Here is the quote from MIL-HDBK-5J:
"The impact strength of 17-4PH, especially large size bar in the H900 and H925 conditions, may be very
low at subzero temperatures; consequently, the use of 17-4PH for critical applications at low temperatures
should be avoided. For non-impact applications, such as valve seats, parts in the H925 condition have
performed satisfactorily down to -320F. The H1100 and H1150 conditions have improved impact strength
so that parts made from small diameter bar can be used down to -100EF with low risk. For critical low
temperature applications, a similar alloy, 15-5PH (consumable electrode vacuum melted), should be used
instead of 17-4PH because of its superior impact strength at low temperature."