Weld Heat Treatment - pwht solutions

Weld heat treatment (PWHT) can enhance weld integrity by melting and fusing metal layers near it, decreasing corrosion or stress accumulation risk in future welds and increasing structural strength.

Heating can also relieve residual stresses by slowly and uniformly warming the steel type, with times and temperatures dependent on its composition.

Forvarmning

Preheating occurs when steel or metal pieces that will be welded are heated prior to being joined by welding processes, in order to decrease their cooling rate, producing more ductile structures with increased crack resistance and decreased crack rates. Preheating also drives out hydrogen which may cause weld-gas embrittlement as well as reduce shrinkage stresses in highly restrained joints.

Preheating can be accomplished using various forms of heating equipment, including gas torches, oxy-gas flames, induction heaters and electric blankets. No matter which method is employed to apply preheat, its temperature must be uniform across all thicknesses as any inconsistency could lead to increased residual stresses, distortion or unwanted metallurgical changes.

Most welding codes stipulate a minimum preheat temperature for any given base metal, welding process and section thickness. When no such minimum threshold exists, however, the welding engineer must decide whether preheat is required and what would be an appropriate setting in any given situation.

Post Heating

Post heating during weld heat treatment is intended to minimize the risk of hydrogen induced cold cracking, typically by increasing and maintaining preheat temperature for some time after welding. This method helps diffuse any hydrogen that has accumulated between parent metal and weld metal and restore it back to an austenitic state, thus eliminating potential cracking risks.

Stress relief heat treatment is used to alleviate and redistribute residual stresses induced during welding by decreasing yield strength of weld metal and increasing melting point of parent material.

Local post heating can typically be achieved with ceramic mat heaters – typically comprised of resistive wires woven through ceramic beads controlled by an electronic power console – that are applied post weld. They’re ideal for large components like circumferential welds on pipes or closure welds on long pressure vessels.

Afhjælpning af stress

PWHT (Post Weld Heat Treatment) is an after welding heat treatment process used to alleviate internal stresses introduced during welding. This is achieved by immersing steel parts in temperatures well above their melting points for an amount of time calculated on an hour per inch of thickness.

Essentially, this causes the material to relax its residual stresses through plastic deformation, leading to reduced peak values of stress and redistribution around notches and stress concentrators.

Certain alloys require this step as part of the weldment process to protect their integrity. Without relieving high levels of stress, distortion may occur during machine work and carbon and low alloy steels could even fracture. As such, many American Welding Society filler metals require classification testing in the PWHT, or stress relieved condition before classification testing can take place.

Ageing

Heat treatment at low temperature immediately following welding can help diffuse hydrogen out of the weld and heat affected zone more rapidly, decreasing the risk of hydrogen-induced cold cracking – especially for highly crack sensitive steels and thick joints.

Heat Treatment after Welding is an essential step to ensure long-term reliability of welded components. It relieves residual stresses while aligning weld properties to those of their base metal substrate – increasing ductility, strength, fatigue resistance corrosion resistance formability tensile integrity.

PWHT is essential when welding under demanding conditions, such as when using filler metals of different forms. Hobart Brothers provides expert metallurgical advice on selecting filler metals tailored specifically to your application, as well as guidance on the most appropriate PWHT process and heating/cooling rates to avoid thermal shock, distortion or uneven stress relief. For further information about weld heat treatment download our Job Knowledge series “PWHT for Welded Construction.”