suitable etchants for 9Cr- steel 1

[sunshine419]

Please would appreciate members help here.
We usually carry out metallographic etching of 9 Cr steels (P91) using Villela reagent. However, one of our customer does not allow the use of this etchant due to restriction on Picric acid.

As an alternative, i have come across Fry's reagent (5g CuCl2, 40ml HCl, 30ml H2O, 25ml ethanol). Using this, i discovered that the exposure time is less than 2s otherwise the sample surface would be destroyed. This condition would not be ideal for on-site investigation and as such, I am looking for an alternative reagent that could be less agressive. Any ideas would be highly appreciated.
Many thanks

 

[metengr]

Use 5% Nital solution, and swab it.

 

[SRPJohn]

I suggest you try the Modified Currans etch. The recipe that we use is 10g FeCl, 30ml HCl, 150ml H20. Practice in the lab to establish the reaction time before you take it to the field. We like this etchant because it gets away from the picric although I think Vilella's is still the best etchant for 91. Good luck..

 

[metengr]

SRPJohn provides good advice for enhanced viewing of Q&T carbide structures in field replication work regarding Grade 91 CSEF steel.

 

[Guest102023]

I am aware of its properties, but at the concentration used in Villela's reagent, what is the risk with picric?

metengr, 5% nital sounds a bit crude, but then I have never used it for anything other than a macro-etch on ferritic steels. How are your results compared with Villela's?

 

[metengr]

bs;
For general viewing, 5% Nital is acceptable in the lab and field especially if other etchants are not permitted. It may be crude but it is easy and safe to use.

As you know, chemical etchants can be tailored to enhance specific features necessary for viewing. To reveal a basic tempered martensite microstructure, 5% Nital works with swabbing for both 9Cr and Grade 91. For metallographic examination regarding carbide morphologies and for TEM, Villela's followed by 3% Nital has been reported to be the best for surface preparation.