Delta Ferrite Measurement in Martensitic Stainless Steels

[mrfailure]

Is the point count method specified by ASTM E562 an accurate means to determine percent delta ferrite in martensitic steels? What alternate method can be used? Any experience is helpful.

 

[EdStainless]

Yes, works fine. There is also a spec that covers using automated (computer vision) for this work.
The real trick to this is the sample preparation (polish and etch).

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P.E. Metallurgy, consulting work welcomed

 

[mrfailure]

So true, Ed. We need to make sure we use an etch that will differentiate the delta ferrite. We're still working on it.

 

[Guest102023]

Yes, microscopy, either manual (tedious!) or automated can determine the fraction of ferrite, so long as the etch clearly differentiates the phases. Ferrite in martensitic SS is white in micrographs I have seen.

Impact testing is a crude, qualitative way to compare ferrite content in samples.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[mrfailure]

Since I posted this, I was able to determine the answer. Delta ferrite will indeed etch as white in tempered martensite, but so does the matrix and the difference is subtle. What appears to work best is a 2-etch solution: First use an etchant that will bring out the delta ferrite like NaOH. Then use a staining etch that attacks the matrix but leaves the delta ferrite untouched - it will show up in white.

A good reference (which I forgot to look at before I posted) is a 129-page slide show George Vander Voort prepared on etching of stainless steels. This has lots of potential etching solutions to bring out the delta ferrite.

 

[EdStainless]

I have a slim little volume from ASM edited by Beraha and Shpigler called Color Metallography.
Some interesting tint etches that would be helpful are listed.

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P.E. Metallurgy, consulting work welcomed

 

[Guest102023]

If Vander Voort can’t figure it out no one can.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"