Investment Casting

the alloy is???

milo

Hi Milo,

The Alloy is EN 1.4517!

Have you discussed your findings with the caster? And, if not, why not? And I mean the foundry that cast the parts, not the supplier.

Hi majordud,

I did already discuss this with the foundry and I'm still discussing, but there is not a clear answer yet. The idea of posting here was also in order to hear if there was any that has experience of something like this before. This way I can better discuss with the foundry and get better answers.

This is a development project so I don't have a continuos supply of material that I can use to do some experiments. So I must have security in my choises for the next orders since they cost extra money.

thanks

Based on my experience as a Quality Manager/General Manager of an investment casting foundry, the defect you are experiencing is due to poor dewax. By the way, it wouldn't hurt for you to use a spell checker before submitting your comments.

I think that both majordud and rorschach have given good advice regarding the process influences.

I would also ask you to determine how much superheat they had when casting, that may be a contributing factor to your latent austenite appearing after heat treat issue. The comments about exogenous (tramp) carbon also ring true.

milo

There is a lot going on with this situation. I am not a metalurgical engineer, so I can't answer questions concerning the outcome of the heat treat, but I do have many years experience in investment casting tooling and processes. Here are some things to consider:

They could be using recycled wax with impurities or good wax that is dirty. They may not be running their autoclave hot enough or long enough.

If it is not the wax, it could be they are mixing their alloys when they recycle gates and runners from previous casting runs. This was a big issue in the 1980s when a cargo door fell off an airplane because of a defective cast hinge. The FAA went over numerous foundries to make sure alloys were not being mixed.

This is just my opinion, but you and your vendor have two different responsibilities. Yours is to cause your drawings and purchase orders to constrain the vendor to supply you with what you want. For example your drawing should require x-ray and magnaflux testing on a certain percentage of the castings as well as maximum size of porosity cavity size. You can require test bars to be cast along with the parts for use in tensile strength testing. You can require chemical anaylisis of the alloy to prove that it meets the specification required on the drawing. You can specify a heat treat process and require testing ro prove that the castings meet the spec. These are all things that should be on your drawings. Your purchase order calling for the casting to be complete to print turns the drawing into a contract. It is then up to the foundry to meet the requirements. If they do not have the ability to do so, then you need to find another foundry.

To Guest - something is basically wrong here. Are you saying that you are buying and paying for castings with obvious defects? That this foundry has inspected these parts and found them acceptable? I agree with Rorschach that the duplex alloys are a royal PITA, but no one can make an acceptable casting with a dirty mold. No doubt the mold you were sent would have seen a preheat oven prior to cast that would have removed the remaining wax (presumably dewaxed in an autoclave, so there would be some visible retained material), but someone should have their head examined at that foundry. To send a customer a mold with retained wax?

I'm not familiar with that specific alloy, Most of the duplexes I've worked with were wrought, not cast. But many duplexes have large additions of nitrogen to shift the austenite/ferrite balance towards the middle of the range, but if the melt is kept molten too long or if it is remelted over and over, the nitrogen will boil off. Filler metals for welding boost the nickel content of the filler to compensate for the lost Nitrogen. I would think Casting alloys would tend towards higher nickel contents as well for the same reason.

You can also have a issue with what is called "hot shortness". Where the solubility of the various elements will change based on temperature and you can end up with elemental segregation as different elements solidify at different rates. That is why fast cooling rates are so important and why re-solution annealing afterwards won't repair the error.

Thank all for your valuable comments.

It seems that most of the opinions have a common point --> the wax: either the quality of the virgin wax or the processing of the wax (re-use and de-waxing process) seem to be a common point. Anyway I also have a suspect about the heat treatment.

As I mentioned before, this is a development project. We have this foundry supplying other alloys and we are trying to approve them to supply this alloy too. That is way we are spending so much effort and time with this supplier. It is an interesting supplier for us but is not a high qualified supplier. But in this times others interests also need to be taken into account ($$$$).

Thank all once again!

Porosity probably comes from retained wax, but could also come from inadequate degassing of the melt combined with poor mold design allowing accumulation of gases in some pockets.

I'm not familiar with the alloy, so the following may be way off.

The austenite layer is interesting. My first thought was they are cooling too slowly, but this would tend to show austenite in the interior whereas I understand it is a surface phenomenon. Perhaps they are not soaking the part long enough during reheating for heat treatment, then cooling too slowly so that austenite forms on the surface but the core didn't reach a high enough temperature to transform. Either way, the post casting heat treatment seems suspect.

Hope this helps.

Good luck