Dissimilar Filler Metals

Date 04-04-2005 03:46

Greetings All,
Have a minor technical problem that I hope somebody might be able to help me with.
The off site fabricators (other contractors) have welded some thermowells (SA 479 316N) onto Carbon steel steam pipe (ASTM A53-B) using ER 309L filler metals.
The maximum Design Temperature for this pipe is 400°C.
Engineering have decided they want them cut out and rewelded with Inconel 82 which they have stated is "Standard Industry Practice".
Is this statement correct?
It will all be variations/extras for us but just thought I had better check to see if all the extra work is necessary before we go ahead with it.
Thanks in advance for any help you may be able to give me.
Regards,
Shane

By MBSims

Date 04-04-2005 03:55

Stainless thermowells are typically welded to carbon steel or low alloy steel steam piping with Inconel fillers. The Inconel filler has a thermal expansion coefficient somewhat between the carbon steel and stainless steel and is less prone to cracking due to thermal stress. We have had a few cases where thermowells and radiography plugs welded with 309 have cracked in service.

By Shane Feder (****)

Date 04-04-2005 04:48

Marty,
Thanks for your response.
Since putting this post on the forum I have located "Guidelines for welding dissimilar metals" by Richard Avery of the Nickel Development Institute.
He states that various 300 series electrodes may be used up till 800°F operating temperature but over 800°F an Inconel electrode is recommended.
As the maximum design temperature for our piping is 752°F it seems strange to be cutting the thermowells out and rewelding them.
Regards,
Shane

By - Date 04-04-2005 12:16

I agree with Mr. Avery. Over approximately 800F, you start entering the range of what is called "embrittlement stage." A 309L, which has a ferrite number of approximately 12-18FN is quite suseptible to this condition. An inconel is a pure austenitic material and has no ferrite so this condition is not in the equation.

CM

By GRoberts (***)

Date 04-05-2005 03:58

Mr. Avery's advise is very good. (I've had him personally help me solve a problem once, and he hit it right on the nose) For constant temperature service, the austinitic filler is perfectly fine at the listed temperatures. I don't think Mr. Avery was talking about cyclic service though. If you have temperature cycling, the inconel is preferred for the reasons MB Simms states. (Thermal stress/fatigue)

By Bonniweldor (**)

Date 04-07-2005 20:29

We regularly specify E,ER 309 for DMW of ferritic to austenitic. In the context of the downstream petrochem industry operators, the max temp limit for 309 is reguarly given as 600F (315C). I intuit the operators have encountered situatiions where higher service temps and/or longer times at temperature are associated with material failure.

309 in 400 C service would be a nonconformance for our construction.