15-5PH Stainless Steel (UNS S15500): Composition, Properties and Heat Treatment

What Is 15-5PH Stainless Steel?

15-5PH is a precipitation-hardening martensitic stainless steel with the UNS number S15500. It combines high mechanical strength, good toughness and moderate corrosion resistance, and it can be supplied in a wide range of aged conditions so that a single grade covers many different strength and ductility requirements.

The grade is a refined development of 17-4PH (UNS S17400). Tighter control of impurities and a lower delta ferrite content give it a finer, more uniform microstructure, better transverse toughness and improved weldability. That combination has made 15-5PH a standard choice for aerospace structures, petrochemical equipment, power generation hardware and other high-performance components.

In the European system the grade is written as 1.4545 or 1.4548, with the designation X5CrNiCuNb15-5. Copper and niobium are the key precipitation-forming additions: copper-rich phases and niobium carbides precipitate during ageing and are responsible for the very high strength obtained without a loss of ductility.

Chemical Composition of 15-5PH

Table 1 shows the typical chemical analysis. Balanced chromium, nickel and copper contents are what allow the grade to be hardened by a simple ageing treatment after solution annealing.

Element	Content (%)
Chromium (Cr)	14.00 - 15.50
Nickel (Ni)	3.50 - 5.50
Copper (Cu)	2.50 - 4.50
Niobium (Nb)	0.15 - 0.45
Manganese (Mn)	≤ 1.00
Silicon (Si)	≤ 1.00
Carbon (C)	≤ 0.07
Phosphorus (P)	≤ 0.040
Sulfur (S)	≤ 0.030
Iron (Fe)	Balance

Mechanical Properties and Ageing Conditions

Table 2 lists typical mechanical properties in two of the most widely used ageing conditions. Higher ageing temperatures reduce strength but increase ductility, toughness and resistance to stress corrosion cracking.

Property	H900	H1150
Tensile strength	≥ 1100 MPa (160 ksi)	≥ 860 MPa (125 ksi)
Yield strength (0.2% offset)	≥ 1000 MPa (145 ksi)	≥ 725 MPa (105 ksi)
Elongation in 50 mm	≥ 8%	≥ 15%
Reduction of area	≥ 25%	≥ 40%
Hardness (HRC)	38 - 42	30 - 36
Modulus of elasticity	approx. 200 GPa (29 x 10⁶ psi)	approx. 200 GPa (29 x 10⁶ psi)
Fatigue strength at 10⁷ cycles	approx. 480 MPa (70 ksi)	approx. 415 MPa (60 ksi)

The heat treatment route has two stages. Solution annealing is carried out at approximately 1038 °C plus or minus 14 °C for about half an hour, followed by cooling to below 32 °C to complete the transformation to martensite. Ageing is then performed at a selected temperature such as 482 °C, 496 °C, 552 °C or 620 °C to develop the required strength.

Condition	Typical tensile strength	Typical hardness
H900	approx. 1380 MPa	approx. 45 HRC
H1025	approx. 1170 MPa	approx. 38 HRC
H1150	approx. 1060 MPa	approx. 32 HRC
H1150M	approx. 860 MPa	approx. 28 HRC

15-5PH Compared with 17-4PH and Other Grades

Both 15-5PH and 17-4PH are precipitation-hardening martensitic stainless steels, and the two are frequently considered together. The differences matter most in demanding applications:

Toughness: 15-5PH has better transverse toughness, particularly in short-transverse directions, which is important for parts loaded in more than one direction.

Microstructure: a lower delta ferrite content and tighter impurity control give a finer and more uniform structure.

Weldability: 15-5PH welds more reliably, with less risk of cracking in restrained joints.

Corrosion resistance: both grades behave similarly in atmospheric and mildly corrosive service, and both need careful attention in chloride-bearing or acidic environments.

Where maximum corrosion resistance rather than strength is the deciding factor, an austenitic grade such as 316L or a duplex stainless steel is normally the better answer, because those families combine molybdenum additions with a structure that cannot be precipitation hardened.

Fabrication, Welding and Surface Finishing

Machining is easiest in the solution-annealed condition, and the grade is usually rough machined before ageing so that distortion during the ageing treatment remains small. Welding is carried out with matching filler metal and a controlled heat input; the welded assembly is then solution treated and aged, or aged directly where the application allows.

Surface finishes are selected to suit the service environment: polishing, sand blasting, passivation, electroplating, chemical conversion treatment and protective coatings are all used to improve appearance, corrosion resistance or wear behaviour. Passivation after machining is a common step because it removes embedded free iron from the surface and helps the passive chromium oxide film to reform.

Typical Applications and Frequently Asked Questions

Aerospace structural fittings, actuators, flap tracks and landing gear components

High-pressure valves, pump shafts and compressor parts in petrochemical plants

Turbine and power generation hardware requiring strength at moderate temperatures

Fasteners, gears, splines and other precision parts needing high strength plus moderate corrosion resistance

Moulds, tooling inserts and components exposed to mildly corrosive media

Q: What is 15-5PH stainless steel?
It is a precipitation-hardening martensitic stainless steel, UNS S15500, supplied in solution-annealed or aged conditions. Its strength comes from a copper- and niobium-based precipitation reaction during ageing.

Q: What is the equivalent of 15-5PH?
The UNS number is S15500 and the European designations are 1.4545 and 1.4548, written as X5CrNiCuNb15-5. It is closely related to 17-4PH (UNS S17400, EN 1.4542).

Q: What is the maximum strength of 15-5PH?
In the H900 condition the tensile strength is typically around 1380 MPa with a hardness of approximately 45 HRC. Higher ageing temperatures give lower strength and higher ductility.

Q: Will 15-5PH rust?
It resists atmospheric and mildly corrosive environments well, but it is not immune to corrosion. In chloride-bearing or acidic service, pitting and stress corrosion cracking can occur, so the ageing condition and surface finish should be selected carefully.

Q: Is 15-5PH magnetic?
Yes. The structure is martensitic, so the grade is magnetic in both the solution-annealed and the aged condition.

Q: What is the difference between 15-5PH and 17-4PH?
15-5PH offers better transverse toughness, improved weldability and a finer, more uniform microstructure thanks to lower delta ferrite and tighter impurity control, which is why it is often preferred for critical aerospace parts.