Calucating post weld for P91 2

[jimwaa]

I'm trying to create a procedure for welding P91. I can't find any method to calculate the "bake-out" temp and time. I figure the temp will equal the preheat temp.

Here is a quote from Robert Swanekamp, PE, HRSG User's Group.

"A post-weld hydrogen bake, at 600 F for a minimum of 20 minutes, at least three inches on each side of the weld."

Is there a formula or rule to calucate bakeout time? Is 20 minutes @ 600 F adequate? How does wall thickness affect this hold time?

jimwaa

 

[metengr]

Bake out is to ensure the removal of hydrogen from the surrounding base metal and weld metal. The minimum temperature for the bake out is normally 450 deg F because this is the threshold to actively diffuse hydrogen atoms dissolved in the metal.

Bake out procedure for this joint would be 500 deg F minimum for 2 hours (1 hour/inch of nominal thickness). The time duration serves to two purposes; to provide for a uniform temperature gradient through wall and to diffuse hydrogen atoms.

 

[jimwaa]

After 2 Hours of Bakeout, should it be dropped to 194 F at a controlled rate? What rate of cooling?

Or

Should it be air cooled to room temp from 450F, uncontrolled?

jimwaa

 

[stanweld]

Dehydrogenation is function of time and temp. As metengr has stated, it begins at approximately 450 F and can be done at any temp generally between 450 F and 650 F. Less time will be required at 600 F than at 500 F to permit Hydrogen to diffuse/evolve from the solid. In general we hold at 550 F to 600 F for one hr to two hours depending on thickness, except for small bore, thin wall pipe.

 

[jimwaa]

On the topic of P91. We are planning on installing gamma plugs which will require a 3/8" seal weld around the plug. After the seal weld, we plan on PWHT. If we PWHT a couple of times, what effect does this have on the material? Should this be avoided?I haven't seen anything in B31.1 that would prohibit multiple PWHT.

 

[davefitz]

I would have to defer to others more qualified, but if the 3/8" pplug is near a major butt weld such that that major weld is re-heated over 1400 F, there may need to be calculated a tempering parameter to ensure the time-temp at PWHT temp does not impact the long term creep strength of the major weld zone.

 

[stanweld]

Because RT is required after PWHT, there will be two post weld heat treatments - one for the butt weld after its PWHT followed by RT and one after welding the gamma plug followed by MT/PT examination of the fillet weld. Or you may wish to examine by UT and avoid the gamma plug problem. There should be no problem with overheating the butt weld during PWHT of the gamma plug weld.