Preheating and post-weld heat treatment services

Certain welding procedures require preheating or heat treatment after welding, depending on the metal, joint thickness and applicable requirements. Preheating and post-weld heat treatment help achieve the required weld quality and manage the risk of welding-related defects. ITVC Global provides specialist preheating and post-weld heat treatment (PWHT) services.

Service scope

• Heat treatment of welded pipes, structures, vessels and tanks to the applicable ASME, AWS, BS or other project requirements.
• Materials: Carbon steel and alloy steel.
• Heating capability up to 900°C, as stated in the source; the actual treatment temperature is determined by the approved procedure and material.

Preheating before welding

Depending on the welding procedure, preheating can serve the following purposes:

• Reduce moisture and help control hydrogen-related cracking. The required temperature depends on the material, thickness, restraint and welding procedure; it is not selected solely by reference to the boiling point of water.
• Control cooling of the weld and heat-affected zone and reduce the risk of excessive hardening and cracking in susceptible materials.

Preheating can use gas, electrical resistance, induction or furnace methods. The specified heating zone and through-thickness temperature must be achieved in accordance with the procedure.

Temperature sensors, displays and controls allow modern heating equipment to monitor and regulate the process.

Heat treatment after welding

Depending on the material and procedure, heat treatment after welding may be used to:

• Reduce and redistribute residual welding stresses, helping manage distortion and brittle-fracture risks.
• Achieve the required mechanical properties in different regions of the welded joint. The treatment must suit the steel’s previous condition and the specified strength and impact-toughness requirements.
• Refine a coarse-grained structure where a suitable treatment, such as normalising, is specified, for example for certain electroslag-welded steel joints.
• Restore or develop specified properties in suitable heat-treatable alloys, including some aluminium alloys, through the appropriate solution treatment and ageing cycle.

Personnel must understand the material, technical requirements and approved procedure. Heating and cooling rates, holding temperature and time must be controlled; incorrect treatment can seriously impair product quality.

TWI — Heat treatment of welded joints

Photographs and temperature record from the source article