Hafnium effect on Inconal 625 castings 1

[A M Contractor]

I want to produce CW6MC Castings with virgin metal and so I have been offered Niobium-Hafnium Alloy (90% Nb and 10 Hf) by the seller. To add 3.25% of Nb the metal will have Hf - 0.36% in the metal. I doubt if this level of impurity might affect the castings. I am not sure what the effect it may have on such an alloy. If it's good then all is good but if it's not then pls tell me the consequences of Hafnium on the 625 alloy. Fe% are restricted to less than 1%.

 

[EdStainless]

Nb+Hf? Where is it from?
It is usually Nb+Ta because they are very difficult to separate.
Personally, I would look for another source of Nb and perhaps mix them.
I would hesitate to add 0.36$ of an unspecified element.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[A M Contractor]

Dear EdStainless
I have the same concerns but simultaneously I am curious to know if someone has experienced the effects of Hf. I read an article which says that Hf is used in turbine rods of a nuclear submarine as it is good absorbent of protons. Also an article claimed that an addition of 0.01% wt or more can increase metal corrosion resistance. I am producing castings which will be used in an oil refinery. I agree to you not to take risk of unknown. Might be a great knowledge if somebody can share the technical pros and cons.

 

[EdStainless]

Do you have small trial furnace?
Pour a 50# heat and test samples.
But it would take a lot of testing to assure me that this was OK.
Since you would be deliberately adding a non-specified element the burden is 100% yours.
Small additions of Hf should be no issue.
It should behave similar to Ta and W and help corrosion resistance.
The problem with this alloy is that people expect certain strength, creep, stress rupture, and metallurgical stability out of it.
I know of many applications where this alloy is used at 1500-1800F for years on end.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[MagBen]

Hf is a reactive element, its function in Ni-based superalloy is similar to Ti, Nb, Ta to provide precipitation strengthening by forming ordered L12 crystal structure Ni3Al type γ′ precipitate. Hf is also formed ideal morphology of Carbides to prevent GB cracking and improve the creep and stress rupture. Hf could also form B2 NiHf for solid solution strengthening.

Earlier and famous benefit might be that Hf is also reported to improve the oxidation resistance, especially in directional soldification applications.

However, Hf leads to the formation of inclusions and lower the incipient melting temperature, which limits the max solution temperature for further strengthening. Too much Hf can form harmful carbides too.

However, in your case if you are making products for customers. Extra 0.36% Hf is probably not allowed regardless of all the protentional benefits it could bring. In the spirit of QA/QC also, Quality will not let you go.

 

[A M Contractor]

Dear EdStainless and MagBen
Sorry for the late reply. I had a busy schedule last week. Agreed that 0.36% of impurity will never be accepted by the client. To tkae samples for a trial is not possible at will cost me dear. As mentioned that it could form harmful carbides has convinced me to drop the idea of using Nb-Hf alloy at all.