SST 17-4 Machining H900 vs other H conditions

I have a SST 17-4 bar that is 6" x 7" x 8" in which I need to bore a series of inline holes the largest dia will be a 3.625-12 UN-2B thread going to a 3.375" dia then a 1.625-12 UN-2B thread and to a 1.25" dia hole. I would like to use the H900 condition for it's high strength properties. We have not machined 17-4 in any heat treated condition. I would like to know what to expect concerning the machinability of H900 vs. H1025 or H1150. How much of a difference in machining time will there be? What types of tool materials would be used with the different heat treat conditions?

17-4Ph heat treated machining

If you want to get good surface finish you better machine it in age hardened condition. Since the part got thread you need good elongation so you better go with H1025 or H1150 (at H900 the elongation is around 17% so the material is brittle and the threads have a tendency to break easily).
The material machine well in age hardened condition with carbide (in solution annealed condition material is sticky and surface finish poor) the difference in machining time will be minimal.

Check on the property and test charts that you can find by googling around. H900 may be strong, but its also unpredictable. The hardness can also vary by quite a large amount, I don't know how that translates into the machining of it.

I wouldn't go any lower than an H950.

If I had to machine it, I'd prefer an H1150, H1050 isn't bad either. I haven't touched anything lower than that in a long time.

Avoid at all costs machining before heat treat, its gummy, kind of like 304. After HT it behaves more like a Heat Treated alloy steel (4140 etc..). Makes your life a lot easier, and you end up with a shinier part.

Just a thought, with that sized chunk of material, you may want to open up the center before heat treat. Soak time, surface area and all that...(I'm not a heat treater for a reason) It would suck to get to the middle of the block and find it all nasty and gummy in the center.

I've machined a gazzilion tons of h900 but nothing as big as you're doing. I usually run around 220 sf and it works just fine. I wouldn't worry too much about it.