Weld Overlay & Back Cladding for PV

[AnEngineer]

Could some one please explain me (in few lines) or refer to a http link, weld overlay and back cladding procedures (with advan. & disadvan.) for PV application. Many thanks in advance.

 

[metengr]

The following web site is for general information regarding strip cladding of pressure vessels;

http://www.chemicals-technology.com/contractors/heattransfer/spf_europe/

SA 263 Specification for Corrosion-Resisting Chromium Steel-Clad Plate, Sheet, and Strip

The difference between weld overlay and cladding is basically the following - with cladding you are relying on a metallurgical bond that is formed between the corrosion resistant sheet and the base metal of the pressure vessel.

For weld overlay, you are fusing the corrosion resistant weld deposit to the base metal of the pressure vessel. This is a fusion weld because you have actual melting between two alloys.

Advantages of cladding in comparison to weld overlay - more efficient process to apply versus welding, no heat affected zone to the base metal of the pressure vessel, and no PWHT.

Disadvantages of cladding - only a metallurgical bond is formed between the cladding material and the base metal that could delaminate over time with thermal or pressure shocks to the vessel walls. It could be difficult to inspect the condition of internal cladding from the OD of the vessel using ultrasonic test methods because of an interface (gap).

 

[AnEngineer]

Metengr,
Thank you. So, back cladding and roll bonding are the same thing or different ??

 

[metengr]

No. In my response to you, I treated "cladding" as an option to provide for a corrosion barrier and compared it to weld overlay, as a corrosion barrier. Roll forming of cladding is one method of installing cladding to the ID or OD surfaces of a vessel. There are other methods to apply sheet, plate or strip cladding - explosive, brazing or in some cases extrusion.

Back cladding, as I know of it, refers to manually applying cladding over a vessel weld.

 

[jte]

AnEngineer-

To provide some further explaination, think of cladding (roll, explosive, etc. bonded) as the stuff (say stainless steel) which is applied to the base metal (say carbon steel) before the plates get to a fab shop. The fab shop then removes the cladding as req'd to weld the plates together or weld internals (tray support ring, bed supports, etc.) to the base metal. You do not want to weld structural attachments to the cladding. At that point, you have exposed CS which kind of destroys the value of the clad plate from a corrosion perspective. So you go back and weld overlay with SS to regain the corrosion protection for the entire vessel.

Cladding: Applied (bonded to the plate) before the plate gets to the fab shop.
Weld overlay: Applied at the shop or in the field.
Weld buildup: A base metal buildup to regain tmin plus some CA or sometimes covered with a weld overlay.

jt

 

[Rich2001]

Cladding is a very generic term .

Weld overlay is a specific type of cladding

Cladding can be accomplished by explosive bonding, welding overlay or by apply sheets and then attaching them by resistance welding, etc.

There is a process called strip cladding where the corrosion resistant layer is overlayed welded to the base metal, by ESW or SAW processes. Here are a few links on this process (note how liberally they exhange the words clad with overlay, in these articles)

SAW or ESW strip cladding.

http://www2.sandvik.com/sandvik/0140/SM/se01240.nsf/0/6d8fb18dca8952bf41256880004611a4?OpenDocument

http://www.esab.se/index.asp?item=104297

 

[AnEngineer]

Gents,
Thank you all.