What Is Knife-Line Attack?

Knife-line attack (KLA) is a specialized form of intergranular corrosion that occurs in a very narrow band (as thin as a knife cut) immediately adjacent to the fusion line of welded stabilized austenitic stainless steels such as Type 321 (Ti-stabilized) and Type 347 (Nb-stabilized). Despite the stabilizing elements intended to prevent sensitization, the extreme temperatures near the weld fusion line dissolve the protective TiC or NbC carbides, leaving chromium free to form Cr23C6 during subsequent thermal exposure.

When Knife-Line Attack Occurs

KLA requires a specific sequence of thermal events:

1. Welding: The narrow zone adjacent to the fusion line is heated above approximately 1250 deg C, which dissolves the stabilizing carbides (TiC in 321, NbC in 347) back into solution
2. Subsequent heating in the sensitization range: A post-weld stress-relief treatment or a second welding pass heats the same zone to 450-850 deg C, allowing chromium carbides (Cr23C6) to precipitate at grain boundaries before the stabilizing elements can recombine
3. Corrosive service: The chromium-depleted grain boundaries corrode preferentially in aggressive media

The attack zone is extremely narrow (often less than 1 mm) because only the metal very close to the fusion line reached the high temperature needed to dissolve the stabilizing carbides.

Comparison: Weld Decay vs Knife-Line Attack

Feature	Weld Decay	Knife-Line Attack
Affected grades	Standard grades (304, 316)	Stabilized grades (321, 347)
Location	HAZ, 2-5 mm from fusion line	Immediately at the fusion line (<1 mm)
Width of attacked zone	Several millimeters	Extremely narrow (knife-line)
Cause	Cr23C6 precipitation due to high carbon	Dissolution of TiC/NbC followed by Cr23C6 reprecipitation
Triggering thermal event	Single weld cycle	Welding + subsequent reheat (PWHT or multipass welding)
Prevention	Use L-grades or stabilized grades	Solution anneal after welding; avoid unnecessary PWHT

Prevention Strategies

Strategy	How It Works
Avoid PWHT on stabilized grades	Eliminate the secondary heating that triggers Cr23C6 precipitation
Solution anneal after welding	Heat to 1050-1100 deg C and water quench to redissolve all carbides and re-form stabilizing carbides on controlled cooling
Use L-grade + stabilized (e.g., 316L)	Low carbon reduces the volume of carbide available for reprecipitation
Control multipass welding heat input	Minimize the time the fusion line zone spends in the sensitization range during subsequent passes
Use Nb-stabilized 347 over Ti-stabilized 321	NbC is slightly more resistant to dissolution than TiC at high temperatures

Pro Tip

If post-weld stress relief is mandatory (e.g., for thick-walled pressure vessels), consider using a low-carbon, non-stabilized grade such as 316L instead of 321 or 347. The L-grade avoids KLA entirely because there are no stabilizing carbides to dissolve, and the low carbon content minimizes conventional weld decay.

Detection

KLA is difficult to detect by standard non-destructive testing because the affected zone is extremely narrow. It is typically identified during in-service inspection when a fine corrosion groove appears along the weld fusion line, or through metallographic examination of test coupons exposed to the service environment. ASTM A262 (Practice E, Strauss test) can reveal susceptibility in laboratory samples.

Read the full guide to fillet welds