Intergranular corrosion

By gsi

Date 12-30-2005 18:59

I have a 24” pipe with a wall thickness of 2”(516 gr70). I’m attaching a ½’’ thick 304L plate to the OD.
The weld joint is a full penetration weld, with a single bevel groove. The bevel is on the 304 plate only (tee joint).
Because of the thickness we are required to preheat the pipe. Our client recommends a 200-degree preheat.
We are also using a 309LSI .045” GMAW electrode with a 98/2 shielding gas.
After we have the root pass in the groove the heat transfers into the 304 plate and stays warm (150-200) about 3” adjacent to the weld joint.
Is there any reason to be concerned with Intergranular corrosion?

By jon20013 (*****)

Date 12-30-2005 20:00

I would say you have no problems, but defer to those with a better knowledge of stainless metallurgy such as Chuck Meadows.

By - Date 12-30-2005 20:04

Hi Dave,
I don't think you have any reason to worry about intergranular corrosion. IGA is dependent of time, temperature, and C content. During normal welding cycles, the time at temperature is normally too short to have sensitization to occur. Plus, using a low carbon filler metal will delay the onset of IGA. Preheating the pipe to only 200F (or even 200C) is still below the heat range of sensitization range. Based upon what you have told us, I think you are in good shape and without the worry of IGA. Your choice of filler metal (309LSi and gas of 98/2) is exactly what I'd recommend if I were asked. The reason the SS stays warm longer than the CS is the difference of the electrical conductivity of the 2 metal, which is perfectly normal. You seem to be doing everything right and I would not worry about IGA.

Chuck

By gsi

Date 12-30-2005 20:20

Thanks Chuck. I was a little curious about the preheating delaying the cooling and the weld to stay in that sensitization range longer. Thanks again and enjoy your New Years!

By - Date 12-30-2005 20:31

Dave,
Without knowing all the details, I think the preheat is more beneficial to the resistance of solidification cracking than sensitization in this case. With the difference of electrical conductivity between a CS and a SS, the cracking problem, or possibility of such, is a greater concern since a normal welding cycle is normally too short to incur sensitization or sigma. Actually, when welding 2 materials with such a greater thickness, it is advisable to retard the cooling rate to benefit the thinner SS. I would advise that you proceed and not worry about cracking or IGA. Thank you, and also I wish you a very Happy and prosperous New Year.

Chuck