Steels_ Metallurgy and Applications, Third Edition

Stainless steels 339

were observed when steels of nominally the same composition were welded
under identical conditions. Alternatively, there was a tendency for the weld to
deflect to one side of the joint when steels from different casts were welded
together. For this reason, the problem is also known as cast-to-cast variability.
Although problems of this kind had been observed previously, and also in
materials other than stainless steels, they could be accommodated in manual
welding by adjustments to power or welding speed. Whereas factors such as
arc-metal interactions may be contributory, it is now generally agreed that
variable peneration is due to composition-induced, surface energy effects.
Llewellyn et al. 23 investigated the problem in commercial tube and plate
samples of Type 304L and found a reasonable correlation between current to
penetrate and sulphur content. This effect is shown in Figure 4.22 and regression
analysis on the pooled data for tube and plate materials yielded the following
relationship:

Current to penetrate (amps) = 47-686 (%S)

which explained 60% of the variance. No other factors were statistically
significant at the 95% confidence level. When similar work was carded out
on laboratory casts of Type 304L, sulphur had no effect. However, these
materials had oxygen contents of 190-330 ppm compared to 20-70 ppm in the
commercial steels. Following the work of Heiple and Roper, 24 it was anticipated
that both sulphur and oxygen would be beneficial in promoting good weldability.
However, the lack of a sulphur effect in the laboratory casts of Type 304L may
well have been due to the overriding effect of oxygen in these materials.
Changes in surface energy across the temperature gradients of the weld pool
surface give rise to Marangoni convection which has a marked effect on the
direction and velocity of fluid flow. Thus direction of fluid flow can be changed
by surface active impurities which reverse the negative temperature dependence
of surface energy for pure materials to a positive temperature dependence when

9 Tubes

50 -'~ ~ o Plates

* 9 o~

45

e9

• ~
  ~ , ,g o

= ~oo 9

0

o

oe ~

35

0 50 1 O0 150

Sulphur (ppm)

Figure 4.22 Effect of sulphur on the current required to give penetration of a 2-ram-thick
sheet or tube wall with a 5 second arc duration (After Llewellyn et al. 23)