Local heat treatment: a practical solution for reliable welded joints

Local heat treatment is an important process that ensures the high quality and durability of welded joints in industry. By using controlled heating and precise temperature control, local heat treatment effectively relieves residual stresses, improves the metal structure and extends the service life of equipment.

In this article, we will look in detail at what local heat treatment is, what problems it solves, in which industries it is used and why on-site heat treatment is the best solution for many companies.

What is local heat treatment?

Local heat treatment is the heating of a limited area of a metal structure (usually the weld and the adjacent heat-affected zone) to a set temperature, followed by controlled cooling. Its main purpose is relieving residual stresses and improving the service properties of welded joints.

The process includes:

• Heating the metal to temperatures from 250 to 750 °C, depending on the type of heat treatment.
• Holding at the set temperature.
• Slow, controlled cooling to prevent thermal distortion and cracking.

Main types of local heat treatment

Depending on the purpose, several types of local heat treatment are distinguished:

1. Post-weld heat treatment (PWHT)

Carried out after welding to relieve residual stresses and improve the structure of the weld and the heat-affected zone.

• Used in power generation and in the oil, gas and chemical industries.
• Reduces the brittleness of the heat-affected zone and the risk of cracking, increasing the reliability of welded joints.

2. Tempering of welded joints

Carried out to temper the hardened structures that form in the weld and heat-affected zone of hardenable steels during welding, and to reduce internal stresses.

• The tempering temperature depends on the steel grade and is usually 550–750 °C.
• Often used after hardfacing to reduce the hardness and brittleness of the deposited metal and the heat-affected zone.

3. Heat treatment of hardfacing deposits

Helps to eliminate unfavourable structures and prevent cracking in the hardfacing zone.

• Important when repairing worn parts.
• Makes the structure and properties of the material more uniform.

4. Heat treatment of pipelines

Often used during installation or repair, especially when welding thick-walled and large-diameter pipes.

• Carried out at the installation site using mobile equipment.
• Avoids dismantling and transportation.

Advantages of on-site local heating

Industrial companies are increasingly choosing on-site heat treatment of welded joints. This is due to a number of significant advantages:

Time and cost savings

• No need to dismantle equipment.
• Treatment is carried out directly at the installation site.

Flexibility and mobility

• Mobile units and electric heaters are used.
• Work is possible in almost any weather and at sites of any complexity, provided the heating zone is protected from wind and moisture.

Controlled heating

• The temperature profile is set precisely and monitored with electronic recorders.
• Heating data can be recorded for the heat treatment report.

Safety and reliability

• Unlike gas-flame heating, electric heating of metal does not contaminate the surface with combustion products.
• Thermocouples and automatic control systems are used.

How is local heat treatment carried out?

The process includes several key stages:

1. Equipment preparation

• Installing electric heaters on the weld zone and covering it with thermal insulation.
• Connecting thermocouples and temperature controllers.

2. Heating the metal

• Gradually raising the temperature to the set level (for example, 600 °C).
• Holding at this temperature for a set time (usually 1–4 hours, depending on the wall thickness).

3. Cooling

• Gradually lowering the temperature at a controlled rate.

Example: during heat treatment of a welded Ø 500 mm pipe, the weld zone is heated at a controlled rate to 620 °C, held for 2 hours and cooled at no more than 50 °C/h to 300 °C, after which it is left to cool under the thermal insulation.

Where is local heat treatment used?

Local heat treatment is used in various industries:

• Power generation – heat treatment of pipelines and boiler equipment.
• Oil and gas industry – post-weld heat treatment at production and processing facilities.
• Chemical industry – tempering of welded joints in reactors and vessels.
• Mechanical engineering – local heating of parts after hardfacing and repair.

Why is local heat treatment of welds important?

Skipping heat treatment of welds where it is required can lead to serious consequences:

• Increased brittleness of the metal.
• Formation of microcracks.
• Premature equipment failure.
• Losses due to accidents and downtime.

That is why timely heat treatment at the installation site is not an optional extra but a necessity for ensuring reliability.

Equipment for mobile heat treatment

The following is used for local heating:

• Flexible electric heaters (ceramic pad heaters, heating cables).
• Temperature control units.
• Temperature monitoring and recording systems.

Most modern units are easy to transport and can be set up on site within 1–2 hours.

Local heat treatment is an effective way to improve the reliability of welded joints, ensure safety and extend the service life of equipment. Modern technologies such as electric heating of metal and controlled heating make it possible to carry out local heat treatment directly on site, quickly and to a high standard.

If you are looking for a solution for stress relief after welding, heat treatment of pipelines, tempering of hardfacing deposits or other heat treatment tasks, choose professionals who will provide mobile heat treatment in compliance with all standards and requirements.