Precipitation Hardening

Yes, age hardening and precipitation hardening are the same process—just different names used interchangeably in the industry. You may hear customers, engineers, or specifications reference either term, but they’re describing the identical heat treatment operation.

The term “precipitation hardening” refers to the fine precipitate particles that form during the process, while “age hardening” emphasizes the time and temperature used to develop these strengthening phases. Both terms accurately describe how the process works: holding the material at elevated temperatures to allow hardening to occur through precipitation.

At Paulo, we use both terms depending on customer preference and specification language, but the process and results remain exactly the same.

No, precipitation hardening only works for specific materials that have the right alloying elements to form strengthening precipitates. The material must be specifically designed with elements that can dissolve at high temperatures and then precipitate out at lower temperatures to create the hardening effect.

Non-ferrous materials that are commonly precipitation hardened:

Aluminum Alloys:

• Aluminum 2024, copper-based alloy for aerospace applications
• Aluminum 6061, magnesium-silicon alloy for structural applications
• Aluminum 7075, zinc-based alloys for high-strength applications

Superalloys:

• Inconel 718 (aerospace and high-temperature applications)
• Other nickel-based superalloys with precipitation-forming elements

Non-ferrous alloys that CANNOT be precipitation hardened:

Many non-ferrous alloys simply lack the necessary alloying elements to form strengthening precipitates. If an aluminum, copper, or nickel alloy doesn’t contain the necessary precipitation-forming elements (such as copper or magnesium in aluminum alloys, or aluminum and titanium in nickel superalloys), it cannot be strengthened through this process regardless of the heat treatment applied.

Ferrous (iron-based) alloys commonly precipitation hardened:

• 17-4 stainless (most common) – aerospace tooling, engine parts, tool and die applications, firearms
• 15-5 stainless – aerospace applications, dimensional stability requirements
• 17-7 stainless – aerospace applications
• 13-8Mo stainless – high-strength aerospace and tool applications
• A286 stainless – aerospace and specialty automotive fasteners, high corrosion and heat resistance

These precipitation hardenable stainless steels combine the corrosion resistance of stainless with the ability to achieve high strength through aging, making them extremely valuable for demanding applications where both properties are required.

Precipitation hardening produces very minimal distortion compared to other heat treatment processes—this is one of its key advantages. The aging temperatures are relatively low (900°F-1,300°F range), which means you don’t get significant thermal distortion. The only dimensional change comes from volume changes as precipitates form in the material’s structure.

Typical shrinkage rates: Approximately 0.0004 to 0.0008 inches per inch of length, depending on the material grade and part length. While this is a very small amount, it’s important to account for during design and machining.

Both air and vacuum processing can achieve excellent results for precipitation hardening—the choice depends on your part’s finish requirements, specifications, and cost considerations.

When to specify vacuum processing:

• When specifications require “bright and shiny” finish or “no scale”
• For finished parts where surface appearance is critical
• When subsequent operations would be complicated by oxidation
• When specifications explicitly call out vacuum processing

When air processing works well:

• For bars, plates, or material processed before final machining (any oxidation will be removed during subsequent operations)
• When surface appearance isn’t critical to function
• For cost-sensitive applications where surface finishing isn’t specified

Important note on distortion: There is no difference in dimensional movement between vacuum and air processing for precipitation hardening. Both methods work at the same temperatures and produce the same minimal distortion—the only difference is surface appearance.

Some customer specifications will explicitly state whether vacuum or air is required. When specifications allow either method, we recommend air processing unless you have specific cosmetic or cleanliness requirements.

Precipitation hardening (also called age hardening) is a heat treatment process that increases the strength, hardness, and high-temperature performance of certain alloys. This process is commonly used for stainless steels like 17-4, 15-5, and 17-7, as well as superalloys such as Inconel 718.

The process works in two main steps:

• Step 1 – Solution Treating: The material is heated to ultra-high temperatures to dissolve strengthening elements (like copper in stainless steels) completely into the crystal structure. The part is then rapidly cooled (quenched) to lock this uniform structure in place. Many customers purchase material already solution treated in what’s called “Condition A,” allowing them to complete most machining before heat treatment.
• Step 2 – Aging (Precipitation Hardening): The material is reheated to a lower temperature—typically in the 900°F to 1,300°F range—and held for an extended period. During this aging step, fine precipitate particles form throughout the material’s structure, significantly increasing its strength and hardness without requiring the severe quenching used in other hardening processes.

At Paulo, we can perform both steps or just the aging step for materials already in Condition A. Our multiple thermocouples ensure uniform temperatures across entire lots, and our one-cycle solution treat and age capability decreases handling time while improving turnaround.