US2105439A - Method of cleaning the surface of solid metals - Google Patents

Description

Patented Jan. 11, 1938 UNITED- STATES PATENT OFFICE METHOD OF CLEANING THE SURFACE OF SOLID METALS Clarence H. Lorig, Columbus, Ohio, assignor to The Battelle Memorial Institute, Columbus, Ohio, a corporation of Ohio I No Drawing. Application May 23, 1935,

Serial No. 23,071

4 Claims. (Cl. 148-4) My invention relates to method of cleaning the surface of solid metals. It has to do, more particularly, with a method of removing scale, defects and detrimental surface film from hot steel ingots, billets, blooms, sheet-bars, forging stock,

' and other iron and steel products, and from the ing heated. Large ingots of steel or large steel products that are being heated to a uniform temperature throughout their mass require many hours of heating, while even ,the smaller products may require from one to several hours in the furnace to heat them uniformly. In consequence of these more or less prolonged heating operations, the scale formed is generally of material thickness. i

Materials whose surfaces are marred by scale of this type have heretofore been cleaned by immersion in acid pickling solutions, by chipping or by grinding to remove the imperfections. Sometimes, they are cleaned with an oxygen-acetylene torch before final fabrication into finished products. These methods are time-consuming and expensive and usually do not entirely correct the damage caused by the scale. For instance, where the steel is to be subjected to hot-rolling or forging, the scaleremaining thereon is often worked into the surface of the steel to mar its appearance.

In some instances, the oxidation of the steel during these more or less prolonged heating operations causes formation beneath the scale of a film of metal which is different from'the main body of the steel. This is illustrated with the steels containing copper or nickel, which,,when

ing and constant care is required in the heating of them to overcome the tendency whichthese steels have to flake or become rough. Surface films of metal which are difierent in composition from the main body of the steel are in many instances detrimental to the quality of the finished product, especially if the products are hot-formed at high temperatures.

This oxidation of steel in soakin -pits or in heating furnaces contributes to another serious obstacle in making high-quality finished steel products. This obstacle is the decarburization of the surface of carbon-bearing steels with the formation of a zone of relatively pure iron and a loss in molybdenum and other alloying elements of high vapor pressure from the surface of steels containing these alloying elements. The disadvantages of the decarburized zone are often met with in attempting to harden the surface of such decarburized steels through heat-treatment,

where surface resistance to abrasion and wear are requisite properties of the product. Owing to the lack of carbon, the surfaces cannot be hardened and the wear or abrasion lbss'is therefore excessive.

In addition, some difiiculties in acid pickling of 3 sheet-bar and steel have also been traced to'the existence of the decarburized layer. Likewise, the loss in molybdenum on heating and oxidizing molybdenum bearing steels through the volatilization of the oxide, M003, quickly produces a considerable de-molybdenized shell which seriously impairs their properties. These are only -a few of the many disadvantages of decarburized or de-molybdenized zones in steels which serve to illustrate the need for-preventing occurrence of such zones or for removing them in the event of their formation.

There are other defects .in hot-worked steel which are traceable to the surface condition of the ingot, as originally cast. Depressions or cavities in the ingot surfaces, due to gases arising from the mold wash, from the molten metal or from the molds themselves, to irregularities in the mold surfaces, or to other causes, are sometimes deep and, when the ingots are rolled or forged, these depressions precipitate the formation of laps, slivers and seams which must later be removed by grinding or chipping.

Metallic articles such as ingots, billets and castings of chromium, nickel, copper, manganese, zinc, silicon, tungsten, molybdenum, lead, cadmium, cobalt and their alloys, as well as articles of some of the less abundant metallic elements, are. characterized by effects when they are heated in oxidizing atmospheres which compare similarly with the effects described above in connection with ferrous metals. Thus, copper, zinc,

molybdenum, tungsten, et cetera, oxidize readily in heated atmospheres; copper-zinc alloys, copper-cadmium alloys, copper-silicon alloys, and many others changetheircomposition at the surface through volatilization and oxidation of some of the alloying 'constituents. For example,

the surfaces of cast,products made from chro-' mium, aluminum and their alloys are, frequently poor in appearance, et cetera. Thus, discussions applicable to iron and steel, in various degrees,

are applicable to other metals and their alloys.

In the prior art, some effort has been'made to obviate the formationof scale upon certain metals during subjection to heating. For example, it has been suggested that iron, steel, and the like be subjected to a reducing atmosphere during the heating of the product. Also, some effort has been made to bring about the removal of these oxide films or coatings which -'occur duringthe heating operation and to protect the metal thus cleaned until its temperature has been lowered to a point at which protection is no longer required. One suggestion along this line involves the submergence of the scale covered metal in a molten bath of slag capable of dissolving the scale and of then adhering to the cleaned metal until it is cooled to such an extent that no scale will form thereon.

,These prior art methods, however, fail to solve the difficulty arising as a result of the formation of a film of metal different from the. main body of steel beneath the scale. In other words, though the scale has been removed, the underlying films, such as copper-rich and nickel-rich films occurring upon copper and nickel steels: during heating, still remain as a potential source of trouble. Likewise, these prior art methods fail to adequately prevent decarburization of the surface of carbon-bearing'steel, de-molybdenization of molybdenumsteel and the loss of other alloying elements of high vaporpressure from the surface of steels containing such alloying elements." Also, they do not adequately reduce or and seams.

One of the objects of this inventioriis to remove or prevent the formation of oxide or scale on the surfaces ofv heated metals.

Another object of this invention is to remove or prevent'the ,formation' of a film of metal at the surface of the metal which is being heated or which has been heated. I

A third object of my invention is'to prevent the formation of or :remove decarburized zones from the surface of steell' Another objector my invention is to reduce Still another object of my invention is to eliminate entirely or materially reduce the defects appearing' on hot-worked metals as the, result of scale, surface films or depressions and cavities existing in the metals prior to the hot-work'- ing operatiom- 3 Still another object of my invention is to, reduce the cost of preparing the surfaces of metal ingots, billets, or shapes prior to hot-working.

' or the like at any hot-working temperature, withresult of the concentration of the ,copper or similarly acting metal at the surface of the steels.

Various other objects will appear as this description progresses. The-preferred embodiment of my invention comprises the use of molten metallic baths for cleaning solid metals. In most cases, Iuse a metal or an alloy thereof which contains the same metal as the metal or alloy being subjected to treatment. This will insure not only the removal of the oxidized surface but will render available a solvent which will remove surface defects.

For instance, should I wish to remove the oxide and the zone of dezincification from a. copper alloy containing zinc, I may use a molten bath containing substantial quantities of copper.- This bath shouldpreferably be of a lower melting point than the copper alloy; yet, this requirement is not always essential. This copper-containing bath will remove the zone of dezincification by solution.

While for steel and iron products, my baths may be principally iron, they preferably, though not necessarily, contain other elements. which lower the melting point of the bath. For example, carbon used in sufiicient quantities lowers the melting temperature of pure iron from about 2785 F. to about 2100 F., when it contains 4.3

g per cent carbon, the eutectic composition. Other alloying elements, such as silicon, manganese, phosphorus, sulphur, boron, chromium, et cetera, affect both the eutectic composition of ironcarbon alloys and the eutectic temperature. In

' general, the presence of these elements reduces the quantity of carbon for a eutectic mixtureand lowers the eutectic temperature. Thus, steel, pig I metal, I mean an alloyw of carbon and iron with all other elements substantially removed in processing.

I have found that such baths are effective in I removing scale from steel, either mechanically by fiuxing orby reduction, and for dissolving from the surface of thesteelmetallic films and layers that are decarburized or de-molybdenized. Renioval of the scale is rapid.- While it is still g obscure, as to exactly what occurs, it is conceivable that through the reduction of the iron oxide by carbon in the metal bath, carbon monoxide is expelled at the surface of the steel and that this carbon monoxide mechanically helps to remove the scale. It is also conceivable that some silicon from a pig iron castiron bathis used to reduce the scale and that the silicon dioxide formed'unites with some of the scale to produce fusible slag. Silicon may act therefore to remove some scale by fiuxing. or completely eliminate surfaces on metals which differ in composition from the metal itself.

Metallic films, such as the copper-rich and the'nickel-rich layers that form beneath the scale of alloy steels bearing these metals are soluble in molten iron, as are pure iron and the steels themselves. Hence, with due precaution with regard to the bath and steel temperatures and the time that the steel is submergeddn the bath, it

- is possible to prepare steel, for hot-rolling or forging or for any of the fabrication operationawhich has no scale on its surface and which is free from the deleterious surface layers of metal that differ in composition from the-main-body of steel. This metallic .bath, through solution, also removes many of the surface defects on steel, suchas laps,

.75 out, causing surface imperfections to form as a seams, and burned areas, and tends to smooth out the depressions and cavities so that, on subsequent hot-working, they do not develop into defective surface areas on the steel.

I have demonstrated that the surface of hotrolled steel may be improved by the use of cast iron bath. Thus, a steel containing 0.2 per cent carbon and 2.0 per cent copper was treated by me in this manner. A bath of cast iron was melted under a layer of coke and graphite to maintain its carbon content as high as possible. Bars of steel, 1%" square, were heated for two hours in the same furnace with the molten iron but exposed to the furnace atmosphere. The

- temperature of the furnace ranged between 2320 F. and 2400 F. In that period of time, the scale on the steel became slightly more than 4 in thickness. a

Two of these bars were dipped in the cast iron for about eight to ten seconds and were then immediately rolled in six passes to round bars. The brief immersion of the bars of steel in the molten cast iron at the same temperature cleaned them of scale and removed from their surfaces the copper-rich film which had formed beneath the scale. The quality of the surface thus produced was excellent.

A third bar of the same steel with the scale thereon was rolled, without first cleaning it. Its surface, after such rolling operation, was so badly cracked and so rough,.due to the rolling in of the scale and the effect of the copper-rich film beneath the scale, that it would be rejected for any commercial application.

In practice, molten pig iron from the blast furnace maybe taken directly to the rolling mills and used for washing .or cleaning the ingots or billets before they are rolled. Not only does the molten ferrous bath remove the scale but it may be so employed that it will dissolve the surface layers of dissimiliar metal, that has formed during heating of the steels, to any desired extent.

' The operation can be so adjusted that the steels are held in the bath only for a time sufiicient to insure removal of the scale and correction of the surface composition. As indicated above, for a billet of steel 1%" square, I have found that an immersion period of eight to ten seconds in a bath of cast iron at 2400" F. was sufiicient. ;,However,

the time required for cleaning is dependent upon r -a number of factors.

These include the steel composition, the thickness of scale, the tem perature of the bath, the temperature of the steel,

the size of the steel article, the composition of the metal bath and other factors. Under some circumstances, it is desirable tohold the steel in the molten ferrous bath for as long as fifteen minutes-before removing. A molten steel bath,

for instance, would act very rapidly, while a cast iron bath at 2150 F., for example, would act v plied from the carbonaceous covering.

The use of such an overlayer of slag or carbon is also advantageous on other metallic baths. It

mayserve to protectthe moltensurfaces against oxidation. It may function as a reducing agent. Also, it may serve to counterbalance pick-up in 'oxidewhich occurs during the use of the bath.

The slag covering the molten metal bath may be of such a selected composition that, as the solid metal article to be cleaned isbeing submerged into the molten metal, the slag will vigorously attack any oxidized surface, thus aiding materially in the cleaning of the article. A slag containing 60 per cent, by weight sodium carbonate and 40 per cent, by weight, boraxwill effectually serve the purpose. Furthermore, when the article is being withdrawn from the bath of metal, .the slag may be of such character as to form a sufficiently tenaciousand impervious film on the article, thereby reducing or eliminating any tendency for the hot article to again scale when exposedto the atmosphere.

In my process of cleaning solid metals I may therefore use a combined slag and molten metal bath or I may proceed with the cleaning in two separate steps, namely, remove some or all of the oxide from the -surface of the article in a slag bath and then transfer the article to a molten metal bath where the removal of the remaining oxideis completed and where other detrimental surface films or surface imperfections are removed. Furthermore, I may make use of a slag film to protect the cleaned metal surface against further oxidation.

Composition of the baths may be maintained by periodically adding new metal. In many operations, contaminated bath metal may be returned to the reduction plant for refining. In this .way, relatively. little metal will be lost and the maintenance of the molten metal bath will be I inexpensive.

tice maintains uniform temperatures throughout the metals while they are being cleaned.

Where close hot-working temperature control is unnecessary, the temperature-range of the bath may 'have considerable latitude.

One of the advantages of this invention arises from the fact that the steels and other metal and alloy products may becleaned easily and 8001101111.- cally before they are hot-worked, thus improving the quality and surface appearance of the products formed therefrom. Another advantage is that certain steels which have hitherto not been hot-workable with any degree of success, can be rendered hot-workable by my cleaning process. It/is also possible, with my invention, to heat metals to any desirable temperature without oxidizing their surfaces, so that a loss of metal through scale formation is avoided. Furthermore, steels and other metals can be cleaned without resorting to such practices as acid pickv.ling, grinding, burning or chipping. Likewise,

my method makes possible the removal of many defects which would be too costly to remove by prior art methods and which would bring about rejection of the steel for 'useful application. In addition, the use of a metal bath makes possible the recovery of the metal which is dissolved by the bath. I

It is a well-known fact that rolled-in scale causes numerous defects in steels. Salt is often sprinkled on the hot steel while it is being rolled to aid the removal of the scale, but for certain types of steel this practice does not give satisfaction. Steels containing nickel, for'example,

have a tenacious dry scale which is dimcult to remove by present methods and which, if allowed to remain, spoils the appearance of the hot- .worked article. My method for removing the scale is positive and avoids the shortcomings of present practices.

Furthermore, my invention permits the hotworking of copper steels at temperatures employed for hot-working the carbon-grade steels. This is because the copper-rich layer that forms beneath the scale of copper steels, due to preferential oxidation of the iron andthe rejection of the copper to the steel surface, can be prevented from forming when thesteels are heated in slag baths or can be dissolved ofi inthe molten ferrous bath. When this layer is removed the copper steels roll equally as well as carbon steels and show no surface roughening.

It should be understood, however, that my invention is not limited to any type of iron, steel or other metal but may be employed for all types.

which comprises dissolving any copper-rich outer layers existing thereon by immersion in a bath of molten ferrous metal.

2 A method of removing de-molybdeniz ed layers from solid molybdenum steel or iron which comprises immersingthesolid steel or iron in a bath of molten ferrous metal and retaining it in such bath for a time sufficient to dissolve the demolybdenized layers. f

3. A method of treating solid iron or steel to remove therefrom any outer layer or layers differing in structure or composition or both from the main body of the iron or steel which comprises comprises heating suchobjects to a temperature approximating that required for hot-working such as rolling or forging, cleaning the surface of the object by immersing such surface in a continuous body of molten ferrous alloy until, the oxide scale resulting from heating is substantially removed and then removing and, without further heating, hot-working the thus cleaned object.

. v CLARENCE H. LORIG.