Ferrous casting with porosity

[dogbural]

Hi,

I am new to ferrous casting (or just casting).
Lately we had some samples from our foundry vendor. They cast using foam pattern.

First, the overall profile is oversized by 2mm

Second, after machining, we only found porosity.

I took that issue to the vendor and ask them for their assessment and rectification.
This time i shared the machining drawing as well on their request.
After their assessment, they simply ask to add 15mm stock on machining face.

Is it common practice, just increasing the stock? Would it have more chance to have porosity?

How usually tackle porosity issue? Or should i ask vendor to try (although i think they should be the expert)?

Regards
Mk

 

[weldstan]

Some porosity is to be expected in steel castings. Castings are often weld repaired to coorrect for a variety of casting flaws.

 

[EdStainless]

But I would expect localized porosity.
If it is general then I would be concerned about metal degassing and mold curing issues.

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P.E. Metallurgy, consulting work welcomed

 

[Tmoose]

So your company made and supplied the casting drawing, not the foundry?
Was the foundry given the chance to review the casting drawing ( if your company made the casting drawing) ?

Is the the "overall profile is oversized by 2mm" comment describing the machining allowance, or the size of some feature or profile that is supposed to remain un-machined?

There are a bunch of Standards that talk about expected tolerances of as-cast parts. They vary by material, casting mold material ( sand, etc), and for short run production vs long runs when the patterns and molds can be tweaked for improved results.

The various sections of ISO 8062 have several grades reportedly based on practical limitations of industry capabilities. I think recommended added material allowances for machining are also included.

There are also Standards for "acceptable" defects like porosity. The requirements vary from cosmetic to structural depending on the use of product being cast.

 

[dogbural]

Hi,

Yes, we are designing casting drawing, not foundry.
Is it a common practice that the foundry actually design the casting drawing?

the size of some feature or profile that is supposed to remain un-machined.

As for the foundry, what could be meaningful information for improvement?

- could be a 3D heatmap view showing location of porosity and its frequency?
- information of area that allow presence of porosity?

I could ask them directly, but I want to get some general ideas before I approach to the foundry.

 

[bobjustbob]

It is difficult to diagnose a "porosity" issue without a lot more information. There are many possible causes of the indications. The deox practice, pouring temperature, pouring speed, gating system, sand quality, etc.. Adding some machine stock is not an uncommon fix for some instances, particularly localized porosity in an area of the casting that would be difficult or costly to rectify in another manner.

Bob

 

[casting supplier]

the normal porosity layer is 3-5 mm undernearth the casting surface. if your machining had cut more than 5mm off the casting surface and you still found the porosity, that was the issue of casting process;

Pike Li
uxitech.net

 

[geesaman.d]

In my work, we provide a casting drawing and the foundry designs the supporting features. If they have concern about a particular cross-section or transition, they use their mold flow analysis tools to suggest solutions which we would approve.

Porosity can just happen sometimes. Sometimes it's from the beginning or end of the crucible where they're trying to get too many pieces per pour. Sometimes it's persistently in the same place, in which case the design and/or tooling needs to change. If they want to add 15mm in order to give gas a place to rise and cool, then I would probably allow that because there is an explanation and this porosity will be plainly visible when the machining happens.

As for the extra 2mm, did you not find this on the first article inspection? Is this normal for foam casting process? If there are critical cross sections that need to be a specific dimension without finish machining, you may have the wrong casting process.

David