Difference betwen Inconel 625 and 626lcf

[Vinzzzz]

Hi to all,

I have a question for materials based on Nikel Alloys.
I have to select a nikel alloy to construct bellows (thermal expansion joint) and i found that there are two different type of Inconel 625:

- grade 2 as per ASTM A443 (hot worked in sheets;
- 626 lcf (worked in special condition in order to make hi performancies for fatigue);

so my question is which are the differencies between the two materials?

Thanks a lot, regards Vincenzo.

 

[btrueblood]

A few minutes with google gave the following. Looks like it is a low-carbon controlled version of regular 625.

http://www.specialmetals.com/documents/Inconel alloy 625LCF.pdf

U.S patent 4765956.

from the patent:

"SUMMARY OF INVENTION

It has now been discovered that the low cycle and thermal fatigue life of alloys described herein can be markedly improved provided the carbon, nitrogen and silicon contents are controlled and correlated such that the sum of the % carbon+% nitrogen+1/10% silicon does not exceed about 0.04% and is preferably not greater than about 0.035%. Moreover, low cycle and thermal fatigue is further enhanced if the alloys are processed by vacuum induction melting followed by electroslag refining. "

 

[EdStainless]

lcf is a low residual alloy with ESR melting. It also has a highly controlled grain size. It does not have much better high cycle fatigue strength, but it has much better low cycle fatigue properties.
The alloy has to be made correctly and fabricated very carefully.

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Plymouth Tube

 

[metengr]

Most if not all expansion joint bellows are designed are for low cycle fatigue (thermal/mechanical) applications in process service conditions. I have never seen bellows used for high cycle fatigue service. I would run a cost benifit analysis and compare with Inconel 625.