888A - Metal surface treatment

Info

Publication number: US2650888A
Application number: US109380A
Authority: US
Inventors: Pottberg Rolfe
Original assignee: Freeport Sulphur Co
Current assignee: Freeport Minerals Co
Priority date: 1949-08-09
Filing date: 1949-08-09
Publication date: 1953-09-01
Anticipated expiration: 1970-09-01

Description

? Se t. 1, 1953 R. POTTBERG 2,650,883

METAL SURFACE TREATMENT Filed Aug. 9, 1949 A T B 0 SURFACE v- HEAT SCALED TREATMENT W PRODUCT 5 5 CHEMICAL MECHANISM FOR TREATMENT GRIPPING AND DRAWING STRIP THROUGH'W'AND'B FIGZ SURFACE TREATMENT HIHH HII HIIHHHHHI ///A 'STBIPETQE cou'mun 0 mm 7 MAJORITY OF CRACKIING OOGURS OVER THIS ANGLE INVENMR.

Patented Sept. 1, 1953 METAL SURFACE TREATMENT Rolfe Pottberg, Loch Raven, Md., assignor to Freeport Sulphur Company, New York, N. Y., a

corporation of Delaware Application August 9, 1949, Serial No. 109,380

1 Claim.

This invention relates to the conditioning of metal surfaces particularly in connection with the removal of oxide films formed, for instance, by strip or sheet rolling operations at elevated temperatures.

The object of the invention is to provide a method of modifying such oxide coatings so that subsequent pickling or other treatments will be rendered more promptly and efficiently effective in removing these coatings.

Other objects of the invention, particularly in the special procedures and apparatus employed in the treatments, will appear from the following specification taken in connection with the accompanying drawing in which:

Fig. l is a schematic outline of a surface cleaner system employing the invention;

Fig. 2 is a diagrammatic view of the surface treatment; and

Fig. 3 is an enlarged view of a portion of Fig. 2.

In the system outlined in Fig. 1 a metal strip having an oxide surface layer 6 is passed through the primary surface treatment stage A and on to the secondary surface treatment B preferably by drawing means C. Stage A involves the application to the strip of physical forces in a manner concentrating this effect on the surface and in particular disrupting the continuity of the oxide films so as to condition them for ready response to subsequent removal treatment indicated at B. Treatment A may be at room temperature or any temperature substantially below that at which such oxide films are formed.

As diagrammed in Fig. 2, the primary surface treatment at A impresses the metal strip by rollers l under adjustable pressure at 8 so that the forces applied to the strip surfaces 6 may be adjusted and regulated. Preferably the rollers l are idlers, the strip being drawn through this stage in any desired manner. The pressure applied by rollers I compresses the strip 5 thinning it somewhat and so causing an angle bend e and rippling Ill of the metal surface in advance of the roll contact as illustrated, for instance, in Fig. 3. The pressures involved and surface distortions are not suflicient to clear the oxides from the surface and substantially no oxide is removed here.

I have found that a much improved over-all removal and descaling is attained by disintegrating the surface oxide layers in situ under the action of this bending and rippling effect and completing the actual removal at the later stage B. Such preliminary modifications of the physical nature of the surface may be attained at no loss of time and while the strip is en route to the removal stage B. Here, at stage B, the time of treatment is greatly reduced by the modified structure of the surface films and this permits a higher rate of feed, or conversely shorter length of the removal stage B. Such shorter timing at stage B, usually a chemical cleaning, also reduces the action on the metal itself and minimizes the losses involved.

To attain the desired condition of the surface the compression effect is regulated to limit it to a .disintegrating of the scale without removing it, thus retaining the surface oxides while developing within them multitudinous microscopic fractures honeycombing the entire film and reacting right down to the metal itself. Any concomitant permanent thinning of the strip from thickness d to thickness b (Fig. 3) is at most a small percentage, 2% to 4% being characteristic, and in some cases the reduction is imperceptible, the elastic limit of the material not having been substantially exceeded. Apparently the invisible network of microscopic fractures, left in tightly closed position by passage between the rolls, serves as a maze of latent paths of attack and entry for the usual pickling solutions and it has been found that wetting agents are not necessarily added.

In attaining this modification of the surface oxide film, the bend 9 and the radius of the crest of the metal ripple or Wave [8 at the surface of the metal entering the rolls are functions of roll diameter and are of an entirely different degree of magnitude in comparison with the radii achievable by bending the entire shape over a pulley, for instance. The smaller the diameter of the roll l the less the pressure as determined by the percentage reduction in thickness will have to be. This is of practical significance for two reasons. By the use of very small diameter work rolls bearing upon heavier back-up rolls, the amount of cold working to which the bod of the metal section must be submitted may be held to a minimum and, in some cases, the portion of the cross section in which the elastic limit is exceeded may be confined to the immediate metal surface. Secondly, the amount of work which must be done on the metal becomes so small that it is often possible to pull material such as a rod or strip through the rolls by use of the existing traversing equipment without the necessity of driving the rolls.

It has been found advantageous to employ the rolls 7 having diameters in the range between 5 and 50 times the thickness of the material being 3 treated and effecting a reduction of less than in the thickness of the metal between the rollers.

The following examples of the working of the process will make clear the very radical change in the character of the oxide film under the action of the surface rolling treatment:

Example I.- H ot.rolled alloy steel strip, in thickness containing 12% of chromium, required two hours of treatment in a 10% sulphuric acid solution at 150 F. to effect all scale removal. Severe acid attack of the base metal o'ccurred. The same strip, passed between 3" diam ter rolls set to effect a 2% thickness reduction required only 10 minutes in the same acid solution to effect all scale removal, there being negligible attack on the base metal.

Example II.Box-annealed stainless steel plate, in thickness, required the following procedure to effect scale removal: minutes in fused caustic alkali made oxidizing by addition of oxidizing salts, followed by water quench, followed by 10 minutes in5% sulphuric acid at 160 followed by 5 minutes in 15% by volume nitric acid solution at 150 F; the whole of the above procedure having to berepeated again before complete descaling was effected, When the same strip was given a 4% reduction by passing between rolls as above described, it was possible to completely clean it according to the following procedure; 5 minutes in the salt bath above described, followed by a water quench, followed by two minutes and'one minute respectively in the acid solutions above cited.

Example III.Scaled, high carbon steel strip required lz minutes to effect complete descaling in an 8% sulphuric acid solution at 150F. After a thicknes'sreductionof 2%, the same strip re- .8 is 'achievedjwhichf results in the production of successive f minutely 'juxtaposed, microscopic fractures. These fractures'or cracks are immediately closed again when they pass between the jrolls butthey nowrepr'e'sent mechanical discontinuities in jthes'cale which serve as lines of ac- ,oess forsubsecluent liquid treatment. The roller action has no "physical effect upon the scale film other than sometimes to render it somewhat shinier in appearance and results in substan- "tially noscale removal. By its use,however, the

area of scale surface'available to be attacked is very greatly increased. Furthermore, the added areas are inlarger part at right angles to the scale surface and extend to the base metal. This has the effect of exposing to chemical attack the less resistant oxide components occurring in the layers nearer the base metal. Such deeper oxide films are generally more reacting particularly with acids due to their lower state of oxidation. The accessibility of the base metal itself at the bottom of the fractures also results in acceleration of the electrolytic influences tending to aid in removal of scale films by acid action.

The amount of cold working produced in the body of the metal is very slight in comparison, for

instance, with scale-breaking treatments which both by stretching and by bending operate to impart'a substantialdegree of permanent stretch throughout the metal, the present method concentrating the majority of base metal distortion at the surface.

I claim:

In the treatment of. a metal article of sheet, plate or strip form having an adherent surface film of scale thereon, subjecting the scale-coated article at a temperature below that of scale formation to rolling pressure between opposite rollers having their axes in a plane normal to the article surfaces and intersecting them at substantially the ar ea'of contact'of the rollers therewith, the diametero f-at least'one of said'rollers being more than five times and less than fifty times the thicknessof the strip, and f orcin'gjsaid rollers against the article surfaces'under sufilcient pressure to develop 'a ripple in th'e scalecarrying surfacein advance'of said rolling pressure'cont'act, progressively disintegrating the-said film of scale by saidroller'of limiteddiameter and preserving the'disint'egratedfilm in place on the article surface and simultaneouslyeffecting a reduction of less than 10% in the thicknessof the metal between the rollers, delivering the article with the disintegratedscale film-adhering thereto, and subsequently treating thesaidj'adhering disintegrated scale film with a fluid reacting therewith toloosen'and remove it.

ROLFE POTTBERG.

References Cited in the file of this patent UNITEDYSTATEES PATENTS Number Name Date 425,880 HOWBH Apr. 15, 1890 1,143,916 Raymond June22, 1915 1,171,757 speller Feb. 15,-1916 1,392,780 Marsh -Oct. 4,1921 1,689,512 Worton Oct; 30, 1928 1,817,707 Rees Aug. -4,1 93l 1,874,080 Brislin Aug. 30,1932 2,133,231 Schermer Oct.'11,'1938 2,234,153 Herber Mar. 4, 1941 2,259,277 Theiss "Oct.'14, 1941