US2199326A - Method of annealing metal - Google Patents

Description

April 30, 1940. H. UNCKEL 2,199,326

METHOD OF ANNEALING METAL Filed May 3. 1937 Patented Apr. 30, 194i) UNITED STATES 2,199,326 l METHOD OF ANNEALING METAL Hermann Unckel, Finspong, Sweden Application May 3, 193i, Serial No. 140,565

. In Sweden May 5, 1936 i Claim.

This invention relates to a method of producing smooth surfaces on metals and metal articles in annealing them to restore their ductility subsequent to a cold working process by which they have been rendered hard. The usual practice up till now in annealing metal articles with a view to restore their ductility has been to anneal the articles by heating them to a temperature above the recrystallisation point of the metal and, after cooling, remove the oxide film formed duringthe annealing process by pickling the articles in an acid bath. -By virtue of the oxidation taking place on the metallic vsurface during the annealing and by the action of the strong pickling agent required for removing the oxide film, the metallic surface becomes dull.

Sometimes, however, it is ,desired to give the annealed article a smooth and bright surface.

' In the recent years several brightening methods brass, none of the brightening methods hitherto commercially employed, has given really smooth or bright surfaces. In the annealing of brass according to known practice the'zinc which forms one of the constituents of this alloy is oxidized exceedingly easily so that the small amounts of oxygen which are present in the furnace atmosphere, even in case of a closed furnace, form surface oxides. Moreover, the zinc, when heated to high temperatures, has a rather high vapor pressure so that zinc will evaporate and, in part, again condense on the metallic surface, making the same dull,

According to the present invention it is possible in annealing processes to obtain completely smooth and bright surfaces even in case of very sensitive metals and alloys, as for instance, brass. The invention resides in the combination with the well-knownannealing process which is carried out in a substantially inert atmosphere, from which the oxygen of the air is excluded, of a pickling process carried out by the use of weak pickling agents the acidity of which is suflicient to clean the oxides from the metallic surface but insufficient to materially attack the, metallic surface proper. The novel method wil hereinafter be described as applied to brass,.

th ugh it is to be noted, that it is not restricted and furnaces to carry said methods into effect- (Cl. 148-4) q to this alloy .but may be applied to advantage to other metals and allows which are of such a nature as to be easily attacked by the oxygen of the air and strong pickling agents. 3

"In the accompanying drawing, Fig. 1 is a diagrammatic vertical sectional view ofa plant, including a muffle furnace, for carrying out the method in a discontinuous way. Fig. 2 is a side elevation, partly in section, of a plant including a muflle furnace for continuous operation.

In Fig. 1, reference numeral I indicates the furnace wall, 2 is the material to be annealed, and 3 indicates. the end of the mufile, immersed in a water mass 4. After the annealing process is completed, the material 2 is immersed in the water 4 by means of a raisingand lowering or immersion device indicated at 5. This furnace allows only a discontinuous operation. Fig. 2 shows a furnace for continuous operation. The muffle furnace is indicated by the numeral l. The muflle ends 3 3 are bent and each immersed in a water mass 4 4 In this case, the material under treatment is conveyed continuously through the water seals and the furnace by means of a conveying chain or a conveying band 6.

In carrying out the annealing process-accorde ing to a preferred form of theinvention, predetermined'charges of the material to be treated are placed in a closed mufile furnace, as illustratedin Fig. 1 of the drawing, filled with a dry, inert gas or with steam, if desired, mixed with an inert gas so that under,all conditions the oxygen of the air will be excluded as perfectly as possible. In many cases, however, it is preferred to employ a type of furnace in which the ends of the muffle are bent downwards and immersed in water, as illustrated in Fig. 2, in

order to close the muille proper, within which the annealing process is carried out, against the atmosphere by water seals. In this latter case, the material under treatment is conveyed continuously through the water seals and the furnace by means of the conveying chain or conveying band 6. In case of drawn or rolled material of great lengths, the material maybe pulled directly through the water seals and the muflle. Brass will not be as bright as glass by such an annealing treatment but will present a discolored surface which may be caused by a certain oxidation resulting from'reactions with mineral constituents of the water used as water seals or by oxygen dissolved in the water or liberatedtherefromand to some extent also by a slight evaporation of zinc. It was. therefore,

up till now considered impracticable to give brass a smooth and bright surface by an annealing process. According to this invention, the discoloring film formed on the annealed material is removed by pickling the material in a very weak pickling bath containing, for instance, tartar or tartaric acid, or extremely dilute hydrochloric acid, acetic acid, sulphuric acid or other very dilute acids (of a concentration of 2 per cent., for instance, or less), and then washing the material and drying it in the usual-Way. By this treatment, the surprising effect is obtained that the surface is perfectly smooth and as bright as glass. Instead of effecting the pickling process after the discharge of the material from the furnace, the pickling agent may be added to that portion of the water which forms the water seal at that end of the muflie where the material is discharged, whereby a separate pickling operation is eliminated.

In the oxidizing annealing of brass and similar materials hitherto commonly employed, an effective pickling treatment must always be efiected subsequent to the annealing treatment, to remove the comparatively deep oxide layer. In case of brass, a 20 per cent. sulphuric acid solution is usually employed at a temperature of about 50-'70 C. If a weaker bath wereused, then the pickling time would be uneconomically long and still no smooth and bright surface would be obtained, because the metallic surface covered by the oxide layer would already be rough and uneven as a result of the extended oxidation caused by the annealing, and the unevenesses thus produced cannot be removed by a pickling process.

The activityor acidity of the pickling liquor' employed according to this invention, is only about a tenth or a twentieth of that hitherto used in effecting a pickling subsequent to an oxidizing annealing process. As an example, a

bath containing 2 per cent. of tartaric acid, the

.and the temperature of the pickling bath and the duration of the pickling action so as to only remove the miscolouring surface layer of the annealed material while not attacking the metallic surface proper. By this means the smoothness and brightness of the metal, after the picklingprocess is finished, will be very close to that of the material prior to the annealing process. In order to express this degree of smoothness or brightness by figures it might be mentioned that investigations made by Messrs. Kenworthy and Waldram (Journ. Inst. Met., of 1934, page 247 if) and by Mr. R. Toussaint (Rev. dAluminium, of 1934, page 2403) have proved that the highest smoothness and brightness (as measured by the reflexion from a brass surface at a 45 angle of incidence of light) at the most perfect polishing amounts to about 95 per cent. of the incident light. On the contrary, a dull reflecting surface has a coefficient of reflexion not higher than up to 10-20 per cent. Smooth rolled brass might have a reflexion coefiicient up to about 50-60 per cent. An annealing and pickling treatment according to the hitherto employed methods yields, when applied to brass, a surface reflexion of 10-20 per cent., whereas a treatment according to the present invention will give the brass surface a smoothness and brightness very close to that of the smooth rolled material prior to the annealing treatment.

The technical advantages gained by the invention may be briefly summed up as follows. Hitherto, in order to obtain a smooth and bright surface, a brightening annealing treatment according to one of the well-known methods has been employed whereby satisfactory results may be obtained in case of certain materials. As to other kinds of material, as brass, the bright annealing treatment is entirely abandoned, as by this treatment it was impossible to obtain a glassy surface, and instead thereof a usual oxidizing annealing treatment and a subsequent strong pickling .has been adopted. According to the invention I use a bright annealing treatment and a subsequent weak pickling process in case of .and brightness which up till now was not obtain- The method of producing bright surfaces in the annealing of metal containing a component having a strong tendency to oxidize at annealing temperature and which has been rendered hard by a cold working process which comprises passing the material to be annealed continuously through a water seal into a muflie furnace and through a practically inert atmosphere therein, free from oxygen to reduce oxidation as far as possible, heating thematerial to the annealing temperature during this passage, and passing the annealed material out of the furnace through a hydraulic seal formed by a weak pickling agent of an acidity sufficient to dissolve metallic oxides but insuflicient to attack the metallic surface proper.

HERMANN UNCKEL.

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