PWHT Procedure Asme B31 3

Post-weld heat treatment (PWHT) helps alleviate residual stresses in the welding region that could threaten brittle fracture.

ASME B31.3 Clause 331.1.1 and Table 331.1.3 establish specific PWHT requirements based on pipe material. However, certain pre-heat temperatures can exempt PWHT requirements in certain situations.

1. Welding Temperature

Temperature plays an essential part in welding. It regulates how fast heat escapes and thus how quickly welded joints cool, so it is vital that welders know the temperature of their steel before starting their welding processes. Many factors affect welding temperatures including type of metal used, thickness of material being welded on, welding process used and time needed.

Typical welding codes specify a minimum preheat temperature that must be clearly stated in a welding procedure specifications (WPS) document. Furthermore, it should be compared with actual target weld temperatures to ensure that proper preheat has been achieved.

An additional key factor is ambient temperature. Welding in cold conditions can result in subpar weld properties such as cold cracking. This is caused by heat loss occurring more quickly since thermal expansion differences between welds in cold environments and parent metal are greater.

Conventional post-weld heat treatment (PWHT) aims to minimize residual stresses, soften by tempering and resist hydrogen damage; however, specific PWHT requirements vary widely across codes depending on factors like weldability and material thickness.

2. Welding Time

PWHT testing is required by numerous standards to ensure welded structures can withstand extreme temperatures and environments, particularly those found within nuclear power plants. As a result, considerable effort must be spent creating and maintaining suitable PWHT procedures.

PWHT involves heating the weld area above its critical point, increasing toughness of weld metal and decreasing residual stresses. This technique is especially important for large diameter pipes where an arc must travel further to reach the fusion zone in order to produce equal weld quality.

Welding duration plays a vital role in the effectiveness of PWHTs. Prolonged welding duration can lead to weld defects such as hydrogen delayed cracking and lack of ductility; while shorter welding durations typically produce greater weld metal toughness and decreased residual stresses.

Post-weld heat treatment requirements vary based on the material being welded, such as carbon steel and certain low alloy steels used in nuclear power plants. PWHT may be mandatory, while requirements may differ between codes – ASME B31.1 exempts P-4 or P-5A materials with wall thickness of 19 mm or less from mandatory PWHT treatment.

3. Welding Process

Welding is an intricate process used to join two pieces of metal together using heat and electrical current. It has multiple applications across industries and creates highly strong bonds between metals; however, this technique takes an immense amount of skill and time-consuming dedication.

Resistance and laser welding are both available as welding processes, offering various degrees of weld strength depending on their use. Resistance welding utilizes constant electricity flows to melt base material with filler material together for an enduring bond, ideal for thick sections of pipe. Laser welding uses focused beams of energy that melt the surface material for rapid weld speeds that work for narrow or deep joints.

Post-Weld Heat Treatment (PWHT) reduces residual stresses in welded steel components and prevents stress corrosion cracking, thus being necessary in sour service piping as well as many codes and specifications.

PWHT is typically performed within the temperature range outlined by relevant codes and standards, usually tabularly. For instance, table 132 of ASME B31.1 Code for power piping specifies 1300-1375 degF as an ideal range for PWHT for lengths of time depending on weld thickness; exemptions exist. This paper seeks to change this requirement through B31.1 as well as other boiler and pressure vessel Code sections in order to lower this required PWHT temperature range.

4. Post-Weld Heat Treatment

Post weld heat treatment (PWHT) reduces and redistributes residual stresses to enhance ductility, resistance to brittle fracture, tempering precipitation or ageing effects in certain steels, while providing some tempering, precipitation or ageing effects on others. Unfortunately, PWHT may result in mechanical property degradation which should be evaluated and controlled carefully in order to avoid distortion, temper embrittlement or reheat cracking effects that could occur as a result of its changes.

PWHT temperatures must be carefully managed to prevent material from being heated beyond its critical transformation temperature or remaining at elevated temperatures for extended periods. To obtain superior results, heating must be distributed uniformly throughout the weld area and cooling rate/tolerances must also be managed to prevent strength losses.

PWHT must be conducted correctly and on schedule in order to avoid excessive residual stresses that exceed a material’s design limitations, leading to weld failure and cracking. Furthermore, maintaining thorough documentation and adhering to industry standards for PWHT can be daunting, particularly in highly regulated industries; fortunately there are tools available that can assist companies meet industry requirements while remaining compliant.