The selection of etchant is recommended to be done based on the phase to be investigated/highlighted. Long time back (~2012) I had explored use of nital etchant (two types: 2% and 4%) to see microstructure of 9Cr steels. Nital helps in highlighting the prior-austenite grain boundary area and those with carbides or MX type precipitates. I had tried to do heat tinting (i.e. after etching sample with 4% nital, the samples were subjected to heating in oven at 260C for 1 hour) so as to get phase contrast in microstructure. In other words, it does not etch the carbides and MX type precipitates. A publication (of 2012) indicating this is attached herewith for your reference.
However, if you want to see the martensite phase, then you will need to use Villela's reagent. Have a look at the attached paper wherein we tried to identify delta ferrite within P91 microstructure (2019 paper). You may use ASTM E-407 which will give you a detailed idea of which etchant to be used for emphasizing which phase in microstructure.Hope this helps.with best regards,Nirav Jamnapara, Ph.D.Chairman - ASM Gujarat Chapter
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