Post Weld Heat Treatment (PWHT) As mentioned in Part 1, PWHT is a specific term that encompasses both stress relief and tempering and is not to be confused with heat treatments after welding. Such treatments may comprise ageing of aluminium alloys, solution treatment of austenitic stainless steel, hydrogen release etc. PWHT is a mandatory. In my experience, PWHT of low carbon steel for structural purposes is very seldom used. The only exceptions would be if the steel is to be machined after welding, or in rare cases to inprove the fatigue life. I do not believe PWHT would be required per AWS D1.1 By - Date 04:44.

Due to the steel pipe transporting gas or liquid need to bear a certain pressure, internal stress produced in the cold rolling process must be eliminated. Welding process, because local overheating will cause the pipe deformation and shrinkage stress, so steel pipe return to heat treatment after welding to eliminate internal stress.

PWHT is based on the hardness of the steel, the thickness of the welding parts, structural rigidity, welding methods and conditions of use. General carbon steel welding performance is good, the welding can be without preheated and heat treatment. But for the high carbon content and high stiffness of the welding parts, it is required to preheat and heat treatment, such as the wall thickness of 30mm carbon steel pipe.

Method of PWHT

The method of PWHT is electric heating. The caterpillar electric heater and linear electric heater should be fixed on the parts which are needed to PWHT; they connect with the automatic controller through cable. And the thermocouple should be fixed on the parts needed to PWHT through the compensating conductor. It connects with the computer automatic controller. Then the controller will work automatically according to the process parameter that is being inputted into the controller.

General rules for PWHT

• On the stress corrosion of the weld, should be carried out post weld heat treatment.
• Prone to delayed crack in steel welding, it should be timely carried out heat treatment after welding. When the heat treatment is not timely, it should be heated to 200℃~300℃ immediately after welding, and the heat preservation and slow cooling, the heating range should be the same as the PWHT.
• After welding heat treatment and before the pressure test, appearance inspection and non-destructive testing should be carried out.
• PWHT qualified parts, should not engage in welding operations, otherwise it should be re-heat treatment.
• Welding inspector should be professional trained and welding heat treatment personnel should be in accordance with the norms and welding work instructions to handle heat treatment.
• When heat treatment is carried out, the temperature should be measured and recorded, the position and quantity of the temperature measuring point should be reasonable, and the temperature measuring instrument should be qualified and effective.

Steel Grade	Preheat Before Welding		Post Weld Heat Treatment
	Wall Thickness (mm)	Temperature (℃)	Wall Thickness (mm)	Temperature (℃)
C	≥26	100~200	＞30	600~650
C-Mn	≥15	150~200	＞20
Mn-V				560~590
C-0.5Mo				600~650
0.5Cr-0.5Mo				650~700
1Cr-0.5Mo	≥10	150~250	＞10
1Cr-0.5Mo-V	≥6	200~300	＞6	700~750
1.5Cr-1Mo-V
2.25Cr-1Mo
5Cr-1Mo		250~350
9Cr-1Mo			750~780
2Cr-0.5Mo-WV
3Cr-1Mo-VTi
12Cr-1Mo-V

Post weld head treatment process

Whole heat treatment

Pipe assembly which should be post weld head treatment treated in the heat treatment furnace as far as possible, but the component should not be welded with the valve and other components.

Subsection heat treatment

When the welded pipeline components cannot add heat treatment furnace, heat treatment allows segmentation. In the subsection heat treatment, the length of the repeated heating is at least 300mm, at the same time, it is necessary to protect the outer part of the furnace, in order to prevent harmful temperature gradient.

When Pwht Is Required For Carbon Steel Alloy

Local heat treatment

When local heat treatment is applied weld joints, a circumferential band of the run pipe, and of the branch where applicable, shall be heated until the specified temperature range exists over the entire pipe section, gradually diminishing beyond a band which includes the weldment or the bent or formed section and at least 25mm beyond the ends thereof.

Safety regulation of PWHT

• The safety warning line must be set to prohibit the entry of irrelevant personnel in the process of heat treatment.
• The operator must go through the heat treatment operation training, familiar with the job technique.
• Flammable materials with 30m range of heat treatment should be cleared.
• Operating personnel must be equipped with personal protective equipment, such as cotton gloves, dark glasses, etc.

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 is in creating a dished head. Stresses can cause cracking to occur under certain operating conditions, especially the heat affected zone and therefore the steel must be relieved of those stresses.

When Pwht Is Required For Carbon Steel Welding

PWHT, also known as stress relief, is a process used to relieve stresses 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 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 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 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 -20F. 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 which 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 in order 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 any welding is to occur on these vessels, either as repair, alteration or addition of external parts, make sure that the PWHT requirements are met. Following these rules will assure 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 that have been stress relieved.