PWHT on T91 (9Cr) Material

[loneinspector]

Looking for some help re PWHT on T91(9Cr) material. We are building a boiler to ASME section 1 and i am looking for some help with timescale on the PWHT of the T91. I have always worked on PWHT must be carried out as soon as practical possible. e.g. 7 days. My client can't do this. ASME doesn't have a specific timescale. I don't like this new best engineering practices wording it will not hold up.

 

[davefitz]

I think you will need to follow the practice as learned from the experience from other boiler mfrs that have used P91 for the last 2 decades- some rules of practice work in their shop environment, and some don't. If you guess wrong, then you will have a costly repair bill to address when final hydro occurs and cracks galore ( younger brother of pu$$y galore, who starred in an early James Bond film) shows up.

First of all, for thick welds, there is not permitted to be any cooling below 170F prior to PWHT, so you must be addressing thinner welds.

The 2 issues that I am aware of that affect this time delay are SCC stress corrosion cracking and plain old shop stresses. I know of P91 pipes that have been left out in the Florida rain for months prior to PWHT, and no issues arose. And there are other cases where P91 bundles stored indoors had SCC , likely related to condensation whose droplets included concentrations of sulphuric compounds. As far as shop stresses are concerned , any loads placed on these welds ( and just pushing the bundle around will stress some welds) while they are in the extremely brittle state prior to PWHT can lead to cracks. It is my understanding that the initial use of higher Ni filler was related to adressing this last issue, as it improved the low temp ductility of the weld prior to PWHT, to reduce shop re-work.

 

[metengr]

There is good information on PWHT of Grade 91 material on the internet. Look for publications by EPRI, if possible. You should search out this information and review it. Next, you should develop a PWHT procedure so the process for heat-up, hold time and cooling rates are consistent.

That is correct, ASME Section I does not provide specific details regarding PWHT of material other than general guidelines because these are process details. PWHT for Grade 91 should be performed as soon as practical to avoid hardened material exposure to moisture or handling.

 

[stanweld]

Is your Client a boiler component Manufacturer or an Assembler? Is he welding tubes to headers and headers to pipe and pipe to pipe? As an Assembler, we have no real problem with time limits. Interruption of welding preheat after weld completion and prior to final PWHT is no real problem provided a dehydrogenation heat treatment at approximately 600F to 650F is perfomed.