Vzorec PWHT

Heat Rate

PWHT processes typically reach temperatures above 450 degrees Celsius for one hour per inch of thickness, making it essential that heating and cooling rates match those required by material type, component size/configuration and desired soaking temperatures. Excessively fast heating or cooling rates could result in unacceptable distortions and thermally-induced stresses if they exceed appropriate levels.

For large and unwieldy fabrications, using trestles or supports may be necessary during local plasma wand heat treating (PWHT). When designing such supports, take into account differential expansion/contraction capabilities of components as well as construction of these trestles from materials which won’t be damaged by high PWHT temperatures.

Auxiliary heating widths play an essential role in the efficacy of local PWHT. Their influence can be felt both during precipitation production and on residual stress reduction in BM and WZ after local PWHT; in fact, as they increase, so too will residual stress reduction levels. The higher an auxiliary heating width is set at, the greater its effect will be.

As part of a welding procedure specification, in addition to specifying the soaking temperature and heating rate, a minimum PWHT time and duration must also be defined. Furthermore, requirements should be included for soak bands, heated zones, gradient control bands and control zones – with gradient control bands acting as areas in which thermal insulation or additional heating elements can be applied in order to maintain acceptable axial temperature gradients.

Temperature

As part of their compliance with code specifications, local PWHT’d areas must meet several requirements in order to remain compliant. These requirements include satisfying soak bands, heated bands, an acceptable axial temperature gradient control band and control zone requirements. Soak bands encompass welds and two HAZs while heated bands use thermal insulation or additional heating elements to maintain an acceptable axial temperature gradient from PWHTed zone to ambient. Typically the edge temperature for heated bands must be half that of soak bands.

Pressure

Molten metal pressure exerted during PWHT can distort components. To avoid this from happening, items must be properly supported during this process – for instance using trestles shaped specifically to fit their contour and made from materials with similar thermal expansion properties as the component being heated (in case of pressure vessels this would include internal supports in addition to outside supports).

Different design codes impose varied PWHT requirements depending on the chemical makeup and thickness of a material, including its chemical makeup or thickness. Some require post weld heat treatment when exceeding certain values; other require it based on its ductility or toughness.

However, there are some similarities in the PWHT requirements of different code bodies; for instance, the thickness restrictions in both PD5500 and ASME B31.3 are roughly equivalent; yet it remains difficult to reconcile these disparate requirements.

PWHT requirements depend on both a material’s intrinsic hardness and strength as well as susceptibility to stress corrosion cracking, with some steels having limits as low as 32mm while others requiring up to 150mm for proper function. Such large variations can present significant issues for components that were not designed correctly.