Chances are good that if you have ever specified materials for use in aerospace parts, you have run into 17-4 precipitation hardening stainless steel. It’s everywhere: landing gear parts, fasteners, valve bodies, actuator parts, and countless other structural components. Why? It’s not because of some sort of inertia or because everyone in the aerospace industry has simply decided to use this material by rote. No, there is a very good reason why 17-4 stainless steel has become ubiquitous in aerospace components. It has a combination of properties that is hard to beat, particularly in those cases where corrosion resistance is required in combination with high strength in a material that can be machined before heat treating.
Why 17-4 Stainless is Everywhere in Aerospace Components
The name “17-4” simply means that the material contains 17% chromium and 4% nickel. Of course, there are other elements present: 3-5% copper, small amounts of niobium and tantalum, and the balance iron. The copper content is the key. Copper allows for precipitation hardening of the steel.
The material is very soft in condition A, solution annealed. It can be machined to final dimensions in this state, then aged hard. Condition H900 has a hardness of about 44 HRC, ultimate tensile strength of 1380 MPa, and a yield strength of 1310 MPa. That’s comparable to some of the high-strength steel alloys on the market, plus corrosion resistance.
The H1025 and H1075 conditions have some loss of peak strength but gain in toughness and stress corrosion cracking resistance. This is often a better trade-off for aerospace use since the failure modes of components are not limited to simple overloads. The H1150 condition sacrifices even more strength but provides maximum corrosion resistance and is useful for components subjected to particularly aggressive environments.
Why Aerospace Engineers Keep Specifying It
The high level of corrosion resistance alone makes 17-4 an attractive alloy for use in aircraft components. Commercial air travel is conducted in an environment that is remarkably aggressive. Coastal flights expose components to salt spray, winter flights expose components to de-icing fluids, and fluctuations in humidity expose components to conditions that cause corrosion in poorly ventilated areas. A component that would corrode quickly in a carbon steel will last for years or even decades in a 17-4 stainless steel.
However, the combination of high levels of corrosion resistance and high strength makes this alloy particularly attractive for use in components subjected to aggressive environments. Landing gear fittings are subjected to enormous cyclic stresses during every take-off and landing cycle. Hydraulic system components are subjected to high pressure and are in an environment where leaked fluids will cause corrosion. Engine components are subjected to thermal stresses and vibrations and are also in an environment where fluids may cause corrosion.
17-4 performs better than most other materials in these conditions. Aluminium alloys have better strength-to-weight ratios but have a greater tendency to corrode and also have less absolute strength. Conventional stainless-steel alloys have good resistance to corrosion but do not have the level of strength provided by precipitation hardening. High-strength steel alloys require the application of a coating that increases cost, weight, and number of possible failure modes.
The advantage of machinability is greater than it might seem. Machining hardened steel is very tool-intensive. With 17-4, you machine in the soft condition where conventional carbide tools are very effective and complex geometries are possible. The age hardening is done at a temperature that does not induce distortion problems—important when the part is already machined to close tolerances.
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