Reheat cracking and knifeline corrosion are two completely different failure mechanisms. The knifeline corrosion has more to do with base metal regions adjacent to the weld where carbides dissolve and re-precipitate as chromium carbides in lieu of TiC or NbC or TaC. This local band of chromium carbide precipitation can be susceptible to corrosion attack in aqueous service.

Reheat cracking is caused by precipitates that form within the grains of the 347 base metal heat affected zone during welding. The precipitates increase the creep strength of the grains themselves and render the grain boundaries susceptible to creep damage during relieving of residual tensile stresses from welding or even cold working in service.

Thanks a lot. You are right. I knew these but a bad article confused me.

By the way, do you know where I can find more information about 316Ti welding?

I am not sure what you are looking for exactly. However, try two of my favorite web sites below

http://www.avestawelding.com/?msg=404

http://www.ssina.com/publications/index.htm

Hello,

I am trying to find out some differences between Titanium stabilized stainless steel (321, 316Ti) and Niobium Stabilized stainless steel(347, 316Nb). It seems Nb stabilized grades are more susceptible to hot cracking than Ti stabilized grades. Am I right? If it is, what is the reason?

Thanks   

Johnae;
What you mean is reheat cracking, not hot cracking. The NbC precipitates in Type 347 stainless steel can significantly increase the creep strength of this material. Whereas, titanium carbides serve to tie up the carbon and avoid the formation of harmful chromium carbides. Hot cracking has more to do with cracks in weld metal formed during solidification of primary austenite in the weld deposit.

Sorry, metengr, I meant reheat cracking.

I think TiC precipitate in 321 has same function with NbC precipitate in 347, strengthening grains and causing grain boundaries tearing when cooling. Also, both of them can serve as stabilizing elements to tie up carbon and avoid CrC formation.

Quote:

I think TiC precipitate in 321 has same function with NbC precipitate in 347, strengthening grains and causing grain boundaries tearing when cooling.

No. Be careful, they are different carbide morphologies. Niobium enhances creep strength properties. In my dealings with 347 stainless, the reheat cracking is more common in heavier section thicknesses, and has not been reported for 321 stainless steel.

There can also be cases with 347 of the alloy being hot-short.  That is cracking during hot working.  This is more common with Nb grades than with Ti grades.
My experience is similar to metengr, in that reheat cracking is more of an issue with 347, but I feel that it is mostly driven by coarser, less distributed, carbides.

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