Note: Descriptions are shown in the official language in which they were submitted.
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_ LEA~-ANTIMONY ALLOY SUITABLE FOR MAKING B~ITERY GRIDS
BACXGROUND OF THE INVENTION
This invention relates to an improved lead-
antimony alloy which is ~trengthened by an extremely
rapid heat treatment method and which is capable of
being processed into battery grids on a continuous
production line.
Lead-acid storage batteries have been used for
many years as starter batteries for internal combustion
engines. Pure lead is a &oft material however, and
oxtensive re&earch has developed a number of alloys to
provide specific physical properties desired by the
battery manufacturers to i~prove the battery and battery
making proce~s.
In general the conventional method of
preparing grids by cagtinq is relatively inefficient and
it is now preferred to use an automated continuous
m thod~which produces grid~ by expanding or punching a
wrought lead alloy strip as de~cribed in U.S. Pat. No.
`
4,443,918. For example, expanded plates can be obtained
by continuously supplyinq a lead alloy strip, expanding
it, pastinq the thus produced ~esh-like strip, drying it
and cutting it to form individual grids. U.S. Pat. Nos.
3,945,097 and 4,271,586 describes methods and machines
; 25 for making expanded battery plates, the patants being
incorporated her~in by reference.
Antimony i~ a common alloying material
desirable in battery grids and amounts up to about lIt
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hav~ been employed to improve the strength and
castability of the lead. Unfortunately, antimony, aside
from being relatively exp~nsive, increases the water
loss of the battery and i8 of limited use in a
maintenance free battery and continuous battery making
process.
U.S. Patent Nos. 4,629,516 and 4,753,688 which
are assi~ned to the same assignee as the present
invention and which are hereby incorporated by
reference, disclose a number of lead-antimony alloys
useful for making battery grids. The patent notes
however that the low antimony alloys require long times
to heat treat the alloy for increasing thalr ~trength
and a new alloy and short-term heat treatment were
developed to enable continuous grid production.
While the alloys and heat treatment methods of
the prior art continuous battery making processes
produce grids suitable for use in batteries, mechanical
working of the alloy durinq the process after the heat-
treatment step, particularly in the strip expansion
st~p, may recrystallize the alloy under certain
circumstances and d~creas~ its physical properties.
It is an ob~ect of the present invention to
provide high strength antimonial lead strips or battery
grids which exhibit enhanced resistance to 1085 Or
physical properties, in particular, strength and
hardness due to mechanical working in the process of -~
making the grids. It is a further ob~ect o~ the present
invention to provide a continuous process for providing
battery grids from a lead alloy strip. Other ob~ects
will be readily apparent from the following description.
SUM~RY OF THE_INVENTION
It has now been found that the physical
propertie~ Or the lead-antimony-arsenic alloys of U.S.
Patent Nos. 4,629,516, and 4,7S3,688, which alloys have
been worked and heat treated according to the patents,
may be significantly reta$ned after further mechanical -~
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wo9o/os462 PCT/US90/003tO
working by incorporating an effective amount of
tellurium in the alloys. Broadly stated, the process
~omprises working the alloy and rapidly heat treating
(which includes quenching) the alloy for a 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.
The alloy comprises, by weight, ~bout 0.5%-6% antimony
lo about 0.002-1% arsenic, up to a~out 1%, or more
tellurium, up to about o.5% t~n and 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 preferàbly greater than
80% or 90% and is preferably reduced by rolling in
several successive stages of substantially equal
percentage reductions.
DETAILED DESCRIPTIQ~ OF THE I~y~TIQN
The lead-antimony-arsenic-tellurium alloys
which may be strengthened by thQ process of tho
invention comprise, by weight, antimony in an amount
greater than about O.S%, e.g., about 0.5-6%, preferably
about 0.75-3% and most preferably 1-2.5%, arsenic in an
amount of about 0.002% to 1%, preferably 0.05% to 0.25%,
and most proferably 0.1% to 0.2% and tellurium in an
; ~ amount of about 0.0005% to 1%, preferably 0.003% to 0.5%
and most preferably 0.007% to 0.14%. Tellurium in
combination with the antimony and arsenic, has been
found to be essQntial to provid~ enhancQd maintenance of
the physical properties of the worked, rapidly h-at-
treated alloy after further working of the alloy to
; form, for example, battery grid~ by expansion of strips
of the heat-treated alloy.
Working and heat treatment of the new alloy is
as described in the above-referenced U.S. Patent Nos.
4,629,516, and 4,753,688, With the importanoe of the
processing steps and parameter~ being to produce an
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wosu/os462 ~s~ P~T/US90/00310
alloy having a microstructure such as F~g. 2 of the
patents, i.e., a completely recrystallized structure
having 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
therein. In general, less than 50%, usually less than
25%, and typically less than 10%, e.g., 5% or 1~ or less
of the soluble antimony (black region~-antimony-rich
eutectic phase bands) of the as-rolled alloy is
dissolved.
Working of the alloys may be performed using
conventional procedures well-known in the art, and by
working or rolling, extrusion, etc. i8 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 schQdules 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 ll rolls wherein each roll in
succession reduced the thickness of the billet by about
25%. Other rolling schedules can suitably be employed.
~eat treatment o~ the alloy i~ performed under
time and temperature cond$tion~ which do not result in a
conventional solution treatment ~ffect. The heat
treatment of the pre~ent invention, which includes the
quenching step, when applied to worked lead-antimony
alloy~ containing a correlated amount of arsenic,
àntimony and tellurium, activates a strengthening
reaction and a maintenance of the alloy's strength by -
means not yet clear.
Solution heat treatment as defined in ASTM
Desiqnation: E 44-83, means heating an alloy to a
suitable temperature, holding at that temperature long
enough to cause one or more con~tituents to enter into
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Wogo/09462 PCT/US90/003tO
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solid solution and then cooling rapidly enough to hold
these constituents in solution. The heat treatment of
the present in~ention 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.
The temperature of the heat treatment is
between about 180-C. and the alloy liquidus temperature,
preferably, 200-C. to 252-C., and most preferably 220-C.
to 245-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-C. for about 15 seconds provided excellent
strengthening, and retention of the strength after
further working, for a 0.040 inc~ thick strip of alloy.
An equivalent heating time for a muf~le furn~ce would be
about 2.5 minutes. Por an alloy about 0.25 inch thick,
over the broad range of heating workinq, 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. AB 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 to
30 seconds to less than a~out 1 minute, and for a muffle
furnace, about 1 minute, preferably 2 minutes and most
preferably less than about 5 minutes. Longer times may
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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, infra-red, and the like. Resistance
or infra-red heating, for example, would provide almost
instantaneous heating thus requiring very short heating
times of 30 seconds or less, although longer times could
be employed if desired.
Any method and machine may be employed for
making thQ worked alloy and/or battery plates and U.S.
Patent Nos. 3,310,438: 3,621,543; 3,94s,097; 4,035,5s6: ~ -
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. Patent No.
4,271,586 shows, for example, a ribbon of lead being fed
into an inlin~ expander, followed by pasting, drying,
cutting and accumulating into ~tacks. U.S. Patent No.
4,035,556 disclose~ 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 tb) 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
~anuf~cture of the alloy or battery grid. Preferably,
th- alloy can be continuou~ly cast, worked into strip,
heat treated, expanded or punched into the grid, pasted,
and assembled directly into the battery. The worked
strip can also be coiled for storage and then heat
treated and made into grids or it can be heat treated,
coiled and stored for proce~sing 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.
woso/os462 2 ~ ~ PCT/US90/~310
The following exampleq will further illustrate
the present invention. It will be understood that
throughout this specification and claims, all parts a~d
percentages are by weight and all temperatures in
degrees Centigrade unles~ otherwise specified.
EXAMPLE I
A series of alloys having varying tellurium
levels were prepared in a heated graphite crucible by
melting a base lead, antimony, arsenic, tin alloy to a
final composition containing 1.3~ antimonyl 0.16%
arsenic and 0.23% tin, balance essentially lead.
Tellurium metal was added to the crucible at the le~els
indicated in TABLE I. Four alloys were prepared by
casting into a graphite book mold at 400-C.-500-C. to
produce a cast block approximately 10 inch x 3.5 inch x
0.75 inch.
The casting~ were m$11ed to remove surface
defects and then rolled at room temperature to 0.045
inch in eight-twelve pas~e~ taking about a 20-30%
reduction per pass. Sample~ for che~ical analysis were
cut from the resultant ~trip. Blanks 4 ineh x O.5 ineh
for machining to test bara were eut from the strip in
the rolling ~lonqitudinal) direction. A Tensilkut
Machine was used to eut the test bars to a 1 ineh gage
}ength and 0.25 ineh width. Heat treatment wa~
performed by immersion of the bar~ in a molten ~alt bath
at 230-C. for 15 seeond~ and quenehed by plunging into
room temperature water irmed~ately upon removal from the
salt bath. The samples were then stored at room
temperature for 10 days. Tensile test~ representing the
ultimate ten~ile strength ~UTS) were perforJed on an
Instron Maehine using a eros~head ~peed of 0.2
inch/minute and mierohardness testing was employed to
obtain Viekers Hardn-ss Numb~rs (VHN) at the grip
seetion (un~trained) and the gage length seetions
(strained) of the speeimen.
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TABLE I
As-Rolled
Alloy Tellurium _ VHN
No.(ppm) UTS(psi)l ~ UTS(psi~ ~ (Strained)
170 4050 9.4 5450 13.2 13.7
2200 4150 9.9 5280 11.6 13.3
3340 4150 9.4 4870 12.7 12.8
4l 1400 l 42Z0 6380 13.7 13.6
The results in TABLE I clearly show that the
tellurium containing alloys retain their heat treated
hardness after being pulled in tension to failure (27-
40% elongation to fracture). The results als~ indicate
the increase in strength of the heat treated alloy
versus the as-rolled alloys.
Similarly prepared alloys which contained no
tellurium were tested in an identical manner. None of
the alloys exhibited the permanent hardness retention in
the strained region of the tensile specimens as clearly -~
shown in TABLE I, with the hardness decreasing to values ;~
near the as-rolled condition.
Whlle 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
thi~ invention without departing from the scope of the
invention defined by the appended claims.
