US2412186A - Method of bright polishing stainless steel tubes - Google Patents

Description

De@ 3, 1946 l. wHrrEHousE x-:rAL 2,412,186

METHOD OF BRIGHT POLISHING STAINLESS STEEL TUBES Filed Oct. 24, 1942 INVENTORS HPV/N6 wH/ fHol/SE 4 v/c 702? s. CHAMBERS 'ing of. inner surfaces of pipes and tubes.

Patented Dec. 3, 1946 METHOD F BRIGHT POLISHING STAINLESS STEEL TUBES Irving Whitehouse, south Eucua, and victor s.

Chambers,

Cleveland,

Ohio, assigner: to

Republic Steel Corporation, Cleveland, Ohio, a corporation of New Jersey Application October 24, 1942, Serial No. 63,216

Clalms.

This invention relates ,generally to the art of polishing stainless steel articles and particularly to the bright polishing of the inner surfaces of stainless steel tubes.

A demand has existed for a considerable time for stainless steel pipes and tubes for uses where the maintenance of sanitary conditions is essential, for example, in the milk industry. Altho efforts have beenmade by others to meet this demand none of those efforts have been entirely satisfactory so far as we are advised. To be sat-y isfactory for uses requiring a high degree of sanitation, it is important that the inner surfaces of the pipes and tubes should be bright polished, that is, they should be free from scratches, pits and, in general, any surface roughness in which foreign material may lodge and from-which it cannot be removed readily.

Heretofore efforts have been made to bright polish the inner surfaces of stainless steel pipes and tubes by the use of endless abrasive belts. In addition to being quite expensive .this method is not entirely satisfactory for, no matter how fine the abrasive material on the belts was, the belts made a multitude of minute scratches on the inner surface of the tube from which foreign material could not readily be removed. Altho the exterior surfaces of stainless steel tubes and pipes have been bright polished satisfactorily by the process of electrolytic polishing in which the tubes were immersed in or drawn through the electrolyte, that process, when applied to the inner surfaces of suchpipes and tubes, resulted in pitting and otherwise roughening .those surfaces so that they were unsatisfactory for uses where a high degree of sanitation was required.

We have discovered the causes of unsatisfactory l obtained when the procedures which are satisfactory for polishing exterior surfaces are applied to inner surfaces are traceable to certain factors and conditions which are peculiar to the polish- Apparently one of such factors is that the ratio of amount of electrolyte to the area of the sur- `a sheet or strip. The use of a smaller amount of electrolyte seems to result in more rapid depletion of the active ingredients in the electrolyte, rapid rise in electrolyte temperature and liberation of more gases per unit of electrolyte. Seemingly gases formed in a tube adhere to the tube and create theproblem of detaching and removing them whereas gases formed on exterior sur-y faces, either dislodge themselves or are readily dislodged and, once separated from the metal surface escape thru the electrolyte. Gases which are mechanicall trapped in the electrolyte as distinguished from 'those which may adhere to the tube must also be removed quickly to avoid unequal pickling and resulting roughness of the tube.

'I'he present invention is predicated on the dis.-`

covery of the foregoing factors and on the provision of means and steps by which those factors and their disadvantages may be avoided. Briefly stated, the present invention consists of the steps of flowing a suitable polishing electrolyte, preferably under pressure, thru a pipe whose inner surfaces are to be'bright polished, and passing polishing current in substantially uniform amount between each unit of the inner surface of the tube and a cathode disposed on the longitudinal axis of the tube, and preferably extending beyond eachy end of the tube. The temperature and concentration of the electrolyte are controlled within predetermined ranges and gases are allowed to escape from electrolyte leaving the tube. Preferably the tube is polished while in a vertical position and while the electrolyte is being forced upward therethru under pressure somewhat greatei` than atmospheric pressure.

In the drawing accompanying and forming a part of this specification,

Figure 1 is a somewhat diagrammatic view of one form of apparatus in which the present invention may be practiced; and,

Figure 2 isv an enlarged fragmentary view, partly in section, of one end of the tube being .treated and associated parts of the apparatus of Fig. 1.

In Fig. 1, l designates a receptacle for electrolyte which4 has a capacity several times that of the tube whose inner surface is to be polished. A heat exchange coil 2 serves to control the temperature of the electrolyte 3 in receptacle I. A pipe line 4 serves to conduct electrolyte from the lower part of receptacle l to a pump 5 which forces the electrolyte under pressure thru pipe 6 and into and thru the tube 'l whose inner surface is to be polished. The electrolyte discharged from tube 1 passes thru pipe 9 and is discharged into receptacle I, the gases escaping into the air above the surface of the electrolyte in receptacle I whence they may be withdrawn by a fan or other suitable means (not shown) if desired. Along opposite sides of tube 1 electrically conductive metallic strips 9 are held in close contact with the tube 1 and serve to distribute throughout the length of the tube 1 current sup'- plied to strips 9 thru a cable I0 from a sourceof current (not shown). A cathode II in the form of a rod is connected by cable I2 to the source of current. This cathode Il is disposed centrally in tube 1, i. e., preferably on the longitudinal center line of the tube and, as shown, is long enough to project from each end of tube 1.

In Fig. 2 is shown the construction and combi-l nation of parts at either end of the tube 1 of Fig, l. There the tubular metal fitting I3 has a beveled surface I4 at one end to engage the end of tube 1. Between its ends fitting I3 has an renlarged chamber I5 which communicates with the opening defined by tapered surface I4 and with either pipe 8 or pipe 6 depending on whether the fitting is at the upper or lower end of tube 1. An annular inwardly projecting flange I6 adjacent to the chamber I5 serves as a stop for a gasket I1. A nonconductive ring I8 engages the gasket I1 opposite to the surface engaged by flange I6 and a tube I9 projects from ring I8 to slightly beyond the end of fitting I3. The gasket I1 and tube I9 are preferably composed of rubber material but any other material may be used which has the property of deforming lunder endwise pressure with coincident reduction in internal diameter.

The fittings I3 with gaskets I1, rings I8 and tubes I9 serve to locate the cathode II on the axis of or concentrically with tube 1 and to prevent loss of electrolyte at the ends of the tube and cathode. When nuts 20 are screwed onto cathodes Il and exert endwise pressure on tubes I9 the surfaces I4 of fittings I3 are pressed against the ends of tube 1 and the gasket I1 and rings I8 are deformed into sealing engagement with the fittings I3 and cathode II, Thus screwing nuts toward one another, or turning one while the other is xed, results in not only positioning cathode Il centrally in tube 1 but also in sealing the ends of the tube against loss ofv electrolyte.

It will, therefore, be seen that by the above described parts and arrangement of parts the cathode II is disposed and maintained concentrically in tube 1 and that liquid may circulate freely thru tube 1 from the fitting I3 at one end of the tube to a similar fitting at the other end. It will further beseen that the fittings I3 do not afford any pickets in which gases may be trapped in contact with the inner surface of the tube1 and do have, in chamber I5, a space which is well above the upper end of tube 1 and in which gases may collect and from which they may be removed without, coming into contact with the tube.

When the apparatus is assembled as shown by the figure, it may be operated substantially as follows: Actuation of pump 5 withdraws electrolyte from receptacle I and forces it under pressure, preferably greater than atmospheric pressure, thru tube 1 and back to receptacle I. When a source of current is connected to cables I0 and I2, current flows between strips 9, substantially Such flow of current thru the electrolyte polishes the inner surface of tube 1, liberating gases which are dislodged from the inner surface of the tube 1 by the moving electrolyte and are carried out of the tube 1. The major amount of these gases escapes from-the electrolyte when the latter is discharged i'nt'the air above the surface of the electrolyte in receptacle I. Most of the remaining gases escape from the upper portions of the electrolyte in the receptacle. As the'polishing action progresses, the temperature of the electrolyte increases. Cooling fluid is passed thru the cooling coil 2 to maintain the temperature of the electrolyte within the predetermined temperature range.

The following examples of various operating conditions which have been found to be satisfactory will make it possible for those skilled in the art to apply the present invention to tubes of different sizes and compositions without the necessity of experimentation.

A tube of 16 gage, 18-8 type, `stainless steel 11/2" in diameter and about 12 feet long was satisfactorily polished on its inner surface by being.

assembled in the apparatus shown in the drawing and treated as follows: While the tube was maintained in an approximately vertical position about 5 gallons of electrolyte per minute were circulated upwardly therethru for about ten minutes and the vtemperature of the electrolyte was maintained between about A F. and 140 F. and a current of about 3 amps. per sq. in. at about 5 volts was passed between the Walls of the tube and the cathode thru the electrolyte.

For the 18-8 type steels, the temperature range of the electrolyte may extend from about 100 F. to about F. and the amperage may range from about 3A amp. to about 6 amps., while the voltage ranges from about 5 to about 5.5 volts. Satisfactory polishing has been obtained under these conditions in about ten minutes. The 18-8 type stainless steels have, as a typical analysis, carbon'.08% to .20%, manganese 1.25% maximum,`silicon .75% maximum phosphorous and sulfur .03 maximum each, chromium 17.5% to 20%, nickel8% to 10%, the remainder being iron with minor amounts of ordinary impurities.

When tubes of the type AA stainless steel are to be polished somewhat higher electrolyte temperatures may be employed, preferably ranging between about F. and about 200 F. A typical analysis of the AA type stainless steels is carbon .12% maximum, manganese .50% maximum, silicon .50% maximum, phosphorous and sulfur .03% maximum each, and chromium 14% to 18%, the remainder being iron with minor amounts of ordinary impurities. A somewhat sho-rter polishing time is required with the AA type stainless steel tubes than with the 18-8 types. The curre'nt may be about the same as with the 18-8 types of steel. e

The currents specified hereinabove are, for the sake of brevity, referred to in the appended claims as "polishing currents, that is. currents which when employed with a suitable electrolyte and for the times stated will polish the inner surfaces of stainless steel tubes of the 18-8 and AA types the full length of tube 1 in substantially equal amounts between each unit area of the inner surface of tube 1 and the cen'trally disposed cathode to a bright finish.

There are various electrolytes which are suitable for use with the present process. One such electrolyte is composed of about 40% by weight of 96% sulfuric acid, about 5% by weight of tartaric acid, the remainder being water. Another suitable electrolyte consists of about 50% by weight of 96% sulfuric. acid, about 10% tartarlc acid, the remainder being water. Electrolytes of this general composition may contain from about 40% to about 70% of 96% sulfuric acid by Weight and from about 5% to about 20% tartaric acid by weight, the remainder being ordinary tap water. These tartaric and sulfuric acid-containing electrolytes may be used at temperatures between about 100 F. and about 200 F. Another electrolyte which has given fairly satisfactory results consists of about 60% of glycerine and 40% of an 85% solution of phosphoric acid. This electrolyte seems to operate more satisfactorily at. temperatures at approximately 200 F. Other electrolytes which are capable of polishing stainless steel surfaces to a bright finish when used in conjunction with polishing currents may also be employed in carrying out the present process. All such electrolytes are collectively referred to in the appended claims as polishing electrolytes, for the sake of brevity.

It is important, as noted above, that there should be a substantially uniform flow of current between each unit area of the tube and the cathode. A satisfactory flow of current may be obtained when the strips 9 and cathode I l are composed of good electrically conductive material, such as copper, and when the cathode is disposed centrally in the tube and the strips are clamped against opposite sides of the tube 'l at a plurality of places -so that the strips will press against the tube thruout substantially their full lengths. In this manner the metallic part of the circuit is highly conductive, with the exception of the part composed of the stainless steel tube. However, that part of the circuit can not disturb the substantially uniform flow of current at each area of the inner surface of the tube for the current does not Dass lengthwise thru the tube, as would be the case if the strips 9Y were not employed.

Having thus described our invention so that others skilled in the art may be able to understand and practice the same, we state that what we desire to secure by Letters Patent is dened in what is claimed.

What is claimed is:

1. 'I'he method of polishing which includes the 'steps of passing thru the annular space between an elongated stainless steel tube and a cathode disposed axially therewithin a polishing electrolyte which iills said space and ilows at a rate sufcient to dislodge gases from the inner surface of the tube, and passing thru the flowing electrolyte and between the tube and cathode a polishing current which is of substantially uniformdensity r on every unit area of the inner surface of the tube.

2. The method of polishing which includes the steps of passing a polishing electrolyte up thru a stainless steel tube, and passing thru the owing electrolyte and between the tube and a cathode disposed axially therewithin a. polishing current which is of substantially uniform density on every unit area of the inner surface of the tube.

3. The method of polishing which includes the steps of passing thru a stainless tube several feet long a polishing electrolyte which ilows at the rate of several gallons per minute and iills the tube, and passing thru the flowing electrolyte and between the tube and a cathode disposed axially therewithin a. polishing current which of substantially uniform density on every unit area of the inner surface of the tube.

4,. 'Ihe method of polishing which includes the steps of passing thru a stainless steel tube several feet long a polishing electrolyte which ows at the rate of several gallons per minute and fills the tube, and passing thru the flowing electrolyte and between the tube and a cathode disposed axially therewithin for approximately ten minutes a polishing current which is of substantially uniform density on every unit area of the inner surface of the tube and which ranges between about .75 and about 6 amps. per sq. in.

5. 'I'he method of polishing which includes the steps of passing thru a stainless steel tube a polishing electrolyte which fills the tube, passing thru the flowing electrolyte and between the tube and a cathode disposed axially therewithin a polishing current which is of substantially uniform densityV on every unit area of the inner surface of the tube, liberating gases from the electrolyte after it has passed out of the tube, cooling suchv electrolyte to between about F. and200 F. and returning the cooled electrolyte to the tube.

6. The method of polishing which includes the steps of passing a polishing electrolyte at the rate of several gallons per minute up thru a stainless steel tube several feet long. Passing thru the electrolyte between the tube and a cathode disposed axially therewithin for approximately ten min-- utes a polishing current which is of substantially uniform density on every unit area of the inner surface of the tube and which ranges between about .75 amp. and about 6 amps. per sq. in., liberating gases from the electrolyte after it haspassed out of the tube, cooling such electrolyte to between about F. and 200 F. and returning the cooled electrolyte to the tube.

7. The method of polishing which includes the steps of passing thru a stainless 18-8 type steel tube several feet long a polishing electrolyte which flows at the rate of several gallons per minute and lls the tube, passing thru the ilowing electrolyte and between the tube and a cathode disposed axially therewithin for approximately ten minutes a-polishing current which is of substantially uniform density on every unit area of the inner surface of the tube and which ranges between about .75 and about 6 amps. per sq. in., liberating gases from the electrolyte after it has passed out of the tube, cooling such electrolyte.

to between about 120 F. and about 140 F. and returning the cooled electrolyte to the tube.

8. 'I'he method of polishing which includes the steps of passing thru a stainless AA type steel tube several feet long a polishing electrolyte which flows at the rate of several gallons per minute and illls the tube, passing thru the electrolyte and between the tube and a cathode disposed axially therewithin for not over ten minutes a polishing current which is of substantially uniform density on every unit area of the inner surface of the tube and which ranges ybetween about .'75 and about 6 amps. per sq. in., liberating gases from the electrolyte after it has passed out of the tube, cooling such electrolyte to between about F. and about 200 F. and rei turning the cooled electrolyte to the tube.

IRVING WHITEHOUSE. VICTOR B. C.

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