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Post-Weld Heat Treatment (PWHT) of Steel Structures

What’s It?

Post-weld heat treatment (PWHT) of steel structures is a process that involves reheating the welded joints to a certain temperature and holding them for a specified time. The purpose of PWHT is to improve the mechanical properties and reduce the residual stresses of the weld and the heat-affected zone (HAZ). PWHT can also prevent hydrogen-induced cracking (HIC), which is a type of brittle fracture that can occur in high-strength steels due to the presence of hydrogen, stress, and a susceptible microstructure.

There are different types of PWHT, such as stress relieving, normalizing, tempering, and annealing. The choice of PWHT depends on the material type, thickness, welding procedure, and service conditions of the steel structure. The PWHT parameters, such as temperature, time, heating rate, and cooling rate, are usually specified by codes and standards, such as ASME, AWS, API, etc.

Using induction heating for post-weld heat treatment (PWHT) of steel structures

Induction heating is a method of applying PWHT to steel structures using electromagnetic induction. Induction heating can provide fast, uniform, and controlled heating of the weld and the HAZ, without the need for direct contact or combustion. Induction heating can also reduce energy consumption, environmental impact, and safety hazards compared to other methods of PWHT.

Induction heating PWHT can be performed using different types of equipment, such as coils, blankets, or pads, depending on the shape and size of the structure. The equipment is connected to a power source and a temperature controller, which regulate the heating cycle according to the specified parameters. The heating cycle typically consists of three stages: ramp-up, soak, and ramp-down. The ramp-up stage is the period when the temperature of the structure is increased from ambient to the desired holding temperature. The soak stage is the period when the temperature of the structure is maintained at the holding temperature. The ramp-down stage is the period of time when the temperature of the structure is decreased from the holding temperature to ambient.

Induction heating PWHT can offer several advantages over other methods of PWHT, such as:

  • Faster heating and cooling rates, which can reduce the total time and cost of PWHT.
  • More uniform heating and cooling, which can improve the quality and consistency of the weld and the HAZ.
  • Less distortion and shrinkage, which can preserve the dimensional accuracy and alignment of the structure.
  • Less oxidation and scaling, which can improve the surface finish and corrosion resistance of the structure.
  • Less noise and emissions, which can enhance the working environment and comply with environmental regulations.

Induction heating PWHT can also have some limitations and challenges, such as:

  • The need for specialized equipment and skilled operators can increase the initial investment and training costs.

Induction heating heat treatment of rail welds

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