Aircraft landing gear and gas turbine shafts have one thing in common; they both utilize 15-5 PH stainless steel and withstand extreme loads while maintaining form and integrity.
15 5PH stainless steel is defined as a precipitation-hardening martensitic stainless steel alloy and is used across a broad range of industries. There exist multiple designations for the alloy including AMS 5659, AMS 5862, and UNS S15500 in the United States, while in Europe it is defined as W.Nr 1.4545. Stainless steel 15-5 PH contains 14-15.5% chromium and 3.5-5.5% nickel, and is designed with Cu of 2.5- 4.5% and Nb to assist with heavy aging.
15-5 vs 17-4
If you have experience with 17-4 PH stainless steel alloy, you will see many similarities with 15-5 PH stainless steel alloy. 15-5 is an improvement to the original starting point of 17-4. The design goal for 15-5 was to decrease the delta ferrite significantly. While it may seem insignificant, it actually results in much better transverse properties, and explains the popularity of 15-5 in bar stock and forgings.
The same goes for 17-4 as the stainless steel respects the original design philosophy, with the added benefit of geometry. 15-5 excels in shafts, valve bodies, gears and structural components where load is applied in varying and often inconvenient directions.
Heat Treatment
As Condition A, 15-5 exhibits little to no mechanical properties prior to ageing, and the chosen ageing temperature impacts everything.
These conditions, H900 to H1150, indicate the various temperatures of ageing each in Fahrenheit. H900 creates the highest strength, but results in low ductility and toughness. H1025, H1075, H1100, and heats above provide higher ductility and toughness, but low strength. Strucutural aerospace applications tend to operate in the H1025 to H1075 range.
Due to its toughness and strength, H1150 has been used in the strain ageing temper of tough chemical and marine environments.
Applications
Corrosion resistant, tight-tolerance, and defense or aerospace system demanding applications incorporate this alloy within the parts.
Gas turbine applications in power generation are hot and hard applications, so 15-5 holds up well in shafts, valve internals, and steam-side fittings.
Motorsport teams use 15-5 in valve parts and fasteners to ensure strength to weight ratio requirements are present and corrosion between racing sessions is not an issue.
A note on melting practice
In order to have double-melted material, which can be ESR or VAR, both Aerospace and other safety-critical livelihood sectors require this. There is a major difference in cleanliness, inclusion, and ultimately fatigue performance, based on which melt route you decide to take. Single-melted bar is more cost-efficient and deemed fit for a variety of applications, but you can’t use it to specify a turbine disc.
Sourcing
You can get the material in standard AMS sizes in bar, plate, and sheet. Dynamic Metals carries AMS 5659 and AMS 5862 and can cut and convert 15-5 PH stainless steel to shape before it leaves their facility.
It is an unattractive alloy, and for that reason it’s most likely underappreciated, but for industries that are in the field of aerial or rotational mechanics and/or under a load-bearing application, it is most likely the material being used.