Heat Treating of Lead and Lead Alloys,Heat Treating of Lead and Lead Alloys,Heat Treating of Lead and Lead Alloys
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 recrystallize
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 met with some success.
Solid-Solution Hardening
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 overaging 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--Pb3Ca in lead-calcium alloys and Pb3Sr in lead-strontium alloys--
as grain boundaries move through the structure.
Solution Treating and Aging
Useful strengthening of lead can be attained by adding sufficient quantities of antimony to produce hypoeutectic leadantimony
alloys. Small amounts of arsenic have particularly strong effects on the age-hardening response of such alloys,
and these effects are enhanced by solution treating and rapid quenching prior to aging.

Additional comments :