TIG Welding Heat Transfer Question

[Marcus388]

I have an application where I have a 3/8" thick plate, mild carbon steel. One side is painted with an industrial enamel rated around 250 deg F, the other is unpainted. On the unpainted side I am performing some stitch welds running about 200A. Using 1/8 Tungsten, and 3/32 filler. The bead size is between 1/8-3/8" and about 2" long. I am having trouble because the paint on the opposite side is bubbling and distorting from the heat, causing unneeded rework of the paint. I need a method to capture the heat and prevent the paint on the other side from getting ruined. The opposing side must be painted before welding due to our process and limited painting requirements post weld.

Thanks so much for any and all advice.

 

[fegenbush]

Are these fillet welds you are making?

It may be possible to use wet cloths on the back side (with the enamel) to act as a heat sink while you make your welds.

 

[Marcus388]

Yes these are fillet welds. I have considered that option, and even found some gels that act as heat sinks in this type of application. Is there anyway the heat dispersion from the heat sink affect proper fusion of the welds?

 

[fegenbush]

I have not encountered fusion problems using this technique, but you could always verify with destructive testing if you are concerned about it.

 

[DekDee]

Marcus,
Something doesn't seem right.
You say bead size is between 1/8 and 3/8" - that is quite a difference.
What is the filet size called for on the drawing ?
200 amps seems pretty hot for GTAW - are you doing theses fillets in one run ?
If so try doing two or three small beads instead of one large - see how that goes.
Hope that helps,
Cheers,
DD

 

[MikeHalloran]

It may be more effective to partially submerge the part in a shallow tray of water. A layer of cotton cloth or shop rags on the bottom of the pan should minimize scratching of the paint and allow enough circulation of the water to keep the painted face cool. ... or at least not too hot.








Mike Halloran
Pembroke Pines, FL, USA

 

[Marcus388]

I guess I should've mentioned that this part is a 60,000 lb tank being used for the power industry. The bead size varies based upon the gap that's required to fill between the material being fixed and the base material(tank), the variation in flatness causes the gap to fluctuate. We are running so hot because we have to arc through a coating on the interior of the tank( again something that has to be done) . Thanks all for the quick comments, it's got my creative juices flowing. Stay tuned for results of heat sink ideas.

 

[gtaw]

Anytime you have to weld through coats, quality is going to suffer. While thin coatings may be volatized, thicker coatings will typically cause problems with porosity. In the case of a lap joint or any joint where the coating is entrapped within the region of he weld root, porosity will be a problem. Generally the degree of porosity is proportional to the coating thickness.

When welding carbon steel, rapid cooling can result in welds that are not as ductile as they would be if they were allowed to cool slowly. There are application where there is little alternative, such as repairing a leaking hull in a ship while underway. Generally, slow cooling is best when welding carbon and low alloy steels.

You may save the paint, but at a loss of ductility and an increased opportunity to experience hard, brittle welds, delayed cold cracking, and porosity.

Best regards - Al

 

[Tmoose]

Would MIG or even stick welding be acceptable?
In my hobby welding experience workpieces/weldments simply don't get nearly as hot as when TIG welding.

 

[gtaw]

In any case, fusion welding has to bring the base metal up above its melting point. That's pretty darn hot in my neck of the woods.

Best regards - Al

 

[Tmoose]

But, isn't it the paint on the outside of the 3/8 inch thick plate ( vessel too?) that is blistering and turning to (non-arctic) char?

In my strictly hobby welding experience it takes me a while to get a puddle going using TIG or oxyacetylene, and while welding I need to be mindful about resting my hands or arms on the ever hotter workpiece, or when picking it up when finished.
Zapping the OP's 2 inch long fillet stitches wouldn't take long at all with SMAW.
I'm not sure about the suitability of any (DCEP) SMAW electrodes for Marcus388's application but they offer some advantages on thinner materials due flipping the 70/30 heat relationship of the DCEN electrodes

 

[weldstan]

I would also recommend short circuiting or pulsed transfer GMAW to lower heat input. You may still need to place in cooling water or use a 1/2" to 1" thick by 1" minimum width copper bar as a heat sink.