Porosity in threaded holes

[sjwarren]

Howdy -
I was wondering if there is a standard or spec for the size of porosity allowable in a threaded hole.

For example, if I had a cast part that then had threads machined into it, and that machining exposed a porosity hole. What would be considered too big of a hole?

Thanks for any help.

 

[israelkk]

The casting spec should define the maximum allowed pores.

 

[MintJulep]

You are the designer or user of the part. You, and only you, are in a position to determine how much, if any, porosity is tolerable for this part.

There is no way for any standards writing body to know what your part is, or what its application is, or what it is made from, or how strong it needs to be. So how could they create a standard for your part?

There are general standards that define and characterize casting defects, and methods to inspect for them. But it remains up to you to determine how they are applicable to your part.

 

[CorBlimeyLimey]

That would be entirely dependent on the intended use of the casting and in particular the threaded hole itself.

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[unclesyd]

One work around is to use a Heli-Coil or thread insert in that hole or if there is any question about the other Heli-Coils or inserts all around.

 

[arunmrao]

The accepted norm is to reject the casting if you get a porosity in a thread hole. Alternately if you desire to be generous with the foundry,relocate the hole and see if you are lucky.What is the material being machined and the application ?

 

[Cockroach]

I have modelled porosity as a discontinuty in wall.

Suppose you had a certain population of porous holes of diameter d. If they touched eachother, then the discontinuty is obviously greater, so the wall is effectively thinner at that point. From Thick Wall Pressure Vessel Theory, compute the stress on a section of wall under triaxial loading. You'll find the resulting expression to be purely a function of vessel geometry (i.e. OD and ID) and internal pressure. There is a scalar multiplier, SQRT(3).

So given the OD and internal pressure P, you can effectively compute the minimum ID given a yield strength of the material. This means that you can theoretically compute the number of porous holes of diameter "d" which when magically perfectly aligned, contributes to the change in wall. This is your answer.

Clearly in the real world, porous holes are not aligned. So what you are looking at is a statistical approach where the proximity of one porous hole would be of influence to it's nearest neighbor. You will find that in the Kepler Packing Problem, the theory lends itself very well to your particular porous hole problem.

That will then lead you to a density of porosity acceptable under Von Mises-Hencky triaxial stress, the material strength of your vessel in question and porosity density allowed in a section of wall of thickness "t". You track you allowable porous population as a limit to shrinking safety factor, i.e. the partial derivative of wall thickness to number of holes.

The result is a linear equation. Surprise!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Heckler]

Porosity in a casting is usually grounds for rejection especially after it has been machined. I should add that if it's flight hardware like a pump housing or some other critical component. This type of defect should be red flagged in inspection or NDT.

What is this parts intended use? is it a flange or is it something critical like a pressured pump housing?

Best Regards,

Heckler
Sr. Mechanical Engineer
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Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.

 

[Heckler]

MIL-STD-276 - testing procedures for castings and powdered metal components.

 

[Heckler]

Two more specs our level three tests our casting to

MIL-STD-1907
MIL-STD-1949

 

[sjwarren]

Thanks you all for the responses and I apologize for not getting back you faster.

The intended part is an aluminum engine block and the hole in question if for the long bolts that hold the head onto the block. So, obviously a pretty high stress joint.

As of right now, we are getting porosity holes of about 4 threads in length on M10x1.25 threads, so the holes are about 5mm long. The print calls for 48mm of full thread.

I don't believe we are getting this problem 100% of the time, but it is frequent enough to be a concern.

Thanks again.

 

[arunmrao]

Al engine block should be a die cast component. Is it possible to shift the hot spot from the current location . What is the current practice to solve this problem. Is it a gas porosity (smooth and rounded) or shrinkage porosity (rough and angular). On a first observation it will give you the cause of defect, to carry out further investigation in association with the foundry.

 

[swall]

Porosity is inherent with high pressure aluminum die castings. The surface layers of the casting solidify first and trap the center layer, which contains entrained air from the injection shot. This is the source of porosity in die castings and you have to deal with it. When you machine threads, you cut through the outer skin, exposing the porous inner layer. If you want threaded holes with high structural integrity, you will need to use thread inserts, either installed after machining or cast in place.