99 Diseases of Pressure Equipment: Welding to Weld Overlayed or Clad Equipment

A myriad of issues need to be considered before welding to or repairing weld overlayed or clad equipment. (By clad we mean roll-bonded or explosion bonded, i.e. basically 100% metallurgically bonded, and not a loose or seam-welded liner, e.g., not strip lining.) We’ve listed many of the QA/ QC issues here along with items to inspect or check. This is not intended to be an all-inclusive treatise on the topic and the items that apply to your situation will depend on your specific application. The best advice we can provide is to make sure that a welding or metallurgical engineer that is familiar with the base metal, the overlay/ cladding material, and the service environment is involved in developing the welding plan. 

There are typically two situations where you want to weld to the existing weld overlay or cladding in a vessel. One is when you need to repair the damaged overlay/cladding and the other is when making revisions to vessel internals e.g. you need to alter the existing design or add internal attachments. 

Issues when adding/altering attachments:

• Will the overlay/cladding hold the added load, i.e., not disbond or crack and peel away from the base metal due to the added shear and tensile stresses? This needs to be checked with an engineering analysis.
• Does the existing overlay/cladding have full integrity?
• Has it already disbonded from service conditions or was it disbonded from original manufacturing in the area associated with the attachment? This is typically checked with straight beam ultrasonic testing (UT). The amount of disbonding, if any, may need to be part of the analysis mentioned above.
• Is it cracked in the area of interest? Many austenitic overlays/claddings suffer from intergranular corrosion or stress corrosion cracking, chloride stress corrosion cracking, or if the overlay/cladding embrittled in service it could be full of tiny thermal fatigue cracks. Magnetic particle testing (MT) or penetrant testing (PT) may be needed depending on whether the overlay/cladding is ferritic or austenitic
• Is it embrittled? Ferritic claddings such as the 12 Chrome type (Type 405 or 410 stainless steels) are susceptible to 885F (475C) embrittlement. 300 series stainless steel overlays are susceptible to sigma phase embrittlement (from service or during fabrication heat treatment of the vessel). These embrittled corrosion resistant layers may crack or disbond under the thermal stresses induced by welding. A short bead on-plate welding test may be advisable to check the weldability of the existing overlay/cladding.
• Will the welding procedure cause damage to the base metal beneath the cladding/overlay? This is typically a concern when the equipment is in high-temperature and/or high-pressure hydrogen service.
• Is the base metal (typically a ferritic, low alloy steel, or carbon steel) temper embrittled and/or hydrogen embrittled? If so, there is a concern of cracking from the thermal stresses of welding. A metallurgical/ welding engineer needs to be involved in this evaluation and can determine if the steel can be de- embrittled or if the welding parameters (e.g., heat input and preheat) can mitigate concerns.
• Is the hydrogen concentration in the base metal such that there are concerns of hydrogen stress cracking during welding or delayed hydrogen cracking after welding? The need to involve a metallurgical/welding engineer and control of the welding parameters applies here.
• Will the heat input from the welding process cause a new heat affected zone or hard spot in the base metal, making it susceptible to hydrogen stress cracking in service? The best mitigation here is to qualify the welding process, and the welder that is to perform the work in the field, with mock-up simulating the field or shop conditions. Keep in mind that the mock-up sample that is to be tested will typically be done in an ideal shop environment, thus there needs to be an appropriate amount of QA/QC on the job to assure that welding parameters used to qualify the mockup are adhered to during the job. Qualifying the welder(s) that will actually perform the work is equally as important as qualifying the welding procedure.

Issues when repairing the existing weld overlay or cladding:

Many of the concerns and issues listed above for adding attachments also apply when repairing the existing weld overlay or cladding. I’ve repeated some of these concerns below in a summary list, but I did not repeat the remediation or QA/QC, as they are the same as stated above. The typical issues for repairing existing weld overlay or cladding are: 

• Is the surrounding weld overlay/cladding that is to be repaired intact, i.e., disbonded in the area of the repair, cracked, or embrittled?
• Will the repair cause damage to the base metal beneath the cladding/overlay?
• The only real additional concern here is dilution. Has the welding procedure, including filler metal, been selected and adequately qualified to assure that dilution effects will not cause deterioration of the corrosion resistance. Have you thought through all the potential issues with repairing or welding on overlayed or clad equipment, including: metallurgical effects, changes in corrosion resistant properties, needed welding QA/QC issues and potential problems before welding and during welding.

Have you anticipated all the potential in-service damage mechanisms (e.g. hydrogen stress cracking, corrosion of diluted welds, etc.) so that you have the needed QA/QC during the repair or alteration?