Heat treatment is an important operation in welding. It affects the strength, toughness, corrosion resistance, and residual stress of a welded joint. Although it can be time-consuming and costly, it is often mandatory under many welding codes and standards and is an essential variable in welding procedure qualification.

This guide explains the main types of heat treatments used for welded joints and their purposes, with emphasis on post weld heat treatment (PWHT) for carbon and low alloy steels. Fig. 1 illustrates the common effect of heat treatment on ferrous alloys. Increase in temperature reduces the 0.2% proof stress for low-alloy steel, austenitic stainless steel, and carbon steel. 

Understanding Heat Treatment Terms

Before diving into specific processes, it’s important to understand the basic terms used in welding heat treatment.

Solution Treatment

• Performed at a high temperature to dissolve certain elements and compounds into a solid solution.
• The metal is then rapidly cooled (quenched) to retain these elements in solution.
• Used to:
  • Improve corrosion resistance, or
  • Reduce or adjust the strength of a welded joint.
• In some alloys, it is followed by a lower-temperature heat treatment (called ageing or precipitation hardening) to reform precipitates in a controlled manner and restore strength.

Annealing

• Involves heating the metal to a high temperature where recrystallisation and/or phase transformation occur.
• The metal is then slowly cooled, usually inside the furnace.
• Purpose:
  • To soften the metal after it has been hardened (e.g., by cold working).
  • To reduce internal stresses and improve machinability.
• A full anneal produces the softest microstructure possible.
• Effects:
  • Reduces yield strength and tensile strength.
  • In ferritic steels, it may also reduce toughness.

Normalising

• Applied only to ferritic steels.
• The steel is heated 30–50°C above the upper transformation temperature
  • For example, about 910°C for 0.20% carbon steel.
• After heating, the steel is cooled in still air (not in the furnace).
• Purpose and effects:
  • Refines grain size.
  • Improves strength and toughness.
  • Helps produce a uniform microstructure.

Quenching

• Involves rapid cooling from a high temperature.
• For ferritic steels, the material is:
  • Heated above the upper transformation temperature, then
  • Quenched in water, oil, or by air blast.
• Purpose and effects:
  • Produces a fine-grained martensitic structure.
  • Results in very high strength and hardness.
• Important note:
  • Steels are never used in the quenched condition alone.
  • They are always tempered after quenching to improve ductility and toughness.

Tempering

• A low-temperature heat treatment applied to ferritic steels.
• Performed below the lower transformation temperature:
  • Typically 600–650°C for structural carbon steels.
• Purpose and effects:
  • Reduces hardness.
  • Lowers tensile strength slightly.
  • Improves ductility and toughness.
• Application notes:
  • Most normalised steels are tempered before welding.
  • All quenched steels are used in the quenched and tempered condition.

Ageing or Precipitation Hardening

• A low-temperature heat treatment used to create fine precipitates within the metal.
• Purpose:
  • To achieve the correct size and distribution of precipitates.
  • To increase yield strength and tensile strength.
• Usually performed after solution heat treatment.
• Typical temperature ranges:
  • Steels: 450–740°C
  • Aluminium alloys: 100–200°C
• Effects of time and temperature:
  • Longer times or higher temperatures → larger precipitates.
  • Larger precipitates lead to lower hardness and strength, but higher ductility.

Stress Relief

• A heat treatment used to reduce residual stresses caused by weld shrinkage.
• Principle:
  • As the temperature increases, the yield strength of the metal decreases.
  • This allows residual stresses to redistribute through creep in both the weld and parent metal.
• Purpose:
  • To minimise distortion, cracking, and stress-related failures in service.
• Cooling:
  • Carefully controlled cooling is required to prevent harmful thermal gradients or new stresses from forming.

Post Heat

• A low-temperature heat treatment applied immediately after welding.
• Procedure:
  • The preheat temperature is increased by about 100°C.
  • This temperature is maintained for 3 to 4 hours.
• Purpose:
  • Promotes hydrogen diffusion out of the weld and heat-affected zone (HAZ).
  • Reduces the risk of hydrogen-induced cold cracking.
• Application:
  • Used only on ferritic steels.
  • Especially important for:
    • Crack-sensitive steels.
    • Very thick weld joints.

Post Weld Heat Treatment (PWHT)

• The term “Post Weld Heat Treatment” (PWHT) refers to a specific heat treatment applied after welding.
• While some engineers use it broadly for any post-weld heating, codes and standards such as BS PD 5500, EN 13445, and ASME VIII define it precisely.
• PWHT is distinct from other heat treatments like annealing, tempering, or stress relief and must be applied according to the relevant code requirements.

Purpose of PWHT

PWHT may be required for one or more of the following reasons:

• To achieve dimensional stability, maintaining tight tolerances during machining or service conditions.
• To develop specific metallurgical structures that ensure the desired mechanical properties.
• To reduce residual stresses, thereby minimising the risk of:
  • Stress corrosion cracking, or
  • Brittle fracture during service

Main Mechanisms of PWHT

1. Stress Relief – Reduces high residual stresses caused by welding.
2. Microstructural Modification (Tempering) – Refines hardness and improves ductility and toughness.

Scope

• PWHT requirements vary based on material type, thickness, and application standards.
• Most emphasis is placed on carbon and low alloy steels, though other ferrous alloys may also require post-weld heat treatment under specific conditions.

Stress Relief

Why Stress Relief is Necessary

• Stress relief is often required after welding to reduce high residual stresses trapped in the joint.
• Although it is an expensive process—since it requires heating large components to high temperatures—it is essential for ensuring long-term integrity.
• Without stress relief, residual stresses can cause:
  • Distortion or deformation during machining or in service.
  • Brittle fracture in carbon and low alloy steels by driving crack propagation.
  • Stress corrosion cracking, especially in:
    • Carbon and low alloy steels exposed to caustic environments.
    • Stainless steels exposed to chloride environments.

What Causes Residual Stress

• During welding, molten metal is deposited between cold parent metal faces.
• As the weld cools, it contracts but is restrained by the surrounding material.
• This restraint leads to the buildup of tensile residual stresses in the joint.
• As the temperature drops further:
  • The yield strength increases, preventing further plastic deformation.
  • At room temperature, the residual stress can be nearly equal to the proof strength of the material.

How Stress Relief Works

• The welded component is reheated to a controlled temperature high enough to reduce yield strength.
• As yield strength decreases, creep deformation occurs, allowing residual stresses to redistribute and relax.
• The component is held (soaked) at this temperature for a defined time to achieve a stable stress condition.
• It is then cooled slowly to room temperature under controlled conditions.
• The remaining residual stress corresponds approximately to the proof strength at the soak temperature.

Typical Stress Relief Temperatures

Material Type	Soak Temperature Range (°C)
Carbon–Manganese Steels	550–620
Low Alloy (Creep-Resistant) Steels	650–750
Stainless Steels	800–850

Note:

• Residual stress in carbon–manganese steels begins to fall steadily up to around 600°C.
• Creep-resistant steels require temperatures above 400°C before stress starts to reduce.
• Stainless steels show significant stress relief only above 500°C.

Heat Treatment of Welded Joints Part 1 FAQs

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What is heat treatment in welding and why is it important?

Heat treatment in welding is a controlled heating and cooling process applied to welded joints to modify their mechanical and metallurgical properties. It affects strength, toughness, corrosion resistance, and residual stresses. It is often required by welding codes and is essential for long term service performance.

What are the main types of heat treatments used for welded joints?

• Solution treatment
• Annealing
• Normalising
• Quenching
• Tempering
• Ageing also called precipitation hardening
• Stress relief
• Post heat
• Post Weld Heat Treatment PWHT

Each process serves a specific purpose such as improving ductility, relieving stress, refining microstructure, or restoring mechanical balance.

What is solution treatment and when is it applied?

Solution treatment heats a metal so that alloying elements dissolve into a solid solution followed by rapid cooling. It improves corrosion resistance and can adjust joint strength. It is common for stainless steels and some non ferrous alloys. In some alloys it is followed by ageing to restore strength.

What is the purpose of annealing in welded joints?

Annealing softens metals, reduces internal stresses, and improves machinability. The metal is heated to a high temperature where recrystallisation or phase transformation occurs and then cooled slowly in the furnace. A full anneal gives the softest and most ductile condition and reduces yield and tensile strength. In ferritic steels it may also reduce toughness.

How does normalising differ from annealing

Normalising applies only to ferritic steels. The steel is heated about 30 to 50 ^oC above the upper transformation temperature and then cooled in still air. It refines grain size, improves strength and toughness, and helps produce a uniform microstructure.

What happens during quenching and tempering?

Quenching uses rapid cooling from high temperature to form a hard strong martensitic structure. Tempering reheats quenched steel typically to 600 to 650 ^oC to improve ductility and toughness while reducing brittleness. Quenched steels are never used without tempering.

What is ageing or precipitation hardening?

This low temperature treatment forms fine precipitates to raise yield and tensile strength and usually follows solution treatment. Typical temperatures are 400 to 740 ^oC for steels and 100 to 200 ^oC for aluminium alloys. Longer times or higher temperatures give larger precipitates which lower hardness and strength but raise ductility.

What is stress relief and why is it necessary after welding?

Stress relief reduces residual stresses caused by weld shrinkage during cooling. It helps prevent distortion, brittle fracture, and stress corrosion cracking especially in carbon and low alloy steels in caustic service and stainless steels in chloride service. What causes residual stresses in welded joints

Molten weld metal contracts as it cools but is restrained by the colder base metal. This restraint creates tensile stresses. As temperature drops the yield strength rises and prevents further plastic deformation so residual stress at room temperature can be close to the proof strength of the material.

How does stress relief heat treatment work?

The joint is reheated to a controlled temperature high enough to reduce yield strength. Creep allows the residual stresses to redistribute and relax. The component is soaked at temperature then cooled slowly to room temperature. The remaining residual stress corresponds roughly to the proof strength at the soak temperature.

Residual stress in carbon manganese steels falls steadily up to about 600 ^oC. Creep resistant steels need temperatures above 400 ^oC before stress starts to reduce. Stainless steels show significant stress relief only above 500 ^oC.

What is post heat in welding?

Post heat is a low temperature treatment applied immediately after welding. The preheat temperature is raised by about 100 ^oC and held for 3 to 4 hours. It promotes hydrogen diffusion from the weld and heat affected zone and reduces the risk of hydrogen cracking. It is used only on ferritic steels and is most important for crack sensitive steels and very thick joints.

What does Post Weld Heat Treatment PWHT mean?

PWHT is a controlled heating and cooling cycle applied after welding to relieve residual stresses and modify microstructure. Codes such as ASME section eight, BS PD five thousand five hundred, and EN thirteen four four five define it precisely and treat it as distinct from annealing tempering or simple stress relief. Why do welding codes and standards require PWHT

• To achieve dimensional stability for tight tolerances in machining and service
• To develop specific metallurgical structures for the desired properties
• To reduce residual stresses and lower the risk of brittle fracture or stress corrosion cracking

What are the main mechanisms of PWHT?

1. Stress relief which reduces high residual stresses from welding
2. Microstructural modification also called tempering which improves ductility and toughness by refining hardness

How do material type and thickness affect PWHT needs?

PWHT parameters depend on composition thickness and the applicable standard. In general thicker and higher strength materials need tighter control and often higher temperatures or longer soak times within the limits of the standard. What happens if PWHT is omitted or done incorrectly

Possible outcomes include high residual stresses, cracking during or after service, reduced toughness, and increased risk of stress corrosion cracking or brittle failure.

Is PWHT applicable to all metals?

No. PWHT is mainly required for carbon and low alloy steels. Stainless steels and other alloys may need it only under specific service or fabrication conditions.

How long should a component be held at PWHT temperature?

Soak time usually scales with section thickness and follows the relevant standard. A common rule of thumb is one hour per twenty five millimetres of thickness subject to minimum and maximum limits in the code.

What is the difference between preheat, post heat and PWHT?

• Preheat is applied before welding to slow cooling and lower hydrogen cracking risk.
• Post heat is a short term low temperature hold after welding to promote hydrogen diffusion.
• PWHT is a controlled cycle at higher temperature to relieve stress and refine microstructure.