A335P91-PWHT-maximum creep properties

Date 09-15-2008 18:13

I thats an exact quote from your customer they're lazy. Thats lousy specification language. Creep under what conditions? If they really wish to maximize or optimize, creep properties then they should do their own homework not to mention provide you, their supplier, with the temp with which the material should be PWHT'd which is tied into their design.
However, this also ignores the balance that needs to be maintained between creep and toughness when considering PWHT. Current thinking is that the best balance is achieved at ~1400deg F.
If you want to "garantee maximum creep properties" then theoretically you don't heat treat it at all. That way all of your Cb, V, and N stays in solution. However, this would be insane. You have to consider toughness as well.

By locolito Date 09-16-2008 05:53

Many thanks js55 for your reply. It's actually the first time I'm trying this forum and I'm realizing it's a big help !
You are recommending the PWHT to be done at 1400°F which is the upper limit given by the code (B31.1) for 1hr./in., is that correct ?
In order to avoid any bad surprise at the end we would do the extended hydrogen baking (acc. ASME B31.1, Ch. V, 131.6.1 (C) > intermediate heat treatment (hydrogen bake / soaking) as to allow
the weld to cool down to ambient temperature for NDT and then go ahead with the PWHT as such.
What happens if in worst case I however have to re-do the weld completely (e.g. because of a reject-able NDT indication after PWHT (crack) or as I know of a situation where after hydrotesting a cap will be removed and a tie-in spool will be fitted, performing two golden welds at the very same location).
Am I right assuming that if I qualify the WPS with impact testing so I would need to apply/qualify two PWHT cycles in order to cover such a case.
Please give me an input. Thanks

By js55

Date 09-16-2008 12:20

Keep in mind the upper limit for PWHT, if your working to B31.1 is not mandatory. The transformation temp is mandatory. You need to be aware of where the transformation temp is. This is the reasosn for Ni + Mn to be less than 1.5% in most specs. Ni + Mn reduces transformation temp bringing it closer to most recommended heat treat temps.

You do not need to qualify additional HT cycles. But many specs require some relationship of time at temp between the qualification and production. This is the important variable. Time at soak temp.

The requirements for impact testing are only imposed if they are code required. For B31.1 they are not. Any impact requirements pertaining to welds being used under B31.1 have to be specification driven, or come from sound engineering judgement, the biggest concerns being moving the stuff around before PWHT and the stresses involved in start up.

I would highly recommend, iof your going to be doing Grade 91, getting a copy of the Mannesmann P91 book and perhaps visiting the EPRI website.

By locolito Date 09-18-2008 14:52

Many thanks again js55 ! You are really helping me to get the point.
I have registered on the EPRI website and I'm waiting for the access password. What's the shortest way to order the Mannesmann P91 book ?
Let me ask you another question, please;
If any client spec. calls for impact requirements, then the supplementary essential QW-407.2 (T&T range) (Sec. IX) becomes for both the GTAW and SMAW process essential.
The text of QW-407.2 says: "A change in the postweld heat treatment temperature and time range".
How is this to be understood, how narrow (what temp./time tolerance plus/minus) would "a change in the postweld heat treatment temperature and time range" have to be in order not to exceed the "range" ?
My thinking is that since a WPS can be qualified for quite a range of base metal thickness, the holding time would for 1" (1 hour) however be different than for 2" (2 hours) (B31.1, Table 132 - 1hr/in.).
I will be very glad about your clarification. Thanks and regards

By gwg

Date 10-21-2008 02:34

"A change in the postweld heat treatment temperature and time range".
How is this to be understood, how narrow (what temp./time tolerance plus/minus) would "a change in the postweld heat treatment temperature and time range" have to be in order not to exceed the "range" ?

Reply;
The time and temperature range normally stated for PWHT on the WPS is based on code of construction PWHT minimum temperature, and 1 hr/inch for x nominal thickness (followed by hr/inch for each additional thickness over nominal thickness of x). The maximum PWHT temperature for the WPS is specified as an absolute max to avoid exceeding the lower critical transformation temperature of the base metal or weld deposit (if applicable). For Grade P91 material the key is to know the Mn and Ni composition of the filler metal to avoid overshooting the lower transformation temperature. If you use the range of 1380-1420 deg F, you should be fine for PWHT for Grade 91 material.