Process for strengthening lead-antimony alloys

A process is provided for increasing the strength of antimony-lead alloys by specially treating an alloy which contains an effective amount of arsenic in the alloy, the process comprising working the alloy, rapidly heat treating the alloy, which includes quenching, for a period of time sufficient to activiate a strengthening mechanism in the alloy. The process is especially useful for the manufacture of battery grids on a continuous production line.

BACKGROUND OF THE INVENTION 
This invention relates to a process for the strengthening of lead-antimony 
alloys and, more particularly, to an extremely rapid heat treatment method 
which strengthens specially correlated alloys and enables the alloys to be 
processed on a continuous production line into storage battery grids. 
Lead-acid storage batteries have been used for many years as starter 
batteries for internal combustion engines. Pure lead is a soft material 
however, and extensive research has developed a number of alloys to 
provide specific physical properties desired by the battery manufacturers. 
Antimony is a common alloying material and amounts up to about 11% have 
been employed to improve the strength and castability of the lead. 
Unfortunately, antimony, aside from being relatively expensive, increases 
the water loss of the battery and is of limited use in a maintenance free 
battery and attempts have been made to decrease the antimony level in lead 
battery alloys. 
U.S. Pat. No. 3,993,480 discloses a low antimony-lead alloy containing, by 
weight, 0.5-3.5% antimony, 0.01-0.1% copper, 0.025-0.3% arsenic, 
0.005-0.1% selenium, 0.002-0.05% tin, the balance lead. Other low 
antimony-lead alloys are disclosed therein and show, in general, the 
effect of the different alloying elements on the properties of the alloy. 
U.S. Pat. No. 3,912,537 shows a highly castable lead alloy for producing 
battery grids containing 0.002 to 0.5% selenium, 0.25 to 0.5% arsenic and 
up to 4.0% antimony. An improved low antimony-lead alloy for use in the 
manufacturing of grids for maintenance-free storage batteries is disclosed 
in U.S. Pat. No. 4,158,563 and contains about 1.3-1.9% antimony, 
0.05-0.45% arsenic, 0.02-0.5% tin, 0.02-0.09% copper and 0.003-0.012% 
sulfur. These alloys are stated to have sufficient hardness, good 
castability and pasteability, excellent corrosion resistance, good grid 
growth characteristics and a low drossing rate. 
While the alloys of the prior art have solved many of the problems with low 
antimony-lead alloys in cast grids, modern grid technology presents a new 
obstacle. The conventional method of preparing grids by casting is 
relatively inefficient. An efficient automated continuous method is now 
preferred which produces grids by expanding or punching a wrought lead 
alloy strip as described in U.S. Pat. No. 4,443,918. For example, expanded 
plates can be obtained by continuously supplying a lead alloy strip, 
expanding it, pasting the thus produced mesh-like strip, drying it and 
cutting it to form individual grids. U.S. Pat. Nos. 3,945,097 and 
4,271,586 describe methods and machines for making expanded battery 
plates. The disclosure of the above patents are hereby incorporated by 
reference. 
Although superior in performance in many aspects of battery grid behavior, 
wrought antimonial leads have been excluded from continuous grid 
production means. It has been shown in J. Electrochemical Society, Vol. 
128, Part II, No. 8, July-December 1981, pages 1641-1647, that grids 
prepared from such alloys, as worked, are inherently soft and result in 
short lived batteries although it is indicated that the grids can be 
hardened to tensile strengths in excess of 6000 psi with very short term 
heat-treatments. It is noted, for example, in "Lead and Lead Alloys" by W. 
Hofmann, Springer-Verlag New York, Heidelberg, Berlin, 1970 on page 89 
that heat treatments of wrought antimonial lead alloys at 250.degree. C. 
for as short as 10 minutes provide a hardening reaction. Cited in Hofmann 
(footnote 239) is an article by Dean et al. entitled "The Lead-antimony 
System and Hardening of Lead Alloys" which discloses heat treatments as 
short as 1 minute in an oil bath. Unfortunately, a short term heat 
treatment does not, by itself, provide sufficient hardening and the need 
still exists for alloys and a heat treatment method which will provide a 
hardened material under the time constraints of a continuous production 
process. 
It is an object of the present invention to provide a continuous process 
for providing high strength antimonial lead strip or battery grids. 
It is a further object of the present invention to provide high strength 
antimonial lead alloys. 
Other objects will be apparent from the following description. 
SUMMARY OF THE INVENTION 
It has been unexpectedly found that the strength of low antimony-lead 
alloys can be increased by specially treating an alloy which contains an 
effective correlated amount of arsenic, the process comprising working the 
alloy and rapidly heat treating (which includes quenching) the alloy for 
sufficient time at an elevated temperature to activate a strengthening 
mechanism in the alloy, the time of the heat treatment step being 
substantially less than that used to conventionally heat treat 
lead-antimony alloys. Broadly stated, the alloy comprises, by weight, 
about 0.5%-6% antimony and about 0.002-1% arsenic, the balance being 
essentially lead. The alloy may be worked, e.g., reduced, by an amount 
greater than about 15%, preferably greater than about 50% and most 
preferably greater than 80% or 90% and is preferably reduced by rolling in 
several successive stages of substantially equal percentage reductions.

DETAILED DESCRIPTION OF THE INVENTION 
The lead-antimony alloys which may be strengthened by the process of the 
invention can contain many of the elements normally used in these type 
alloys, such as tin, copper, silver, cadmium, selenium and tellurium, with 
the proviso that antimony be present in an amount greater than about 0.5%, 
e.g., about 0.5-6%, preferably about 0.75-3% and most preferably 1-2.5%, 
and the arsenic in an amount of about 0.002% to 1%, preferably 0.05% to 
0.25%, and most preferably 0.1% to 0.2%. Arsenic, in combination with the 
antimony, has been found to be essential to provide strengthening of the 
alloy when using the novel heat treatment process of the invention. Of 
particular note is not only the significant difference in Ultimate Tensile 
Strength (UTS) after 24 hours aging, but that the UTS continues to 
increase substantially compared with alloys containing levels of antimony 
of, e.g., about 1-2%, but which contain low levels of arsenic outside the 
invention. 
While it is known that conventional heat treatment, e.g., solution 
treatment, which typically comprises heating the alloy in the single phase 
region of the phase diagram for periods of about 1 hour or more and 
quenching, strengthens the alloys, it has been discovered that such a 
solution heat treatment is not necessary if the alloy contains a special 
correlated amount of arsenic and antimony, is worked, heated rapidly to 
the desired temperature and quenched, which procedure activates a 
strengthening mechanism in the alloy. It is hypothesized that 
strengthening of the alloy occurs by precipitation of a hardening phase 
and that nucleation of the hardening phase is facilitated by the presence 
of correlated amounts of antimony and arsenic and the heat treatment step. 
This mechanism is distinct from a conventional solution treatment which 
strengthens the alloy by a time consuming diffusion controlled 
solubilization of antimony at high temperature and precipitation of the 
super-saturated solution at room temperature. The novel rapid heat 
treatment of the invention provides little or no strengthening at low 
levels of arsenic. 
Working of the alloys may be performed using conventional procedures 
well-known in the art and by working or rolling, extrusion, etc. is meant 
mechanical plastic deformation of the metal and includes cold and hot 
working. In general, the alloy is cast into a billet and reduced to the 
desired size strip by passing it through successive rolls, wherein each 
roll in succession further reduces the thickness of the alloy. Constant 
reduction rolling schedules in the same rolling direction are preferred 
whereby, for example, a 0.75 inch thick billet is reduced to a 0.04 inch 
thick strip by passing it through 11 rolls wherein each roll in succession 
reduced the thickness of the billet by about 25%. Other rolling schedules 
can suitably be employed. 
Heat treatment of the alloy is performed under time and temperature 
conditions which do not result in a conventional solution treatment 
effect. Solution treatment requires diffusion controlled dissolution of 
the already precipitated antimony rich phase. Such processes are slow 
depending on the solid-state movement of individual atoms from one crystal 
site to the next. Strengthening occurs after quenching when the 
super-saturated solution precipitates in a form which strains the alloy 
crystal lattice and inhibits dislocation motion. 
The heat treatment of the present invention, which includes the quenching 
step, when applied to worked lead-antimony alloys containing a correlated 
amount of arsenic and antimony, activates a strengthening reaction by 
means not yet clear. With out being bound to theory it is believed that 
antimony in low or arsenic-free lead-antimony alloys has difficulty in 
precipitating and therefore substantially remains in solution through the 
casting, working process and aging period. In fact, it has been found that 
worked alloys, even containing the correlated amounts of arsenic and 
antimony, do not strengthen appreciably on aging or standing. Only when 
the alloys are heat treated according to the invention do the alloys 
strengthen on aging and it is hypothsized that the heat treatment forms 
meta stable arsenic bearing nuclei which facilitate the antimony 
precipitation process. 
Referring to FIGS. 1, 2 and 3, all three photomicrographs are of samples 
from the same sheet of cold rolled alloy, approximately 0.08 inch thick, 
comprising, by weight, about 2% antimony, 0.2% arsenic, 0.2% tin, the 
balance essentially lead. The alloy sheet produced by cold rolling a cast 
alloy to a reduction of about 90% through nine successive reductions of 
about 25% each, is shown in FIG. 1. FIG. 2 shows the microstructure of the 
cold rolled alloy heated in a molten salt bath at 230.degree. C. for 30 
seconds and water quenched and FIG. 3 the cold rolled alloy heated in a 
molten salt bath at 230.degree. C. for 1 hour and water quenched. All 
samples were mounted in resin and polished using standard mechanical 
metallographic procedures immediately after quenching. They were etched 
using a mixture of acetic acid and H.sub.2 O.sub.2. The photomicrographs 
show the longitudinal rolled direction at 200X at approximately 24 hours 
after quenching and were taken using Polaroid Type 55 film on a camera 
mounted upon a metallurgical microscope. 
FIG. 1 shows recrystallization of the lead matrix proceeding (though 
incomplete) at room temperature. The black bands are the antimony-rich 
eutectic phase resulting from rolling a nonequilibrium solidified cast 
block. It should be noted that the as-rolled alloy as characterized by 
FIG. 1 shows very little strengthening, if any at all, on aging at room 
temperature. FIG. 2, however, representing an alloy prepared according to 
the invention, shows a completely recrystallized structure with the 
antimony-rich bands still present and the volume fraction of the 
antimony-rich regions being approximately the same as the as-rolled alloy 
of FIG. 1. In contrast, FIG. 3, showing a solution treated microstructure 
has a structure which is recrystallized with increased grain growth, with 
the antimony-rich bands almost completely in solution. The white dots 
visible on all three Figures are a tin arsenide phase which does not 
appear to play a significant part in the hardening process. 
Solution heat treatment as defined in ASTM Designation: E 44-83, means 
heating an alloy to a suitable temperature, holding at that temperature 
long enough to cause one or more constituents to enter into solid solution 
and then cooling rapidly enough to hold these constituents in solution. 
The heat treatment of the present invention comprises only requiring the 
alloy to be heated to the desired temperature. In general, heating the 
alloy at the desired temperature does not dissolve any appreciable amount 
of soluble antimony, e.g., less than 50%, usually less than 25% and 
typically less than about 10%, e.g., 5% or 1% or less. For example, as 
shown in the Figures, the as-rolled alloy of FIG. 1 contains approximately 
the same amount of coarse precipitated antimony (as shown by the black 
bands) as the heat-treated alloy of the invention of FIG. 2. This is to be 
contrasted with a conventional solution heat treatment as shown in FIG. 3 
wherein there is very little coarse precipitated antimony remaining. The 
soluble antimony is shown as the black regions (bands) in the figures and 
may be measured using quantitative metallurgical techniques. Antimony is 
soluble in lead up to about 3.5% by weight and amounts in excess of 3.5% 
would not be considered soluble antimony for the purposes of defining how 
much antimony may be dissolved according to the process of the invention. 
In general, the temperature of the heat treatment is between about 
180.degree. C. and the alloy liquidus temperature, preferably 200.degree. 
C. to 252.degree. C., and most preferably 220.degree. C. to 245.degree. C. 
The time required to bring the alloy to the desired temperature varies 
according to the thickness of the alloy and the temperature and method of 
heating, with thinner strips of alloy, higher temperatures and/or higher 
heat transfer heating means requiring shorter times. It is preferred that 
the alloy be brought substantially completely to the desired temperature 
to realize the full effect of the heat treatment on the strengthening of 
the alloy. In a preferred embodiment, employing a molten salt bath at a 
temperature of about 230.degree. C. for about 30 seconds provided 
excellent strengthening results for a 0.040 inch thick strip of alloy. An 
equivalent heating time for a muffle furnace would be about 2.5 minutes. 
For an alloy about 0.25 inch thick, over the broad range of heating 
temperatures, a heating time using a salt bath is less than about 2 
minutes, and even 1 minute and for a muffle furnace, less than about 8 
minutes. As noted above, heating times will vary depending on the 
temperature and the thickness of the alloy and, in general, for a strip of 
alloy about 0.025 inch to 0.1 inch thick, a heating time using a salt bath 
is about 1-3 seconds, preferably 5 or 30 seconds to less than about 1 
minute, and for a muffle furnace, about 1 minute, preferably 2 minutes and 
most preferably less than about 5 minutes. Longer times may be employed, 
if desired, although the longer times will not typically result in any 
substantial increased operating efficiencies. Other heating means can 
suitably be employed such as oil, induction heating, resistance heating, 
infrared, and the like. Resistance heating, for example, would provide 
almost instantaneous heating thus requiring very short heating times of 5 
seconds or less, although longer times could be employed if desired. 
Any method and machine may be employed for making the worked alloy and/or 
battery plates and U.S. Pat. Nos. 3,310,438; 3,621,543; 3,945,097; 
4,035,556; 4,271,586; 4,358,518; and 4,443,918 show representative methods 
and machines, the disclosures of the patents being hereby incorporated by 
reference. U.S. Pat. No. 4,271,586 shows, for example, a ribbon of lead 
being fed into an inline expander, followed by pasting, drying, cutting 
and accumulating into stacks. U.S. Pat. No. 4,035,556 discloses forming of 
finished storage battery grids from rolled sheet material by (a) slitting 
and expanding to form an open grid, (b) punching out an open grid, (c) 
forming an interlocked type of grid and (d) combinations of (a) or (b) 
with (c). 
It will be appreciated by those skilled in the art that heat treatment of 
the alloy may be performed at any convenient interval during preparation 
or manufacture of the alloy or battery grid. For example, the alloy can be 
continuously cast, worked, heat treated and expanded or punched into the 
grid and assembled directly into the battery. If desired, the strip can be 
coiled for storage and then treated or it can be treated and then coiled 
and stored for use at a later time. The alloy can also be heat treated 
after preparation of the grid. Regardless of the method of heat treating 
and preparing of the grid, it is important that the alloy be worked before 
the heat treatment. 
The following example will further illustrate the present invention. It 
will be understood that throughout this specification and claims, all 
parts and percentages are by weight and all temperatures in degrees 
Centigrade unless otherwise specified. 
EXAMPLE I 
The alloys listed in TABLE I were prepared in a heated graphite crucible by 
alloying corroding grade lead with elemental antimony, arsenic and tin. 
The melts were cast into a graphite book mold at 400.degree. C. to produce 
a cast block approximately 5 inch.times.4 inch.times.0.75 inch. 
The castings were milled to remove surface defects and then rolled at room 
temperature to 0.045 inch in eleven passes taking about a 25-30% reduction 
per pass. Samples for chemical analysis were cut from the resultant strip. 
Blanks 4 inch.times.0.5 inch for machining to test bars were cut from the 
strip in the rolling (longitudinal) direction. A Tensilkut Machine was 
used to cut the test bars to a 1 inch gage length and 0.25 inch width. 
Heat treatment for samples in TABLE I were performed in a molten salt bath 
at 230.degree. C. for the times indicated and quenched by plunging into 
room temperature water upon removal from the salt bath. The samples were 
then stored at room temperature for aging. Tensile tests were performed on 
an Instron Machine using a crosshead speed of 0.2 inch/minute. 
TABLE I 
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Ultimate Tensile Strength (psi) 
30 Second Heat Treatment 
1 Hour Heat Treatment 
Alloy 
Sb Sn As As Rolled 
24 Hours 
10 Days 
24 Hours 
10 Days 
__________________________________________________________________________ 
1 2.0 
0.19 
0.005 3,500 5,100(45) 
6,600(56) 
9,500(170) 
10,000(183) 
2 1.90 
0.19 
0.05 4,000 6,100(51) 
7,800(95) 
11,000(174) 
11,500(187) 
3 1.86 
0.2 
0.22 4,000* 
6,300(58) 
7,500(88) 
10,900(172) 
11,400(185) 
4 0.98 
0.19 
0.2 3,500 4,200(19) 
4,800(37) 
4,100(18) 
5,200(48) 
5 1.4 
0.19 
0.21 3,900 5,000(29) 
6,500(67) 
6,400(65) 
8,500(120) 
A 0 0.2 
.19 3,090 3,600(16) 
3,600(17) 
3,800(21) 
3,500(13) 
B 0.49 
0.19 
0.2 3,500 3,800(7) 
4,000(14) 
4,000(14) 
4,000(14) 
C 1.8 
0 2-6 ppm 
3,400 3,850(14) 
4,200(24) 
7,600(124) 
8,500(152) 
D 2 0.18 
2-6 ppm 
3,400 4,080(19) 
4,000(18) 
5,400(58) 
8,300(144) 
E 2 0.18 
3-12 ppm 
3,600 4,090(13) 
4,300(18) 
7,700(111) 
10,000(181) 
__________________________________________________________________________ 
() = percent increase compared to the as rolled material. 
*4140(4) after about 6 weeks aging at room temperature. 
The data in Table I clearly shows the increase in Ultimate Tensile Strength 
(UTS) when employing the heat treatment process of the invention on lead 
alloys containing antimony and arsenic in correlated amounts. Thus, a 
comparison of Alloys 1, 2 and 3 with Alloys C, D and E show the importance 
of arsenic to provide an increase in UTS for a 30 second heat treatment 
period. Alloys A and B show the need for having levels of antimony above 
about 0.5%, with the preferred alloys containing about 1.8-2% antimony. 
While this invention has been disclosed in terms of specific embodiments 
thereof it is not intended to be limited thereto and it will be understood 
that modifications may be made in the improved process of this invention 
without departing from the scope of the invention defined by the appended 
claims.