aerospace heat treating: processes, alloys, and certification requirements

why precision matters in aerospace heat treating

The pursuit of faster, lighter, more efficient aircraft has spurred the development of countless complex alloys. And for those alloys to perform as designed, equally complex heat treatments have emerged.

The development of these parts and the advanced heat treatments necessary for them to achieve superior mechanical properties have made precision critical at every point in the supply chain.

what are common alloys used in aerospace components?

Aircrafts feature a wide variety of complex materials. Manufacturers must balance important design and material selection concerns to produce long-lasting parts at reasonable costs.

aluminum alloys

Aluminum alloys have been crucial to aerospace material and design advances for decades. As production techniques have matured, aluminum alloys like 6061 helped make aircraft lighter and sturdier.

Today, modern aluminum alloys remain integral in some aerospace applications due to their high strength-to-weight ratio.

Common types include:

• 5xxx alloys featuring silicon and manganese that are relatively workable and feature high hardness and high tensile strength despite low density.
• 2xxx alloys commonly used for structural service and as fasteners and rivets.
• 7xxx alloys that also perform well in structural applications and which boast high toughness and resistance to stress corrosion cracking.

titanium alloys

Titanium and titanium alloys are commonly used in critical aircraft applications where resistance to constant stress is necessary.

Ti6Al4V, a titanium formula containing aluminum and vanadium as alloying elements, is the most popular. 

It’s known for its performance under constant severe motion in the cold sections of jet engines. Ti6Al4V is often specified for discs, ducts, turbine blades and bearing housings. Slight tweaks to the formula result in excellent performance in cryogenic temperatures. It’s also sometimes used to make fasteners.

Titanium performs well in many aerospace settings, but it’s not as widely used as many in the industry would like. That’s because the element is hard to mine and process. It may become more widely used as mining and processing techniques improve.

Common aerospace nickel alloys include:

• WASPALOY and its variants, which are very strong at high temperatures.
• Inconel 718 and Inconel 625, each of which boasts high strength in hot environments; 718 is stronger than 625 but 625 is more corrosion-resistant.
• Alloy 36, which retains dimensional properties in high temperatures and is often specified for complex aircraft instrumentation.
• Alloy 230, which resists corrosion under high stress and is suited for cold section service.
• PWA 1484, a formula which is very resistant to high temperature creep. Alternatives include the René alloys N5 and N636 among others.

cobalt alloys

Cobalt alloys such as L-605 exhibit good oxidation resistance and high strength at elevated temperatures. They’re commonly used in turbine rotors, nozzle diaphragm valves, springs, and other components.

stainless steel alloys

Stainless steel alloys are among the most versatile in modern industry. From automotive to power generation to pharmaceutical applications and beyond, stainless steel alloys are widely specified for their excellent corrosion resistance.

The following stainless steel formulas are widely used in a broad range of aerospace settings:

• 304, used in fuel tanks and some structural components not subject to very hot temperatures.
• 309, a heat-resistant formula containing extra chromium and nickel.
• 316, 316L, 321, S30415 and S30815, all of which are more heat-resistant and suitable for use in exhaust systems, engines and some high-temperature structural service.
• 17-4Ph, a formula containing copper and niobium alloys that is comparable to 304 stainless but chosen when extra corrosion resistance is desired.

Many aerospace components sustain constant strain and motion and serve in extremely hot or cold environments. For parts like these, complex and precise heat treatments are necessary.

Vacuum furnaces are ideal for hardening and strengthening aerospace components for a couple of reasons: 

• Parts treated in vacuum furnaces cannot oxidize or become otherwise contaminated when atmosphere is removed from the chamber. 
• These furnaces are built to include process controls that allow metallurgists to design the complicated treatment cycles that fulfill the industry’s strict specs.

Some projects come with specs that dictate the exact treatment parameters we must follow. Other orders specify treatments based on the properties parts must exhibit, and it’s up to us to develop the treatment that makes it happen. Either way, we rely on our experienced team of metallurgists and the industry’s most advanced equipment.

Vacuum furnaces operating in AST-Paulo’s Cleveland Division routinely execute complex day-long treatments that top 2,400 degrees Fahrenheit. 

There’s more to thermal processing for the aerospace industry than an in-depth understanding of materials and metallurgy. Earning and maintaining key certifications is equally critical. Read on to learn about the industry’s most rigorous certification and the benefits of working with providers who’ve earned it.

what are the requirements for nadcap-approved heat treaters?

Nadcap requirements are quite prescriptive. Other industry standards programs give suppliers varying amounts of leeway en route to achieving the benchmarks they set. Nadcap features no such liberty.

Becoming a Nadcap-approved heat treater requires proving you have the equipment and processes in place to treat safety-critical aerospace and defense components. Keeping the certification is all about maintaining the mountain of documents showing you’re continually able to meet the strict requirements.

what are first article inspections and AS9102 requirements in heat treating?

One of the industry’s most important quality assurance principles is first article inspection (FAI)—the systematic inspection of new parts to ensure they’ll perform as designed.

Aerospace, automotive, medical, and other industries all have FAI requirements. In general, these inspections entail thorough investigation of parts pulled from the first-ever batch to verify that:

• The exact material specified for a given part was used.
• Manufacturing or finishing methods detailed in the specs were followed.
• Part characteristics that result from manufacturing or processing are within tolerances stated in the specs.

In the aerospace industry, SAE AS9102 standardizes the documentation that industry suppliers must provide to meet OEMs’ FAI requirements.

FAI reporting for heat treaters

Because heat treatment of aerospace components is so critical to their end use, heat treatment documentation for FAI parts is quite extensive. Every aspect of heat treatment is documented, including:

• The inspection of parts before they’re even processed to verify they meet initial tolerances stated in specs.
• Heat treatment parameters including type of treatment, how long the treatment lasts, how hot the furnace gets (and even how quickly or slowly that temperature is reached), and the way the part is quenched.
• The verification of post-treatment variables like hardness, tensile strength, yield strength, and critical dimension characteristics against tolerances stated in specs.

That last point is extremely important. Heat treatment almost always distorts parts—it’s the price that comes with enhancing mechanical properties. First article inspections help heat treaters and customers determine whether specified processes will result in acceptable amounts of distortion or if design, material, manufacturing and processing specs need to change.

reduce risk with the right nadcap-certified heat treater

When it comes to managing quality for aerospace components, every detail must be documented and scrutinized. Nothing can be left unplanned. 

At AST-Paulo, our cross-functional team applies its heat treatment expertise to the information we gather about part design, material and manufacturing. This intensive review allows us to flag potential problems earlier in the process and, when possible, recommend upstream tweaks that will result in parts that pass inspection. 

Our advanced process tracking system ensures nothing slips through the cracks by recording process information in real time and alerting operators if furnace conditions such as pyrometry slip outside of the tolerances defined in treatment specs. 

Together, with a team of expert metallurgists and advanced equipment, we provide heat treatments that are successful and highly repeatable. 

Ready to make your heat treating more precise and repeatable? Partner with AST-Paulo on your next project.