Hi Jignesh and others,
In an earlier response to this query Mr Jeffrey Sowards, Ph.D. nicely explained how important is Delta Ferrite in Austenitic Weld Metals and what’re the common modes for formation of this phase.
If you’re interested for more details, the attached paper which was published in IIW-Welding in the world- in 1993 would be of help. The original IIW document is IIW-1695-2, title as mentioned above. This document is the guidelines for many design specifications related to ferrite in austenitic welds.
The following areas are of interest:-
· Benefits of Ferrite (of-course known to all)
· Compositional effects, Cr/Ni Eq ratio and the effects of ferrite. Fig-3 shows , as this ratio goes up, more and ferritic mode of solidification sets in (corresponding Ferrite No- 0,1,2.5,7, 50).
· Effects of heat treatment on ferrite, 475, 550-900 and over 900 deg C.
· What ferrite No is good for what applications and why
· Welding processes and the effects of welding variables on ferrite.
· Ferrite measurement issue.
The starting block in ferrite controls the chemistry, Cr/Ni Eq, which decides the solidification mode, cooling rates/ solidification rate, welding process variable all these helps to limited degrees in controlling ferrite.
Also in ASS if elements such as S, P and other tramp elements are very less (≤ 0.0025 or less), then the chances of their segregation @ grain boundary are greatly reduced, and so is resultant hot cracking. Since Mr Jeffrey Sowards, Ph.D explained this earlier, I am not elaborating the same.
Original IIW documents are Bilingual (English and French), hence you have to read it twice??? 
Thanks
Pradip Goswami,P.Eng.
Welding & Metallurgical Engineer/Specialist
Ontario Power Generation Inc.
Email-pgoswami@sympatico.ca,
From: pgoswami [mailto:pgoswami@quickclic.net]
Sent: Thursday, July 22, 2010 6:32 AM
To: GOSWAMI Pradip -THERMAL
Subject: FW: Re: [MW:6071] How Delta- Ferrite prevents Solidification Cracking/ Hot Cracking in SS??
Pradip Goswami,P.Eng.
Welding & Metallurgical Engineer/Specialist
Ontario Power Generation Inc.
Email-pgoswami@sympatico.ca,
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of manpreet
Sent: Thursday, July 22, 2010 3:09 AM
To: materials-welding@googlegroups.com
Subject: Re: Re: [MW:6071] How Delta- Ferrite prevents Solidification Cracking/ Hot Cracking in SS??
Hi
A minimum ferrite content is necessary to avoid hot cracking in stainless steel welds. The amount of ferrite in the weld
metal also controls the micro structural evolution during high temperature service. Moreover the amount of ferrite
controls the corrosion and stress corrosion resistance. The low temperature toughness of the weld metal is also related to
the weld metal ferrite content
The exact amount of it can't be specified, depends on different materials. for eg-
-When joining ASME P-8, Group 1 austenitic stainless steels, the filler metal must contain at least 1 FN or 1 percent ferrite. However, 347 electrodes shall contain at least 4 FN.
-When joining 310 or 330 stainless steels and for cryogenic and special corrosive service, 0 FN filler metal may be used.
-For service temperatures exceeding 430°C (800°F), the deposited weld metal shall not exceed 10 FN.
-For cryogenic service with temperatures of –100°C (–150°F) and lower, the ferrite content of all austenitic stainless steel welding material shall be in the range of 2 to 5 percent.
Hope Mr. Jeff has solved your quarry, this is just an example to demonstrate that ferrite content can't be specified.
Regards
Manpreet Singh
Welding Engineer
Spiecapag
On Wed, 21 Jul 2010 12:58:22 +0530 wrote
>
Hi,
We have come across, with very low delta ferrite ( 0.5- 1.5%) in the weld, leading to cracking & failure of components. With high % of delta ferrite also (10%), cracking is observed (even during storage).
Minimum 3% is required to avoid fissuring. Low and high FN-both are not advisable. I can explain afterwards, how it prevents.
Regards,
Vaidehi Ganesan.
-----------------------------------------------------------------------------------------------------------------------------
----- Original Message -----
From:
jignesh
makwana
To: materials-welding@googlegroups.com
Sent: Wed, Jul 21, 10 9:43
Subject: [MW:0] How Delta- Ferrite
prevents Solidification Cracking/ Hot Cracking in SS??
>
Dear All,
Good Morning
I have query regarding the solidification Cracking. Generally, FeS the compound responsible for solidification cracking.
Also Arsenic, Sulphur shows same phenomenon. Now Delta- Ferrite in 3-8 FN proportion prevents hot cracking.
But how ?? In most of the books, it is written that, " Delta- Ferrite has tendency to lock the impurities".
But how Delta- Ferrite can be helpful for the prevention of hot cracking by locking the impurities??
Also, what happens if Delta- Ferrite is very low of the order of 0.5 FN or so or what happens if Delta-Ferrite is high of the order of 15 FN or
so.
Dear All,
Please help me to find the answers.
Thanks in advance.
Jignesh
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