Large sand steel casting discontinuities

LMO

Involved In Discussions
#1
Casting (discontinuities)

I have rather large sand steel castings (800 to 900) lbs. Some surfaces have an extra 3mm per side to allow for machining operation. Someone decided to change the standard to say [Linear discontinuities ?None Acceptable?] such as, cracks, porosity, hot tears, etc.

Can this specification be met (100%) defect free on discontinuities?

What does it take to meet this specification?

:confused:

:thanx::thanx:
 
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#2
It might need a change in process, from sand casting to some other technology. For a given component - which may not have been designed with the sand casting process in mind - it might be a practical impossibility to produce castings with out these features. If the part designer was consulted about how the mold is configured for runners, risers etc. then it doesn't matter how goo you are at controlling pour, temps etc. the casting will likely exhibit these defects...
 

normzone

Trusted Information Resource
#3
Casting (discontinuities)

Can this specification be met (100%) defect free on discontinuities?

What does it take to meet this specification?
Ah, the casting fantasy. Somebody didn't like the voids inherent in the casting process, so they changed the spec.

That's okay - all you have to do to meet that is machine your part out of a 900 pound piece of solid stock.

(Princess Bride - Wesley's voice) " Castings ARE flawed, princess. Anybody who tells you different is selling something ".

This falls under what I refer to as " unrealistic expectation of process capabillity".

:nope:
 

John Broomfield

Staff member
Super Moderator
#4
Sound castings from sand.

Do we have any bell foundry experts on the Cove?

Most bells do not end up cracked and displayed as tourist attractions.

Must the casting material be steel?
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#5
My father spent several years in the iron/ steel casting industry, and I have some engineering knowledge of metallurgy.

Generally all sand iron/steel castings will have some level of pores and flaws. You basically do have to go to forging if you want less flaws. However, castings have been successfully used for years in many applications, so one really needs to look at what the application is and what are acceptable defects.
 

normzone

Trusted Information Resource
#6
Yeah, in my limited experience the flaws either don't matter, represent structural or dimensional problems, or are only cosmetic challenges. Some of those can be worked around by welding and machining/grinding.
 

LMO

Involved In Discussions
#7
I thank you all for you input. My thoughts exactly.

Welding and some others have unrealistic expectations.

:agree:
 

ncwalker

Trusted Information Resource
#8
Let's talk just general steel billets - they have terms like capped, rimmed, killed, semi-killed all of which are used to control defects. Basically pull them to the top of the billet. Then if you need "clean" steel, you can hog off the top where all the impurities are.

And a billet is nothing more than a large, rectangular casting. But all the process I mentioned above either reduce the defect or move the defect to where it does not matter.

And that's the game in casting a part.

Because you are mentioning a 3 mm stock for machining, I am going to assume that it is only a problem once you machine the part, exposing the defect and because these defects are much more rare on the surface. If you're getting these on the "as cast" surface, you really have gating/riser problems.

So the first answer is minimize your machining stock. This can be done by carefully choosing datums in the casting and machining so the "sensitive" areas have the best controls - stackups of follow on ops needing more stock. But 3 mm isn't a lot for a casting of this size.

The second answer (in processing) is control your temperature on solidification. There are two approaches, both can work.
a) Make the surface right near the problem area cooler - causing this area to solidify sooner which drives the porosity away from this surface.
b) Make the surface right near the problem area hotter - in this scheme, you attempt to pull the porosity into the machining stock to be machined away.

This is going to come down to your sand mold - and the spectrum of additives you can choose from, which in most cases come down to application of graphite (better conductor of heat, cooler sand in that area) or possibly a sand core insert, like a shell core. But that's money.

I have had most success with a) above. For b) you may have to have more stock, to provide more room for the defects to go. Most people don't like this option - it's not steel safe in the pattern, it slows the machining process removing more stock.

And I forgot option c) make the porosity acceptable. Make samples and functionally test them. Again, given the size of your castings - yeek. That's an expensive testing program.

Final thought on the metal chemistry side: you could play around with your Moly content or your Silicon or both. An experiment with those at the high/low range may be fruitful. But be careful with this, you will wind up with a "special" batch recipe for this part. And that's easy to mess up in production - your lab releases a billet because it is in spec for the "book value" of this steel, but forgets this one is a special case.
 

Englishman Abroad

Involved In Discussions
#9
Hello,

Yes, it is not possible to have defect free steel sand castings. (It is not possible either to have defect free continuously cast steel). In both cases you need a clear definition from the customer about what is and what is not acceptable.

In a previous life, I dealt with steel (and iron) sand castings for the railway industry. We specified acceptable surface and volumetric flaws as detected by Magnetic Particle Inspection (MPI), and X Ray.

For surface defects we accepted rework by grinding, MPI inspection of the bottom of the grind (to ensure removal of the crack) and then welding, followed by repeat MPI. (against a national standard for steel castings).

Needless to say that MPI, X Ray and welding were all special processes and the persons performing had to be certified, and the processes qualified, and approved by the Customer.

You will need to investigate with an NDT specialist what level of defect is detectable for your industry, and, discuss with the customer what is acceptable in terms of defect level, and explain that to guarantee this level of defect you will probably need some extra NDT inspection and welding rework.
 
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