Copper and copper base alloys

Patented May 21, 1940

2,201,555

UNITED STATES PATENT OFFICE

2,201,555

COPPER AND COPPER BASE ALLOYS

Michael G. Corson, New York, N. Y., assignor to Union Carbide and Carbon Research Laboratories, Inc., a corporation of New York

No Drawing. Application April 13, 1938, Serial No. 201,725

Claim. (Cl. 148—11.5)

The invention relates to copper and copper base alloys in which the matrix or continuous phase is an alloy of the alpha copper type, such as an alpha brass, an alpha bronze, a cupro5 nickel, or a high nickel brass.

Copper and copper alloys of the alpha type are ordinarily cast and then heat treated and wrought in the following fashion. The metal when cast does not have a homogeneous structure, but con10 tains some portions which have a copper content lower than the average. Hence, the metal is usually homogenized by heating at an elevated temperature below 850° C. Mechanical working is generally effected while the metal is 15 cold. Because these metals harden when they are cold worked, if the total amount of mechanical workings is substantial it is done in several working steps with intermediate annealing steps at a temperature between 300° and 850° C., usuy ally between 480° and 660° C.

Such metals have a strong tendency for rapid grain growth at elevated temperatures above 300° C. Grain growth during the annealing or homogenizing steps adversely affects the strength and : ductility of the metal and its rolling, drawing, and other working properties. Furthermore, the amount of cold work done on a given section has a marked effect on the rate of grain growth, and annealing will usually produce grains of differSQ ent and unpredictable sizes in different parts of the same article. A coarse grained material has a greater tendency than a fine-grained material to split, tear and roughen.

It is the primary object of the invention to 30 improve the rolling, drawing, and other physical properties of copper and alpha alloys of copper by inhibiting grain growth at annealing temperatures.

I have found that the presence of a relatively ₍ small amount, between 0.05% and 0.5%, of finely divided precipitated chromium in copper and copper alloys of the alpha type effectively inhibits grain growth in this material at temperatures up to about 850° C. After heating at annealing /temperatures below 850° C. the grain size of the chromium-containing material is only about 10% to 20% of that of material containing no chromium but otherwise of similar composition. The addition of chromium provides a uniformly fine ) grain throughout the metal, regardless of wide differences in the amount of cold work done on the metal and regardless of the specific annealing temperature and time used. Thus, conditions of the cold working and annealing opera> tions need not be limited, as they ordinarily are when treating alpha brasses, by the necessity to control grain growth. Because the grain size . changes but slightly over a wide temperature range, drawing conditions and grain size can be readily controlled. 5

For most purposes, 0.2% chromium is sufficient to impart the full benefits of the invention, and in no instance should enough chromium be added to affect materially the properties of the copper base metal in any respect other than those de- io rived from the inhibition of grain growth.

The chromium may be added as chromium metal or ferrochromium or, preferably, cuprochromium, to the molten copper base material to be treated, and the melt then cast. The chro- 15 mium is precipitated by maintaining the alloy within the temperature range of 350° C. to 700° C. for a time, suitably a few minutes at the higher temperatures within such range or several hours at the lower temperatures. This step 20 may coincide with the customary homogenizing anneal. Thereafter, the material may be mechanically wrought and annealed in conventional manner, but the alloy must not be permitted to reach temperatures above 850° C., because at gg such elevated temperatures the chromium will rapidly dissolve and permit grain growth. There is some tendency for the chromium to dissolve at temperatures between 700° and 850° C„ but the rate of solution at such temperatures is rela- jq tively slow.

It has been proposed previously to add chromium to copper and copper base alloys, but all of such proposals have been for the purpose of producing a hardenable alloy. According to _3δthese proposals, chromium is added, usually in an amount greater than 0.5%, to copper or a copper base alloy, and such chromium is then fully dissolved by heating the alloy at an elevated temperature above 850° C. for a short time or at a temperature between 700° C. and 850° C. for a very long time. As a general rule, silicon is also added to intensify the hardenability. The alloy containing chromium wholly in solid solution is then mechanically wrought and annealed, the annealing being carried out at temperatures high enough to keep the chromium in solution. After being wrought, the alloy is hardened by heating within the temperature range of 350° C. to 700° C. to precipitate the chromium. In such processes, the chromium, since it is in solid solution during the annealing steps, is ineffective to inhibit grain growth. According to the present invention, to the contrary, the chromium is precipitated before mechanical working is com- 1

2,202,566 menced and it is thereafter kept in uniformly distributed, finely precipitated form throughout the working and annealing steps.

This application is in part a continuation of 5 my application Serial No. 64,884, filed February 20, 1936.