[MW:17617] RE: 17614] PHWT for P91

a)   Yes

 

b.1)  Since alloy 91 is fully air hardening, some authorities recommend a protracted hold at around 600°F or slightly higher to enhance hydrogen escape from the finished weld. This is probably essential for field work.  My experience with this alloy is that for welds made in shops, under controlled conditions, with good low hydrogen practices, this is not necessary.  However, I would recommend that the welds be wrapped with insulation after welding to slow the cooling rate.

 

b.2)  Cooling to room temperature after this low temperature soaking (or close to it) is required to ensure that as much retained austenite is transformed to martensite as possible.   Typical TTT diagrams show the martensite finish temperature (M_f) to be marginally above 200°F.  However, small amounts of austenite are “retained” even at temperatures less than the M_f.   Any austenite that has not transformed may transform to martensite after PWHT or subsequently,  forming hard, brittle martensite islands with obvious detriment to the weld.

 

For a brief introduction to this alloy you might want to read the piece http://www.sperkoengineering.com/html/Grade%2091%20R%2010-05.pdf

 

John A. Henning

Welding & Materials

 

From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of Binoy Soman
Sent: Tuesday, May 07, 2013 1:08 AM
To: materials-welding@googlegroups.com
Subject: [MW:17614] PHWT for P91

 

 

Dear experts,

 

Kindly explain following queries

(a)   PWHT holding time for P91 mentioned in B 31.1 is 1 hr/25 mm; minimum 30 minutes. Does it mean that if the thickness is 6/10/12.5 mm the holding time is 30 minutes & for 15 ,20,30,40 mm the holding time is 36  48,72,96 minutes respectively.

(b)    I got some special heating curves for P 91 from our group. But Kindly explain about the reference /standard used for preparation of this heat curve ( eg- why the preheat maintenance is 4 hours & again free cooling).

I raised these doubts earlier in [MW:17585], but I didn’t receive any response. Please give your comments.

 

 

S.Binoy

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