Precipitation or Age Hardening is a process strengthens metals by forming fine precipitates within the structure.

Basic Concept

• There are several ways to increase the strength of metals, such as:
  • Alloying – adding alloying elements to improve properties.
  • Quenching – rapid cooling to form hard microstructures.
  • Work hardening – strengthening through plastic deformation.
  • Precipitation (Age) Hardening – a controlled heat treatment process.

Precipitation (Age) Hardening is a special type of heat treatment that strengthens metals by forming fine precipitates within the metal structure. Applicable to many ferrous and non-ferrous alloys. The process requires precise control of time and temperature to achieve the best mechanical properties.

Basic Metallurgical Principle

• The process depends on how the solubility of an alloying element changes with temperature.
• The solubility of alloying elements (solute) in the base metal (solvent) increases with temperature.
• As the metal cools, solubility decreases, and the solute forms solid precipitates within the structure.
• These precipitates block dislocation movement, increasing the strength and hardness of the alloy.
• This behaviour is shown by the solvus line on a phase diagram.

Analogy: Salt in Water

• When temperature rises, more salt dissolves in water.
• As the solution cools, salt crystals precipitate out.
• A similar process occurs in metals, except it happens in the solid state, making it much slower because atoms move more slowly in solids than in liquids.

Solution Heat Treatment Stage

• The alloy is heated to a high temperature (above the solvus line on a phase diagram) to dissolve existing precipitates into a solid solution.
• The metal is then rapidly cooled (quenched) to prevent precipitates from reforming.
• The result is a super-cooled, unstable solid solution that is ready for ageing at a lower temperature to form fine, controlled precipitates.

A schematic of such a heat treatment cycle is also given in Fig. 1 for alloy N.

Ageing Process

In the solution heat treated metal the atoms of the alloying element, the solute, are randomly distributed throughout the matrix but once the temperature is raised the precipitates begin to form by a nucleation and growth process. That is, When the metal is reheated to the ageing temperature, the solute atoms begin to cluster and form precipitates through a process known as nucleation and growth.

Guinier–Preston (GP) Zones

• At low ageing temperatures and over a short time, very small and finely dispersed precipitates form, called Guinier–Preston (GP) zones.
• Named after the two metallurgists who first identified them.
• GP zones are:
  • Extremely small, invisible under a normal optical microscope.
  • Can only be seen using electron microscopy (≈ ×100,000 magnification).
  • Coherent with the base metal, meaning they have the same crystal structure as the surrounding matrix.

Effect on Metal Properties

• GP zones distort the crystal lattice of the metal.
• This hinders dislocation movement, which is the primary mechanism of metal deformation.
• The result:
  • Increased tensile strength and hardness.
  • Reduced ductility and toughness.

Further Ageing and Coarsening

• As ageing continues or the temperature increases, the precipitates grow larger but remain coherent for a while.
• Tensile strength continues to increase during this stage.
• Eventually, the precipitates lose coherency and become incoherent particles:
  • These have a different crystal structure from the base metal.
  • They become visible under an optical microscope.
  • This marks the transition to the overageing stage, where strength begins to decrease.

Overageing and Control of Mechanical Properties

Peak Strength and Overageing

• The highest tensile strength occurs just before the precipitates lose coherency with the base metal.
• As incoherent precipitates form and grow larger, the tensile strength gradually decreases.
• When this happens, the alloy is said to be overaged.
• Even in the overaged condition, the precipitates still contribute to overall strength, though less effectively.

Example – HSLA Steels

• High Strength Low Alloy (HSLA) steels are a good example where incoherent, overaged precipitates are intentionally used.
• This controlled overageing provides a balance between strength and ductility, leading to substantial strength improvement without excessive brittleness.

Optimising Ageing Parameters

• To achieve the best combination of properties, the precipitates must be:
  • Evenly distributed within the alloy grains.
  • Of optimum size, not too small (brittle) or too large (soft).
• Temperature and time during ageing can be adjusted to tailor properties:
  • Higher temperatures or longer times → larger precipitates, lower strength, higher ductility.
  • Lower temperatures or shorter times → finer precipitates, higher strength, lower ductility.
• Overaged structures exhibit maximum ductility but minimum tensile strength.

Typical Ageing Conditions

Alloy Type	Ageing Characteristics	Temperature Range (°C)	Notes
17/4PH Stainless Steel	Slow precipitation allows air cooling	480–620	No quenching required
A286 Stainless Steel	Requires long ageing times	700–760	Used in high-temperature applications
Al–Cu Alloy (e.g. 2219)	Can age naturally at room temperature	~25 (ambient)	Hardens over several days
Al–Si–Mg (6000 series) & Al–Zn–Mg (7000 series)	Can age naturally or artificially	100–200	Artificial ageing accelerates hardening
Ferritic & Nickel-based Alloys	Used in the overaged condition for better ductility	700–850	Maintains toughness for service applications

Alloy Type	Heat Treatment	Temperature Range (°C)	Notes
17-4PH Stainless Steel	Artificial ageing	480–620	Air cooling sufficient due to slow kinetics.
A286 Stainless Steel	Artificial ageing	700–760	Requires long ageing times.
Aluminium–Copper Alloy 2219	Natural ageing	Room temperature	Hardens over a few days at ambient temperature.
Aluminium 6000 & 7000 Series (Al-Si-Mg, Al-Zn-Mg)	Natural or artificial ageing	100–200	Can age at room or elevated temperatures.
Ferritic & Nickel-Based Alloys	Artificial ageing	700–850	Typically used in overaged condition for ductility.

Typical ageing heat treatments and properties of a range of age hardening alloys

Natural vs. Artificial Ageing

• Natural ageing: Occurs at room temperature (e.g., Al–Cu alloys like 2219).
• Artificial ageing: Occurs at elevated temperatures in a controlled furnace (faster and more uniform).

Practical Considerations

• Tight control of time, temperature, and cooling rate is crucial for consistent results.
• Aluminium alloys:
  • Often solution-treated in salt baths.
  • Artificial ageing typically done in a forced-air furnace.
• Nickel and ferrous alloys:
  • Require higher ageing temperatures and longer times.
  • Heat treatment performed in gas-fired or electric furnaces with multiple thermocouples to maintain uniform temperature.

Control of Heat Treatment Parameters

• Precise control of time, temperature, and cooling rate is critical to achieve the desired mechanical properties during precipitation (age) hardening.
• Small deviations in these parameters can cause significant variations in tensile strength and overall performance.

Heat Treatment of Aluminium Alloys

• Solution treatment: Commonly performed in a salt bath to ensure rapid and uniform heating.
• Artificial ageing: Conducted in a forced-air circulation furnace for consistent temperature control.
• Even a temperature difference as small as 40°C during ageing can cause a major change in tensile strength, as shown for an Al–4%Cu alloy (e.g., 2025) in Fig. 2.

Fig.2 shows typical trend of the effect of the aging temperature and times on UTS and hardness.

Effect of Temperature on Strength (Example: Al–4%Cu Alloy)

• Even a small temperature difference (≈ 40°C) during ageing can cause a significant change in tensile strength.
• This highlights the importance of precise temperature control during heat treatment.

Mechanism

• Alloy is solution heat treated at a high temperature to dissolve alloying elements.
• Rapid quenching traps them in a supersaturated solution.
• Reheating to a lower temperature allows fine precipitates to form, increasing strength.

Example Alloys:

• Aluminium alloys (2000, 6000, 7000 series)
• Nickel-based alloys
• Precipitation-hardening stainless steels (e.g., 17-4PH)

Heat Treatment of Nickel and Ferrous Alloys

• These alloys require higher ageing temperatures compared to aluminium alloys.
• Typically heat-treated in gas-fired or electrically heated furnaces.
• Multiple thermocouples are used to monitor and maintain uniform temperature throughout the component.

Key Takeaways

• Precipitation hardening improves strength through controlled formation of fine precipitates.
• Accurate time–temperature control is essential for desired mechanical properties.
• Overageing increases ductility but reduces strength.
• Natural and artificial ageing methods depend on alloy type and production requirements.
• Common applications include aerospace alloys, stainless steels, and high-strength aluminium.
• PWHT is essential for safety, dimensional stability, and mechanical integrity.
• Always follow the applicable code requirements (ASME, EN, PD).
• Control temperature, heating rate, soak time, and cooling rate precisely.
• Document thermocouple positions, temperature charts, and procedures for traceability.
• For local PWHT, maintain uniform heating and verify soak temperatures using surface thermocouples.
• Accurate temperature monitoring and strict control of heat treatment conditions are essential to:Ensure uniform mechanical properties. Prevent underageing or overageing. Maintain repeatability and quality across production batches.

Key Standards Referencing PWHT and Heat Treatment in Welding

ASTM Standards

• ASTM A53 / A53M – Specification for Pipe, Steel, Black and Hot Dipped, Zinc Coated, Welded and Seamless.
  Includes weld seam heat treatment requirements.
• ASTM A671 – Electric Fusion Welded Steel Pipe for Atmospheric and Lower Temperature Service.
  Includes notes on post weld heat treatment (PWHT).
• ASTM A488 / A488M – Practice for Steel Castings, Welding Qualification of Procedures and Personnel.
  Addresses qualification when castings will undergo heat treatment after weld repair or fabrication.
• ASTM A991 / A991M – Test Method for Temperature Uniformity Surveys of Furnaces Used for Heat Treating Steel Products.
  Used to verify furnace capability for PWHT operations.

ASME Codes and Sections

• ASME BPVC Section VIII Division 1 – Rules for the Construction of Pressure Vessels.
  Contains PWHT requirements and exemptions by material P Number and thickness (see paragraphs UCS-56 tables).
• ASME Section IX – Welding Procedure and Performance Qualification.
  PWHT is an essential variable for procedure qualification (see QW-407 series).
• ASME B31.3 Process Piping – Provides PWHT tables and exemptions for piping welds (see Tables 331.1.1 through 331.1.3).
• ASME Section I Power Boilers – PWHT requirements for boiler parts (see PW-39 for temperature, time, and exemption guidance).

AWS Structural Welding Codes

• AWS D1.1 Structural Welding Code – Steel
  When PWHT is required, it must be specified in contract documents. Clause 7.8 gives stress relief heat treatment requirements and holding times.
• AWS D1.6 Structural Welding Code – Stainless Steel
  References heat treatment requirements for qualification and procedure control when applicable.
• AWS D1.2 Structural Welding Code – Aluminum
  Post weld heat treatment is performed only when specified and must follow approved procedures.

ISO and EN Standards

• ISO 15614-1:2017 – Specification and Qualification of Welding Procedures for Metallic Materials, Part 1: Arc and Gas Welding of Steels and Arc Welding of Nickel and Nickel Alloys.
  Defines PWHT as an essential variable requiring requalification if changed or omitted.
• ISO 17663:2023 – Welding Quality Requirements for Heat Treatment in Connection with Welding.
  Provides guidance for PWHT planning, execution, thermocouple placement, measurement points, and documentation, including local PWHT for pipework.
• EN 13445-4:2021 + A1:2023 – Unfired Pressure Vessels, Part 4: Fabrication.
  Contains fabrication and heat treatment rules aligned with European practice.

Guidance and Reference Resources

TWI Job Knowledge Series – Heat Treatment of Welded Joints (Part 2)
Provides comparisons of PWHT temperature ranges used in pressure vessel codes and explains metallurgical effects and service considerations.

Practical Notes for Contracts and WPS Preparation

• PWHT as an Essential Variable:
  Under ASME Section IX and ISO 15614-1, any change in the PWHT range or omission of PWHT requires requalification of the Welding Procedure Specification (WPS).
• Determining Applicability:
  For pressure vessels and piping, always refer to the governing code (ASME Section VIII or ASME B31.3) first to determine whether PWHT is mandatory or exempt, then align your WPS and Procedure Qualification Record (PQR) accordingly.
• Structural Applications:
  Under AWS D1.1, PWHT is not automatically required. It is applied only when specified in the engineer’s contract or project documents.
• Verification:
  Always confirm the current edition of each standard and whether it is normative for your project or regulatory region.
• Temperature and Time Tables:
  Some standards, such as ASME Section VIII, include detailed PWHT temperature and soak time tables by material and thickness.

Heat Treatment of Welded Joints Part 3 Frequently Asked Questions

What is precipitation or age hardening in metals?

Precipitation or age hardening is a controlled heat treatment that strengthens metals by forming fine precipitates inside the structure. These particles hinder dislocation movement which raises strength and hardness.

Which alloys can be strengthened by precipitation hardening?

Common families include aluminium 2000 6000 and 7000 series, nickel based alloys, and precipitation hardening stainless steels such as 17 4PH. Some high strength low alloy steels also benefit.

What are the main stages of the precipitation hardening process?

1. Solution heat treatment to dissolve alloying elements into a solid solution.
2. Quenching to retain a supersaturated solid solution.
3. Ageing to form fine strength enhancing precipitates.

How does precipitation hardening increase metal strength?

Fine uniformly dispersed precipitates block dislocation motion within the crystal lattice. This makes plastic deformation harder which increases tensile strength and hardness.

What are Guinier Preston GP zones?

GP zones are very fine coherent clusters that form at early ageing. They are too small for an optical microscope and are visible only with electron microscopy. Coherency with the matrix gives strong lattice strain and high strengthening.

What is the difference between natural ageing and artificial ageing?

• Natural ageing occurs at room temperature for example aluminium copper alloys such as 2219.
• Artificial ageing is done in a furnace at elevated temperature for faster and more uniform results.

What is overageing and why does strength drop?

As ageing continues precipitates grow and lose coherency. Once incoherent they strengthen less effectively so tensile strength falls while ductility rises. This is called overageing.

How do ageing temperature and time affect properties?

• Higher temperature or longer time produces larger precipitates which lower strength and raise ductility.
• Lower temperature or shorter time produces finer precipitates which raise strength but lower ductility.

Why is tight temperature control during ageing important?

Even a change of about 40°C during ageing can cause a large change in tensile strength for some alloys. Accurate control is essential for consistent results.

What are typical ageing temperatures for common alloys?

Alloy type	Temperature range °C	Notes
17 4PH stainless steel	480 to 620	Air cooling is sufficient due to slow precipitation
A286 stainless steel	700 to 760	Requires long ageing time
Aluminium 2219	~25 ambient	Natural ageing at room temperature
Aluminium 6000 and 7000 series	100 to 200	Natural or artificial ageing depending on alloy
Ferritic and nickel based alloys	700 to 850	Often used overaged to maintain ductility

How is precipitation hardening applied to aluminium alloys?

Solution treatment is often done in a salt bath for rapid uniform heating. Artificial ageing then uses a forced air furnace with close temperature control to achieve consistent strength and hardness.

Why are some alloys used in the overaged condition?

Controlled overageing can give a better balance between strength and ductility which improves service performance for families such as HSLA steels and nickel based alloys.

What equipment and controls are recommended for ageing?

Use gas fired or electric furnaces with stable temperature control and multiple thermocouples on the component. Record soak temperature time and cooling to ensure repeatability.

Read more: Heat Treatment of Welded Joints  – part 1 and part 2