B31.1 has changed in recent years. PWHT is not required per general notes (a) and (b) to Table 132.
General Note (a) is the basic historical exemption and applies where the minimum value of the weld thickness or the thicker base metal (nominal thickness per 132.4.1) is < 19mm and preheat of 95C is required if one of the base metals exceeds 25mm in thickness. [as pre previous]
General Note (b) is relatively new exemption, and exempts carbon steel materials with a carbon equivalent < 0.5 (using the IIW formula) with a "nominal material thickness" per 132.4.4 (i.e. the thicker of the materials being joined at the weld) when 19.0mm < Tnm <38mm. Two caveats: (1) the preheat must be > 121C and (2) individual weld pass thickness < 6mm.
In practical terms Note (a) allows welding relative thin pipe to pipe and thin pipe to valve bodies w/o PWHT, while Note (b) allows limited hardenability carbon steel pipes to be welded to greater thickness (2X) than previously allowed with just Note (a).
Please take care to note the difference in definition of Nominal Thickness (132.4.1) and Nominal Material Thickness (132.4.4).
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From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of Bathula Raghuram (Mumbai - PIPING)
Sent: Tuesday, April 14, 2009 3:21 AM
To: materials-welding@googlegroups.com
Subject: [MW:1945] RE: 1944] PWHT per B31.1
It depends on your joint configuration, thickness of both basemateials, weld thickness etc.
Not mandatory when both the conditions are met, not just one.
1) the nominal thickness is ≤19mm
2) and a min preheat of 95 °C applied, when either of base metal exceeds 25mm
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of Torkaman
Sent: Tuesday, April 14, 2009 1:38 PM
To: materials-welding@googlegroups.com
Subject: [MW:1944] PWHT
Dear All.
Acc. to ASME B31.1 for material's with P No.1 & THK>19 mm, PWHT is required. But when min. preheat 95oC is applied, PWHT is non mandatory. Is it true?
Best regards,
Mehrdad
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