Stainless steel 15-5PH - 1.4545 - X5CrNiCu15-5 - EZ5CNU15.15

15-5 PH Stainless Steel: Properties, Heat Treatment, and Applications

15-5 PH stainless steel is a precipitation-hardenable alloy featuring a martensitic structure. It is designed to provide both excellent corrosion resistance and very high mechanical strength. It results from efforts to improve another well-known grade: 17-4 PH. Compared to the latter, it has a more homogeneous microstructure and better toughness.
Its name can be broken down as follows: approximately 15% chromium and 5% nickel are included in its composition (with the possible addition of copper), and the abbreviation “PH” stands for “Precipitation Hardening” (precipitation hardening).

Metallurgical and Mechanical Properties

The hardness of the martensitic phase in 15-5 PH is enhanced by the precipitation of fine particles during heat treatment. This dual characteristic gives it high mechanical strength even at temperatures up to around 315 °C. Its main mechanical properties are detailed below

Regarding its corrosion resistance, it performs best in the fully hardened state, nearly matching that of 17-4 PH. Among all martensitic steels, it offers the best corrosion resistance, particularly in marine environments. Nonetheless, they are generally less resistant to corrosion than duplex (austenitic-ferritic) steels.

Heat Treatment

15-5 PH can be precipitation-hardened, and here is the process:

• The process begins with an initial heating (solution annealing) between 1025 °C and 1050 °C
• Followed by a rapid cooling (in water, oil, or air depending on the thickness of the piece). This quenching transforms austenite into martensite, which is harder and stronger.
• Next, a second heating (aging) is carried out at a lower temperature to allow the formation of precipitates within the metal, further increasing its hardness and strength.
• Finally, depending on the final aging temperature (H900, H1025, H1075, etc.), the balance between strength, ductility, and toughness is adjusted according to the intended application.

In simple terms, the higher the final temperature, the greater its ductility and toughness, and the lower its hardness. For example, H900 provides maximum strength and hardness, with slightly reduced ductility, whereas H1025 offers a balance between the two, and H1150 maximizes ductility and toughness while reducing its strength.

In some cases, an additional heat treatment is applied to optimize the resistance to stress corrosion cracking, particularly for high-temperature applications. To ensure dimensional accuracy and limit deformations, manufacturing operations (cutting, shaping, etc.) are generally performed after heat treatment.

Forming and Machining

15-5 PH is forged between 950 °C and 1180 °C and then air-cooled, which refines the grain. After welding, a solution treatment followed by aging is recommended to minimize residual stresses and preserve its mechanical properties.

It can be machined in either the hardened or solution-treated state, with saw cutting being preferred over thermal cutting to limit undesirable deformations and maintain a homogeneous structure. Mechanical cutting also allows direct transition to finishing machining (drilling, milling, turning) without the need to reheat the piece.

Applications

15-5 PH is used for components subjected to extreme conditions, whether in the civil or military sectors. Its use spans numerous industries: oil, gas, and marine, as well as chemical installations and paper manufacturing. It is particularly employed in the production of shafts, landing gear, valves, gears, fasteners, gas turbine components, and even nuclear reactor equipment. Its overall toughness plays a critical role in sectors where even the slightest cracking can have severe consequences.