Crack like indication after machining of centrifugal casting

[lidbasky]

Does anybody had experience with machining of castable super invar. I am working on centrifugal castin ring. Material is special super invar containing 28Ni-14Co-0.5C. After machinig and PT I saw some crack like indications. Is it possible that cracks occure due to bad machining?
Does super invar needs special care for machining?
Thanks

 

[EdStainless]

Has it been stress relived?
How much stock did you remove?
Is this the ID or OD?
I have seen some significant flow and lap defects in CC high Ni alloys.
You might need to a little local stock removal and see how deep this goes.

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P.E. Metallurgy

 

[lidbasky]

Dear EdStainless
Thanks. It is stress relieved.
It is inside of ring. More than 5 mm was removed.
You can see PT result in attachment.

 

[arunmrao]

I have experienced cracks being observed after machining in Invar castings. Though, they were not visible in as cast condition. Radiography would sometimes reveal secondary shrinkage defects but no cracks.

As the castings were small, about 5-10kilos, I made die moulds and cast the parts. Rejection was reduced but not eliminated.

Please check and confirm, if the defects are observed under the feeder or near feeder zone.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[lidbasky]

Dear Arunmrao
It is centrifugal casting cylinder. There is no feeder.
We cut rings from cylinder and machined them.
The image shows ring after cutting and machining.
I think it is hot tear due to excess centrifugal force than shrinkage defects.
But casting supplier says that maybe they are machining cracks.

 

[arunmrao]

Sorry my mistake. I overlooked the fact it was centrifugally cast.I have no reason to believe that machining a fully austenitic material can generate cracks.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[EdStainless]

I'll second that, I can see you getting tears, laps, and smearing from machining but not cracks.
No stresses from machining can easily open cracks, but not likely to cause them in this material.
This looks to be a sizable casting so I would be concerned about differential thermal effects in casting.
Was this ring cut from one end of the casting? Because it looks like the indications are concentrated at one end of it.

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P.E. Metallurgy

 

[lidbasky]

Deararunmrao
I should explain more.
Due to presence of Cobalt and also high amount of Carbon in this alloy (0.45 to 0.55 %) it is not fully austenitic
and as I check in the phase diagram it is dual phase (austenite+ferrite)and maybe high amount of carbides.
On the other hand the amazing thing is rings are semi magnetic as I checked by magnet.
Dear EdStainless
We will cut it from one end.
Regarding differential thermal effect The alloy is special one with very low CTE up to 350 C.

 

[EdStainless]

But the contraction on solidification is considerable, if the solidification and cooling are not uniform then trouble can ensue.
This alloy is actually Kovar isn't it? Super Invar is 30Ni 5Co, or something like that.
Why isn't this austenitic? Both C and Co are austenite stabilizers along with Ni. At low temp I can see martensite forming, but not at normal temps.
Now I am wondering about the C solubility in this alloy, and what carbides would be forming...
Should this material be solution annealed to remove the carbides?
I would love to see the Co-Fe-Ni phase diagram.

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P.E. Metallurgy

 

[lidbasky]

EdStainless
Really I don't know this is a Kovar or Super Invar. I am working on this alloy for
a long time. But it is a castable very low CTE alloy up to 350 C. I don't know why designer used such huge amount of Carbon.
I have checked before Co-Fe-Ni phase diagram and it is shown the region that you have both ferrite and austenite.
You can see diagram in attached paper and diagram that I prepared by Thermocalc for this alloy.
I am not sure about my calculations.
Rings are heat treated about 750 C for several hours without any solution. Due to large grain size and lack of Cr it is prone to
grain boundary oxidation at high temperature.

 

[EdStainless]

There are about 8 chemistries of Kovar that I know of. These are controlled expansion alloys designed to match the thermal expansion of various glasses and ceramics. These alloys all have K94xxx UNS numbers. I didn't see any that allow over 0.10%C, with some of them having a 0.02%C max.
I would expect these to be solution annealed after rough machining. Usually this would be done in a protective atmosphere (neutral to slightly reducing) and with a fairly short hold (to reduce grain growth) such as 30 min at 850C. It would then be stress relived after machining.
If this is going to be bonded to glass or ceramic it is often give a very quick surface oxidation treatment of just a few min at 900C in air.

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P.E. Metallurgy

 

[lidbasky]

Thanks Ed for useful informations.
It is a gas turbine part to make stable clearance during working. It is used in cold section of turbine and up to 350 c.
So specially designed by gas turbine manufacturer.
As a conclusion the indications that revealed in PT test after machining (see attached pic in my second post) are casting defect (Shrinkage or hot tear) or occured due to bad machining parameters?

 

[MagBen]

The composition is between super invar and Kovar, and structure is most likely austenitic, no chance to get ferrite. There is, however, a possibility for martensitic formation, especially at low temperature. The high C may be served as that purpose (to prevent martensitic at low temperature).
Super Invar machines similar to, but not as well as, Type 316 austenitic stainless steel.
Parts should be degreased and cleaned as soon after machining as possible to remove any residual sulfur which can cause grain boundary embrittlement.
Did you do stress relief after machining to minimize thermal expansion coefficient? I can imagine you have to machine off lots of staff to get to the final shape.

 

[lidbasky]

@Dear MagBen
Thanks for comments.
As said before when I check it by magnet it is semi magnetic so it has other phase than austenite.
We do a lot of machining from centrifugsl casting cylinder to final ring.
What is your idea about defects in attached picture what is the cause of this defects? Casting defects or machining defects?
Please see attachments.
They are located on inner surface of ring and reveled in PT after machining.

 

[MagBen]

How can you say it is semi-magnetic? This alloy is magnetic although it has a fully austenitic structure (note this is different austenitic stainless that non-magnetic. pure Ni, for example, is austenitic, but magnetic). If the invar alloy lose its ferro-magnetism, it will lose most of its low expansion characteristics, the expansion curve will be parallel to regular steel.

As for the cracking, my opinion is it was from casting/solidification, not from machining. I do not see much of cracking (bar stock) and edge checking (strip) issue even experienced with a large cold working. However, this is my wild guess, a metallurgical investigation is needed to find reliably the cause.

 

[RolMec]

The cracks are like equally distributed on the inner surface: Any more cracks subsurface further within the body?
Are there handling operations involvimg a turning up of the ring, or even a sitting on its outer diameter?

You say, you cut rings from cylinders:
Then, due to ring form (inner vs. outer dia.) and cooling from inside diameter and outside diameter, there will be inner tensions. Typically, these release during machining operations. Adapted stress relieve is imperative, perhaps repeated when removed thickness is larger.

I. m. exp.: Encountered cracks in ring form pieces were due to differential action of heat treatment related inner tension states and subsequently released / cracks triggered during machining. So it was both: Material and machining in combination.

Roland Heilmann