Fig. 1. Ms temperature for selected precipitation-hardening stainless steels[4]

Question:
As 15-5 Mo and 17-4 PH are martensitic precipitation-hardening stainless steels, I assume they have the typical critical temperatures where austenite begins to form and completes formation. I know the upper critical is below 1800°F, the solution temperature. But I am trying to get a handle on the lower critical. Can you help?

Answer:
This is an excellent question since most often we purchase these materials in Condition “A” (solution heat treated) and only perform an aging treatment or we are told how to perform the heat treatment and given mechanical properties for properly heat-treated material. In either case we fail to understand the underlying metallurgy.

The precipitation-hardening (PH) stainless steels are divided into three types: martensitic (maraging), semi-austenitic and austenitic. The difference between them is in the metallurgical microstructure that exists after heat treatment. There is no major differentiation based on the precipitate formed. The alloying elements in these steels are balanced either to produce a martensitic structure at room temperature, a structure of meta-stable austenite that can be converted to martensite (typically via an “austenite-conditioning” process) or completely stable austenite.

Here is some key metallurgical information about 17-4 PH. The papers referenced have additional information about 15-5 Mo and other PH stainless steels.

For 17-4 PH, the AC1 point occurs between 550-600°C (1025-1110°F) while the AC3 line is at 900°C (1650°F).

For martensitic PH stainless steels such as 15-5 Mo and 17-4 PH, the alloy content has been adjusted so that the Ms temperature is high enough to transform the elevated temperature austenite phase to martensite on cooling to room temperature. The Ms point for 17-4 PH is shown in Fig. 1. Note that 17-7 PH and AM 350 are semi-austenitic grades. The referenced source for more information is given in Reference 5 below.