Casting Porosity and how much should be "expected" 2

[Loosehead]

I have a sealing system that has a casted (ductile iron) housing. This housing has one area that has always had porosity problems. Lately it seems (to me) that it's getting worse. The foundry that we deal with claims that it is the nature of the part and that they cannot improve on it. My question is, does anyone else have porosity issues and how do you "gage" the levels to ensure the quantity is not getting worse. Also, my part is not complex at all. Should I be sourcing other suppliers or is casting porosity really that hard to control?

Thanks,

Loosehead.

 

[CWIC]

Loosehead:
Casting-quality has many variables which contribute to the quality of a part. The type of casting (sand, investment, etc.), the type of material being cast, the foundries own internal QC program (or lack thereof), etc. can all play into answering your post.

The foundries QC program may help do determine if the raw materials going into the mold are per your specifications and do not exceed any ranges you may have specified.

In a nutshell, gating and their locations within the mold often contribute to the the presence (or lack) of porosity. Well designed molds will reduce the presence and probability of pores. "Dead ends" or locations in the mold that confine or trap hot gases, air or sand often contain unwanted pores.

 

[TVP]

CWIC provided good information. Porosity is often a problem in thick-to-thin transitions. Have you investigated NDT methods for evaluating porosity? Radiography is the most common test method. ASTM E 802 is a set of reference radiographs by which you can compare your castings. Your vendor should be capable of suggesting alternatives for gating, overflows, vents, etc. to improve the situation. Good vendors don't need to be coerced into providing high-quality castings.

 

[CWIC]

Ditto to TVP's final statement regarding your vendor.

 

[arunmrao]

Porosity in a ductile iron casting can be contributed by shrinkage(during solidification) or mold wall movement(low strength of the molding that region due to inadequate ramming etc). One of these features is contributing to your defects.

These can be detected by radiographing those regions which will indicate the extent and nature of defects.

I suppose you are needing hydraulic soundness in your casting. This is your requirement and the vendor must satisfy this. It is not possible to say that accept a leaky castings. No it is not done.

If there are no good ductile iron founders in your region,then convert this into a steel casting,you will have controlled some of these defects.

 

[RunnerMan]

The porosity in question could be a volumetric loss due to insufficient compensatory liquid metal in that particular area of the casting. Changes in section and / or incorrect 'feeder head' or 'feeder neck' dimensioning could also contribute. Casting geometry could play a part also, does this defect always occur in a pocket or tight re-entrant angle ?
Is the porosity shrinkage related or is it gas porosity ?

Should you decide to source other suppliers, you may wish to seek out any foundries that utilise 'solidification simulation' software. MAGMAsoft is one such package - and if in the hands of an experienced operator should be able to identify and therefore remedy your porosity problem.
The foundries who do use this type of software will probably also use 3D modelling software eg Pro-Engineer, Solidworks etc. They may then be able offer suitable design change options for your casting, that may facilitate easier castability.

Good Luck

 

[met1]

Porosity, specifically shrink porosity, can be expected in cast components. The key is to let it form in non-critical areas or in other words manage it through design. There is one thing that is known about shink porosity and that is - it: alwyas, always, always, forms in the last place to solidify. If the last place to solidity is in a critical area, then this area will need to be: chilled, fed or re-designed in order to change the solidification characteristics in that area - by making it the first place to solidify. This can be done by "gut feel" or through the use solidification software as suggested above. Shrink porosity is gauged by a variety of techniques but most often by radiographic methods. All porosity problems can be solved. It just takes: imagination, work, time and money. If your source cannot or is not willing to solve this problem then it's time to resource the job.

 

[arunmrao]

Please ask the foundry to use a graphite chill and a zircon sand patch in the area that you are consistently observing the defect. Also if he is using green sand molding practice let him change over to CO2/Na2SiO3 system. My gut feeling is that the problem should disappear. Also what is the CE of the ductile iron poured. Is there sufficient graphitization or is there any carbide formation. If this happens then you might need more feed metal. Hence check the post innoculation techniques too.

Porsosity control is a common problem and as metman says that an experienced caster with gut feeling can control this defect.

 

[Loosehead]

Thanks for all the responses. The fact is that we go through close to one hundred of these per month so Radiography is not really a feasible option. When we began constructing these 5 years ago we informed the Foundry that they are allowed minor porosity in this "trouble" area. Lately our quality department has rejected 30 pieces because the porosity is extreme. The foundry is saying that we changed the rules by rejecting them and we are saying that these are considerably worse than previous orders. The trouble is that we don't have a benchmark or really anyway to create one. Quality department wants everything black and white as to what is an "acceptable" amount of porosity but I don't really know how to achieve that. I thought about demanding zero porosity but the foundry says there costs will increase then and in a very competitive market can't allow that. I am not really sure how to gage the levels of porosity and set a mark as to what is acceptable.

 

[CWIC]

Loosehead:
I would consider the reference radiographs noted by TVP as your reference standard. Define the maximum size pore, how much porosity is acceptable in an area (clusters) then define how much total (accumulated) porosity is acceptable. This acceptance criteria can be implemented on a random/percentile basis, then increase the frequency of testing if rejectable parts are discovered by your QC personnel. Any through-pores would be rejected.

Also, it could help those replying - to help you if the specific type of discontinuity is known such as porosity, shrink, etc. noted by others who have replied. Voids are not always porosity with castings, but I won't preach to choir.

IMHO:
I do agree with some of the statements made previously by some of the others, it sounds like your casting house is coming up with excuses when they should be trying to figure a solution as to keeping a valued customer satisfied.

On the other hand, if there was no acceptance criteria standard specified when the project was awarded to this casting house, then they have a righteous beef regarding "...that we changed the rules..."

 

[TVP]

I completely agree with CWIC. You need to have some radiography performed on a representative sampling of the production parts. I am not sure that I understand your objection based on one hundred parts per month. There are automotive components that are manufactured in the 10's or 100's of thousands per month that are 100% x-ray inspected. If the porosity is a problem, then you need to inspect for it either destructively or non-destructively.

 

[Loosehead]

Sorry, I should state that it is not the radiography that I am opposed to but this particular part is not the "money maker" of the company. The market for this particular part is so competitive that if our costs increased by even $30 we would be priced out of the market. To x-ray would likely not be feasible for this particular part.

I have just met with the foundry. He is stating that they could make a porosity "free" part but would require new tooling to do so. He claims that the cost of our part would substantially increase. I have a hard time believing and have decided to persue other foundries while working with existing to get us through until we start a steady supply from somewhere else.

I apprieciate all your help.

Loosehead.

 

[jetmaker]

Loosehead,

A company I was working for switched from machined shaft couplings to casted couplings due to the cost savings. However, not long after the change, several couplings were breaking. It was attributed to a poor batch of couplings due to high porosity.

I simple porosity check that I did was as folows: a representative part, that was of know quality... obtained through x-ray... was weighed. The part was also placed it in a water jug, to measure the displaced volume. These numbers were then used as our benchmark. After all this, it was sectioned it to verify its quality. All subsequent parts recieved from the foundry were then weighed.. and if a significant discrepancy on the light side was noted.. its volume was checked. In some cases, we found parts with nearly equivalent volumes.. but 5-10% less weight... these we chose to x-ray... and found unacceptable porosity... similar to those that had failed in service.

Hope this helps with your problem.

jetmaker