Page 229 - Battery Reference Book

Important physical characteristics of antimonial lead battery grid alloys 18/13
Increase of age-hardening by further additives
The age-hardening is also increased by further
additives, e.g. arsenic. The influence of arsenic is
more effective when the supersaturation of antimony
in lead is relatively low (e.g. for grids not subjected to
special heat treatment). Figure 18.12 shows this effect;
arsenic not only increases, but also accelerates the age-
hardening process.
18.3.2 Castability

In addition to the mechanical strength required for fur-
ther treatment of the grids, castability is an important
factor for producing sound grids at tolerable cost. The
main difficulty with casting low-antimony alloys is
that the decrease of antimony content is accompanied
by the appearance of hot cracks if no special precau-
tions are taken. For example, Figure 18.13 shows the
cast structure of grids containing 2% antimony. The
solidification takes place in a coarse dendritic structure
containing cracks along grain boundaries. Inadequate
grid quality (caused by these cracks) was one of the Figure 18.13 Coarse dendritic cast structure with cracks
main reasons why low antimony alloys were formerly (lead-2% antimony alloy). (a,b) Industrial grid (x100) (Courtesy
not used on a large scale in the battery industry. of the Swiss Post Office, Bern)
The poor grid quality of low antimony alloys can be
overcome by the addition of selenium. This addition
forms a fine globulitic solidification which results in
fewer casting faults (Figure 18.14). The grain size of
an alloy containing 2% antimony decreases from about
100 pm to about 60 pm when 0.02% selenium is added.
With respect to grain refinement, selenium proved to
be superior to all other additives. The formation of

Age-hardening time at room temperature
until HE = 14 kgf/rnmZ are achieved

.-m Ir

;
I
;
I
!
I
)
Without With 1 I ,
arsenic aneni? Figure 18.14 Fine globulitic cast structure (lead-2% anti-
mony-0.02% selenium alloy). (a) Automotive grid (x200),
(b) industrial grid (x200) (Courtesy of the Swiss Post Office, Bern)
mould during casting and leads to casting faults, is
” dendrites, which disturbs the feeding capacity of the
As cast (without Heat treatment almost fully suppressed. With fine globulitic solidifi-
heat treatment) 1 h/240 ‘C/H,O ageing cation, uniform mechanical properties are achieved in
at room temperature all directions, and hence ductility is increased.
Figure 18.12 Influence of arsenic on the age-hardening of Using the combination of alloying elements of
lead-2% antimony alloy (Courtesy of the Swiss Post Office, Bern) antimony (1.5-3.5%) arsenic, selenium (and tin to