Does anyone know how to heat treat titanium (6al4v) for maximum hardness?

Answer 1

Conventional heat treatment processing limits Ti 6AL-4V to about 160-KSI.

Fastener vendors claim up-to 180 KSI for "titanium bolts"... but, from various discussions... it is obvious they are using a proprietary blend of materials, thermal treatments and strain hardening of the raw-stock to "get there". Conventional machined parts would have to be fabricated from this highly processed raw-stock to have close to this strength... then risk low fracture toughness due to the extreme hardness (there will be a very small difference between FTU & FTY. If You positively need these properties, then talk with titanium fastener manufacturers that claim to have Titanium fasteners performing at the at 180 KSI sstrength level... and have them give you a fabrication proposal.

Note: hogging parts from this material would probably be a nightmare due to the incredibly hard-machining nature of titaniun... agravated by the increased strength levels.

Perhaps another Titanium alloy would be more appropriate for Your needs: there are several "newer" alloys [or with better processing] that have higher properties [+170-KSI] becoming available in plate, sheet & bar.

If you're looking to get 180 ksi ultimate strength from a titanium alloy, consider 38-6-44. 6-4 isn't going to get you there.

Answer 2

Heat to 1650F and hold for 1 Hour.
Quench.
Age for 4-6 hours at 1000F.
Let cool in furnace to 300F.