Both preheating and post-weld heat treatment are processes where the application of heat to metals is essential. Although they may seem similar, their objectives and timing are completely different.
Understanding this difference is key to knowing what is necessary in the thermal treatment of materials within industrial environments.
Why are heat treatments applied in welding processes?
Heat treatments help improve the quality, durability, and safety of welded joints. Applying them correctly prevents deformations, cracks, or service failures.
Thermal effects in poorly managed welding
- Accumulated internal stress.
- Brittleness or loss of ductility.
- Cracks in heat-affected zones.
- Premature failures under operation or pressure.
What is preheating and when is it used?
Preheating is the controlled heating of a part before the welding process. Its main objective is to reduce thermal shock when applying the welding arc to a cold metal.
Benefits of preheating in industry
- Prevents a sudden temperature difference between the weld area and the rest of the metal.
- Reduces cooling speed, avoiding hydrogen-induced cracks.
- Improves weld quality, especially in thick materials or sensitive alloys.
When is it necessary?
- In materials with high carbon content or special alloys.
- In high thicknesses.
- In equipment exposed to very demanding operating or extreme conditions.
- When required by regulations or client technical specifications.
- In low ambient temperature environments.
![industrial tank under construction where welding heat treatments are applied]()
What is post-weld heat treatment (PWHT)?
It is a thermal process applied after welding, with the objective of relieving residual stresses generated by the joining process.
What does PWHT achieve?
- Redistributes the internal stresses of the material.
- Increases ductility and fracture resistance.
- Stabilizes the mechanical properties of the assembly.
In what type of structures is it applied?
- Pressure pipes.
- Equipment subjected to constant thermal load.
- Critical structures such as pressure vessels, reactors, or industrial columns.
- Heavy boiler components or with multiple welds.
![pipe preheating process with thermal resistances and control cables]()
Key differences between preheating and PWHT
Comparative table of their characteristics
| Characteristic | Preheating | PWHT (Post Weld Heat Treatment) |
|---|---|---|
| Application timing | Before welding | After welding |
| Main objective | Reduce thermal shock | Relieve residual stresses |
| Influence on welding | Improves penetration and quality | Stabilizes and homogenizes its properties |
| Type of thermal control | Uniform pre-weld temperature | Heating and cooling curves |
| Equipment used | Resistances and thermal blankets | Heating systems and recorders |
What happens if these processes are skipped?
- Increased risk of cracking.
- Reduced equipment lifespan.
- Risk of accidents due to structural failure.
- Rejection in inspections or certifications.
![weld with crack in pipe due to lack of proper heat treatment]()
How is the quality of these treatments ensured?
The key lies in the use of proper equipment and a well-defined thermal plan.
Best practices when applying both treatments
-
- Use type K thermocouples for temperature control.
- Monitor the entire process using multi-channel recorders.
- Apply certified heating and cooling curves.
- Perform subsequent inspections (NDT) if necessary.
As you can see, understanding the difference between preheating and post-weld heat treatment is essential to ensure safe, durable, and standard-compliant joints. Each process serves a specific function, so skipping them can seriously compromise the integrity of a welded structure.