What is post weld heat treatment (PWHT) or stress relieving?


Post Weld Heat Treatment (PWHT) or Stress Relieving

Post Weld Heat Treatment (PWHT), or also called stress relief (SR) is a method for reducing and redistributing the residual stresses in the material that have been introduced by welding.

Post weld heat treatment is a controlled process in that a welded components (welded material) is reheated in a furnace or localized heating arrangement to a temperature below its lower critical transformation temperature (AC1), and then it is held at that temperature for a specified amount of time. This is followed by slow & controlled cooling in the furnace.

Why Post Weld Heat Treatment or PWHT is required?

The development of residual stresses approaching or even exceeding the yield stress is possible when welding thick sections. For certain industry sectors, eg. Petrochemical, Chemical, Oil and Gas, etc. the existence of residual stress of this magnitude is completely unacceptable. With pressure equipment operating at 200° C and below a variety of stress corrosion cracking mechanisms under the general term “environmental cracking” become prevalent.
There is also the problem of fatigue to be considered and the effect that residual tensile stresses have in that regard.

Effect of Post Weld Heat Treatment or PWHT on material

Tempering effect of the PWHT

Post weld heat treatment will generally result in a modification of the microstructure of both the weld metal and heat affected zone. With the exception of the 9Cr1Mo and 12Cr1MoV materials, the microstructure of all other materials should contain a mixture of ferrite and iron or alloy carbide. The effect of short-term (1 to 2 hours) post weld heat treatment on the carbide is generally beneficial, whereas longer times result in a reduction in toughness due to spheroidising effects. The normal microstructure for the parent, weld and HAZ for the 9Cr1Mo and 12Cr1MoV materials is martensite, and post weld heat treatment is absolutely essential in these materials to temper the martensite phase.

PWHT Effect on Mechanical Properties

Generally, following are the effects of post weld heat treatment compared with the as welded condition in a material:
1. Yield strength is decreased slightly, the effect falling off fairly rapidly with time.
2. The tensile strength is decreased.
3. The ductility is increased.
4. Hardness levels are reduced.
5. Toughness is slightly reduced at short times but the effect can be significant over longer times.

PWHT Effect on Creep Properties

For creep resisting material, post weld heat treatment is required in order to fully develop the creep strength. This is especially true for thicker components such as headers. There has been a tendency in recent years to allow waiving of the post weld heat treatment stage for thinner materials used typically for superheater and reheater coils in the Power Generation industry, but a variety of conditions have to be met.

Benefits of Post Weld heat Treatment or stress relieving (SR)

1. Improving the diffusion of hydrogen out of weld metal
2. Softening the heat affected zone and thus improving toughness (although not weld metal toughness)
3. Improving dimensional stability during machining.
4. Improving ductility.
5. Improving the resistance to stress corrosion cracking.
6. Reducing the effects of cold work.

Difference between post heating & stress reliving

What is Post Heating

  • Hydrogen induced cracking (HIC) often occurs when high levels of ambient hydrogen permeate into a material during welding. By heating the material after welding, it is possible to diffuse hydrogen from the welded area, thus preventing hydrogen induced cracking. This process is known as post heating and should start immediately after the weld is completed. instead of being allowed to cool to room temperature, the material needs to be heated to a pre-defined temperature depending on the material type and thickness of the material. It should be held at this temperature for a number of hours dependent on the thickness of the material thus allowing the hydrogen to diffuse. AS heating is performed after welding it is called post heating in welding.

Stress Relieving

  • By the time welding is completed, due to the effect of welding thermal cycles in the material, the welding process can leave a high residual stresses in the material, which can lead to an increased potential for stress corrosion and hydrogen induced cracking. PWHT can be used to release these residual stresses and reduce this potential. This process involves heating the material to a specific temperature and then gradually cooling it is known as Stress Relieving.

How to perform Post Weld heat Treatment (PWHT)

Most of the requirements for post weld heat treatment can be found in the fabrication Standard to which the vessel or other structural parts are constructed. For example, AWS D1.1 for structural parts, ASME Section VIII for pressure vessels and ASME B31.3 for Piping. Different techniques are used to perform the PWHT as explained further in this post.

Fixed Furnace PWHT

This method is traditional and well known to most people in the fabrication industry. A
fixed furnace usually consists of a rectangular box made from heat resistant materials in which are embedded electric resistance elements. Doors open at each end and a travelling bogie allows for loading and unloading of the charge. Furnaces such as these are often capable of heating to 1200° C and can normalize and anneal as well as stress relieve. Some furnaces are gas fired, with two or even four nozzles at each end.
Fixed furnaces tend to be large and expensive to operate. They often have fixed thermocouples that measure the furnace atmosphere temperature and not the
temperature of the article being heat treated. This is usually satisfactory up to around 300°C, but beyond, thermocouples physically attached to the article must take over both temperature control and temperature measurement. Furnaces such as these must be equipped with correctly calibrated temperature controllers/recorders with at least 12 recording points. Care must be taken when cooling after post weld heat treatment. Most manufacturing Codes specify a controlled rate of cooling until a certain temperature is reached (typically 300 – 400°C depending on the thickness), so it is normal to control cool in
the furnace before opening the doors.

Temporary Furnace

These are custom-built around a vessel, rather than transport a vessel to a fixed furnace. The idea is to minimize the air space between the vessel and furnace walls, and they allow for faster heating and cooling. The basic structure of the furnace should be creep resisting piping (if the pipes are to be continually re-used) with heat resistant materials attached to them. Heating can be through resistant heating mats placed on a concrete floor or via gas burners placed at each end. In the case of gas burners, care must be taken to avoid direct flame impingement on the vessel. Temperature control is again through a 12 point recorder/controller, but atmosphere thermocouples are not generally used. The same heating and cooling rate restrictions apply as with fixed furnaces.

PWHT using Internal Firing

Vessels of suitable dimensions and arrangement of openings can be post weld heat treated by gas firing through nozzles or manways. Manways are large enough to accommodate the gas burners, but care needs to be exercised with the diameter and position of nozzles and expert opinion should be sought. Care must also be taken to place deflector plates inside the vessel and opposite the burner entry points to avoid direct flame impingement on the shell. It is not advisable to post weld heat treat vessels that contain internals in this manner.
The outside of the vessel must be completely encased in insulating material, and again, at least a 12 point temperature recorder is advisable.

PWHT using Local Heating or Localized PWHT

Circumferential weld seams can be post weld heat treated by heating a band around the weld. Although not specifically stated, such heating is essentially limited to resistance or induction heating, mainly because of the controls required on heated band width, width of insulation and temperature measurement requirements. The use of gas ring burners and other methods should be carefully documented, proposed to the Client and accepted before being contemplated for use.


Typical restrictions on the width of the heated band in both AS 4458 and EN 13445
involve the use of a formula, ie. 5(Rts
where the weld is in the center and:
R = inside radius of the shell, in mm
ts = nominal shell thickness, in mm


This means that with a vessel of say 2 meter inside diameter and 50mm wall thickness,
the minimum width of the heated band would be:
5 x (1000 x 50)½ = 1118 mm
or 560mm each side of the weld.

In addition to this, the width of the insulated band is recommended to be 10 (Rts)½, which
in the above example would be 2236mm. This is to ensure that the temperature gradient of the portion next to the heated band is not harmful. There may be other restrictions or different formulae involved, and the relevant Code must be checked.

Partial Post Weld heat Treatment

There are occasions, for example with very long vessels, when the entire vessel will not
fit into a fixed furnace. This has been catered for in most Standards, and it is permissible to post weld heat treat section of the vessel first, then turn the vessel around and heat treat the remaining section. As with local heating, there are restrictions in this case as well over the degree of overlap and the longitudinal temperature gradient.
Generally, there are better ways to post weld heat treat vessels than to use this partial method.

Heating & Cooling Rates in PWHT

These are specified in most of the construction Standards, and are reasonably similar. For example, As per ASME section Ⅲ NX-4623,Above 800°F(425℃) the rate of heating and cooling in any hourly interval shall not exceed 400°F(222℃) divided by the maximum thickness in inches of the material being heat treated, but shall not exceed 400°F(222℃) and need not to be less than 100°F(56℃)in an hourly interval.

As per ASME Sec VIII Div. 1  code rate of heating / cooling  which is mentioned in UCS56 as below ” The rates of heating and cooling need not be less than 56°C/hr. However in all cases consideration of closed chambers and complex structures may indicate reduced rate of heating and cooling to avoid structural damage due to excessive thermal gradient”

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