ASME P number 15E to ASME P number 8 weldability , Technical query

[Zakaria ghrab]

Dear experts,

 i have a doubt and need your help.

We have a piping dissimilar material P91 to SS 304 grade, Normally should be welded by ER82 GTAW filler metal.

Actual it's already welded but using ER 316 ss filler metal without PWHT;

Construction code : ASME B31.1

Design condition : T= 571°C and  P = 18 MPa

Operation condition : T = 566 °C and P= 16.5 MPa

Could you please explain to me what's the risk for the actual weld in process condition and more advise .

Regards.

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زكرياء غراب
GHRAB Zakaria
Jorf Lasfar Units 5&6 Power Plant construction Project
EA2C/ SDEM Entreprises
Site QA/QC Manager
Mobile : Morocco (+212) 6 61 07 00 11
             Tunisia (+216) 96 635 805


Yesterday is the pass, tomorrow is the future; and today? Today is the present, the gift: Enjoy it!

[Octane Mac]

Austenitic stainless steel filler at this high temperature is not advised to be used. The main issue with this welding is the carbon migration from the base metal (P91 in this case). Chromium has a greater affinity for carbon than does iron. A significant number of carbon is migrated from the base metal. This carbon migration result in low rupture strength of the weld joint during elevated temperatures. Or fatigue failure may occur in the decarburized band.

Secondly, PWHT should not be waived off as P91 must not be taken in service without PWHT after welding is performed.

For dissimilar welding, you may also consider reading the following short document:

http://www.scribd.com/doc/167909514/What-to-Consider-When-Welding-Dissimilar-Materials

Octane

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[pgoswami]

Hi  Zakaria,

 

Grade-91 steel should always be welded to any austenitic stainless steel as follows:-

• Welding consumable high Ni type, preferably,  E-NiCrFe-3/ ER-NiCr-3(In-182/82) or alternatively E-NiCrFe-2 also would be accepted.
• Thinner sections say T-91(tubes), typically 4-5 mm and around could be directly welded to austenitic S.S. Following welding the welds have to be PWHT-ed typically @ 740-760Deg C.
• PWHT at this high temperature is bad for  300 series steel, as it could  be sensitized. However a smaller soak cycle of 15-30 mins (based on weld  thickness) @ above temperature is the only choice. 
• Sensitization is time & temperature dependent issue . A shorter soak time may result in lesser damages. As least this would be better than leaving the grade-91 steel in Non-Heat treated condition  following welding.
• Thicker sections,  say Pipes-(P-91)  should always be buttered with  E-NiCrFe-3/ ER-NiCr-3(In-182/82) , PWHT-ed at above temperature , then welded with the same consumables without further PWHT.
• If this buttering technique is feasible in the field for tubes, it could be a good option also and would be preferred than direct welding and then PWHT.
• The other feasible option is to insert , a shop welded Pup- piece(P8-Grade 91 transition piece) in the field and weld individual alloy # same.

In this case using 316 S.S consumable+NO PWHT  was a gross mistake. The possible consequences  of this mistake would be:-

• failure of the hardened Grade-91 HAZ (typically 350 HV or above), during service or hydro test
• the service induced failures could be thermal fatigue if the plant is cycled (quick start up and shut down)
• 316 S.S has higher coefficient of thermal expansion , hence would during service would exert  more thermal/tensile  stress on the weld during the service. Such higher stress coupled with hardened  Grade-91 HAZ  could cause creep failures also.

 

The suggestion would be :-

• perform PMI on similar welds  under doubt
• if found that 316  or 309 filler was used, please cut it out and reweld.
• the sooner it's done , it's better.

Also have the walk-downs  with the welding crew and explain them the consequences of such mistake. I've seen many experienced welders get thoroughly convinced to do a better and makes the life of the Q.C guys "less miserable" next time.

 

Many fabricators treats Grade -91 steel as like any other Cr-Mo steel, without being aware of the proper welding practices and resultant consequences. The example stated by you is a classic example.

 

Thanks.

 

Pradip Goswami, P.Eng,IWE

Welding & Metallurgical Specialist

Ontario, Canada

pgos...@quickclic.net

pgos...@sympatico.ca

 

 

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From: material...@googlegroups.com [mailto:material...@googlegroups.com] On Behalf Of Zakaria ghrab
Sent: Saturday, September 14, 2013 4:13 AM
To: material...@googlegroups.com
Subject: [MW:18654] ASME P number 15E to ASME P number 8 weldability , Technical query

[george....@gr.bureauveritas.com]

If your construction code is ASME I or B31.1, then you should know that P91, regardless of thickness, has to be PWHTed
You should not weld directly the 304 to P91. It is advisable to butter the P91 with a high nickel consumable, then perform the PWHT and then weld the 304 without PWHT.
After the PWHT of the buttered piece, it is advisable to perform a corrosion test, since there is a risk of sensitization of the 304.

An other way would be the following:
Use a transition section as follows: P21 to P91, then P11 to P21 and then 304 to buttered P11
Even if the thickness of P11 is such to require a PWHT, the risk for sensitization will be much lower
 

best regards

Dr. Georgios Dilintas

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Zakaria ghrab ---14/09/2013 11:25:50---Dear experts,  i have a doubt and need your help.

From: Zakaria ghrab <zakari...@gmail.com>
To: "material...@googlegroups.com" <material...@googlegroups.com>
Date: 14/09/2013 11:25
Subject: [MW:18654] ASME P number 15E to ASME P number 8 weldability , Technical query

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