PWHT in Cr-Mo alloys How many times allowable? 2

[ak1965]

Dear Forum,

Few questions regarding Cr-Mo alloys are lingering in my mind....putting up the same to the forum seeking guidance:-

1. How many times a Cr-Mo alloy steel can be subjected to PWHT operation without affecting the material properties, owing to repair of recurring weld defects?

2. Should an alloy steel weld joint be necessarily subjected to radiogrpahy "before & after PWHT"? if yes, what are is the philosophy behind this concept?

3. What kind of changes occur in the microstructure during PWHT? I believe that no microstructural changes occur in the weld joints & only residual stresses are relieved.


Regards

 

[DVWE]

The number of cycles depends on your PWHT time, and temperature. It will also depend on the quality of steel that you buy. We have qualified some of our 2.25Cr-1Mo weld procedures for 12 hours at 1275 F. That gives us 4 cycles if our PWHT single cycle is 3 hours maximum. One of the purposes of WPS qualification is for you to determine how many cycles you can do at a certain temperature.

I don't necessarily know about RT before and after, but many customers require some type of NDT before to ensure all defects are removed before PWHT - that way you don't have to expend another cycle fixing something that was there before PWHT. Then RT after to ensure nothing happened (such as cracking) during PWHT.

 

[ak1965]

DVWE

I could not exactly correlate as how the no. of cycles depend on PWHT time and temp.?

My question was simple....due to repeated defects in welding, we need to carry out PWHT again and again....how many times PWHT of a weld joint isallowed without adversaly affecting metallurgical properties of material?

 

[DVWE]

ak1965,

Answer me a few questions:

1. Do you have a qualified WPS?
2. What is the PWHT temperature for the WPS?
3. How many hours for the WPS?

 

[ak1965]

DVWE

1. Yes I do have...!

2. PWHT temp. was 725 deg c soak for 2 hrs.

3. Heating at 75 Deg. C per hr and colling also 75 deg. c. loading and unloading temperatures at 300 Deg.C

 

[DVWE]

Then you are only qualified to do 725 C for 2 hrs!

If you need more time, you have to qualify your WPS for the maximum foreseen time needed for production. You will need a new WPS for this amount of PWHT time.

Here's an example:

WPS is qualified at 725 C for 8 hrs. You can do 4 cycles of 2 hrs. So you can use 1 cycle for fabrication, and 3 cycles may be used for repairs if needed.

 

[DVWE]

One thing I forgot to add:

If your WPS fails the mechanical tests, then you know that your temperature is too high, or your time is too long. You will have to either decrease time, or temperature, or both.

Does that help?

 

[Metalmeister]

AK1965,

In response to your questions:

1. None. Everytime you PWHT you affect the material properties. ASME III requires that test coupons be tested with at least 80% of the time that the fab shop uses in production and field installation.

2. The code typically specifies what NDE is necessary and the sequential timing of same. Since welds can crack during PWHT, you need to do it after. On the other hand you don't want to time and energy PWHT a weld with defects. So you might want to consider both.

3. Microstructural changes indeed occur during PWHT - lots of them: tempering of Martensite/bainite; carbide growth, carbide spheroidization. That is why metal slowly softens, gets weaker after many hours of PWHT.

Hope this helps.

 

[metengr]

2. Should an alloy steel weld joint be necessarily subjected to radiogrpahy "before & after PWHT"? if yes, what are is the philosophy behind this concept?

It depends on the alloy composition. For carbon and low alloy, no. RT can be performed before PWHT provided a surface NDT is performed (this is not a code requirement it is my requirement) after PWHT. Also, low hydrogen electrodes should be used if RT is performed before PWHT.

3. What kind of changes occur in the microstructure during PWHT? I believe that no microstructural changes occur in the weld joints & only residual stresses are relieved.

It depends on the alloy composition. The Cr-Mo grades that contain carbides will see some coarsening of carbides in the ICHAZ in addition to the reduction in residual stress,.