Arc Strike on T91 reheater tube.

[replica]

Hi all,

I need to know whether heat input due to arc strike on the T91 reheater tube is detrimental to creep properties.

We had various spot on T91 reheater tube that was accidentally arc strike with welding set. The region had left the hot spot and after replication it revealed martensite structure with hardness value of 320 Hv. The hardness at the surrounding region is 199 Hv (tempered martensite). This hard region is deep and could not simply be removed without deep grinding that may involve a lot of metal removal.

My question is this local high hardness region (untempered martensite) can be detrimental to creep properties and develop a high stress region which may affect the performance of the tube.

What is the step to be taken in order to use the affected tube?

Comments are highly appreciated.

 

[EdStainless]

Yes, they are just like welds that have not been PWHT.
If it is just a couple of individual tubes I would consider plugging them.

Of course I would also ban the welder for life from my site, and inform his employer that they now have a large liability.

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P.E. Metallurgy, Plymouth Tube

 

[replica]

EdStainless

Thank you very much for a prompt reply. We may consider replace it.

 

[metengr]

Why not remove the arc strike by grinding IF minimum wall thickness has not been violated?

 

[mrfailure]

Why not just do a Dutchman repair and replace the length containing the arc strike? I would not leave it in service at all.

 

[metengr]

If you are going to install a dutchman, PWHT is necessary, so you can do the same with the arc strike, perform PWHT and perform a surface NDT to ensure no cracks.

 

[mrfailure]

The reason I prefer a Dutchman is the certainty of fixing the problem. It is just as easy to replace as it is to grind and test to make sure wall is not damaged and minimum wall is maintained. (Of course it is easy for me to say - I am not the one who actually has to perform the work).

 

[replica]

Thank you very much for the valuable comments. I may consider all the options. We had identified the culprit. The management had made the rule that all the arc strike on T91 reheater tubes should be used. Some arc strikes were also found on T23 tubes but I supposed this was not as an issue compared to T91.

 

[pradipgoswami]

Hi Replica,
If the inventory level permits and if not installed yet, just replace the tubes.
Or else, painstakingly grind each arc strike area and do weld build up by GTAW and perform PWHT by either blanket heating or induction heating method. Re heater tubes may not have enough wall thickness left after grinding, hence you may have no other options but to repair should you decide to use the existing tubes.

Leave aside the creep issues, arc strike on thinner walled Non- heat-treated reheaters could cause cracking due to the differential stresses imposed on the reheater walls during operations.

Attached are some extracts from an Alstom Presentation for your information.

Thanks.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299
All provided answer are personal opinions or personal judgements only. It's not connected with any employers by any means.

 

[replica]

pradipgoswami

Thank you very much for the info. There was a mistake in my previous posting. The management had made the rule that all the arc strike T91 reheater tubes should NOT be used. We will replace the tubes.

EdStainless ,metengr,mrfailure,

Thank you very much for your comments.