Steels_ Metallurgy and Applications, Third Edition

338 Steels: Metallurgy and Applications

24 0

2

4

6

20 8

~ 16 9

12

18 20 22 24 26

Chromium equivalent

Figure 4.21 De Long diagram for prediction of microstructure in stainless steel welds
(After De Long 22)

calling for low magnetic permeability, which precludes the introduction of delta
ferrite.
The problem of chromium carbide precipitation at grain boundaries and sensi-
tization to intergranular attack was discussed earlier in the section dealing with
the corrosion behaviour of stainless steels. However, this problem can now be
avoided with the use of stabilized grades such as Types 321 and 347 or with the
L grades where the carbon is restricted to 0.03% max.

Ferritic stainless steels

Again, the absence of transformation in ferritic grades eliminates the potential for
cold cracking and, due to their moderate thermal expansion characteristics, these
steels are generally free from solidification cracking. On the other hand, ferritic
stainless steels are prone to grain growth and this leads to low levels of toughness
in the weld, particularly in thicker sections. However, sound autogenous welds
are produced in Type 430 (17% Cr) steel in thin gauges in applications such
as domestic sinks, washing machines and dish washers. Where thicknesses of
several millimetres are to be welded, it is preferable to use an austenitic filler
such as Type 316 which produces weld metal of high toughness.
Ferritic stainless steels are susceptible to hydrogen cracking, and in gas
shielded processes, pure argon rather than argon-hydrogen mixtures should be
used.

Variable weld penetration

With the introduction of automatic welding processes, such as orbital TIG for
tube joining, major problems were encountered with stainless steels due to
variable weld penetration. Thus marked differences in the depth of penetration