Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 103

I

/ "x~176176

I--7[,- acceptable!

I "i lz, i

(a) i 8,1 ~ 8. 2

(b)

: i ....

h

Acceptable

conditions

Small \

welds \ ,,

Current

Figure 1.102 Weldability lobes-resistance welding; (a) fixed time Tt; (b)for various

times (After Jones and Williams 152)

50% compared with comparable thicknesses of plain carbon steel. The available
current can also be increased significantly by using larger electrode tip sizes. The
width of the lobe, for example, can be increased by 700-2000 A by increasing
the tip diameter from 4.8 to 6.4 mm.
Alloying elements increase the resistivity of low-carbon ferritic steels and
consequently less current is require to produce a weld of a given size. The
increased electrode forces, however, reduce the contact resistance. The overall
effect, therefore, is generally a reduction in the current required.
The mode of failure can be different in higher-strength steels compared with
low-strength steels. Both carbon and phosphorus can influence the fracture mode
of spot welds and partial plug failures are the most common type of failure
in peel tests on rephosphorized steels with a diameter of 5~/t. 100% plug fail-
ures, however, can be obtained in rephosphorized steel of 0.67 mm thickness by
increasing the weld size to 7.5~/t. 152 The mode of fracture is, however, influ-
enced by steel type, weld diameter, sheet thickness, geometrical factors and test
type. Plug failures are promoted by large weld/high heat conditions. The optimum
balance between strength and weldability is obtained by generally limiting the
combined carbon plus phosphorus contents to a maximum of 0.18%.
It is useful to note that a considerable improvement to the fatigue life of welded
joints may be obtained by increasing the weld size or by increasing the number of
welds. 152 It was found, however, that the increase in strength was generally rather
less than a simple multiplication of the strength of each weld by the number of
welds due to the unequal loading of each weld during testing. It was considered
that weld design, pitch and number need to be considered carefully if adequate
benefit is to be derived from the use of higher-strength steel.