Cronidur 30 Etch

[AidanMc]

I have a part that came in for a metallurgical inspection made of Cronidur 30 (aka N360 Iso-Extra and AMS 5898). We have tried etching in both Nital and Vilella's, our standard etchants for most materials. Neither reveals much of a structure and the part also seems to have scratches afterward. Can anyone recommend a microstructure etchant for this corrosion resistant material?

Aidan McAllister
Metallurgical Engineer
Automotive Enthusiast

 

[metengr]

You might find the excerpt from the book below interesting for background information, and it does provide a description of the microstructure for Cronidur 30 bearings.

http://books.google.com/books?id=VQ...&resnum=2&ved=0CBMQ6AEwAQ#v=onepage&q&f=false

I did not find much information regarding etchants for this HNS martensitic stainless steel. I would recommend you contact Buehler, specifically, George Vandervoort for etchant techniques/recommendations.

http://www.buehler.com/

 

[AidanMc]

Thank you very much. I couldn't find much information myself on Cronidur 30. That helps some. I will try to get in contact with Mr. Vander Voort, I have seen much of his work in various magazines and books before.

Thanks

Aidan McAllister
Metallurgical Engineer
Automotive Enthusiast

 

[TVP]

Aidan,

I'm not sure if you or metengr noticed, but the authors of the article from the above link described the etchant that was used (2nd to last paragraph on page 137). I have not personally used HF + molybdic acid (MoO₃) as an etchant, so I cannot give any advice as to its usefulness for this alloy, but boiling HF is not your everyday lab chemical.

I don't think nital by itself will etch this alloy, but maybe adding some HCl to it would improve the response. I would try 5-10% nital + 5-10 mL HCl. Another option would be to revisit the Vilella's etch, but to use a different ratio, such as the one suggested by Struers in the following Application Note:

http://www.struers.com/resources/elements/12/101820/Application Notes Stainless Steel English.pdf

One suggestion from GVV and Buehler for etching martensitic stainless steel is modified Fry's reagent (150 mL H₂O + 50 mL HCl + 25 mL HNO₃ + 1 g CuCl₂). Here is a link to some of their technical information on etching:

http://www.buehler.com/technical_information/Buehler Book/etching.pdf

By the way, I am not sure that GVV works for Buehler any more. The latest issue of Industrial Heating has an article authored by him, but it shows his affiliation as a consultant to Struers. I haven't tried to contact him to confirm his whereabouts, so if anyone else does, please share with the group.

 

[mighoser]

My color metalography reference text shows the following for stainless steels (all types):

5-10ml HCL (35%)
1-3ml selenic acid
100ml ethyl alcohol (95%)