Recommendation needed to minimize pump case warpage cause by heat due to welding. 2

[Mech5656]

Hello,

We are weld repairing a pump case on IDs/bores and want to minimize case warpage. This is because warpage can cause other issues like more extra machining and additional work will be needed if case warps too much. Especially the case split lines need to be relatively flat (within 0.020") so we dont have to take too much from split lines when planning.

Case base material is Carbon steel and bores are welded with 309 weld overlay. Welding will be needed at location shown in below picture.

Things that I am planning to do to minimize warpage: Use TIG welding instead of stick, weld at different locations and give time to settle (while maintaining inter-pass temp) and minimize weld thickness. Any thing else that you can recommend?

That diameter needing welding is 14 inches.

 

Things that I am planning to do to minimize warpage: Use TIG welding instead of stick, weld at different locations and give time to settle (while maintaining inter-pass temp) and minimize weld thickness. Any thing else that you can recommend?

1) For mitigating distortion you could not do worse than TIG. If available I recommend a wire process, with GMAW in intelligent short-arc mode preferred. Pulsed spray GMAW or FCAW are the next preferred processes. Small diameter SMAW is not good, unless it can get you a desirable thickness in one layer (see 2 below).

2) As a first approximation (all other factors being equal), distortion is proportional to the amount of deposit. That implies minimizing overwelding, so you should devise a layer thickness strategy that exceeds the minimum finished thickness by as little as possible.

3) Location can be more or less significant in terms of the impact on significant dimensions.

4) Stainless weld deposit will cause more distortion than carbon steel.

This casing looks sufficiently massive to resist bending. But again, the critical question is how much you want to do.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[TugboatEng]

If you're going to plane the mating surface of the cases, the bores will not be round and all will need refinishing regardless.

How much thickness to you need to build up?

For low temperature applications I have used hard chrome to replace lost material while keeping machine work to a minimum. Electroless nickel may not need any post application clean up depending on your specific requirements.

 

[btrueblood]

Like Tug, I would opt first for an electroless nickel build, no disortion worries. Alternatively, I would find somebody locally who could flame spray the areas of interest using the alloy of your choice. We've done tribological overlays on steel and bronze to good success using the latter.

 

[gtaw]

Preheat the entire case before welding.

Generally, with carbon steel about 10% of the distortion is from the weld cooling and contracting, the remaining 90% of the distortion is from the thermal gradients from one location to another (weld to HAZ to unaffected base metal). Cutting to the chase, where ever the base metal experiences a thermal gradient of more than 220 degrees F (round numbers), the base metal has undergone some plastic deformation. Heating the entire case reduces the thermal gradient, thus reduces the resulting distortion. Why? Any base metal that has experienced a thermal gradient in excess of 220 degrees F, has deformed because the yield point has been exceeded and there is plastic flow. Heating the metal, expansion, plastic flow. Cooling the metal is contracting and where there has been plastic flow, the residual stress is equal to the yield point of the material. Easily explained with a drawing showing the isothermal lines, tough to show without the drawings showing the weld without preheat and with preheat.

Picture this: No preheat, the thermal gradient from the weld extending out to an area where the temperature is ambient plus 220 degrees F might be 3-inches. All the metal that attained 80+220 or 300 degrees F has undergone plastic flow. All that volume is going to cool to ambient and will contract and will have a unit stress equal to the yield strength of the steel case.

Heat the entire case to 350 degrees F, now the base metal will had to have reached 570 degrees F in order to have experienced plastic deformation. The base metal reaching 570 degrees might only extend 1-inch from the weld centerline. The region that reaches that high temperature is much narrower than that which reached 300 degrees. That means when high preheat was used, less volume of the base metal is contributing to distortion. End result, significantly less distortion.

I hope the short explanation helped.

Best regards - Al

 

[Compositepro]

A thermal gradient would have units of degrees/mm, or equivalent, not just temperature. But the concept makes sense.

 

gtaw,

A monstrous amount of preheat is required to give substantial mitigation, because the starting point of the weld is the solidification temperature. In a part like this one, 600 to 800°F is feasible, but even that would not do much. Since the material is carbon steel, the metallurgical ceiling is quite high.

Permanent distortion is caused by the shrinking of the cooling weld deforming the surrounding metal plastically.
Spray coating is an option worth looking into, but the question with that is always the long term integrity, and pieces falling off inside this equipment could cause serious problems.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Mech5656]

Hello All,

Thank you so much for your replies. They are all very helpful.

I wanted to mention that the pre-heat temperature on welding procedure is 500F and it doesnt require post weld heat treat based on the thickness of the weld we will apply.
Is the temperature too high and will cause distortion?

 

Is the temperature too high and will cause distortion?

Not too high and it will not cause distortion, it will very slightly mitigate it.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Mech5656]

Hello Ironic metallurgist,

Thank you for replying.

I apologize. 500F is the inter-pass temperature not the pre-heat temperature.

 

[thruwall]

May I offer up a different solution from past knowledge and experience dealing with this type of weld repair or weld build-up? I just registered on this site and not sure if I should just jump right in or not?

 

[Mech5656]

Yes, please go ahead and it will be best if you can be as detailed as possible. Thank you!

 

[thruwall]

I notice a couple of things, I would not have it up in the air unsupported in the middle of the casing? The way its supported now is ideal for the attachment of Tie bars without having to weld the tie bars to the casing. You would need to weld some type of plate to the tie bar to fasten them at the 4 bolt locations. 2 tie bars across and 2 down on the face of the split flange If it was returned to the FOUNDRY where it was originally made that would be the first thing to do if its close to final machine on the split flange. You have 2 options as far as welding position. In the position its in now you must support the underside middle of casing. Another way it would be welded if returned to the foundry would be to stand the casing up in a vertical position that way if a thicker build-up is needed you would not have any problem with layout. A no post heatreat welding procedure preheat 175 with interpass temp monitoring most likely would be used. Ive had to do this before not only with weld build-up but also with cracked casings that were close to final machine.