I think Inconel 750 is for more common. Very hard to give specific recommendation when no specific information is given. What constitutes "best" to you?
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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
... click on 'Inconel Alloys' tab... download data for X-750 TDS.
MatWeb Inconel Data sheets do NOT include X-650
INCONEL® Technical Data Sheets
Can't say anything about X-650... I cannot recall even hearing about it! I looked in my Metallic Materials Specification Handbook [4ed] and came-up 'dry'.
EdStainless: You have a clue-card where to find X-650 data?
Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
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To be honest the only X-650 data that I have is not for spring temper material.
As far as I know X650 is not an industrially significant alloy.
It would be common today to use alloys such as A286, 718, or even cold worked 600 or 625.