Post Weld Heat Treatment (PWHT) and Pre-Heat Treatment (PWHT)

PWHT testing is an essential element of quality assurance in industries like oil and gas and power generation, providing improved resistance to thermal fatigue as well as improving reliability.

Cost and expertise are two major obstacles when it comes to fabricating pressure vessels and piping systems, such as those shown in Table 1. Divergences among fabrication standards for pressure vessels and piping can make any attempts at rationalisation very challenging.

Weld Heat Treating

Post weld heat treatment, commonly referred to as stress relief, serves to both reduce and redistribute residual stresses caused by welding. The process involves heating the weldment to an exact temperature before slowly cooling it in order to avoid any reintroduction of new stresses; its exact temperature and duration will be determined by applicable codes.

PWHT also utilizes higher temperatures that induce metallurgical changes like tempering, precipitation and ageing that help increase weldment strength by inducing tempering, precipitation and ageing processes that help lower hardness, improve ductility and minimize risks of brittle fracture in some materials. This process helps lower hardness while improving ductility as well as decreasing fracture risks in these materials.

PWHT testing is essential in many industries and applications, including nuclear power plants, oil and gas pipelines and pressure vessels. PWHT is an essential element of turnaround to ensure that welds on these critical pieces of equipment can withstand high temperatures, pressures and corrosion-ridden environments in which they will operate.

Utilizing controllable resistance heating systems paired with thermocouples to monitor weldment temperatures accurately and consistently is crucial for producing accurate and repeatable results. Soaking time should also be managed so as to minimize thermal gradients across the weldment, so as to not simply move existing residual stresses around. Preheat should also be adjusted according to material type and thickness being treated for maximum effectiveness.

Pre-Heat Treating

Heat treatment is an expensive and time-consuming operation used to improve strength, toughness, corrosion resistance and residual stresses in welded components. It is specified as part of many welding application codes and standards and pre-heat treating is another essential procedure which reduces cracking risk during welding processes.

A welding preheater typically made of ceramic mat heaters or electrical strip heaters can be placed either within or adjacent to the weld area and heated according to a specific temperature profile. Preheat temperatures are controlled electronically through an electronic controller incorporating thermocouples and data recording capabilities.

To determine an ideal preheat temperature, many factors must be taken into account, including code requirements, base metal chemistry, section thickness, restraint level, ambient temperature and filler metal diffusible hydrogen content. In order to avoid cracking of weld seams during welding processes and section thickness combinations. Optimally, however, the minimum value should be exceeded so as not to fail safety requirements for certain bases metals welding processes and section thickness combinations.

For instance, for ASTM A572-Gr50 butt welds that require local post-heat treating of at least 2 inches around their weld, local post-heat treating is necessary in order to ensure that preheat temperatures are sufficient to prevent cracking during welding and to reduce hydrogen-related cracking by driving out any remaining hydrogen from welding immediately postweld.

Post-Heat Treating

Heat treating is a manufacturing process which involves carefully heating and then cooling metals and alloys in order to alter their microstructure, which has a direct influence on its mechanical properties – sometimes leading to significant increases in strength and toughness.

While post-heating can be included in a welding procedure, its purpose is usually specified by a client and may or may not be mandated by code. Client requirements typically dictate soaking temperature and duration. When stipulated as code requirement, the period should cover both weld metal as well as base metal on either side; temperature must remain below the lower critical temperature and controlled cooling must take place afterwards.

Some low alloy steels, such as quench hardened materials found in HAZ environments, require post weld heat treatment in order to achieve an appropriate microstructure with regards to strength and toughness. Not only is this required by code requirements; but also serving as an important safety practice to avoid hydrogen cracking in the future.

Heat treaters who deal with such materials typically prefer solution annealing over quench annealing as it allows higher temperatures to help avoid the formation of damaging phases or particles which would form otherwise in quench annealing.

Exenciones

As PWHT can be costly, exemption from its requirement in fabrication is an important concern. Determining what thickness threshold requires PWHT can be challenging with large steel assemblies that use tough materials; some fabrication standards and inspection codes now stipulate it as required PWHT treatment of weldments exceeding this thickness threshold.

This article evaluates and contrasts various fabrication codes (primarily piping and pressure vessel standards) with regard to PWHT requirements above a certain thickness, toughness tests (Charpy test) requirements, as well as potential rationalisation possibilities between BS 2633 [25], ASME VIII, and ASME B31.1 and B31.3 standards.

Findings indicate that, although some similarities exist amongst various fabrication codes under consideration, alignment by material grouping does not seem an appropriate approach to reaching some degree of rationalisation. Furthermore, testing programs need to be implemented in order to eliminate some of the anomalies observed within this area of fabrication code requirements, something covered further in Part 2 of this article.