Traitement thermique post-soudure

Post Weld Heat Treatment (PWHT) is a controlled process in which material that has been welded is heated above its lower critical transformation temperature for an extended period of time, typically following welding operations, to address residual stresses and microstructural changes caused by welding processes. PWHT may be required due to residual stresses or micro-structural changes caused by welding itself.

Soudage et traitement thermique

Welding is an integral process in construction and manufacturing, yet can introduce stresses and imperfections into welded components. Postweld heat treatment (PWHT) is used to address these concerns and enhance mechanical properties of the weld materials produced during welding processes. PWHT may reduce formation of brittle phases, alleviate welding residual stresses, improve toughness and ductility of both base material and weld zone components while helping mitigate risks such as stress corrosion or hydrogen induced cracking.

PWHT generally involves heating an entire welded structure to below its critical transformation temperature for a predetermined amount of time, to enable weld metal expansion and relieve welding residual stresses. The chosen PWHT method depends on several factors including its composition, end use or service conditions and welding technique.

PWHT can also be used for tempering, precipitation and ageing effects in certain steels, providing extra benefits that extend beyond stress relief such as reduced hardness levels, improved ductility and resistance to thermal fatigue. Although PWHT offers several advantages it should be noted that its application can be costly and labor intensive; furthermore specialized equipment and facilities may be required; accurate temperature recorders need to be calibrated beforehand in order to accurately manage temperature during weldment processes.

Annealing

Annealing is one of the key parts of PWHT that helps enhance metal properties such as ductility, toughness and homogeneity, which aid in fortifying structures such as pipelines, bridges and pressure vessels against forces they’ll face over time.

Annealing can be used on various metals and alloys to change their physical and chemical properties, via heating, holding and slow cooling processes. Annealing requires heating, holding, and then slowly cooling until desired results have been reached.

Accurate temperature regulation is key to the success of any PWHT process, and Libratherm offers several PID ramp soak controllers which provide accurate temperature management of the annealing process and can be configured according to your unique specifications.

Annealing involves heating cold-worked material just above its critical transformation point (CTP) for soaking, which allows stress relief, microstructure refinement, hydrogen removal, decarbonization and reduced work hardening to occur, while at the same time decreasing work hardening from cold working. Once this step has been completed, slowly cooling can begin in order to avoid residual stresses forming; additionally annealing promotes recrystallization by replacing deformed and strained grains with larger and uniform ones, which makes the material more ductile while decreasing work hardening; especially crucial when working steels and alloys.

Normalizing

After welding, parts that aren’t properly normalized or annealed may develop internal stresses that lead to metal failure. PWHT normalizing removes these internal stresses by heating material to temperatures higher than critical temperature line of Austenite region for some time and then cooling it in air – providing an alternative to full annealing when necessary for parts requiring machinability.

Heat treatment of iron-based metals such as Carbon steel, Stainless steel and alloy steel typically includes quenching and tempering processes to achieve maximum performance.

Normalizing steel involves heating it 30-50 degrees Celsius above its Austenite Critical Temperature Line for a certain period, before holding it there for some time and cooling it in air. The process resembles that of annealing except with slightly faster cooling rate and refined crystal grains, producing more ductile structures and increasing toughness (AKV value).

Machinability improvement techniques are typically utilized to increase impact resistance of parts and increase their machinability. Furthermore, low-alloy hot-rolled steel plates, forgings, and castings often utilize these processes to achieve satisfactory strength while decreasing their likelihood to fracture during use.

Stress Relief

Post Weld Heat Treatment (or PWHT) is a thermal process designed to reduce and redistribute residual stresses in welded materials, typically by heating it to an elevated temperature and then cooling back down under controlled conditions before returning it back to ambient.

Stress relief aims to relieve internal stresses which cause cracking and distortion in welded fabrications and castings, while PWHT helps improve mechanical properties that make metals more resistant to fatigue and other factors that lead to in-service failures.

Welding is an essential industrial process, but its inherent stresses may compromise product integrity and performance. To address these concerns, the PWHT process utilizes heated/cooled furnaces to relieve build-up stresses within metal. This ensures that welded components can withstand their intended loads as well as environmental conditions.

PWHT should be carried out in an atmosphere-controlled environment to safeguard weldments against oxidation and hydrogen-induced cracking, which occurs when hydrogen absorbed during welding reacts with oxygen in the air and causes cracking to form. In addition, PWHT may soften components so they can be machined with precision dimensions and surface finishes – something particularly valuable in high pressure applications where fracture can occur quickly.