Main reason of PWHT by carbon steels.

[EuroWeld]

Good day,

I have some questions that relates to whether or not to apply post weld heat treatment on carbon steel headers.
This question relates to the fabrication of pressure equipment.

Can somebody give me the main reasons for applying pwht by for example an carbon steel header of an air cooled heat exchanger.
I know the heat treatments reduces residual stress and lowers the hardness. But is there more to think of?
Is this heat treatment usefull for avoiding stress corrosion cracking and/or caustic stress cracking?

What is the result of pwht related to service, meaning: liftime of equipment etc.?

Most design codes obligates the fabricator to perform pwht; but this related to material (weld) thickness.
My questions mostly related to downstream oil/gas reffineries.

All information will help.
Thanks, Euroweld

 

[Metalguy]

"Carbon steel" has a wide range of compositions that apply to your questions. There's low, med and high C, along with lots of small amounts of elements that can affect hardenability. Even the allowable variations of some elements in low alloy steels can make a big difference.

So yes, a PWHT will reduce the likelihood of caustic SCC by reducing the residual stresses.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[EuroWeld]

Hello metalguy,

Thanks for your reply.
I mean unalloyed carbon steel such as a-516.
Do you know if there are more reasons to apply?

Thanks

 

[metengr]

Euroweld;
In addition to the above by Metalguy...PWHT in Codes and Standards for even plain carbon steel was developed based on practical experience in building boilers and eventually pressure vessels. Residual tensile stress from fabrication is probably one of the main drivers that influences the susceptibility of a component to failure from fatigue and stress corrosion cracking or environmental-assisted cracking.

Thickness of the pressure retaining item plays a significant role in requiring PWHT because of possible plain strain conditions if internal flaws are present and heating/cooling either during forming or welding.

 

[kingnero]

Also it gives any hydrogen more time to diffuse out of the weld.

PWHT and CE are related, do you know how to calc carbon equivalent and how to interpret the value?

 

[EuroWeld]

Hello Kingnero,

Kindly thank you for your reply. I am aware of the carbon equivalent (Cev) and its relation to the maximum hardness of the material.
Next to lowering the resedual tensile stress we can make a list of the following factors/reasons to apply PWHT:

1. Diffusion of hydrogen from the weld;
2. Lowering the chance of Caustic and Stress corrosion cracking during service;
3. Lowering hardness;
4. Lowering the chance of brittle fractures.

Am I correct or can we find more?

Thanks and regards,

 

[Metalguy]

CE isn't used to determine the max. hardness a steel can reach. It is a measure of hardenability, which means how deep/thick that steel is capable of being hardened. It is related to the time a steel takes for austenite to transform by diffusion to pearlite. The higher the CE, the less time.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[metengr]

EuroWeld;
What are you trying to accomplish with the above? Don't try to re-invent the wheel.

 

[EuroWeld]

Metengr

I am not trying to. Just for my interest.
Wanting to get a good view on when to perform or when not!
Also I want to sort out the invluence of the heat treatment towards stress corrosion cracking.
For this view I need a complete situation. I hope this is not against any rule?

Metalguy

Cev does not relate to maximum hardness of the material?
Does it also relates to the invluence of alloy elements on the weldability?

 

[Metalguy]

No on the first question. If you mean the likelihood of HAZ cracking w/o pre or post weld HT, then yes.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[metengr]

EuroWeld;
I had listened to the attached presentation by Dr. Carl Spaeder, and I think you would find the attached presentation beneficial for your endeavors;

http://www.nationalboard.org/SiteDocuments/General Meeting/Spaeder.pdf

Other references

http://www.scribd.com/doc/28548657/PWHT

http://www.kobelco-welding.jp/education-center/abc/ABC_2001-02.html