Etch for Austenite in Maraging Steels

[will123]

Your assistance on a particular problem would be appreciated and I thank you in advanced.

I am examining a cracked component manufactured from Maraging Steel. The manufacturing process is simply finish machine then solution treatment and age. Before, you ask why it was solution treated because that is the usual as-supplied condition, it was simply a miscommunication and has now been corrected for future batches.
On carrying out metallurgical examination on the part after polishing and etching with Nital 2, an area of size approximately 400micron by 5micron close to the crack initial and adjacent to the surface of the part was seen to be unaffected by the etch. The hardness of this area is around 350-400Hv; much softer than the rest of the section (615Hv). Although I currently do not know how it developed I assume this is austenite - as a quick test to confirm whether this is the case can any one suggest a suitable etchant for Austenite.

Once again thank you for any assistance

 

[EdStainless]

"maximum amounts of stable austenite occur on holding 18Ni(250) steel for 1 hour at 1110-1200F. In general, the stable austenite produced by (this) reversion cannot be transformed to martensite by refirgeration."
I actually cannot find any optical pitures of these alloys, everything is SEM or TEM.
The best ref for these alloys that I have ever found is part of the Cobalt Monograph Series, Cobalt Containing High Strength Steels, pub by CIC, Brusseles, 1974.

These alloys can be very sensitive to thermo-mechanical history. Re-heat treatment can be very problematic.

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Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[bruv]

Check this thread - there might be something useful, particularly in the ASM book

thread330-49147

Bruv

 

[CoryPad]

Maybe Beraha's tint etch. It is suitable for austenitic, duplex, and precipitation-hardenable stainless steel grades, but should work for you. The following is from the ASM Handbook: "... 200 mL HCl and 1000 mL H₂O. Add 0.5-1.0 g K₂S₂O₅ per 100 mL of solution (if etching is too rapid, use a 10% aqueous HCl solution). Immerse at room temperature (never swab) for 30-120 s until surface is reddish. Austenite colored, carbides not colored. Longer immersion colors ferrite lightly. If coloration is inadequate, add 24 g NH₄F · HF (ammonium bifluoride) to stock reagent..."

Regards,

Cory

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