SS 347 Welding & Carbide Precipitation

[Guest102023]

I am familiar with the issues around sensitization by welding of stabilized SS347, but I have had no direct experience with metallography or failure investigations.

(1) What features am I looking for in the HAZ?
(2) What do the carbides have to look like in terms of size and distribution for me to know I have a possible sensitization issue?
(3) In the stabilized condition, what do the Nb(C,N) particles look like - anything like the Ti(C,N) particles I am more familiar with?

Any good references on this subject?

Thanks in advance.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[EdStainless]

There are micros in the ASM handbook.
If the carbides are along the grain boundaries you are in trouble, they should be fairly small and well dispersed.
You will be able to identify them in a polished and etched sample.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

There is more to this material than simply evaluating sensitization. 347H can be prone to reheat cracking after welding. If you search the internet there is information on reheat cracking of 347 stainless steel, which I believe needs to be considered especially on thicker wall material after welding.

 

[Guest102023]

The joint configuration is ½" Ø I-800 tubes OFWelded into a 347 tubesheet, about 1" thick. There is a trepanned groove around each tube hole to mitigate the thermal mass imbalance, so it doesn't qualify as thick wall.

Is there an ASTM corrosion test to evaluate possible sensitization?

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[Guest102023]

A couple of pictures, which I'm unsure how to interpret. I don't have access to the real metallographic sample.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[metengr]

First comment - for 347 stainless steel, the amount of sensitization from welding is usually minimal if the material has been properly stabilized.
2nd comment - typically, ASTM A 262 is used to evaluate sensitization of austenitic material which has been in service or after fabrication.

 

[Guest102023]

Thanks Metengr.

I can see a few large inclusions that are possibly Nb(C,N) from stabilization. What would be the likely makeup of the band of fine precipitates?
BTW this section was etched with oxalic acid.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[metengr]

You will have in descending order; Cb(Nb) and Ti carbides followed by CrC.