Trattamento termico post-saldatura

Welding can result in high levels of residual stress that, combined with load stresses, exceed the material’s yield strength and lead to further material deformation. To minimize these stress levels and to decrease post weld heat treatment’s negative impacts, post weld heat treatment must be undertaken as part of its post weld heat treatment process.

Industry codes and specifications typically mandate PWHT treatment on specific materials when service conditions meet them; this practice is frequently seen in oil & gas, petrochemical, and nuclear sectors.

Reduction of residual stresses

Residual stresses resulting from welding can significantly weaken equipment, increasing its susceptibility to stress corrosion cracking. The PWHT process reduces these stresses to help avoid such damage; this is done by heating all structures to a lower critical transformation temperature for an extended period of time, effectively heating everything as an integrated unit.

Pwht is not comparable to normalizing or annealing processes used to improve mechanical properties by changing micro-structural characteristics; however, like these heat treatments it helps reduce and redistribute residual stresses created during welding processes.

In general, higher peak temperature and longer soak time leads to greater reduction of residual stresses. It is important to remember, though, that excessively high peak temperatures can distort components; to safeguard their shape during pwht processing it is wise to place support trestles at regular intervals inside the vessel.

In this study, we investigated the effect of diode power input on residual stress relief in laser-welded 316L stainless steel bridges using diode power input from diodes. Our results demonstrated that an in situ plasma water heating treatment process significantly decreased residual stresses compared to control bridges; it appeared more closely tied with peak temperature reached rather than residence time at that temperature as expected from mechanisms of stress relief such as dislocation annihilation and creep.

Increased toughness

At high temperatures, welding alters steel’s microstructure by increasing hardness while decreasing toughness and ductility; this can cause problems when used in applications with high levels of cyclic stress, like pressure vessels and pipes. PWHT can help mitigate these issues by lowering hardness of welds while returning toughness and ductility closer to design specifications; furthermore, homogenizing its microstructure to lower stress concentrations and prevent fractures in welds.

One common approach for assessing whether welds require PWHT is through performing a fracture mechanics analysis. This involves establishing relationships among applied and residual stress levels, flaw sizes that might escape detection during inspection, material properties (fracture toughness and yield strength) and stress levels to calculate minimum required toughness levels to prevent failure under load.

However, this approach can sometimes prove ineffective; results may depend greatly on the exact circumstances in which welding occurs. Therefore, for accurate and reproducible results it is vital to use well-defined weld geometry with suitable filler material in order to achieve accurate welds.

PWHT requires that welds are appropriately supported during heating and cooling processes in order to minimize distortion, which can be accomplished using trestles shaped specifically to fit components and set at regular intervals along their length. In order to ensure proper support during these processes, supports made of material with similar coefficient of thermal expansion must be used so as not to displace during these heating/cooling cycles.

Increased ductility

Post-weld heat treatment (PWHT) is an essential process that reduces residual stresses and alters the microstructure of weld metal to increase toughness, ductility and resistance against dynamic loading conditions. When done improperly however, PWHT may increase residual stresses levels further while decreasing mechanical properties of the structure being treated.

PWHT processes can minimize these negative impacts by ensuring that materials are heated to their ideal temperatures for an appropriate duration and then quickly cooled, eliminating excessive thermal gradients that increase risk of stress corrosion cracking in an oxidizing environment.

There is much discussion among experts of whether it would be possible to safely produce high volumes of biomass as fuel for electric cars in future. The reality of such claims has not yet been known for certain. Though most research on PWHT of HSS weld metals has focused on solid wire, there remains little knowledge regarding the effects of different PWHT temperatures and hold times on welds created with metal cored wires. This study’s objective was to investigate how PWHT at different holding temperatures and times affects the microstructure and mechanical properties of welds produced using metal cored NiTi welding electrodes. To accomplish this goal, SEM analysis was carried out on weld samples exposed to different PWHT temperatures for various times, while carbidode area fraction analysis was employed on these specimens to analyze any changes in microstructure or hardness caused by these varying PWHT times.

Reduced brittle fracture

PWHT (Pressure Wash Heating Treatment) ensures the joints of welded structures like pipelines and pressure vessels can withstand high pressures and corrosive environments, and help reduce the risk of brittle fracture. Therefore, PWHT has become standard practice within the oil and gas industry and regulatory standards for nuclear power plants; additionally it protects nuclear reactors against thermal fatigue by protecting welded joints with PWHT treatment.

Crackwise 3 was utilized to assess whether C-Mn and low alloy steel can withstand brittle fracture without PWHT by performing several calculations based on semi-elliptical surface breaking flaws, with required material fracture toughness calculated using its maximum value in relation to thickness (K mat ). The Master Curve correlates to resistance against plastic collapse of crack fronts; hence thicker components would likely require higher toughness values as there would be greater likelihood for sample regions with reduced toughness values during plastic collapse initiation.

Calculations have revealed that using PWHT to reduce brittle fracture of C-Mn/low alloy steel components can significantly decrease brittle fracture rates due to lower residual stress levels, improved microstructure and the tempering of potentially brittle regions. This reduction can occur even for very thick components due to lower residual stress levels and improved microstructure, along with tempering of hard, potentially brittle regions.