Why Is High-Strength Steel Important in Aerospace Manufacturing?
When manufacturing components for the aerospace sector, low mass, toughness, corrosion resistance, and strength are important. However, standard aircraft grades of aluminum don’t provide the properties that some parts (like those in landing gear) need, which is why there is a role for high-strength steel in aerospace.
Characteristics & Categories of High-Strength Steel
High-strength steels are characterized primarily by their tensile strength but are also known for their high levels of toughness and corrosion resistance. They are generally divided into three categories:
- High-strength, low-alloy (HSLA) steel
- Advanced high-strength steel (AHSS)
- Ultra high-strength steel (UHSS)
Benefits of High-Strength Steel
High-strength steel has many benefits, but its strength-to-weight ratio can’t match that of 7XXX series aluminum. So, why do aerospace manufacturers use it?
Aerospace application requirements dictate the use of high-strength steel for particular components. The main reasons are:
- High Tensile Strength: The minimum tensile strength for high-strength steel is around 36,000 PSI, but some go up to 270,000 PSI. Less is needed than if lower-strength steels were used.
- Corrosion Resistance: These alloys contain chromium and/or nickel, both of which combat corrosion.
- Good Fatigue Resistance: Strength plus ductility and complex microstructures help prevent cracks that can result from fluctuating loads.
- Impact Resistance: High-strength steels tend to be tough and perform well in Charpy impact tests.
Common Aircraft-Quality Grades
SAE has published Aerospace Manufacturing Specifications (AMS) that address the material consistency and quality needs of that sector. Common aircraft quality steels covered by AMS include:
SAE 4140 (AMS 6349)
SAE 4140 is considered an HSLA steel. It contains small quantities of chromium, molybdenum, and manganese and features a tensile strength of 95,000 PSI and 25% elongation, which signifies good fatigue resistance. Its strength can be increased significantly with heat treatment.
SAE 17-4 PH (AMS 5643)
SAE 17-4 PH is a martensitic stainless steel with high chromium content and excellent corrosion resistance. Its tensile strength is 112,000 PSI, but heat treatment can raise this strength. Ductility is lower than other alloys but sufficient for many aerospace applications.
SAE 4340 (AMS 6415)
SAE 4340 is an HSLA steel whose properties are enhanced by small additions of chromium, nickel, and molybdenum. Its tensile strength is 108,000 PSI, which can be increased through heat treatment. Elongation is high at 22%, indicating good fatigue resistance, and the alloy is readily machinable.
SAE 4340M (AMS 6417)
The M in this SAE designation signifies a modification of the standard 4340 steel. In this case, the modification raises the silicon content to 1.6 to 2.0%, increasing toughness and fatigue resistance.
Maraging 250 Steel (AMS 6512)
Maraging steels, like AMS 6512, don’t have a four-digit SAE designation. Maraging 250 steel is an AHHS alloy containing 18.5% nickel, which provides corrosion resistance. It has a tensile strength in the annealed state of 140,000 PSI, but precipitation hardening will raise strength further. Elongation is 17%, and machinability is good.
Advancements in High-Strength Steel Development
Driven by demand from aerospace and, to a lesser extent, automotive, steel companies and researchers are seeking ways to make stronger and tougher alloys. Significant advances in recent years include:
Maraging Steel Compositions
Maraging steel is produced in a vacuum arc furnace, which reduces the level of impurities. Current development work is addressing aspects such as modified compositions (such as removing cobalt) and the introduction of nanoprecipitates and nanosized austenite.
Embrittlement-Resistant Alloys
As hydrogen is stored and used under high pressure, strength is a priority for the steel used. However, hydrogen embrittlement (HE) poses a potential problem. While not currently a major concern in aerospace manufacturing, this may change as companies like Airbus continue researching the use of fuel cells in aviation.
Austenitic alloys with a face-centered cubic (FCC) structure are particularly resistant to HE. Hence, the use of these high-strength alloys in hydrogen applications will likely rise.
Nitriding & Shot Peening
Shot peening induces residual compressive stresses at the surface that improve fatigue resistance. Gas nitriding is a heat treatment process where nitrogen atoms diffuse into the steel to form hardness-raising nitrides. Current research indicates that combining these treatments can improve the mechanical properties of steels like 4140, thereby extending their use into a wider set of aerospace applications.
High-Strength Stainless Steel
Stainless steel satisfies many aerospace application requirements primarily due to its good corrosion resistance, but usage is held back by high density. Work is underway on microstructure modifications and incorporation of other alloying elements, including aluminum, to produce lighter-weight, high-strength, corrosion-resistant alloys.
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