Pre & Post Weld Heat Treatment

Post-weld heat treatment (PWHT) is a thermal process performed on welded metal components after they have been welded. It involves heating the welded assembly to a specific temperature range, holding it at that temperature for a predetermined period, and then cooling it down at a controlled rate. The purpose of PWHT is to relieve residual stresses and to improve the material properties of the weld and surrounding base metal.

In piping, PWHT stands for "Post Weld Heat Treatment." It's a crucial step in the fabrication and installation of welded piping systems, particularly in industries where high-pressure or high-temperature conditions are present. Such as oil and gas, petrochemical, power generation, and nuclear. It involves subjecting the welded joints and adjacent areas of the piping to controlled heating and cooling cycles "post" welding, to relieve residual stresses and improve the mechanical properties of the welds and base metal.

Watch this video example.

PWHT may be required for carbon steel components when the material thickness exceeds certain specifications, when stipulated by various industry codes, standards, and regulations, as well as for carbon steel components that will be subjected to high-stress or high-temperature service conditions. These conditions can induce significant residual stresses during welding, which can lead to distortion, cracking, or reduced mechanical properties, something that PWHT can help to mitigate.

View our PWHT machines currently in stock here.

Post Weld Heat Treatment (PWHT) for stainless steel is generally not as common as it is for carbon steel. However, there are certain circumstances where PWHT may be required or recommended for stainless steel components. For example, in applications where stainless steel components will be subjected to high temperatures, as PWHT might be needed to relieve residual stresses. It can also aid reducing sensitisation in certain grades of steel, and can also help to reduce defects like cracking and distortion in complex stainless steel welded assemblies, where welding-induced stresses are expected. Sometimes, it can also be as simple as part of a client specification, or code and regulatory requirements!

A few examples of the types of PWHT commonly used in welding and fabrication processes include stress relieving, preheat, postweld, interpass and annealing. The selection of the appropriate PWHT method depends on factors such as material composition, thickness, welding process, service conditions, and desired outcomes for the welded component. See all heat treatment machines currently in stock here.