Matching grade 91 PWHT holding time

[rmillercwi]

Have some down time and researching PWHT hold time for matching grade 91. The majority of what I have read (prior to April 2015 EPRI report Guidelines and Specifications for High-ReliabilityFossil Power Plants, 2nd Edition) lists lengthy PWHT hold times of 2 hours and beyond.

Some recent consultation with a trusted weld engineer suggested cutting this back to PW.39 code minimums (1hr inch/30min min < 5" thk)as standard practice.

As I understand it, this is to limit softening but I cant find any published papers specifically stating that in any language that I understand.

Can anyone offer any insight?

 

You can take the most recent EPRI guidelines as gospel.
The really critical aspect of P91 PWHT is fine tuning the holding temperature based on the base metal Ni+Mn content. This is necessary to avoid going above the critical temperature. I'm certain your 2015 EPRI document addresses this issue.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[rmillercwi]

Unfortunalty it does not. It does detail a lowered heat treatment temp but reccomends detailing hold times based off experience with hardness numbers "The temperature and time at temperature for the PWHT should be
selected to ensure that the hardness at all locations in the area heated is within the specified range".

I did find a paper by the International Journal of Scientific & Engineering Research, Volume 6, Issue 4, April-2015 where they joined p-11 to grade 91 using .5hr, 1hr, 2h soak times using both B9 and B3 filler. Conclusion pointed towards a shorter soak provided higher tensil strength. Not quite the same thing I was looking for but I think the results with 91-91 would be similar


CONCLUSIONS
The effect of post weld heat treatment at 750°C for 0.5, 1 and 2 hours on microstructures, hardness and Tensile of TIG and SMAW weldment between P11 and P91 Pipe using E9018B3/ER90SB3 and E9015B9/ ER90SB9 as filler metals was studied. The following conclusions can be drawn.
1. Post weld heat treatment provided more homogeneous mi-crostructures after welding process and reduced hardness dif-ferences in welded microstructures, which could lead to a de-crease in weld cracking.
2. the most suitable post weld heat treatment condition for these TIG and SMAW weld joints is at 750°C for 0.5 hours for Sample 1 and at 750°C for 1 hour for Sample 2. This condition provides the Tensile Strength and the minimum hardness of the weld zone between P91 steel and weld metal as well as minimum hardness difference between P11 and weld metal.
3. Tensile strength decreased with increase in temperature and soaking time