chromium depletion in the surrounding area. Corrosion is therefore limited
to zones where niobium carbide has been re-dissolved, i.e. zones in the immedi ate vicinity of the weld. These zones are rather narrow and this type of inter granular
corrosion attack is thus called knife line corrosion. Its appears only in stabilized aus tenitic materials in both the HAZ of the base metal and the HAZ of the weld metal
in multi-layer welds or where there are two or more parallel weld deposits.
Fig. 88 shows the appearence of some typical welded joints where intergranular
corrosion has taken place according to Schabereiter [253]. After welding, these
specimens were annealed in the temperature range of550-600° C and then subject ed to the Strauss test. In the unstabilized, corrosion susceptible materials (upper
micrographs), intergranular corrosion attack occurs over the whole area of the weld
metal (Fig. 88a) and the base metal (Fig. 88b). The specimens were taken from the
top run of a single V but weld of 20 mm thickness. Intergranular corrosion attack is
only visible in the deformed part of the bent specimen, where the area attacked by
intergranular corrosion has opened up. In niobium stabilized weld metal (ratio of
stabilization- % Nb : % C = approx. 10 : 1), intergranular knife line corrosion
Fig. 88. Different types of intergranular corrosion in weldments after prolonged annealing in
the temperature range of around 5500 C obtained in the Strauss test according to Schabereiter
[253]. Upper micrographs-Unstabilized chromium-nickel steels of the type AISI 308; a inter granular corrosion in the IC prone weld metal, b intergranular corrosion in the heat affected
zone of the IC prone base metal. Lower micrographs-Niobium stabilized chromium-nickel
steel of type AISI 347. c knife line corrosion in the heat affected zone between the three beads
of the top layer ofintergra.nular corrosion prone weld metal, d knife line corrosion in the HAZ
of the intergranular corrosion prone base metal
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4.1 Carbide Precipitations in Stainless Steels 123
appears in the heat affected zone between the three beads of the top layer of the
weld (Fig. 88c). The bead in the middle which had been deposited last, produced
knife line corrosion in the heat affected zone ofthe previously deposited outer beads
of the top layer. If the weld metal is sufficiently resistant to intergranular corrosion
attack, but the base metal is not, corrosive attack will be limited to the heat affected
zone of the base metal on both sides of the weld (Fig. 88d).
The occurrence of knife line corrosion in the HAZ after annealing in the
temperature range of 500-700° C can be avoided by lowering the carbon content to
below 0.04% with a simultaneous overstabilization to at least one and a halftimes
the normal stabilization ratio (with titanium> 8 X % C, with niobium> 15 X % C).
Heat treatments of 10-15 hours in the temperature range of 500-600° C are most
critical. If annealing takes place at higher temperatures, a smaller degree of over stabilization will normally be suffice since re-diffusion of chromium in the depleted
zones is already apparent in such cases.
Fig. 89 shows the areas of grain decay caused by intergranular corrosion
in austenitic weld metal with different chromium contents and varying stabi lization ratios which clearly shows the influence of alloying elements. The behav iour of stabilized austenitic weld metal is mainly governed by the combined effect
of the three elements carbon, niobium and chromium which are interdependent
with regard to their effect on knife line corrosion. An additional factor to be consid ered in this context is the fact that niobium contents above 1% will increase the hot
cracking sensitivity and chromium contents above 20% will increase the tendency
towards sigma phase precipitation.
Therefore, an increase in these two elements is limited and for a change in
content we must always consider the overall property requirement of the weld
metal.
mOr-----,------,------.------,
500~----~------~----~~--=-~~ 0,1
Time in hours
Fig. 89. TTC diagram of the beginning of inter granular corrosion attack with consideration to
knife line corrosion in austenitic weld metals with different contents of chromium and nio bium. Chemical composition in %: No.1: C= 0.027, Si= 0.63, Mn= 1.55, Cr= 17.97, Ni=
9.00, Nb = -; No.2: C= 0.027, Si = 0.74, Mn = 1.71, Cr= 20.07, Ni = 10.02, Nb = -; No.3:
C = 0.033, Si = 0.70, Mn = 1.75, Cr = 21.20, Ni = 11.08, Nb = 0.30; No.4: C = 0.035, Si =
0.85, Mn=1.58, Cr=17.98, Ni=8.97, Nb=0.55; No.5: C=0.031, Si=0.62, Mn=1.78,
Cr = 19.96, Ni = 10.14, Nb = 0.65