holding period for PWHT of a buttering layer

By jrsh Date 06-02-2010 06:43

We need to weld an austenitic stainless steel , set-on, nozzle (316L) to a carbon steel barrel of a compressor. This will be done using austenitic stainless steel filler material, SMAW. A 309/ 316 buttering layer will be deposited onto the carbon steel, where the nozzle is to be located, followed by PWHT, and the final weld completed using 316L; this will be done in accordance with QW 283, ASME IX.

The carbon steel barrel is about 150mm thick and the nozzle approximately 90mm, the buttering layer will be 5mm thick. The nozzle will be set on.

How long does the holding period for the PWHT of the buttering layer need to be? There are no through thickness welds in the barrel.

By js55

Date 06-02-2010 11:57

Why butter?
As for PWHT you should have a code imposed regime. I'd follow that.
What code?

By jrsh Date 06-02-2010 12:19

Buttering will be done to prevent the solid 316L nozzles from being subjected to a PWHT to prevent any possibility of sensitization. The design code is ASME VIII Div. 1. It is not clear which thickness to use as the ruling thickness to determine the time required for PWHT. Do I use the thickness of the buttering layer, approximately 5mm thick or do we use the thickness of the nozzle, approximately 90mm?

By js55

Date 06-02-2010 14:32

Is there a concern for sensitization of the stainless nozzle when it is directly attached to a sensitized butter weld and carbon steel.
However badly the nozzle would be sensitized it will still have far greater corrosion resistance than the carbon steel.
I suppose my question is a philosophical one. I have never understood the concern for sensitization of SS to CS dissimilars.
Even a highly unlikely 90% sensitization will give you almost ~2% more chrome than the carbon steel.
Not only that, but your butter layer is going to be even more highly sensitized in that through dilution you will gain carbon from the carbon steel.
More carbon, more carbon to bond with Cr. More bonding, more sensitization.
Buttering in this case makes little sense, and even more so buttering with 309.

You would use the thickness of the carbon steel base metal, and/or any exemptions based thereon. It is rare there is a requirement for the PWHT of an austenitic weldment such as would be the case when welding your nozzle to the butter layer.

By jrsh Date 06-02-2010 14:48

The whole internal of this vessel will be weld overlaid with 316L. The barrel will be of carbon steel, weld overlaid with 309MoL/316L and the nozzles will be of solid 316L. No carbon steel will be in contact with the process.

By js55

Date 06-02-2010 15:20

Ah.
That makes sense.