Steels_ Metallurgy and Applications, Third Edition

._o C

r

O9 ::3

(a)

High-purity

steels

~ negative

Low-purity

steels

dt

(b)

Temperature Temperature

Cold

340 Steels: Metallurgy and Applications

Cold

(c)

(d)

Figure 4.23 Marangoni convection in welds: (a) surface energy dependence; (b) tempe-

rature distribution in welds; (c) fluid flow on surface; (d) bad and good weld penetration

impurities are present. The effects are illustrated schematically in Figure 4.23(a).
The temperature distribution in the weld pool is similar in both 'good' and
'bad' materials (Figure 4.23(b)) but the different relationships between surface
tension and temperature in the two materials cause a marked difference in fluid
flow (Figure 4.23(c)). Thus in high-purity steels, the surface liquid flow is from
the centre to the outside, giving an upward axial flow and poor penetration
(Figure 4.23(d)). On the other hand, in steels containing surface active impurities,
the flow is towards the centre of the liquid pool, giving downward axial flow and
good penetration.
Leinonen 25 has also demonstrated the beneficial effect of sulphur on the TIG
weldability of Type 304 steels. He showed that the maximum welding speed
that could achieve through-penetration in a steel containing 0.13% S was 74%
higher than that for a 0.003% S steel, when using 100% argon shielding gas. The