[MW:7148] MW: 7143] ISR of P-91 pipes.

Hi Ravinder,

Grade 91 demonstrates substantial harden ability when welded. Although the hardness of the base material is around 220 Hv, hardness of weld metal as fabricated prior to PWHT can reach 450 Hv. This as welded high hardness results in substantial strength differences between the base material and the weld metal, which can lead to fatigue notches for equipment loaded by cyclic forces. Additionally, weld metal with hardness of this level generally does not exhibit adequate toughness to resist crack initiation or propagation. High hardness welds may also be susceptible to stress corrosion cracking (SCC) in humid environments.

The figure shown below the default recommendation for welding P-T-91 steel. This steel is fully martensitic when welded with the below recommended cycle, and that’s required for this steel. The major purpose of using this steel is to lower superheater and steam piping thickness for better heating efficiency and superior creep resistance. P-91 steel if treated like other Cr-Mo steels would spell disaster only.

There’s no allowable temperature for ISR of this steel in ASME Code. However I’ve come across some consulting companies specification where the recommended maximum ISR temperature is 427 ⁰ C(800⁰F), with typically 2 hrs / inch of soak at this temperature. Hardness readings require to be taken at this point to get the idea about weld and HAZ hardness. Following ISR the pipe spools shall be wrapped dry. Final PWHT shall be performed ASAP following the ISR , preferably within a week or two. Delaying the PWHT to 60 days could seriously harm the components, especially chances of failure during further fit-ups, hydro test or even later during equipment commissioning. You need to discuss this with your clients.

My organization is involved with EPRI (www.epri.com). In North America EPRI championed the awareness on P-91 steel amongst electric utilities and fabricators.

For your information this alloy was introduced in 1980’s for FAST Breeder Test reactor for US atomic Research program. However once adopted by ASME Sec-I for super critical boilers, it took a decade or more in North America to come to real terms, with respect to understanding the steel metallurgy and fabrication requirements.

In order to avoid ambiguity with other Cr-Mo steels, 2010 ASME Sec-IX changed the P No of this steel to P-15A.

Hope this provides enough guidance in your endeavor. You may find the attachments as useful info.

Thanks

Pradip Goswami,P.Eng.

Welding & Metallurgical Engineer/Specialist

Email-pgoswami@sympatico.ca,

pgoswami@quickclic.net

· Typical thermal cycles observed during welding and PWHT

From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of Ravinder Sharma
Sent: Wednesday, September 22, 2010 6:49 AM
To: materials-welding@googlegroups.com
Subject: [MW:7143] ISR of P-91 pipes.

Hi All,

I have welded joint of P91 of 400DN and 40 mm thickness, due to unavoidable reasons we have to do the heat-treatment after 60 days,

Is there requirement of ISR?

If yes, what should be the heating cycle?

Regards

Ravinder Sharma

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