Brass material and a process for the preparation thereof

A brass material and a process for the preparation thereof, which comprises an alloy of 61 to 65% by weight of copper with the remainder being zinc; the material evidencing a structure in which the recrystallized phases .alpha. and .beta..sub.1 are present in a discrete fine mixture having grain sizes of less than 5 .mu.m. The component of the .beta..sub.1 phase comprises at least 10% of the structure and is arranged in the form of discrete particles in the grain boundaries of the .alpha. phase.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a brass material and a process for the 
preparation thereof. 
2. Discussion of the Prior Art 
From German Published Patent Specification No. 12 28 810 there has already 
become known a process for the preparation of materials, in this instance 
spring materials, which are constituted of copper-zinc alloys. In this 
process, a semifinished material produced from a copper-zinc alloy in a 
method commonly employed for malleable alloys is annealed, cold worked and 
then subjected to a temperature and time-measured heat treatment. Hereby, 
the treatment is regulated so that there will be avoided a 
recrystallization of the material matrix. 
The spring materials which are obtained in this manner evidence an 
increased, extensively isotropic spring flexural limit. However, in 
general, this known process need not improve the mechanical properties of 
commercial brass alloys to a considerable extent so as to render them 
applicable for the increased demands thereon. Finally, this is not only 
documented by the fact that such alloys, in an increasing measure, must be 
replaced by materials which are expensive and difficult to machine or 
process. Moreover, the usual commercial brass alloys are unsuitable for 
further processing through a superplastic deformation. 
The preparation process which has become known from the above-mentioned 
patent publication, which does not in any way provide for a material 
suitable for a superplastic deformation, additionally requires an 
extremely precise maintenance of the temperature as well as of the time 
period for the heat treatment. Thus, even small deviations from the 
predetermined annealing temperature lead to an undesirable reduction in 
the mechanical properties of the material. 
As a consequence, above all also due to the good electrical conductivity of 
the brass, there is thus present a great interest in a simply and 
inexpensively producible brass material which, in contrast with the 
traditional brass alloys, evidences a substantially improved deformability 
as well as occasionally considerably improved mechanical properties. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
inexpensive brass material which, predicated on its structure and its 
mechanical properties, can be further processed as good as is possible, in 
particular through superplastic deformation, and which renders it possible 
to also produce high-strength and highly ductile workpieces. A further 
object of the present invention contemplates the provision of a process 
for the preparation of such a brass material. 
The foregoing invention achieves this object in that it contemplates a 
material which is constituted of an alloy having 61 to 65%, preferably 62% 
by weight of copper, with the remainder being zinc, and which evidences a 
structure in which the recrystallized phases .alpha. and .beta..sub.1 are 
present as a discrete fine mixture with grain sizes of less than 5 .mu.m, 
wherein the .beta..sub.1 -phase component consists of at least 10% and 
this phase is arranged in the form of discrete particles in the grain 
boundaries of the .alpha. phase. 
In its preferred embodiment, the brass material prepared pursuant to the 
invention evidences 10 to 50%, preferably about 30 to 40% of the 
.beta..sub.1 phase in the cohesive .alpha. matrix which is subdivided 
through grain boundaries. In this composition, the superfinely 
recrystallized structure is particularly stable with regard to temperature 
increases, as well as also with regard to an exceeding of the annealing 
time period. This provides a particularly advantageous effect in an 
eventual subsequent further processing through the intermediary of 
superplastic deformation. 
As a result of its extremely fine-grained, crystalline structure, a 
so-called microduplex structure, the inventive brass material is almost 
ideally extensively cold workable (&gt;99%). In connection with the small 
crystal size it is hereby possible to obtain for brass heretofore unknown 
values with respect to hardness and strength. Thus, after a final cold 
working or forming of at least 70%, the inventive material evidences a 
hardness in excess of 220 HV (Vickers hardness), a tensile strength&gt;800 
N/mm.sup.2 and a 0.2% yield strength&gt;600 N/mm.sup.2. Due to its almost 
unlimited deformation capability, this material is hereby particularly 
well suited for additional shaping processes. This good further 
workability is documented in that the material which has been converted 
into the spring-hardened condition, evidences a reduction of area of about 
60% in combination with the above-mentioned mechanical properties. 
Furthermore obtained for this spring material, similarly required through 
the superfine grain as well as by the presence of a second phase, is a 
substantially enhanced fatigue strength. 
The process for the preparation of the inventive brass material makes use 
of the well known fact that the copper-zinc binary system evidences, for 
copper contents of between 61 and 70% in the temperature range of between 
450.degree. and 500.degree. C., a maximum solubility of the 
.beta./.beta..sub.1 phases in the .alpha. solid solution. As a consequence 
of the decrease of this solubility toward lower temperatures there must 
thus result during the cooling a precipitation of the .beta..sub.1 phase 
from the quite supersaturated .alpha. solid solution whereby there is 
theoretically produced the possibility of a precipitation hardening. 
However, in actual practice, the setting up of the equilibrium between the 
.alpha. and .beta..sub.1 phases at low temperatures is so strongly 
hindered through the reduction in the diffusion, also as well as through 
inhomogeneity, metastable conditions and so forth, that it takes place 
over extremely lengthy time spans. Thus, it had heretofore been assumed 
that, at 250.degree. C., there was required an annealing period of about 
one year until the setting up of the equilibrium between the two phases 
corresponding to this temperature. (Compare hereby, for example: T. B. 
Massalski and J. E. Kittl; J. Austral. Inst. of Metals, 8, 1963, 91-97.) A 
technological application of the precipitation of the .beta..sub.1 phase 
from an .alpha. solid solution appeared to be thereby precluded. 
Nevertheless, it is indicated that for bran alloys having the inventive 
composition, a precedently effected cold working or forming of at least 
50% will tend to greatly accelerate the speed of the .beta..sub.1 
precipitation. The annealing periods which are required for the complete 
.beta..sub.1 precipitation and the subsequent recrystallization, depending 
upon the composition and the degree of the previous cold working as well 
as the annealing temperature, now lie at between one minute and 500 hours, 
for the preferred annealing temperatures at between one and eight hours. 
Due to the extremely fine initial distribution of the .beta..sub.1 phase 
in the .alpha. parent phase, after the completed recrystallization there 
will arise a superfine, two-phased structure, in which two phases are 
present with grain sizes of less than 5 .mu.m. Since the two phases will 
permanently inhibit grain growth due to their interaction, this 
microduplex structure will remain stable even at higher temperatures.

DETAILED DESCRIPTION 
Described hereinbelow is the preferred process for the preparation of the 
inventive brass material. 
Proceeding from an alloy with preferably 62% copper with the remainder 
being zinc, through the intermediary of casting and extruding there is 
produced the semi-finished material which serves as the base for 
effectuation of the subsequent processing. Hereby, any kind of suitable 
casting procedure, for instance, such as continuous casting, can be 
employed, but it is also possible to contemplate other methods of hot 
working, such as hot rolling, or also a partial cold forming. 
The thus present semi-finished brass material is thereafter annealed in 
order to ensure that a solid solution of .alpha. only is now available for 
further processing. The annealing is effected in a temperature range of 
between 450.degree. and 500.degree. C., within the range of the .alpha. 
solid solution only. The annealing period consists of about 20 hours. 
Suitable for the subsequent cold working of the material is basically any 
process hitherto known for this purpose, such as rolling, drawing or 
hammer forging. Of importance is only that there is hereby reached a 
degree of deformation of at least 50%, however, preferably in excess of 
80%. In the preferred preparation process, the semi-finished brass 
material is deformed by means of cold rolling at a degree of deformation 
of 90%. Concurrently, the degree of the cold working is herein the measure 
for the intensity of the subsequent heat treatment which is intended to 
effect the precipitation of the .beta..sub.1 phase as well as the 
recrystallization of the matrix. 
At a precedent cold working or deformation of about 90%, the 
recrystallization is completed after an annealing period of four hours and 
an annealing temperature of 250.degree. C. The alloy is now present as a 
superfine two-phased structure with uniform grain sizes of 1 to 2 .mu.m, 
meaning, it is present as a microduplex structure. 
As a result of the heat treatment up to complete recrystallization, a part 
of the material hardness which had been obtained through the extensive 
cold working and the .beta..sub.1 precipitation, will again be lost. 
Therefore, insofar as is intended to obtain a material having a special 
hardness, there is required a renewed cold working subsequent to the 
precipitation and recrystallization annealing, whereby the degree of 
deformation orients itself pursuant to the desired end hardness. Due to 
its extremely fine-grained structure, the brass material of the invention 
evidences a high cold workability so that, at such a final cold working, 
deformation degrees of over 99% are possible without the brittleness of 
the material becoming disturbing in appearance. 
However, on the other hand, it is also possible to submit the obtained 
brass material after the effected recrystallization to a superplastic 
deformation at temperature of up to 350.degree. C., whereby, as a result 
of the good temperature stability of the microduplex structure, no 
substantial grain coarsening is encountered. The super-fine grain affords 
that, with low deformation forces, there may be attained relatively great 
deformations, even into complicated configurations. 
Whereas it is possible for alloys with copper contents of higher than 62% 
by weight to reduce the time period for the annealing in the range of the 
.alpha. solid solution through the selection of correspondingly higher 
annealing temperatures (up to 700.degree. C.), depending upon 
circumstances, to less than one hour, for the preferred composition, due 
to the plot of the equilibrium line .alpha./(.alpha.+.beta.), it is not 
possible to anneal at more than 500.degree. C. However, in a modification 
of the presently described process, for the preparation of the inventive 
brass material it is possible to shorten the annealing period for the 
annealing in the range of the .alpha. solid solution in that the 
semi-finished material, preceding this first annealing, is at first 
subjected to an additional cold working or deformation of about 50%. The 
annealing period for the annealing in the range of the .alpha. solid 
solution at 450.degree. to 500.degree. C. is then reduced to about one 
hour. 
As has already been mentioned, the inventive brass material is particularly 
suited also for the production of high-strength workpieces, in particular, 
springs. For this purpose, in order to convert the material into the final 
spring-hardened condition there is carried out, following the 
precipitation and recrystallization annealing leading to the formation of 
the microduplex structure, a subsequent cold deformation of about 80% 
which, for instance, can be effectuated through cold rolling or drawing. 
When during the final cold working there are employed degrees of 
deformation in excess of 70%, preferably 80 to 99%, it is then possible to 
achieve a hardness of over 220 HV at a tensile strength &gt; 800 N/mm.sup.2 
and a 0.2% yield strength&gt;600 N/mm.sup.2. On the other hand, the still 
remaining capability of changes in configuration facilitates the 
utilization of additional forming procedures, for example, in the 
manufacture of screws, particularly cross-slotted or Phillips-head screws. 
In a further embodiment of the inventive material preparation process, the 
alloy contains a recrystallization retarding additive of nickel in an 
amount of up to 5% by weight. This prevents too rapid a recrystallization 
sequence, occurring especially during heat treatments at higher annealing 
temperatures and which will prematurely disrupt the .beta..sub.1 
precipitation prior to the reaching of the equilibrium condition. For the 
same purpose it is also possible to utilize an addition of zirconium, 
silver, niobium or vanadium in amounts up to 0.1% by weight, wherein each 
of these additives can also be combined with nickel. However, within the 
scope of the invention, it is also possible to include similar 
recrystallization-retardantly effective additives in parts of up to 0.1% 
by weight of the alloy. 
Furthermore, through the addition of up to 0.1% by weight of arsenic, 
antimony or phosphorous or, respectively, a combination of these elements, 
it is possible to improve the protection of the inventive brass material 
against dezincifying, as is the case with the usually employed additives 
for this purpose in the heretofore utilized brass alloys. The discrete 
distribution of the .beta. phase achieved by the precipitation of the 
.beta. or, respectively, .beta..sub.1 phase from the .alpha. phase remains 
intact due to its extremely fine grain final distribution even during 
further processing at higher temperatures, so that the comprehensive 
protection of the .alpha. phase, surrouning the .beta. phase against 
dezincifying by means of the above-mentioned additives concurrently 
prevents a dezincifying of the .beta. phase. 
Finally, elucidated hereinbelow by way of an example is the preparation of 
the inventive brass material, as well as its further processing into wires 
as the finished material for screws and springs. 
Illustrative Example: Production of Wires 
Utilized is an alloy having 62% by weight of copper, with the remainder 
being zinc. After the casting and the hot working through extruding, the 
material is subjected to an annealing in the range of the .alpha. solid 
solution, meaning, annealed for about 20 hours at 500.degree. C. There is 
then formed a .alpha. solid solution only having a median grain diameter 
of about 150 .mu.m. Through cold working, in this instance through swaging 
and drawing, a deformation of 98% is imparted to the material, which is 
possible without intermediate annealing. An annealing is thereafter 
carried out of the cold worked wires at a constant temperature of 
250.degree. C. over a period of 8 hours for effecting the precipitation of 
the .beta..sub.1 phase. After the course of this time interval, there is 
present now a recrystallized structure of two phases with grain sizes of 
from 1 to 2 .mu.m, whereby the .beta..sub.1 phase is embedded finely and 
discretely in the matrix of the .alpha. phase. The hardness of this 
material lies at about 165 HV. 
Finally, the wires are again cold drawn to about an 80% degree of 
deformation. The wires thus evidence the following mechanical properties: 
0.2% Yield strength: 780 N/mm.sup.2 
Tensile strength: 930 N/mm.sup.2 
Hardness: 260 HV 
Reduction of Area: .about.60%