Can the metallography used to determine the ferrite number ?

[stanislasdz]

Hi,

I used the metallograophy and image analysis to determine the ferrite fraction and using the Long and Delong diagram a convert it to FN.

Is it correct ?

Stan

 

[metengr]

stanislasdz;
The ferrite number should be determined by using a ferrite gage for the best accuracy. The Delong and Schaeffler diagrams were developed to correlate directly the Cr and Ni equivalents into a ferrite number and soldification mode(s), so that you don't need to determine the ferrite number nondestructively.

If you are performing metallographic examination of the weld region. I would use the following approach;

- determine the chemical composition of the weld metal
- determine the chemical composition of the base metal(s).

Depending on the welding process, you can determine the effects of dilution between the weld and base metals;

Depending on the welding process, for example SMAW, you can assume 70% dilution of the weld metal with 15% dilution with each base metal (30% total).

Now set up a small table and select the major alloy elements of the weld and base metals, and determine their composition in the diluted regions of the weld region using the above percentages.

Once you have established the weight percentages in the dilution regions of the weld region, you can use the Cr_[eq] and Ni_[eq] equations from the Delong or Schaeffler diagrams to predict phase and ferrite number.

 

[EdStainless]

The meallography will give you the actual volume % ferrite, you don't need to convert this to anything.
Are you talking about the slight missmatch between FN and Vol%? If that is the concern then I guess that you could covert, but I would rather see the actual volume % reported.

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[stanislasdz]

metenger & Essatainless, since i'am a metallurgist, I realy enjoy when i read yours posts....thanks a lot !!!

Why i ask this question

I have read in the Egyptian Solids Journal a paper. The author say that if we ageing the microstructure for 500 hours :

- The ferrite number wil decrease by 7.6 %
- The ferrite fraction will decrease by 30 %

Why this diffrenec betwen the metallograophy and the ferrite gage ,

Ps: sorry for my very poor english !!

Initial microstructure :

Final microstructure after 500 hours aging at 400 °C

link to the paper :

http://www.egmrs.org/EJS/PDF/vo291/151.pdf

Stan

 

[metengr]

A ferrite gage provides the ferrite number directly. I would not use metallographic examination to obtain a ferrite number.

Obtaining the volume fraction of ferrite is tedious and can be subject to error unless you have some type of digital imaging system. The ferrite gage is easy to use and is based on magnetic properties of the weldment based on the volume fraction of ferrite.


You can Google search words like "ferrite gage" to obtain additional information.

 

[stanislasdz]

thanks metengr !

 

[metengr]

stanislasdz;
You might want to review the paper below.

http://files.aws.org/wj/supplement/01-2003-VITEK-s.pdf

Review pages 13S and 14S to see the background on FN and attempts to correlate the FN and % volume fraction of ferrite.

 

[arunmrao]

Metengr,I agree with your views. However it was just last week my son was taught the metallographic way to obtain the ferrite number. It was a long distance call and I could not help him much. They were not using the ferrite meter ,I understand.

 

[kenvlach]

There is a 2-page article 'Etching Stainless Steels for Delta Ferrite' by Dennis W. Hetzner in the Feb. 2007 issue of Advanced Materials & Processes (

http://www.asminternational.org/Template.cfm?Section=AdvancedMaterialsandProcesses&NavMenuID=60).

[Free with ASM membership; non-members can register & purchase]
Color etching and electrolytic etching were compared for metallographically evaluating the delta ferrite in two martensitic stainless steels, Mill Clad 3 (MC3) & (from Carpenter's) Wrought P675.

"The color etch in these examples was a modified potassium metabisulfite etch composed of 1% HCl, 1g K₂S₂O₅, and 1g NH₄HFH [sic, should be ammonium bifluoride, NH₄HF₂] in 100 ml of distilled water. Approximately 15 seconds was required to etch the specimens.
The electrolytic etch was ASTM E407 number 220. This etch is composed of 20g of NaOH in 100 ml of distilled water. The specimens were etched at 6 volts for approximately 30 seconds."

The color etch seems to be a low-HCl version of Beraha's tint etch for stainless steels. It gives impressive results for the P675 alloy.