Knife Line Attack Welding Defect 70383p

Knife Line Attack Compiled and edited by Nasrizal Mohd Rashdi Senior Lecturer Fabrication & ing Section Universiti Kuala Lumpur

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.

Differs from weld decay  Occurs

in narrow band in the area immediately adjacent to weld metal.  Occurs in stabilized SS

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.

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- welding), Ti carbide does not form appreciably since the temperature level is not high enough.

 Consequently,

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

 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

 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

Prevention methods  Addition 



It reduce the rate of intergranular corrosion. (see fig. 18-18). It no adverse effects on mechanical properties.

 Post 

of rare earth metal ( La + Ce).

weld heat treatment.

Heat to 1000OC to 1100OC, to dissolving the chromium carbide and than rapid quenching.

 Use

low carbon grade.

Question  State

the area are possibility to knife line attack

First

second