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Cast Iron pipe tap

Cast Iron pipe tap

Cast Iron pipe tap

Ladies and gents,

I have a cast iron part that was mis machined. There were thru holes drilled into the part that need to be 'removed'. So we are going to pipe tap the part and install plugs.

My question: What is the best type of material to use for the plugs to create a seal? The plugs need to be permanent. Ideally, we would install a plug and then cap weld. We can't weld these parts because they are Cast Iron. (My mind is going towards having a soft male material (i.e. cast iron) in the hopes that the threads form to one another)


RE: Cast Iron pipe tap

If you know the alloy and its heat treat condition you can weld it.
Is this a real large part?

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P.E. Metallurgy, Plymouth Tube

RE: Cast Iron pipe tap

The part is big, but the area is of error is in a small space. It is a bolt circle around a bore. The bore size is within .0005" and is aligned to another bore within .001" about 25 inches away. PWHT is not an option.

RE: Cast Iron pipe tap

I can't find the exact type of material. The engr dwg states cast steel, but the actual part PMI'd as CI

RE: Cast Iron pipe tap

You did PMI with an OE, so that you could get accurate carbon numbers?
The two (CS and CI) are usually not interchangeable, that could raise some issues.

I have seen tapered plugs friction welded into place.
Anything with threads is less than permanent.

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P.E. Metallurgy, Plymouth Tube

RE: Cast Iron pipe tap

The tap is to become an extra connection without any function. You may want to verify if plugging the tapped hole is acceptable by the design or the client.

RE: Cast Iron pipe tap

Yeah, I have seen nickel on nickel (probably all of us have) galling to the point where no amt of torque will remove the bolt. I wonder if something like that is possible with CI. I'm assuming by friction welding, you mean galling or do you mean actual welding of the materials?

I'll look for the carbon numbers. The other thought is to use locktite or similar.

RE: Cast Iron pipe tap

Installing cast iron tapered thread pipe plugs is absolutely fine as a permanent mechanical fix. In fact, ASME B&PV Code, Section IV allows it for cast iron hot water and steam boilers. Maximum size is 1" diameter.

RE: Cast Iron pipe tap

Pipe threads absolutely will leak without added sealant. The peak of the external thread does not touch the root of the internal thread, resulting in a small spiral gap. Be careful not to crack the casting with tapered plugs.

RE: Cast Iron pipe tap

what is the maximum service temperature ?

My limited experiences with PMI have resulted in doubts, similar to EdStainless. Without knowing the carbon content, it is hardly PMI at all.

There are some helpful test that mere mortals can use to make cast iron reveal itself.
- TIG weld melt a small spot on an edge. No filler. File test when cool. Many irons get very hard when allowed to cool quickly like that.
- Polish and etch an area, and look at the structure.
- Drill a small hole or shallow drill point in a safe area with a new drill bit. True cast iron essentially makes dust. Mildish steel makes long curly chips. Ductile iron is somewhere in between.

How are you sure that the errant holes are not in an area that will be highly stressed in service ?

RE: Cast Iron pipe tap

Service temps are below 300 F.

The PMI was done with: PMI Master Plus / Niton XLt 898 XRF.

Carbon content was found to be >1.8%, Fe ~86%, Si ~1.35%

Those are interesting ways to check for CI. Thanks for the info.

The holes are most definitely NOT high stress.

RE: Cast Iron pipe tap

If it is grey cast iron, a simple ring test will help identify . You can use a threaded cast iron plug , and for additional safety use Loctite.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

RE: Cast Iron pipe tap

Did the PMI Master Plus / Niton XLt 898 XRF include Carbon in its report ?

Were several PMI tests done? What method of cleaning was used?

The attached image suggests that " Nitron XRF units are capable of
detecting elements from 12 - 92. "
Carbon's atomic number is 6.

Carbon composition is the main distinction between cast iron and steel. Cast iron typically contains more than 2 percent carbon, while cast steel often contains between 0.1–0.5 percent carbon.

RE: Cast Iron pipe tap

In my experience, I would not trust XRF results for carbon even if the instrument provides them. XRF is usually not terribly accurate on such light elements even though it may detect them.

Aidan McAllister
Metallurgical Engineer

RE: Cast Iron pipe tap

Maybe you should make a replica on the surface to view microstructure - that will provide definitave evidence of cast steel vs. iron, and will also tell you what kind of cast iron you have (i.e. gray vs. ductile).

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