Post Weld Heat Treatment

PWHT (Post Weld Heat Treatment) serves to alleviate and redistribute residual stresses introduced during welding. Furthermore, it tempers weldment microstructure and diffuses hydrogen to reduce cracking or corrosion risks in service.

PWHT at higher temperatures offers not only stress relief, but also tempering or precipitation effects on weld metal resulting in decreased hardness and improved ductility.

Annealing

Tempering is a form of heat treatment which involves heating the weld below its critical transformation temperature for an extended period, in order to redistribute residual stresses that have been generated during welding, which may have resulted in defects such as brittle fracture or improved mechanical properties of its heat-affected zone (HAZ) through reduced brittleness and susceptibility to corrosion.

Annealing makes metal easier to work by decreasing its hardness and increasing ductility, which allows users to use less force when shaping or stamping it. It is often utilized for fabricating complex shapes at lower cost than using traditional machining operations.

Stress relieving is another common PWHT procedure, in which welds are heated below their critical transformation temperature for a specified time, to relieve residual stresses and improve mechanical properties of their weld. It is particularly useful when used on pressure equipment welds.

PWHT may not always be necessary; its necessity will depend on the composition and toughness requirements of weld and base materials used in welding applications as measured using Charpy energy absorption tests. Furthermore, temperature and time need to be carefully managed in PWHT as excessive temperature or extended soak times could cause temper embrittlement or oversoftening which compromise the strength of quenched and tempered materials significantly.

Normalizing

Normalizing is a complex process designed to refine the microstructure of steels and ferrous metals. The goal is reduced carbon content, increased ductility and enhanced strength as well as decreased risk of weld-related cracking or fracture. Normalizing is an integral component of weldment preparation but must be handled by experienced companies for best results.

Conventional welding processes produce significant residual stress distributions in both weld metals and base materials, which may reach levels nearing their yield strengths in certain conditions, leading to trans-granular fracture in weld areas and general structural failure. This may produce significant residual stresses in welds as well as base metals, creating residual stresses with significant magnitude that approach their yield strength or approach yield strength of either material causing general failure of both structures welded.

Post weld heat treatment (PWHT) is necessary to eliminate brittle residual stresses. Current fabrication standards for pressure vessels and piping require PWHT when welding thickness exceeds certain limits, depending on both its Charpy energy absorption properties as well as anticipated service temperature.

PWHT limits are quite conservative and there may be plenty of room for exemption. However, selecting appropriate temperatures and times for heat treatment is of critical importance as this will affect tempering, precipitation or ageing of base metals as well as final tensile strength and toughness of welded assemblies.

Cooling

Conventional welding processes produce significant residual stresses in weldments that, under circumstances of high restraint, can approach yield strength of parent and weld materials. Post weld heat treatment (PWHT) helps alleviate and re-distribute these residual stresses as well as tempering the weld metal microstructure while diffusing hydrogen in order to prevent hydrogen-induced cracking in welded areas.

PWHT (Pressure Wash Heating Treatment) involves heating parts (or all) of welded fabrication to high temperatures for approximately one hour per inch of thickness, usually for at least an hour per weld. Welds, as well as often an area around them on both sides, are immersed in this heated environment to allow residual stresses to relax and redistribute more evenly across their bodies.

Specialist advice should be sought regarding appropriate welding temperatures and times for different steels to prevent tempering or ageing effects that reduce toughness or ductility, which could compromise weldment strength against stress loads and fatigue loads, leading to its fracture due to brittle fracture.

Current pressure vessel and piping codes stipulate that any welded component does not exceed a set thickness limit, generally determined by Charpy test properties of its material. Unfortunately, however, this limiting thickness criterion is based on conservative assumptions; anomalies do exist. In some instances it can even be used to obtain exemption from PWHT requirements; but, to do so safely.

Preparation

Post weld heat treatment (PWHT) is a process in which parts of a weldment are heated to high temperatures for an extended period, to relieve its residual stresses and temper its weld metal microstructure. It reduces hydrogen induced cracking as well as stress corrosion cracking by strengthening strength and ductility – potentially mitigating hydrogen-induced cracking or stress corrosion cracking effects.

Conventional welding processes generate considerable residual stresses within a weldment that often approach its yield strength, creating significant tensional residual stress distributions throughout. PWHT mitigates these tensile residual stresses through plastic deformation mechanisms such as creep and strain diffusion to allow their redistribution more evenly over the joint area.

Most current design codes for piping and pressure vessels mandate PWHT up to a limiting thickness value, depending on steel grade/composition/welding conditions/likely mode of failure. While PWHT will reduce residual stresses significantly, they cannot entirely disappear despite PWHT being applied.

PWHT is well suited to components with an elongated shape, such as circumferential welds on pipes or closure welds on long pressure vessels. To implement this form of PWHT, the component is welded around an element heated via induction technology – this heating method being much faster than traditional burners or electrical resistance heaters.