I find it necessary to interviene at this point in time.
Firstly ferrite content in a weld is expressed as ferrite number (FN) and never as a percentage as no two persons on earth can agree to the percentage ferrite in a weld. There is no direct correlation between the ferrite number and the % ferrite in the weld.
In welding austenitic stanless steel we normally require between 4 to 7 FN ferrite in order to ensure that no hot cracking takes place in theweld metal. Pure austenitic weld metal is extremely susceptabe to hot cracking.
Numerous studies have show that the heat inut during welding does not have a significant effect on the ferrite number in the normal welding ranges but a very low and very high heat inputs there are some affects.
There are a number of diagrams which can be used to calculae ferrite numbers based on the chemisty. Previously the Delong diagam was mentioned but there ae others including Schaefler, WRC 92, which is more recent and gives additional data.
The issue with getting different ferrite numbers from the same pack of consumables are mostly related to the welder in terms of arc length control Nitrogen has a significant influence on the Ferrite number and een a very small percentage entering the weld metal through using a long arc length will have significant variaion in ferrite number.
John du Plessis
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From: firstname.lastname@example.org [mailto:email@example.com] On Behalf Of suneel gore
Sent: 21 July 2009 01:36 PM
Subject: [MW:2736] Re: 2733] RE: 2729] Ferrite Number (FN) for the deposited weld metal
Dear Rishi RAJ date 21.07.2009
Yes you are right , The heat in put(Or the cooling rate ) have definately have effect on some grades of SS Mainly 304 & 316
Rishi Raj Singh
From: firstname.lastname@example.org [mailto:email@example.com] On Behalf Of PB HEGDE
Austenetic SS used in chemical /petrochemical plant require good Corrosion resistance. For which delta ferrite plays the major role.
i)Increase in Delta Ferrite will reduce the corrosion resistance . Therefore in severe corrosion resistance requirement , ferrite to be kept below 8% (Generally it is kept between 3-7 %)However very low ferrite causes defect like hot crack, fissures ,There is no code requirement but Ferrite requirement is decided by process licenser ,depending upon the service condition
ii)You can check ferrite No by Instrument called Ferrite Meter where you can get Ferrite No directly ,Some time You can check with respect to Delong Diagram ( Given in ASME Sec 2C ) by calculating /plotting the Cr , and Ni equivalent from actual chemical analysis of weld
iii)My experience –some time Ferrite no increases due to work hardening of material If you measure by Ferrite meter
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