Post-Weld Heat Treatment of Pressure Vessels | Blog No. 94

In this article, I will explain the basic principles of Postweld Heat Treatment (PWHT), why it is used in the fabrication of pressure vessels, and how to use this information in pressure vessel inspections.  The rules for PWHT are found in the ASME pressure vessel code, and I caution you that this article is basic information and in no way comprehensive.  If you have a question about a particular vessel, seek advice from the original manufacturer or a repair/alteration organization authorized to work on vessels.  A dangerous result can occur if vessels are not properly heat-treated following any welding, so caution is advised.

Why is PWHT necessary?  When steels are formed at the foundry, their granular structure is usually in a non-stressed condition.  When the steel is welded, the joint is heated to a melting point while the rest of the steel remains solid.  As the weld cools, stresses can be locked into the steel’s granular structure at the weld and also the adjacent area known as the “heat-affected zone”.  Stress can also be induced by forming steel, as it is in creating a dished head.  Stresses can cause cracking to occur under certain operating conditions, especially in the heat-affected zone, and therefore, the steel must be relieved of those stresses.

PWHT, also known as stress relief, is a process used to relieve stress in vessels made with certain carbon steel materials.  PWHT is performed when all fabrication is complete and may be applied to the entire vessel or only a part of the vessel.  The process is accomplished by raising the vessel’s temperature at a controlled rate to a specified temperature and then holding that temperature for an amount of time based on the thickness.  The vessel is then cooled at a controlled rate back down to ambient temperature.  This process causes the granular structure of the steel at both the weld area and the vessel to return to a normal state, thereby relieving stresses that were present.  The heating cycles are specified in the ASME code for pressure vessels based on the type of steel used in fabrication and its thickness.

PWHT may be mandatory or optional.  For most operating conditions that I am aware of, PWHT is optional for vessels in ammonia refrigeration service.  Certain operating conditions, such as lethal service, make PWHT mandatory.  While ammonia is a hazardous chemical, it is not considered lethal in service.  Some operating conditions have been known to cause “stress corrosion cracking,” and PWHT can reduce the chance of this occurring.  PWHT reduces the brittleness of the steel at low temperature, so its application is not uncommon for vessels that operate below -20°F.  Sometimes PWHT is used to provide corrosion resistance.  The designer or user of the vessel may specify if PWHT is to be used for any of these reasons, but as far as I am aware, PWHT is not mandatory for ammonia service.

How do you know if your vessel received PWHT?  There are two ways to determine this, and all such vessels will have both of these methods.  The first is found on the vessel’s data report.  Look for the area of the form that lists “Heat Treatment” or “HT”.  If PWHT was applied, the temperature and time at temperature will be noted on the form.  The other way is to look at the vessel’s nameplate.  If PWHT was applied to the entire vessel, the letters “HT” will be stamped below the U-symbol.  “PHT” is stamped when only part of the vessel received PWHT.

Why is it important to know all of this?

The vessel received PWHT for a purpose, either as a mandatory requirement or by design.  Any welding performed at any time in the future will cause new stresses that must be removed to maintain the vessel’s safety and certification.  Repairs and alterations on vessels are described in the National Board Inspection Code, which includes rules for maintaining the PWHT conditions.   Any welding to the vessel must be performed by an authorized company in accordance with this code.  There are a number of options that may be acceptable.  In some cases, this may involve the use of heating blankets wrapped around the entire vessel.  Other instances may only require special preheating procedures.  But in all cases, the maintenance of the PWHT must be achieved.

I consider the inspector of a pressure vessel to be that vessel’s guardian.  It is your responsibility to see that no harm comes to these components.  If you are aware that welding is to occur on these vessels, either for repair, alteration, or the addition of external parts, ensure that the PWHT requirements are met.  Following these rules will ensure that the vessel continues to operate safely.

So, look for the markings described above on any of your vessels.  And let no harm come to those who have been relieved of stress.

Thanks for reading. Stay tuned for more!

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