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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