Lead is normally considered to be unresponsive to heat treatment. Yet, some means of
strengthening lead and lead alloys may be required for certain applications. Lead
alloys for battery components, for example, can benefit from improved creep resistance
in order to retain dimensional tolerances for the full service life. Battery grids
also require improved hardness to withstand industrial handling.
The absolute melting point of lead is 327.4°C (621.3°F). Therefore, in
applications in which lead is used, recovery and recrystallization processes and
creep properties have great significance. Attempts to strengthen the metal by reducing
the grain size or by cold working (strain hardening) have proved unsuccessful.
Lead-tin alloys, for example, may recrystailize immediately and completely at room
temperature. Lead-silver alloys respond in the same manner within two weeks.
Transformations that are induced in steel by heat treatment do not occur in lead alloys,
and strengthening by ordering phenomena, such as in the formation of lattice
superstructures, has no practical significance.
Despite these obstacles, however, attempts to strengthen lead have had some success.
In solid-solution hardening of lead alloys, the rate of increase in hardness generally
improves as the difference between the atomic radius of the solute and the atomic
radius of lead increases.
Specifically, in one study of possible binary lead alloys it was found that the
following elements, in the order listed, provided successively greater amounts of
solid-solution hardening: thallium, bismuth, tin, cadmium, antimony, lithium, arsenic,
calcium, zinc, copper, and barium.
Unfortunately, these elements have successively decreasing solid-solution solubilities,
and therefore the most potent solutes have the most limited solid-solution hardening
effects. Within the midrange of this series, however, are elements that, when alloyed
with lead, produce useful strengthening.
A useful level of strengthening normally requires solute additions in excess of the
room-temperature solubility limit. In most lead alloys, homogenization and rapid
cooling result in a breakdown of the supersaturated solution during storage. Although
this breakdown produces coarse structures in certain alloys (lead-tin alloys, for
example), it produces fine structures in others (such as lead-antimony alloys). In
alloys of the lead-tin system, the initial hardening produced by alloying is quickly
followed by softening as the coarse structure is formed.
At suitable solute concentrations in lead-antimony alloys, the structure may remain
single phase with hardening by Guinier-Preston (GP) zones formed during aging. At
higher concentrations, and in certain other systems, aging may produce precipitation
hardening as discrete second-phase particles are formed.
Alloys that exhibit precipitation hardening typically are less susceptible to over
aging and therefore are more stable with time than alloys hardened by GP zones.
Lead-calcium and lead-strontium alloys have been observed to age harden through
discontinuous precipitation of a second phase Pb-Ca and Pb-Sr in lead-strontium alloys
as grain boundaries move through the structure.
Solution Treating and Aging
Adding sufficient quantities of antimony to produce hypoeutectic lead-antimony alloys
can attain useful strengthening of lead. Small amounts of arsenic have particularly
strong effects on the age-hardening response of such alloys, and solution treating and
rapid quenching prior to aging enhance these effects.
Hardness Stability. For most of the two-year period, the
solution-treated specimens were harder than the quench-east specimens. Other
investigations have also shown that alloys cooled slowly after casting are always
softer than quenched alloys. The alloys with 2 and 4% Sb harden
comparatively slowly, and the alloy containing 6% Sb appears to
undergo optimum hardening.
Application. Because of the detrimental effect of antimony on charge
retention, the effort to reduce antimony contents of the positive plates in lead-acid
storage batteries has led to the trend of replacing eutectic alloys with a
Pb-6Sb-0.15As alloy. Battery grids made of this arsenical alloy will age harden slowly
after casting and air-cooling. However, storing grids for several days constitutes
unproductive use of floor space and results in undesirable interruptions in
Although large-scale solution treatment of battery grids might be difficult to justify
economically or to achieve without some distortion, quenching of grids cast from
arsenical lead-antimony alloys offers an attractive alternative method of effecting
improvements in strength. The suitability of quenched grids can be assessed by
comparing with the hardness level that battery grids require in order to withstand
industrial handling (about 18 HV, the hardness of the eutectic alloy). The alloy
containing 2% Sb clearly does not respond sufficiently to be
considered as a possible alternative. The 4% Sb alloy, however,
attains a hardness of 18 HV after 30 min, and the alloys that contain 6, 8, and
10% Sb could be handled almost immediately.
Another mechanism for strengthening of lead alloys involves elements that have low
solubilities in solid lead, such as copper and nickel. Alloys that contain these
elements can be processed so that no homogenization results; most of the strengthening
that occurs is developed through dispersion hardening, with some solid-solution
hardening taking place as a secondary effect.
The resulting structure is more stable than those developed by other hardening
processes. Dispersion strengthening also has been achieved through powder metallurgy
methods in which lead oxide, alumina, or similar materials are dispersed in pure lead.