Precipitation Hardening: Process, Benefits, and Comparison with BORINOX®

How Precipitation Hardening Works: Step-by-Step Process

The precipitation hardening process is a multi-step heat treatment method that strengthens metals by forming fine precipitates within the material’s structure. These precipitates hinder the movement of dislocations, making the metal stronger and more resistant to deformation. The process involves three main steps: solution treatment, quenching, and aging.

Solution Treatment

The first step in the precipitation hardening process is solution treatment. This involves heating the metal to elevated temperatures to dissolve alloying elements, forming a homogeneous solid solution.

• The metal is heated just below its melting point, ensuring that alloying elements such as aluminum or titanium are fully dissolved into the base material.
• This step eliminates inhomogeneities and prepares the material for the next phase.
• If the temperature is too low, the treatment will be ineffective, while excessively high temperatures can lead to melting or grain coarsening.

Quenching

Once the alloying elements are dissolved, the metal is rapidly cooled in a process known as quenching. This step prevents premature precipitation of strengthening particles and locks the microstructure in a metastable state.

• Common quenching mediums include water, oil, and gas, depending on the metal type and desired properties.
• The rapid cooling can introduce internal stresses and distortion, requiring additional post-treatment to ensure dimensional stability.

Aging (Precipitation Hardening Phase)

The final step in the precipitation hardening process is aging, where the metal is reheated to an intermediate temperature. This step allows the controlled formation of precipitates within the material’s structure, strengthening it by obstructing the movement of dislocations and enhancing tensile strength. The precise aging temperature and duration depend on the specific alloy composition and desired mechanical properties.

• Aluminum alloys: Aging is typically performed at 250–400°F (120–205°C) to achieve the optimal balance between strength and ductility.
• Stainless steel alloys: Aging is carried out at 900–1150°F (480–620°C) depending on the required hardness and toughness.
• Magnesium alloys: Aging occurs at 320–480°F (160–250°C) to improve strength while maintaining lightweight characteristics.
• Nickel-based superalloys: Aging is done at 1200–1400°F (650–760°C) to enhance creep resistance and high-temperature strength.

The exact conditions must be carefully controlled to optimize tensile strength, toughness, and corrosion resistance while minimizing distortion or undesirable microstructural changes.

Benefits of Precipitation Hardening

The precipitation hardening process is widely used because it enhances the mechanical properties of metals while maintaining relatively low processing temperatures compared to other strengthening methods.

• Increased tensile strength – The process significantly improves yield strength, making metals more resistant to deformation under load.
• Corrosion resistance – Many stainless steel alloys and aluminum alloys retain their corrosion resistance after precipitation hardening, depending on the specific alloy composition.
• Good dimensional stability – Unlike surface-coating or nitriding processes, precipitation hardening strengthens the material internally, ensuring dimensional accuracy.
• Versatility across metals – Precipitation hardening can be applied to stainless steel alloys, aluminum, and magnesium alloys, offering flexibility in various industries.
• Optimized for lightweight applications – Aerospace and automotive industries benefit from the enhanced strength-to-weight ratio provided by this process.

Despite these advantages, precipitation hardening has some limitations, especially for applications requiring extreme surface hardness and superior wear resistance.

Suitable Materials for Precipitation Hardening

The precipitation hardening process is primarily used for metals that can form a solid solution with alloying elements and develop fine precipitates during aging. The most common materials suitable for this process include stainless steels, aluminum alloys, and magnesium alloys.

• Stainless steel alloys – Several precipitation-hardenable stainless steels, such as AISI 630 (17-4 PH), AISI 631 (17-7 PH), and AISI 632 (15-7 PH), respond well to the process. These alloys are widely used in aerospace, medical, and marine applications due to their combination of high strength, corrosion resistance, and good dimensional stability.
• Aluminum alloys – Heat-treatable aluminum alloys, such as those in the AISI 2024, AISI 6061, and AISI 7075 series, are ideal candidates for precipitation hardening. These alloys are known for their excellent tensile strength-to-weight ratio, making them extensively used in aerospace, transportation, and structural applications.
• Magnesium alloys – Certain magnesium alloys, such as AISI AZ91 and AISI WE43, can be precipitation hardened to enhance their mechanical properties. These alloys offer a unique combination of lightweight characteristics and improved strength, making them suitable for high-performance aerospace and automotive components.

Other materials, such as nickel-based superalloys (e.g., AISI N07718 – Inconel 718) and certain copper alloys (e.g., AISI C17200 – beryllium copper), can also be precipitation hardened to improve their mechanical properties. The effectiveness of the process depends on the material’s ability to form a uniform solid solution during solution treatment and develop finely dispersed precipitates during aging.

Comparison: Precipitation Hardening vs. BORINOX® Hardening

While precipitation hardening is effective in improving tensile strength, it does not always provide sufficient surface hardness and wear resistance, which are crucial for industrial components exposed to friction, abrasion, and corrosion. The BORINOX® process offers an alternative by providing a low-temperature diffusion treatment that hardens stainless steel surfaces while maintaining corrosion resistance.

Key Differences Between Precipitation Hardening and BORINOX® Hardening

BORINOX® is particularly advantageous for stainless steel components that require both hardness and corrosion resistance without the risk of distortion.

Conclusion: Selecting the Right Hardening Method

Precipitation hardening is a well-established process for increasing the strength of metals, particularly stainless steel alloys, aluminum, and magnesium alloys. It is ideal for applications requiring high yield strength and good dimensional stability. However, for components that need superior wear resistance, extreme surface hardness, and corrosion protection, alternative methods such as BORINOX® should be considered.

Choosing the right hardening process depends on factors such as wear exposure, operating conditions, and material requirements. Manufacturers should evaluate whether precipitation hardening alone is sufficient or if a more advanced surface hardening technique like BORINOX® is necessary.

For expert guidance on selecting the best hardening process for your application, contact us to discuss your specific requirements.