knife line attack.pdf

1. Knife Line Attack
edited by
Ayman Aydan Mahamoud Abdelhadi Sec 2
Abdulrahman Amer Mohamed Amer Sec 2
Hussien Mohamed Hussien Sec 2

2. 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.

3. Differs from weld decay
 Occurs in narrow band in the area
immediately adjacent to weld metal.
 Occurs in stabilized SS

7. 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.

8. 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 this
area.
 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.

9.  Consequently, Cr carbide precipitates at
grain boundaries, and this area becomes
susceptible to intergranular corrosion
attack.

10.  Because of the high temperate gradient
near the fusion boundary and the high Ti
carbide dissolution temperature, the region
in which Ti carbide dissolves during
welding is very narrow

11.  As a result, subsequent intergranular
corrosion attack occurs in a very narrow
strip immediately adjacent to the fusion
boundary, and thus the name knife-line
attack. Figure 18.15 shows the
transmission electron

13. 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 mechanical
properties.
 Post weld heat treatment.
 Heat to 1000OC to 1100OC, to dissolving the
chromium carbide and than rapid quenching.
 Use low carbon grade.

15. Question
 State the area are possibility to knife line
attack
second pass
First
Pass