Comparison of ASME B31 3 PWHT Requirements

Prior to 2014, ASME B31.3 required carbon steel welds be post weld heat treated (PWHT). However, with recent amendments of this Code allowing exemptions when weld thickness exceeds certain limits, post weld heat treatment becomes optional in certain instances.

This paper will analyze the arguments used to justify exemptions, showing many to be invalid or incomplete. Furthermore, an EPC contractor’s perspective will also be provided along with practical approaches that may help mitigate risks.

PWHT Requirements

PWHT of carbon steel can be an expensive and complex process, yet there has been growing interest in eliminating its requirement from fabrication standards for certain applications. This trend can particularly be found within the petrochemical industry and power generation through EGWP (The Electricity Generators Welding Panel). Part 1 of this series compares and contrasts various fabrication standards regarding when PWHT becomes mandatory for welds with special attention paid to any differences in requirements between industries.

PWHT not only serves to alleviate stress, but it can also induce phase transformations and reduce grain size, increasing strength and toughness while simultaneously relieving stress. Therefore, temperature should be high enough for this process but not so high that weld cracking occurs as a result.

As with any fabrication standard, the PWHT temperature required in a weld will depend on its materials used during welding. For instance, ASME B31.3 requires PWHT temperatures for pipe welds of different thicknesses depending on P-number and group number of pipe material; thicker pipes may qualify for exemption if preheating and multi-pass welding is employed with preheated wire, multipass welding being an acceptable process option; temperatures allowed in these codes range from as little as 1100 degF for P No 4 (Group 1) steel to approximately lower critical temperature for 1-1/2 Cr steels.

Preheat Temperature

Preheat temperatures are an integral component of welding processes. They help decrease cooling rates in weld areas while simultaneously improving their ductility for improved weld quality and heat-affected zone (HAZ) results. But it is crucial that you know how to determine the appropriate preheat temperature for your project.

As an example, when welding low-carbon steel that’s less than 1 in (25.4 mm), 100 degrees Celsius may suffice as the preheat temperature; however, if welding an alloy steel or other high-strength material instead, higher preheat temperatures are recommended.

Preheat temperatures vary based on both base metal chemistry and welding process; welder protection services (WPS) typically set them according to material type and welding procedure requirements specified in project specifications.

Temperature-indicating crayons or infrared digital thermometers should be used to monitor and control preheat temperature, with devices located 4t away (where t is weld line thickness) from both heat sources on either side of the weld line and at an equal distance away from both sources. It’s also important to keep in mind that applying and maintaining uniform temperatures throughout weld length will minimize overheating, burn-through distortion as well as any thermal gradient effects on joint.

Preheat Holding Time

Preheat holding time is an integral component of conducting SW-846 testing for hexavalent chromium in soil samples, as establishing an appropriate holding period will prevent changes to properties representative of material collected at collection time. Chapters 3 and 4 provide guidelines for this process; additional preparation and determinative methods may be specified on a project-by-project basis.

Preheating temperatures that are too low can result in fouling of test specimens, leading to significant lengthening of test duration and decreased overall efficiency of procedure. Furthermore, heat transfer capability may decrease dramatically as a result. To prevent this scenario from arising again in future procedures, test methods must include higher preheating temperatures with shorter holding times – making up for any inefficiencies caused by poor preheating temperatures in advance.

Preheat holding times can be reduced significantly for low-alloy steels with easily dissolvable carbides, with only minutes required for their preheat. When calculating soaking times for alloyed materials (e.g. mild steels containing high amounts of manganese which affect the rate at which preheat can be achieved), however it’s important to take alloying factors such as manganese into account; ASM Handbook Volume 4 offers a general rule of 1 hour/inch of thickness as an approximate guideline when preheat holding times are being calculated.

PWHT Temperature Range

At the core of post weld heat treatment lies its temperature range. To maximize desired material properties and avoid reheat cracking, the target PWHT temperature must be set high enough to promote them without leading to overheating and cracking of material properties. Heating and holding times must also be carefully managed in order to achieve uniform temperature distribution and adequate stress relief; any errors with temperature control or unplanned cooling could cause phase transformations which increase hardness while decreasing toughness, or create additional stresses which lead to cracking.

PWHT requirements tend to differ among various pipe materials and service conditions, and may even vary significantly for individual pipes. Although this variance might not necessarily pose any safety concerns, genuine concerns arise when regulations appear marginal from a technical viewpoint.

As an example, PWHT requirements in B31.1 vary significantly across code sections, with power generation applications necessitating higher PWHT temperatures due to fracture toughness calculations that use different reference flaw sizes requiring a different temperature range for PWHT requirements.

Studies conducted to date reveal that current PWHT temperature requirements for power generation materials do not provide adequate toughening, due to temperatures required by codes that are too low to achieve laser welded NiTi toughening with laser welding processes and short soak times that do not allow enough remelting or microstructure stabilization time.