Knife Line Attack

Knife Line AttackCompiled and edited byNasrizal Mohd RashdiSenior LecturerFabrication & Joining SectionUniversiti Kuala Lumpur1Knife Line AttackOccur 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.2Differs from weld decayOccurs in narrow band in the area immediately adjacent to weld metal.Occurs in stabilized SS3

4

MechanismThe 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.7MechanismBecause of the rapid cooling rate through its precipitation temperature range, titanium carbide does not re-precipitate during coolingThus 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.

8Consequently, 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

10As 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 methodsAddition 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.13

Question

State the area are possibility to knife line attack second passFirst Pass15