Environmental Engineering Reference
In-Depth Information
If now the weldment is heated to 500-800
C for stress-relieving purposes or in
service, chromium carbide forms and sensitization follows. Since niobium car-
bide needs a higher temperature to form, the presence of niobium in the alloy
cannot prevent chromium carbide formation. The same situation prevails for the
titanium-stabilized variety.
°
Remedial Measures
Three effective means are available for avoiding intergranular corrosion or weld
decay in austenitic stainless steels. They are as follows:
1.
Solution annealing
. The sensitized components are heated to a temperature
above 815
C) followed by quenching in water. The high-
temperature treatment dissolves the carbides and quenching prevents their
reformation. While this treatment can be given to small components, it is
not practicable for large components or structures.
°
C (usually 1000
°
2.
Reduction of carbon content
. Austenitic stainless steels with low carbon con-
tent (
0.03%), e.g., 304L, 316L, etc., can be welded or heated in the critical
range without sensitization problems. However, grease or oil should be
cleaned from the surface of such steels before welding to prevent carbon
pickup during welding.
3.
Stabilization
. Austenitic stainless steels are available in ''stabilized'' grades
having alloying additions of Ti (type 321) or Nb
Ta (type 347). These ele-
ments have a higher affinity for carbon than chromium and when added in
requisite amounts they react with carbon to form various carbides at tempera-
tures higher than 815
C. Chromium is prevented from carbide formation at
the sensitizing temperatures as no more carbon is available.
°
3.6.2 Ferritic Stainless Steels
Like austenitic stainless steels, ferritic stainless steels also exhibit sensitization
and intergranular corrosion by a chromium depletion mechanism. However, there
are several differences between the two. First, the solubility of nitrogen being
low in austenitic stainless steels, precipitation of chromium nitride is not the
prime factor for sensitization. In ferritic stainless steels, this contributes signifi-
cantly. The second difference is the temperature range of sensitization. In ferritic
stainless steels this lies above 925
C where the solubility of carbon and nitrogen
becomes significant in ferrite. Because of this difference in the sensitizing temper-
ature range, the zone of intergranular corrosion also differs. In ferritic steels the
attack takes place at areas adjacent to the weld or in the weld itself.
The immunity to intergranular corrosion is restored by heating the sensitized
steel between 650-815
°
C for a short time. As the diffusion of chromium in the
body centered cubic (bcc) lattice of ferritic steel is significantly higher than in
austenitic steel, the uniform chromium composition is reestablished.
Reduction of interstitials, both carbon and nitrogen, provides an effective step
°
