Nital or ammonium persulfate etching is appropriate when checking for overheating of high carbon martensitic steels, such as carburized steels or alloy steels like 4340. Some heating above the tempering temperature will cause the steel to darken when exposed to the acid etch procedure (overtempered), and more heating will cause a white spot surrounded by darkening (re-austenitized and quenched). The two industry specifications I know are AMS 2649, "Etch Inspection of High Strength Steel Parts" and AGMA 2007, "Gears- Surface Temper Etch Inspection after Grinding".
AMS 2649 is being revised by AMD Committee B, by the way. Should you have any questions or want to suggest changes, contact Committee B through the SAE webpage, or send me an e-mail (I'm not hard to find) and I'll pass along the comments. Regarding the AGMA spec, here's the information from their webpage-
ANSI/AGMA 2007-C00 Surface Temper Etch Inspection After Grinding [Same as New ISO 14104]
Explains the materials and procedures to determine and evaluate localized overheating on ground surfaces. Includes a system to describe and classify the indications produced during this inspection. However, does not provide specific acceptance or rejection criteria. Revision of ANSI/AGMA 2007-B92.
ISBN: 1-55589-761-4 Pages: 6
Hope this helps!
