Post Weld Heat Treatment Formula

Post weld heat treatment (pwht) is a stress reliever designed to lower residual stresses near welds. Most governing codes require local PWHT treatments in order to prevent fatigue cracking in process piping systems.

To determine the optimal conditions for local PWHT, a full factorial design with three factors was implemented. Variables investigated included:

Temperature

PWHT should never exceed the tempering temperature of the material being processed and any such attempt must be verified through mechanical testing to show that its strength has not decreased below what was specified for that grade. Therefore, post weld heat treatment needs to take place within regions defined by specific codes that regulate post weld heat treatment such as soak zones, heated zones or gradient control bands.

For optimal PWHT results, items to be processed should be supported during their soak period by internal and external trestles shaped to suit them and placed at regular intervals around their perimeters. This step is especially important with pressure vessels subject to internal and external forces that cause expansion and contraction.

Pressure

Post Weld Heat Treatment (PWHT) is used to alleviate residual stresses that would otherwise combine with load stresses to exceed material design limits, potentially leading to weld failure, stress corrosion cracking or brittle fractures. PWHT reduces these stresses by heating and cooling weld metal at a controlled rate in order to alleviate them.

Dependent upon the type of steel being treated, various PWHT methods exist, which may include annealing, normalizing, tempering and stress relief. When selecting an appropriate method based on chemical makeup or intended application (pressure vessels or pipes for example), proper equipment and facilities with holding temperatures controlled properly as well as quality assurance measures as well as appropriate heating and cooling rates are key elements to achieve results that meet desired expectations.

PWHT may be required by code when weld materials exceed a certain thickness threshold, however it could also be determined based on microstructural analysis or susceptibility to stress corrosion cracking.

Proper PWHT usage is essential to preventing weld failures and other defects in welded structures. PWHT can increase toughness, improve ductility and decrease cracking risks during service; however if PWHT is applied too rapidly or uncontrollably these effects could become reversed, leading to early failure of structures.

Rate of Heat Transfer

Heat transference between materials depends upon their properties; for instance, metal conducts thermal energy more effectively than plastic sheets or wood. Therefore, metal plates will transfer more heat into their surroundings than their plastic counterparts due to temperature difference DT between hot and cold environments and thickness of materials used.

PWHT methods used can have an effect on both heating and cooling rates of components, particularly vessels. If using this process with vessels, support must be provided during PWHT to avoid excessive distortion; this may involve placing trestles tailored specifically for them at regular intervals during PWHT process or using either permanent fixed furnace that bogie loads components onto wheeled furnace beds, or temporary furnace erected on site for PWHTing process.

Code requirements require the welding area of components to be divided into several regions known as soak bands, heated bands and gradient control bands. Thermal insulation or additional heating elements are installed within gradient control bands in order to maintain an acceptable axial temperature gradient between weld temperature and ambient temperature; convection and radiation heating techniques are used respectively.