Τι είναι η θερμική επεξεργασία μετά τη συγκόλληση;

Post weld heat treatment is an integral component of pressure equipment quality assurance, often mandated by industry codes such as ASME Section VIII or nuclear power plant safety standards to ensure it will perform to specification.

PWHT helps reduce and redistribute residual stresses in welding regions while increasing ductility, toughness, and strength to levels that meet acceptable service conditions. Furthermore, its effects can improve corrosion resistance as well as stress corrosion cracking susceptibility.

Τι είναι το PWHT;

Post Weld Heat Treatment (PWHT) is a controlled process in which metal that has been welded is heated and then cooled in order to relieve internal stresses that were created during welding. PWHT is often employed on oil and gas pipelines, pressure vessels, petrochemical plants, nuclear power stations and steel fabrication projects to ensure welds will meet operational conditions successfully.

PWHT not only reduces and redistributes residual stresses, but it can also cause metallurgical changes that improve material strength. For instance, when heated above its critical transformation point temperature for steel pieces to undergo precipitation or ageing processes which decrease hardness while increasing its ductility and toughness.

PWHT is often an essential process in industrial applications, yet can be challenging to execute properly. Requiring special equipment and facilities, it often increases overall project costs while the heating/cooling cycle times add delays to project timelines.

Given these difficulties, it is crucial that when making decisions regarding PWHT for your project, consulting an expert in PWHT is advised. Intertek has extensive experience performing PWHT on various types of metals and alloys so your welding projects will remain compliant with industry standards.

Why is PWHT important?

PWHT is essential to many industries that impose stringent codes and standards, such as pipeline, pressure vessel, boiler or nuclear power plants operating in highly corrosive environments such as pipelines. PWHT reduces hydrogen induced cracking or stress corrosion cracking which may occur if residual stresses aren’t properly relieved.

PWHT involves heating the material to a specific temperature before slowly cooling it, in order to redistribute residual stresses caused by welding processes and increase overall strength of material. Furthermore, phase transformations occur and finer, more uniform grain structures form which improve mechanical properties of material.

Timing of heating and cooling cycles is of utmost importance when performing PWHT, as too-rapid heating or cooling may lead to cracks or fractures in the weld area. To avoid this scenario, heat treatment must take place under controlled circumstances using an electric resistance heater tailored specifically for treating pipes of that size – this ensures that weld areas are heated, cooled and soaked accurately, minimizing stress effects as well as thermal gradients on weld area while giving sufficient time for tempering, precipitation and ageing processes – further reducing hardness while increasing ductility of as-welded materials and improving ductility.

How is PWHT performed?

Post weld heat treatment (PWHT) is a controlled process in which weld metal is heated below its lower critical transformation temperature, held there for an extended period of time and held at this temperature until all residual stresses produced during welding have been alleviated. Stress relieving, also known as PWHT, reduces residual stresses produced during welding that combine with load stresses to exceed material design strength, leading to failure. PWHT mitigates residual stresses as well as microstructural changes resulting from PWHT to create more stable welds with improved mechanical properties.

PWHT employs various techniques, but they all involve heating and then cooling a weld area in order to modify its microstructure and mechanical properties. Annealing is one popular PWHT method; it helps alleviate stress by encouraging more even austenization of welds; normalizing also improves toughness and ductility while quenching promotes dislocation movement within crystal structures for harder welds.

PWHT presents many challenges, such as the requirement of specialized equipment and trained personnel, time consuming process and thermal fatigue resulting from repeated cycles at high temperatures which could hamper its durability and lifespan.

What are the advantages of PWHT?

Welding is an integral component of manufacturing and construction, yet can introduce stresses that weaken its structure. To address this issue, postweld heat treatment (PWHT) is often used to strengthen the integrity and performance of welded components; PWHT helps reduce residual stresses while minimising formation of brittle phases as well as improve both heat-affected zone (HAZ) ductility and toughness in both HAZs as well as base materials.

Thermal gradients during welding create internal stresses in weld metal and parent material, which combine with load stresses to exceed design limits of structures. PWHT reduces these stressors by heating weld metal to an exact temperature for a predetermined period. This treatment redistributes these internal strains and relaxes them into more stable states, helping return structures back into service more rapidly.

PWHT welding processes can also help reduce weld brittleness by encouraging phase transformations and grain refinement, as well as improving corrosion resistance by decreasing formation of oxidation scales on weld surfaces.

As a result of these benefits, PWHT is increasingly utilized by industries such as oil and gas, power generation and mining. Some welding codes and standards even mandate PWHT for certain materials like carbon and low-alloy steels with thicknesses up to 0.75 inches (20 mm). PWHT welding processes may also be mandated on pressure vessels, tanks, storage containers bridges or other large structures.