Dear, Mr Sankaran,
- One of the materials that has spread through the piping and boiler industry recently is an alloy, referred to in various specifications as "T-91, "P-91, "F-91" and "Grade 91." This is a specially modified and heat treated 9% chromium, 1% Molybdenum, Vanadium enhanced (9Cr-1MoV) steel that performs quite well at elevated temperature – usually 1000°F and higher.
- F91 grade Preheat and interpass temperature are very important. A range of 400 to 550°F is recommended, After welding is completed, the joint should be allowed to cool slowly to at least 200°F after welding is completed to be sure that all the austenite has been transformed to martensite prior to postweld heat treatment (PWHT). If this is not done, there is risk of martensite formation after PWHT; this will result in hard, brittle welds. For the metallurgists out there, the Mf temperature is above 212°F, varying some with the grain size.
- Pre Heat literature suggests that 200°C is adequate for preheating P91 and P92 grade weldments. Fabricators typically aim for 200°–250°C, but will go as low as 121°C for root and hot pass layers, thin-walled components, or where GTAW is utilized. Experience indicates that no elevated preheat is required for T23 or T24 grade weldments; however, some code bodies including ASME require preheat or postweld heat treatment (PWHT) for these alloys.
- Typical maximum interpass temperature is 300°C, less is acceptable but no more than 370°C. The interpass maximum helps to prevent the possibility of hot cracking due to the silicon and niobium content of the weld metal. Field operations rarely have problems.
- Finally, postweld heat treatment is required for Grade 91 steels, regardless of what construction codes may permit. The holding range should be 1375 to 1425°F for a minimum of 2 hours. Even on small superheater tubes, a long time at temperature PWHT temperature is necessary to form the required weld structure, to ensure adequate toughness during hydrostatic testing and to ensure adequate service life. The lower transformation temperature can be as low as 1450°F; if this temperature is exceeded during PWHT, the weld should be allowed to cool to below 200°F followed by reheat treating or the condition of the joints should be evaluated by hardness testing.
Thank & Best Regards,
M Veera SAMY- Bureau Veritas.
Coimbatore
9790964530
Date: Fri, 31 Aug 2012 18:03:05 +0800
From: subasankaran@yahoo.co.in
Subject: [MW:15173] Defects in the MIG welding of F91+F91 -reg
To: materials-welding@googlegroups.com
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