Carbide Participation from PWHT

[Metal1956]

We are considering the fabrication of a pressure vessel being lined through the explosion bonding process.
This vessel needs to be PWHT.
Our concern is carbide precipitation into the liner from the PWHT.

Specs:
Vessel shell and heads ,,, SA516
Vessel thickness,,,,,,,,,, 45-57 mm
Liner,,,,,,,,,,,,,, Alloy 59 (N06059)
Liner thickness,,,,,,,,,,,,,3 mm
Liner to nozzles,,,,,,,,,, Alloy 59 TIG welding overlay.
PWHT procedure temp,,,,,,, 570 C
Process,,,,,,,,,,,,,,,,,,,Versatic Acid
Operating conditions: 95 bar, 200 C.

Is Carbide Precipitation into the liner and it's surface a concern?

Thanks in advance.

 

[Metal1956]

The thread was suppose to read Precipitation.

 

[metengr]

You will/should need to have the liner material subjected to a simulated production PWHT, and evaluate Alloy 59 corrosion resistance.

 

[weldstan]

Are you asking whether carbon will migrate into the Alloy 59 from the SA-516? If so it won't migrate any where near the surface (corrosion barrier) of the cladding.

 

[DBreyer]

I agree with weldstan.
I wouldn't be worried about diffusion of carbon from the base metal into the liner because it will only diffuse a very short way into the liner so at the surface won't be any carburization.
Also you plan to have weld overlays in your nozzles performed via TIG. This will certainly bring more carbon into your weld overlay than the PWHT would ever be able to do on it own (mix up of base metal and welding material).
Be sure the chemistry of the top layser of your weld overlay has the correct chemistry (Fe will strongly reduce corrosion resistance). Perform checks on a sample.

 

[EdStainless]

That is why they do weld overlay in these alloys in two or three passes, to minimize dilution of the CRA with base metal.
Given the time/temp of PWHT C diffusion isn't an issue, but any that is there will for carbides and reduce corrosion resistance.

Usually you would do PWHT after nozzle fabrications and before the overlay is applied.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[SJones]

Indeed, but what about intrinsic sensitisation of the material? The TTS curve in the VDM datasheet gives you about 20 hours to play with at 650 deg C. Is the total soak time, including the potential for future post installation treatments, going anywhere near this?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[Guest102023]

Alloy 59 is the most stable of the elite C alloys, mainly due to its not containing things such as W. I would not be concerned about precipitation reactions within the 59 itself at the carbon steel PWHT temperature range. Keep near the lower end of the permitted hold temperature range. Being a nickel alloy it will be resistant to carbon migration during the relatively short PWHT time (and as mentioned that will not be on the process side of the cladding). The maker VDM should be able to advise you.

Considering the expense you are going to it makes sense to retain one of the folks at the Nickel Development Institute who are very well versed in corrosion issues in chemical process industries. Follow best practices!

For the 'back' cladding between clad sections, I have a general comment applicable to all alloys: 3mm is nowhere near enough depth to permit the attainment of full overlay chemistry at the weld surface (assuming conventional weld processes). Aim for three layers to achieve maximum corrosion resistance.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"