Inconel 625 etching 2

[Mett87]

Hi,

Could anybody help with a suitable etchant for revealing grain strucutre for grain size determination in Inconel 625, I'm guessing mixed acids is the way to go? I am aware a bit trial and error would be the best solution but as I am waiting for the components I thought I would ask the question.

Thanks

 

[CoryPad]

Glyceregia and aqua regia will work. Here is a definitive guide:

http://www.georgevandervoort.com/mic_met_pdf/HeatResistingAlloys_Vol_9_MetalsHandbook.pdf

 

[EdStainless]

We use electrolytic oxalic acid, it give enough GB definition to be usable.

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Plymouth Tube

 

[Mett87]

Thanks guys, I done a bit reading up on some of Vander Voorts papers. One strange thing with the samples is MC type carbides are visible in the un etched condition, however, in the areas the material has been deformed they appear to have disperesed very finely/or gone completely! Is this possible!?

 

[Mett87]

also I have tried many of the advised etching techniques with no results

 

[jwhit]

It is extremely nasty and safety gear and good ventilation is a must, but aqua-regia with 30% H2O2 will often work. Mix the aqua-regia then make occasional additions of the H2O2 very carefully. The reaction is extremely vigorous. Good luck and be careful. The first time I used it the fume hood was not up to the task and the lungs took a burning.

 

[Mett87]

Thanks for the advice, I have now had some luck etching but I am seeing strange areas in the microstructure. I have attached a micrograph

 

[EdStainless]

1. etching for precipitates is much different than etching for GB.
2. you can have little to no precipitates, it depends on anneal temp, cooling rate, and cold work

Can you see the acicular phase without etch (polarized or DIC)?

What is the product for and what is its thermo-mechanical history?

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Plymouth Tube

 

[Metaljon]

The structure does not look like inconel 625 or no inconel 625 I have ever seen. I don't think you have etched this structure correctly as I can see any grain boundaries. I think the acicular structure is a polishing artefact. Etching inconel 625 is sometimes difficult and requires some experimentation to get it to etch properly. Try 15 mL HCl, 10 mL acetic acid, 10 mL HNO3, The acid has to be made up fresh as it goes off.

 

[Mett87]

Ed, Unfortunately I do not know the background behind the material, also, i dont have the facility for dic or polarized light. metaljon, I have tried with mixed acids etchant with no luck

 

[Mett87]

Another micrograph showing defects due to cold working?

 

[OGMetEngr]

Not sure if you've tried this combo, but 15mL HCl, 10mL acetic acid, 5mL HNO3, and 2 drops glycerol revealed the grain structure in this 625 micrograph.

Source: ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys.

 

[CoryPad]

That does not look like cold work, looks like solidification structure (e.g. welding).

 

[metengr]

I would agree, this appears to be more of a dendritic microstructure.

 

[Mett87]

It appears only on the outer few mm. Would you agree it coukd be possibly from localised heating and slow cooling?

 

[EdStainless]

Or from serious heavy machining.
Looks more like deformation than structure.

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Plymouth Tube

 

[Metaljon]

Well your sample looks etched to me. Etching is a bit like making a cake you have to experiment with the acid mixture and concentration. A little a bit of glycerol may help. The photo shows evidence of a weld structure. Has this been weld repaired?

 

[Mett87]

i was saying i had no luck with the mixed acid etchant. this was etched electrolytically

 

[mrfailure]

You certainly see weld (dendritic) structure and grain structure in the adjacent base metal (probably heat affected zone) when you zoom in on the #2 picture. Looks like electrolytically-applied oxalic works (in that picture), but you may want to photograph at higher mag.

Aaron Tanzer

http://www.lehightesting.com