US2413929A - Heat-treating bath for aluminum - Google Patents

Description

Patented Jan. 7, 1947 HEAT-TREATING BATH FOR James F. Simpson, Old Greenwich, Conn, as-

signor to American Cyanamid Company, New York, N. Y., a corporation of Maine i No Drawing. Application May 21, 1945,

Serial No. 595,069

The present invention relates to the heat treat- Which is adapted, upon fusion, therefor.

Heretofore, aluminum base alloys have been heat treated to improve their mechanical properties in a circulating gas furnace and/ or in baths of fused nitrates, particularly sodium nitrate.

Heat treatment of aluminum base alloys by means of circulating gas furnaces has included many disadvantages. The heating has not been rapid, uniform, nor evenly maintained. It has been necessary to heat non-productive articles which carry the aluminum pieces such as trucks, carriages, baskets, hangers, and the like. This type of heat treatment is uneconomical and inefficient.

The use of fused nitrate baths involves the maintenance of a hazard, unwarranted in view of the present invention. Although fused sodium nitrate gives physical properties slightly above the minimum requirements set forth in metallurgical specifications, a bath is maintained which can readily cause violent and/or explosive oxidation of any organic materials which may make contact with it.

-The principal object of the present invention is to provide a method and means of heat treating aluminum base alloys which will produce mechanical and physical properties in the alloys treated as good or better than those now obtained by treatment in the usual circulating air furnace or fused nitrate bath, which will have no explosion hazard, which will be chemically inert to the alloy, which will be cheap, and otherwise desirable; I

To this end the invention contemplates the heat treating of aluminum base alloys in a molten *ment of aluminum base alloys and to a salt bath hath comprising KCNO in predominant quantity.

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7 Claims. (Cl. 148-211) desired heat treatment as obviously no treatment in such baths may be had prior to their melting point, which varies considerably. For instance, baths in which aluminum base alloys may be satisfactorily heat treated are given below together with their melting points:

Bath composition gg {KCN H 580 While the above composition represents substantially eutectic mixtures, the proportions may be reasonably varied when'slightly higher temperatures are desired. Such baths may be used at temperatures varying from their melting point up to a temperature just short of substantial decomposition. While the temperature and time of treatment will vary slightlydependent upon the actual composition and mass of the aluminum base alloy, this is a minor variation, as in all cases it has been found that the temperature and time of treatment is substantially that now used for the same alloys in nitrate baths, and some+ what shorter than in circulating air furnaces.

The baths of the present invention for the solution heat treatment of aluminum base alloys are operative between substantially 600 F. and 1050" However, the usual procedure is to treat the piece for varying lengths of time at about 900 to 1010" F. and quench it in water for 10 to 30 amount of aluminum is omitted, but can easily be figured because impurities exist only in traces, it being understood that the alloy has an aluminum base. The common chemical symbols are used in reference to the alloy components.

TABLE I Aluminum alloys Type Composition g igz Casting... 4% Cu;1.5% Mg; 2% Ni 94am 5-l6 hrs.

4.5% Cu 940-970 16-24 hrs. 7% Si; 0.3% Mg 990-1010 12-16 hrs.

Wrought. 4% Cu; 0.5% Mg; 0.5% Mg... 925-950 -90 min; 4.5% Cu; 0.6% Mg; 1.5% Mg 910-930 15 .90 min. 07% Si; 1.3% Mg; 0.25% Cr 060-980 15-45 min. 0.25% Cu; 0.6% Si; 1.0% Mg; 900-980 15-45 min.

Table-II shows the amount of corrosion resulting from the solution heat treatment of aluminum alloy sheets whoseprincipal alloying constituents are 4.5% copper, 0.6% manganese, and 1.5% magnesium. The salt bath which is used is composed of 87% KCNO and 13% KCl and is heated to substantially 930 F. while the test pieces are immersed therein for varying periods of time. The pieces are then quenched in tap water for 5 to 10 minutes.

TABLE II Corrosion of aluminum alloy sheet Ilium; Hours of Average loss in weight Average heat penetratlon, p105 treatment MgJCmI, Mgllcmjlhn inch/year 4 M2 0.0116 0.14 2 it 018 .11 2 %1 (1185 06 2 as .0265 .04 2 1 /5 .062 046 2 2% 115 043 1 4} 21 047 2 20. 45 022 The salt bath described above is typical of those described earlier in this specification. The corrosion of the aluminum alloy is small and any salts which do adhere to its surface are easily washed off with water.

Table III shows the physical properties of specimens of an aluminum alloy of substantially the same composition when they are individually heat treated in the indicated baths for substantially 30 minutes at substantially 930 F., and quenched in water for 10 to minutes. The tensile strength in pounds per square inch and the per cent elongation in2 inches on a standard tensile. test specimen. are. givenfor thestrains exerted on both thetransversal and. longitudinal specimens.

4 TABLE III Physical and mechanical properties of heattreated aluminum alloy 0 Transverse] Longitudinal Remarks Tensile Percent Tensile Percent strength, elongastrength, elonga- 1,000 tion in 1,000 tizon in s. i 2 in. .s. i. in 10 P D Specifications. 62.0 15. 0 64. 0 13. 0

1- NaNOa bath... 65.0 18.0 66.6 18.6 2"... KCNO bath in 641 5 19. 2 66. 5 18. (i l 5 an aluminum surfaced pot. 3. 87KCNO-l3KCl 67. 0 21. 2 69. 5 l9. 7

- bath in an aluminum surlaced pot. 4 87KCNO-13KC1 65. 0 20. 5 67. 0 20. 5

bath in steel 20 pct.

5 Second run of 66. 0 18. 5 68. O 20. 0

No. 4 in a clean steel pot. G 87KCNO-13KC1 67. 0 19. G 69. 0 1S. 0

bath in steel pot.

Average of No. es. a 197 s as. 4 1s. 6

' 3-No. 6 incl.

It is seen that the non-hazardous KCNO bath is as good as a fused nitrate bath. The values obtained by using an eutectic mixture, KCNO-KCI in this instance, are better than obtained by using a fused NaNOs bath.

The duration of the aqueous quench of 10 to minutes is seen to have no appreciable efiect upon the properties of the aluminum alloy. Table IV shows the properties obtained from a bath of predominately KCNO and those from a NaNOa bath are included for a comparison. The baths are. maintained at 920 to 930 F.

TABLE IV Pysical properties of aluminum alloys held in quench water for various times after bath treatment Mechanical properties Minutes in quench- Run Heat-treating No. bath Tensile Yield trength, 1,000

p. s. i. p. s. 1.

Percent elongatron in Specifications.

cnovcmmo Average NaN Oz Average".

of aluminum alloy show no appreciable drop in their good physical characteristics when allowed to stand for extended periods in simulated quench water having a total salt content as high as 10%. Examination for corrosion showed it to be slight, a fact evident from the maintenance of the excellent physical properties of the aluminum al- 103; piece.

It should be remembered that in industrial usage it is only necessary to water quench an aluminum alloy piece until its temperature is below 212 F., preferably below 8090 F., and the adhering salt is dissolved.

A mentioned earlier in this specification, cast aluminum alloys may be heat treated by salt baths of the present invention. The two alloys which follow are typical of those used in the trade.

Alloy A has the following composition, it being understood that aluminum is the base metal:

Composition Percent 4.0 to 5.0. 1.0 max.

1.2 max.

0.03 max. 0.03 max. 0.02 max. 0.02 max. 0.03 max.

Others TABLE V Physical properties of aluminum alloy "A Yield Tensile g f Bath strength, strength, z h

p. s. 1. p. s. 1. in

Specification (min) 21, 000 32, 000 3.

23, 200 37, 900 5. O 87KCNO-13KC1 23, 100 43, 000 6. 0 NSNO; 27, 750 40, 150 4. 5 37KCNO-13KC1 27, 850 43, 350 S. 0

It is seen that the yield strength of tensile test specimens heat treated by the nitrate and cyanate-chloride baths are substantially equivalent per paired specimens, but both tensile strength and the per cent elongation in 2" are higher for specimens heat treated by salt baths of this invention.

Alloy B has the following composition, it being understood that aluminum is the base metal:

Composition Percent Copper 0.20 max. Iron. 0.50 max. Silicon 6.5 to 7.5. Mayne inm 0.20 to 0.40. Others 0.03 max.

Tensile test specimens of alloy B are solution heat treated in both nitrate and cyanate-chloride baths for 12 hours at 990 1 5 F. and quenched in water at 70 F. Table VI shows not only the minimum specification but also the physical properties of several samples. The heat treatment of the aluminum tensile test specimen is performed on pairs cast two at a time in the same flask so that the specimens and results obtained therefrom will be comparable.

TABLE VI Physical properties of aluminum alloy 3 ties are as good or better than those obtained by heat treatment with the hazardous nitrate bath.

While the invention has been described with particular reference to specific embodiments, it is to be understood that it is not to be limited thereto but is to be construed broadly and restricted solely by the scope of the appended claims.

I claim:

1. A method of heat treating an aluminum base alloy work piec which includes the steps of immersing the same in a molten bath predominately of KCNO at a solution heat-treating temperature until the distribution of the alloy components ha been improved, removing the work piece from the bath, and water quenching the same.

2. A method of heat treating an aluminum base alloy work piece which includes the steps of immersing the same in a molten bath predominately of KCNO and also containing KCl at a solution heat-treating temperature until the distribution of the alloy components has been improved, removing the Work piece from the bath, and water quenching the same.

3. The method of claim 2 in which the bath contains substantially 87% KCNO and. substantially-13% KCl.

i. The method of claim 2 in which the bath temperature is from 900 to 1010 F.

5. The method of claim 2 in which a wrought aluminum base alloy is solution heat treated at from 900 to 1010 F. for 10 to 90 minutes.

6. The method of claim 2 in which a cast aluminum base alloy is solution heat treated at from 900 to 1010 F. for 5 to 24 hours.

7. A molten bath including substantially 87% KCNO and substantially 13% K01.

JAMES F. SIMPSON.