“The failure of this vessel weld joint was caused by improper PWHT (Post-Weld Heat Treatment) applied during vessel fabrication!” This is what a well-experienced API Inspector told me when I was just a young engineer. My internal instinct was to challenge the Inspector. How did he know what happened years ago when the vessel was fabricated? Fortunately for me, I didn’t vocalize my thoughts to that salty individual. I held my tongue (a rare event), until I could do some reading and research!
So here is what I learned after reading some articles. That “simple” process of PWHT is far more detailed than I thought. It’s not like putting a cake in the oven and waiting for the buzzer to go off. Things can go wrong and like the very smart Inspector said, improper PWHT can cause failures.
Welding and the Importance of PWHT
Welding is the process of melting base metal and weld metal together. Let me take a moment to elaborate on the three distinct components in the welding process (if you are not already familiar):
- weld metal
- base metal
- heat affected zone (HAZ), which is a narrow area between the weld and the base metal.
With the above in mind, a simple description of the science behind the weld will give us a better glimpse into the importance and potential challenges involved in PWHT:
- Grain structures of the HAZ differ from those of the base metal because it has been heated to a temperature just below the melting point of the material and then cooled quicker than the adjacent base metal. The maximum temperature reached, time at temperature and cooling rate determine the grain structure of the HAZ.
- Grain structures of the weld metal differ from the HAZ, because it too has been heated and cooled at a rate different from the HAZ and because it is a “casting” with a much different thermal and working history. With each successive weld bead, additional heat is applied to the weld below it, so each pass cools at a slower rate.
Unfortunately, mechanical, physical, and chemical properties within the HAZ are not always predictable or desirable and frequently cause problems. For example, in carbon steel and low allow steel, the HAZ can be very hard. As a result, this area can be brittle and commonly succumbs to various damage mechanisms (Caustic Stress Corrosion Cracking, Amine Stress Corrosion Cracking, sulfide stress cracking, stress-accelerated corrosion and many more mechanisms). In stainless steels and aluminum alloys, the corrosion resistance of the HAZ is diminished further than the adjacent base metal or weld metal. This can lead to a phenomenon called weld-decay, wherein the HAZ corrodes rapidly, i.e. stress accelerated corrosion/sensitization.
Proper post-weld heat treatment (PWHT) removes or minimizes the effects of these undesirable properties inherent in the HAZ. PWHT does this by changing the residual stresses and microstructure in the weld area.
3 Popular PWHT Methods
Here are three of the most prevalent types of PWHT for vessels:
PWHT in an Enclosed Combustion Furnace
This is the recommended method of PWHT in the ASME code books. Sometimes this method is impractical due to size or odd configurations of the vessel, but for the most part, if you can build a furnace big enough, you can and should utilize this method.
The benefits of using this method include more uniform control of heat throughout the vessel, quicker setup time, faster turnaround time for finished product, and generally, a lower overall cost.
Negatives of this method include the potential for
- high transportation costs if the furnace is not at your site, and
- difficulty locating a furnace large enough to accommodate large vessels (over 60 feet).
Internal Combustion PWHT
This method involves wrapping the outside of the vessel, as well as all nozzles and attachments, with a ceramic fiber insulation and inserting combustion burners in either manways or nozzles of sufficient diameter to accommodate the burner. This method is used in situations where removal of the vessel requiring PWHT is impractical.
The potential benefits of this method include the convenience of allowing PWHT of the vessel to be done in place, no increased transportation costs, and the owner/operator can maintain custodial control.
Negatives of this method include:
- Cost increases due to additional manpower and consumables items
- Turnaround time is longer due to the amount of time required to wrap the vessel to the completion of the PWHT cycle. All above considerations are dependent on the vessel size.
External Resistance Heating
This method utilizes electrical resistance equipment mounted to either the outside or the inside of the vessel at the weld area, and around the circumference of the vessel. This method is well-suited for closure seams and attachments to vessels in situations where the entire vessel would not require PWHT; or, in cases of a repair that has been made to the vessel.
Benefits of this method include the fact that the vessel can remain onsite, it is easier to control temperature, and it allows for odd configurations to be Post Weld Heat Treated.
Negatives of this method are
- Cost increase due to additional manpower and consumables
- Extended length of time; the set-up times and tear down times are increased
- Not all vessels can benefit from localized PWHT due to code restraints
The sections above are just a few of the items that need to be considered when making a choice of how to proceed with a PWHT cycle on a vessel. Whenever possible, it is best to PWHT a vessel in an enclosed furnace as this is the recommended practice from ASME. The other types of PWHT mentioned are viable alternatives, but they come with potential increases in time and cost.
Let’s go back to the opening statement made by that well-experienced Inspector, “The failure of this vessel weld joint was caused by improper PWHT (Post Weld Heat Treat) applied during vessel fabrication!” That inspector was correct! Just because a piece of equipment has been Post Weld Heat Treated, does not always indicate the procedure was followed and was totally successful. Very common pitfalls that are regularly identified in our audits are:
- Local PWHT using heating elements only applied on one side of shell Remember, the full section thickness must reach the desired PWHT temperature.
- Lack of thermocouple coverage and improper placement to control uniformity of temperature.
- Insufficient temperature and or holding time during PWHT
The process of PWHT appears to be relatively simple - equipment is heated and then cooled in a managed fashion. Detrimental effects include distortion, temper embrittlement, over-softening and reheat cracking, which means that control of heating and cooling rates, holding temperature tolerances, and the times at temperature are extremely important and must be carefully controlled in order to realize the full benefit of the process.
Because the science and effective implementation of PWHT are often more complicated than originally thought, knowledge of the procedures, attention to details and actual experience with PWHT are indispensable. A lesson learned well after meeting our Inspector friend.