Cracks during welding of St.St 347 piping after eight years in-service 2

[engessam2001]

hi every one,
During welding St.St 347 thk45 mm Dia./16inch of transfer line between ammonia converter and waste heat boiler we observed cracks in root toes.
we use ER347Si for root passes and E347-15 for filling passes- and N2 as purging gas .
the specification of line are
-ammonia synthise gas -operating pressure 185 bar.&458 operating temperature-in service since 8 years
have any one subject to this issue before and what the main reason for these cracks and how avoid it in future.

thanks alot

 

[TugboatEng]

Do consider that nitrogen can form compounds with elements in stabilized stainless steel. Titanium nitride and niobium nitride are possible. This is likely detrimental to weld quality.

 

[EdStainless]

You should have used pure Ar for weld, purge, and backing gas.
You can bet that all of your Nb is nitride, and a fair amount of the Cr also.
I will presume that your cleaning and prep work is very good.
Only non-halogen solvents and new abrasives used?
Was any pre-heat used to assure that everything was dry?

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P.E. Metallurgy, consulting work welcomed

 

[Dhurjati Sen]

Carry out spot metallography at the cracked locations.

It seems sensitization has taken place in spite of the SS347 being a stabilized grade.

DHURJATI SEN
Kolkata, India

 

[weldstan]

Check for sigma assumuming 458C.

 

[engessam2001]

hi,
the same happen but with different scenario, 1.25 Cr (P11) 38mm heat exchanger tube sheet has overlay with 347Si electrode ,after PWHT tube 321 ST.ST welded to tube sheet ) tubes welds cracked after 10 years service in N2 fluid content .

 

[EdStainless]

With the stabilized grades if you don't deliberately HT them to for the carbides,
or if your operating temperature isn't in the desired stabilization range ALL OF THE TIME; then they will sensitize.
There isn't anything magical about these alloys.
Operate too cold and they sensitize.
Operate too hot and you undo the stabilization.
Both Ti and Nb can cause hot shortness.
I have long wondered why we don't have a dual stabilized 300 alloy using a little of each.
Many mills try to run the Ti or Nb as low as possible which means running very low C.
This isn't helpful for high temperature operation.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[sahsanb]

I don't believe temperatures in Ammonia Converter outlet line are high enough to initiate sensitization. This looks more like a high heat input related issue at the time of original welding.

In-service cracking issue is quite prevalent in stabilized austenitic steels. At our site we have also faced recurrent leakages issue on SS 321H material piping.

I would suggest to gather the following details to help you in your RCA

1- Carry out insitu-metallography of crack and adjacent base metal
2- Measure Ferrite No. and Hardness on weld joint and HAZ area
3- Try not to repair and reweld over the crack and try to remove the cracked portion of piping for detailed laboratory analysis.