knife line attack welding defect
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Knife Line AttackCompiled and edited by
Nasrizal Mohd Rashdi
Senior LecturerFabrication & Joining Section
Universiti Kuala Lumpur
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Knife Line Attack
Occur in stabilized austenitic stainless
steel such as 321 & 347.
It also inter-granular corrosion but not
same as weld decay.
Also cause by precipitation of Chromium
carbide at grain boundary.
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Differs from weld decay
Occurs in narrow band in the area
immediately adjacent to weld metal.
Occurs in stabilized SS
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Mechanism
The area corresponding to thermal cycle
1 is very close to the fusion boundary
Therefore is subjected to both a very high
peak temperature and a very rapid cooling
rate during welding.
This peak temperature is above the solvus
temperature of titanium carbide, so it will
dissolves in this area.
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Mechanism
Because of the rapid cooling rate through its
precipitation temperature range, titanium carbide
does not re-precipitate during cooling
Thus leaving abundant free carbon atoms in thisarea.
When the weld is reheated in the Cr carbide
precipitation range (for stress-relief or in
multiple-pass welding), Ti carbide does not form
appreciably since the temperature level is not
high enough.
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Consequently, Cr carbide precipitates at
grain boundaries, and this area becomes
susceptible to intergranular corrosion
attack.
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Because of the high temperate gradient
near the fusion boundary and the high Ti
carbide dissolution temperature, the region
in which Ti carbide dissolves duringwelding is very narrow
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As a result, subsequent intergranular
corrosion attack occurs in a very narrow
strip immediately adjacent to the fusion
boundary, and thus the name knife-lineattack. Figure 18.15 shows the
transmission electron
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Prevention methods
Addition of rare earth metal ( La + Ce).
It reduce the rate of intergranular corrosion.(see fig. 18-18).
It no adverse effects on mechanicalproperties.
Post weld heat treatment.
Heat to 1000OC to 1100OC, to dissolving thechromium carbide and than rapid quenching.
Use low carbon grade.
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Question
State the area are possibility to knife line
attack
second pass
FirstPass
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