Welding over thermal spray coating

[dbsmith]

Does anyone have experience welding over a thermal spray coating placed by HVOF (high-velocity oxy fuel) process? I need to do a weld metal build-up repair on 2-1/4 Chrome (SA387-F22) base metal that has been overlayed with 347 stainless. The weld metal build up will also be of 347 SS. I'm wondering what effect welding will have on the sprayed overlay. Will there be any lack of adhesion at the HAZ, or will the sprayed metal simply fuse like a weld metal overlay?

 

[metengr]

dbsmith;
No. Having dealt with weld overlays and HVOF coatings on boiler tubing I would not risk trying to weld over the existing HVOF coating. The deposited weld metal chemistry will be altered and could adversly affect the performance of the weld overlay in elevated temperature service. If you are going to make a local repair to the base metal - remove the coating, weld restore the FP22 and re-apply the HVOF coating or take the coating off completely and weld repair with the 347 stainless overlay on the base metal. Just wondering- why you have a need for weld repairing the base metal?

 

[dbsmith]

Thanks for the insight. We are actually trying to avoid a weld repair to the base metal, and the subsequent PWHT cycle it would require. The application is on a high pressure chemical reactor ring joint flange. During extended service at 2200 psig @ 800 F, stress cracks have developed in the ring groove of the RTJ flange. There are cracks in the corrosion resistant overlay, that do not appear to extend into the base metal. In the course of this repair, we are going to convert the flange from RTJ to raised face. We want to weld on top of the existing stainless steel and avoid affecting the underlying chrome. Where the stainless overlay is cracked, we were considering repairing it with HVOF, then completing the the weld metal overlay for the conversion to the RF flange.

 

[metengr]

dbsmith;
Based on your follow-up, I would NOT attempt local repair of the stainless overlay cracks with HVOF given your service conditions. Are you sure the cracks do not extend or have changed direction/orientation into the base metal? Have you excavated the cracks to confirm depth?

 

[dbsmith]

The vessel is not yet out of service. We are basing our assumption that the cracks extend only through the SS overlay based on previous UT and PT inspections. When the vessel is taken out of service during an upcoming shutdown, we can determine the exact extent of the repair needed, by chasing the crack out.

 

[Carburize]

Weld procedures have been developed to perform repairs to stainless steel weld overlay cladding on F22 without the need for post weld heat treatment. These repairs usually employ a low heat input process such as TIG and are controlled to ensure that the high temperature heat affected zone does not extend into teh base material and in most case require about .125-inch (3-mm) of stainless cladding left between the repair weld and the base material. I would suggest that between now and the shut-down you prepare some test plates with samples of your existing cladding material and base metal and perform some trials to see firsly if there are any adverse effects on the cladding and secondly how close you can get to the F22 without affecting the harness.

 

[GRoberts]

Unless you are talking about a fused thermal spray, any repair that does not weld directly to the underlying base metal would be ill-advised. HVOF sprayed deposits have a bond strength with the substrate of only a few thousand psi. If you weld to the sprayed deposit instead of the base metal, the thermal stresses from welding would likely disbond areas of the coating. You may also run into problems welding to the coating too, as they are not 100% dense. It could cause porosity or other problems if you pick up too much gas or impurites from the sprayed coating. I'm not sure how it would work if your sprayed coating was fused after deposition, but as already advised, a procedure qualification test replicating service conditions would be in order.

 

[metengr]

To add to the previous threads, the National Board Inspection Code (NBIC) has recently passed an alternative welding method 5 that is for "temper bead" welding of dissimlar materials. The welding procedure qualification requirements relate to essential variables listed in ASME B&PV Code Section IX except PWHT is omitted. My reason for saying this is that depending on your location, your Jurisdiction may require weld repairs on the vessel to be performed in accordance with the NBIC.

If hydrogen is generated or used as part of your chemical process reaction, I would attempt to qualify the "temper bead" welding procedure using hardness testing/metallogrpahic examination of the HAZ in addition to standard bend and tensile testing.