controlling ferrite in stanless steel welds

By carlkukol Date 04-10-2007 12:31

my company has a job welding 347 stainless piping. the diameter range is 8" thru 20".all thicknesses are schedule 100.

we are going to use the gtaw process using ER 347 filler metal with an argon purge and argon shielding and a maximum interpass temperature of 350 degrees farenheit.

the construction code is ASME B31.3 and the operating conditions are 824 degrees farenheit at 3200PSI

the job specifications call for ordering filler and 100% ferrite testing of the final weld with a ferrite content between 2 thru 10.

it was suggested to use ER 16-8-2 or ER 308H for better control of ferrite,but it was rejected by the engineering company of record.

my question is in 2 parts as follows

1) are there any specific guidelines to follow during welding to maintain the required ferrite content range?

2) if the ferrite range is exceeded on the completed weld, is there a way to reduce the ferrite number or should it be cut out and re-welded?

thanks
carl kukol

By js55

Date 04-10-2007 14:28

16-8-2 is commonly recommended for high temp high pressure 347. And often it is recommended as ferrite control filler despite the fact that ferrite generally stays pretty low in this stuff. It probably would have been an acceptable choice as it is more crack resistant than 347 fillers.
As for the 347 filler I wouldn't recommend cutting welds out with high ferrite, if that happens considering that the primary threat with high ferrite is sigma fomation. And with 347 this takes place between 1000 and 1700 degrees. 100's of hours required for it to form between 1000 and 1100.
I don't think you'll have a too little ferrite problem. So then you would need your customer to disposition the higher ferrite welds since you will have violated their spec.
As always ferrite is controlled by cooling rate. I would suspect that with a interpass of 350 that your cooling rate is sufficiently fast to ensure retained Delta ferrite.

By js55

Date 04-10-2007 16:05

By the way, I did a quick review of some materials I have on 308H. If, for future reference, you were to consider this choice you may wish to take a look at Mo/high temp ductility curves for CrNi SS's since Mo improves high temp ductility.

By chuck meadows (***)

Date 04-10-2007 18:22 Edited 04-10-2007 18:25

Carl,
347 steel is a stabilized steel used for high temperature properties. ER347 filler metal is used for welding titanium and niobium stabilized steels of type 19Cr 10Ni Ti (Nb) or similar, that provides improved high temperature properties such as creep resistance, compared to low carbon non-stabilized materials. 347 is therefore primarily used where temperatures exceed 400C. The typical FN of the ER347 is around 7, so too high a FN is unlikely. Matching the stabilized 347 base metal with the stabilized 347 filler metal is acceptable, even recommended by many such as Avesta.