I had relatively little experience with P91. With the 2.25 and 5 Cr steels, for field replication, the concern was always to quantify the grain boundaries and cavitation, and we always used nital as the etchant. When temperatures were relatively low we added some H2O2 as an "accelerant"; I am not sure whether the peroxide or just the water content was what increased etching. I believe that we did some P91 with nital.
It is certainly inconvenient when picral-containing etchants are not available for general use. I have never heard of an actual picric laboratory explosion in a metallurgy lab, but occasionally with bio students who weren't caring for their picric staining solutions.
When we first started evaluating the Cr-Mo steels, we followed the practice of a consultant who used electropolishing with perchloric mixes. For various reasons we moved away from that practice to mechanical polishing and chemical etching with nital:
-Perchloric has its own significant safety and handling issues;
-Electropolishing prepared only very small areas, and there is value in getting a larger view of an area;
-Nital is relatively safe to use and transport;
-Picral mixtures are not considered as good for grain boundary structure.
I recommend lab tests on known microstructures for comparative purposes. 9Cr steels may be low enough in Cr to still give good results with nital. You could increase the concentration of nitric to see if it provides an improvement.
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Paul Tibbals
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Original Message:
Sent: 09-18-2020 06:50
From: Waleed Khalifa
Subject: In situ etchant for P91 steel
I am looking for a suitable etchant for field replication of the P91 steel. The picric acid and its reagents are restricted. I would appreciate if you can share some of your experience.
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Waleed Khalifa
Principal and CEO
Arabic Consultancy Center for Engineering Materials, Inspection
Maadi, Cairo
01098163293
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