From 17-4 PH® to PH 13-8 Mo: The Evolution of High-Performance Stainless Steels
How ARMCO’s Stainless Steel Grades Pushed Aerospace Engineering to New Heights
In 1948, ARMCO® (American Rolling Mill Company) introduced 17-4 PH, a precipitation hardening stainless steel known for its tunability. By adjusting the precipitation temperature, engineers could tailor the alloy’s properties; from soft and ductile to strong and brittle, and everything in between.
The grade was further developed for use in the aerospace industry. This was done by adjusting its chemical composition for a lower δ-ferrite content in order to improve its isotropy (its properties are more consistent in different testing directions). It also added remelting to the process route to improve its steel cleanliness and have a better resistance against fatigue cracking. This became known as ARMCO 15-5 PH and was introduced in 1969.
If you ever wondered why 17-4 PH and 15-5 PH are so similar? Now you know.
15-5 PH is, just like its older sister 17-4 PH, tunable. This is shown below, where ultimate tensile strength (UTS) is plotted against Charpy V-notch impact strength. Pick the condition that suits your needs:
Typical aerospace use for 15-5 PH is, for example, in wings, doors, panels, and engine pylons.
But what if 15-5 PH falls short? What if both the tensile strength and the impact strength need an increase? What if the application is more demanding? Or even more safety-critical? Landing gear components maybe? Secure door latches? That’s where ARMCO PH 13-8 Mo comes in, which was introduced in the same time frame as 15-5 PH.
ARMCO PH 13-8 Mo does a few things differently, other than putting “PH” in front the grade designation. Its chemical composition requires vacuum induction melting instead of electric arc melting, and its precipitation hardening is based on NiAl intermetallics (nickel-aluminium form the hardening precipitates) instead of copper. But the result is clear. Below is the same graph as before, but with PH 13-8 Mo added:
As is evident from the graph, PH 13-8 Mo will offer higher typical ultimate tensile strength (UTS) values, while at the same time also offering higher Charpy V-notch impact strength values.
So those really high-performance applications where 15-5 PH falls just short? Or those applications where safety is paramount? From 17-4 PH to PH 13-8 Mo, ARMCO’s innovations have continually pushed the boundaries of stainless steel performance, meeting the evolving demands of aerospace engineering and beyond. There is a premium member in the ARMCO PH family that might fit your application!
AK Steel International markets specialty steel products such as ARMCO® Pure Iron, stainless steel, metal products and composite auxiliary materials for the aerospace, automotive and industrial sectors.