PH is shorthand for "Precipitation Hardening" which is a heat treatment that is a bit different from that of the conventional heat treatment of the Martensitic types.

High strength, excellent corrosion performance, and a simplified heat treatment process are advantages of the PH types as compared to the conventional  martensitic types.

An initial "solution treatment" at high temperature .....typically 1900 degrees F....  assures that all the alloy elements needed for the hardening reaction are uniformly distributed within the metal structure. At these temperatures the structure is austenitic. From this temperature, the alloy is cooled at a rate that retains the distribution of the hardening elements in solution.