Hello Shaji
Ferrite is needed in austenitic weld metal to reduce sensitivity to hot cracking.The minimum ferrite limit necessary to assure freedom from cracking depends, among other factors, on the weld metal composition.
As per IIW, IX-1695-92-Guidance on specifications of ferrite in stainless steel weld metal- For high-temperature applications(up to 700 deg C) for SS 316H, 308H, 347, typically 3-8 FN range suits most applications.As per API RP582:- The minimum FN for type 347 shall be 5 FN. The minimum FN may be reduced to 3FN provided the fabricator submits data verifying that hot cracking will not occur using the lower FN consumable to be used in production and this is approved by the purchaser.
Welding technique , especially arc length can alter ferrite readings in the weld.Longer arc can cause ingress of Nitrogen in the weld and loss of Chromium through oxidation. Nitrogen being a strong austenitie stabilizer would reduce ferrite. However controlling (increase or decrease) ferrite by this technique is not advisable. One can not control the arc length precisely.
The best way to get the correct ferrite values would be through adjusting the correct weld metal composition. Please see the attached document which illustrates the Schaeffler and Delong diagram. The very basic difference between the two is that Delong considers the effect of Nitrogen in the weld metal.
Traditionally for S.S 347 weld metal ASME SFA 5.4 does not allow any Nitrogen.The correct composition, together with welding technique will be helpful to get ferrite between 3-8 FN as stated above.
Hope this clarifies your query.
Thanks.
From: shaji nair [mailto:shajinair70@gmail.com]
Sent: Thursday, March 10, 2011 9:26 AM
To: pgoswami@sympatico.ca; pgoswami@quickclic.net
Cc: Jignesh Panchal
Subject: Effect of ferrite element on 347H material
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