ferrite number at SS 300 series

[ahmadire]

Dear experts,
I wanna know that what parameters cause the ferrite number goes up or down at SS 300 series?
and in clad with material of SS 309 and then 308 what would be predictable? could it be in the range of 5 to 12 percent?

 

[metengr]

Ferrite content is controlled by chemical composition. Typical ferrite number should be around 5 to 9 to avoid hot cracking.

 

[EdStainless]

So you have elements that stabilize austenite (Ni, C, N, Cu, Mn) and elements that promote ferrite formation (Cr, Mo, Si, Nb). 304 and 316 are meta-stable austenitic alloys, so some ferrite will form on solidification.
Alloys such as 309 and 310 are often very stable and have zero ferrite formation. While this helps their performance at long term elevated temperatures it can make welding difficult. Specialty alloys (Nitronic xx, alloy 20, AL-6XN) are also zero ferrite grades.
If you are welding thin material and it is not highly constrained you can get away with welding with low or no ferrite. In thicker more rigid systems I agree, at least FN 4 is needed, and values up to 10 usually don't hurt.

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P.E. Metallurgy, Plymouth Tube

 

[gtaw]

Just a point of information; The benefit of Ferrite in the weld is that it mitigates the probability of micro-cracks because the Ferrite has a higher tolerance for certain materials the will not dissolve in microstructures consisting of only Austenite. However, rather than speaking in terms of the percentage of Ferrite in the weld, industry typical uses Ferrite Numbers (FN). While they are in close agreement at the lower end of the range, there is divergence between percentages and FN as the amount of Ferrite increases.

Attempting to measure the percentage of Ferrite in the weld is difficult because it is not uniformly dispersed in the weld. When several labs evaluate the same sample, the results can have considerable variation because the field of view of an optical microscope is rather small. It is like looking at an elephant in the dark with a flashlight. Depending on where you happen to be looking, you will see a different feature; a leg, an eye, the trunk, etc. Thus several people, each seeing just a small portion of the elephant will form different conclusions.

Where as determining the percentage of Ferrite with an optical microscope provides a very limited view, the instruments used to determine Ferrite Numbers looks are a much larger area which tends to average the amount of Ferrite present and provides more consistent, reproducible results when a sample is evaluated by several individuals.

Best regards - Al

 

[ahmadire]

ferrite check was performed on surface of clad in Nozzle 36 inch. clad material at first layer is 309 and at last layer is 308. why the ferrite check number for 308 was about 1? is it meta stable or not? our desire number has been 5 to 12 and is this possible for 308 or 309??

 

[EdStainless]

I would expect 309 to always be very low.
What is the calculated value for your fillers? It is not uncommon to have the actual welds lower than the calculated value, it is the upper limit.
A value of 1 does not sound unreasonable for these alloys. Did you get good welds (they passed PT)?
Why last pass with 308? It is weld filler (there are no other product forms) and it has less corrosion resistance than the 309...


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P.E. Metallurgy, Plymouth Tube

 

[Guest102023]

The problem with low FN with 309 used for cladding touches on one of my pet peeves - 309 was designed for joining dissimilar metals (e.g. 304 to CS), but has been pressed into service as a butter layer for 308 overlay. Typical 309 composition is not well suited for this, which is recognized by some manufacturers who offer a low Ni, high Cr version just for overlay.

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

 

[EdStainless]

Brim, I agree that this is poor practice. Too low of FN and large CTE mismatch.

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P.E. Metallurgy, Plymouth Tube

 

[ahmadire]

As you mentioned firstly 309 was used as a buttering layer and then 308 was used.Can we consider 308 as a meta stable SS? If so it has to have enough amount of ferrite.Finally I didn't get my answer for this situation.Has the clad been designed correctly or not? Is this ferrite number predictable?