I bet you used a lower hardening temperature, say 1800F, then tempered at <700F to get a very high HRC. the concern of SCC is reasonalbe when the hardness is at high level.
why not hardening at higher temperature, say 1900F, then temper at 700F? This will give you the best of Charpy, and lower HRC as well.
Or, if the high hardness is a big concern, you may use a high temper temperature (>1100F) to get a low hardness, and at the same time still good impact strength.
Avoid tempering between 700-1050F which gives you low impact and decreased general corrosion resistance, although it tends to give you high tension properties.

We ran samples austenitized at 1800, 1900, and 2000 then oil quenched. All austenitizing temperatures gave the same hardness after tempering at 600 or 700 degrees, 43-46 HRC.

We also tested tempering at 1100 degrees. A portion of this part gets carburized and that temper dropped the hardness in the carburized area too low.

Thank you for the advice on SCC.

At this hardness, SCC is a major concern and depends on the fluid environment. Second, how will they avoid softening along the HAZ in butt welds (if they need to join pipe sections)?

43-46HRC itself is kind of "Big" difference. The tendency to obtain lower hardness when increasing austenitizing temp is more obvious for bar products than plate or strip products, but still there is 1-2 HRC decrease when temp increases from 1800 to 1900. For a bar product, the decrease can be up to 5-6 HRC points.
The reason is that higher hardening temp can lead to remaining austenite after quenching, which will decrease hardness, while increase toughness.

Your 1800 and 1900 parts may have similar hardness, but they could have very different toughness.
Oil quench (high volume of well agitated oil) from 1800 for better toughness.
Temper at 1100 is too soft??? This is still 150ksi UTS.
If he wants to use it harder than that then he will have to accept the low toughness and high risk of SCC.
This alloy is susceptible to SCC from both hydrogen and caustics. The hydrogen could be present because of corrosion.

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P.E. Metallurgy, Plymouth Tube

Quote (Lyrl)

A portion of this part gets carburized and that temper dropped the hardness in the carburized area too low.

Can you provide additional details of the carburizing process you used with this 431 stainless?

tbuelna - sorry for the delay in responding, this has been a hectic week.

The plan for this application is to paint carburizing stop-off paint over most of the pipe, since the carbides on the carburized surfaces will significantly decrease corrosion resistance. (I know some heat treaters are able to carburize stainless steel without producing carbides, and I did some tests with this goal in mind. My conclusion is their equipment is capable of carburizing at higher temperatures - probably around 1900-2000°F - compared to my furnaces that have a maximum of 1800°F.)

Then the parts with unpainted bearing races (which are sealed from the corrosive media) would be carburized on basically a very long but otherwise standard carburizing cycle (long cycle due to the chromium and nickel slowing down carbon diffusion).

I really appreciate all the feedback on the SCC susceptibility, our customer was not very clear on this and I hadn't worked with stainless enough to be confident in that myself. We have suspended our testing with 431 material and are currently pursuing testing with a different stainless alloy.

So how were you planing on getting the hardness? Q&T after carburizing?
If you carburize at higher temp you still form a lot of carbides, but you get enough Cr diffusion so that you don't have a Cr depleted region around them. You loose some corrosion resistance due to lower Cr in solution, but you are not sensitized.

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P.E. Metallurgy, Plymouth Tube