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Etchant for inconel 718 after welding and heat treatment  

May 18, 2009

I need to know what are the suitable etchants for inconel 718 (compressor blades) after a welding and heat treatment (760 degrees Celcius). The purpose is to investigate bluish color that appears after heat treatment process is conducted.

Farah Ismail
process engineer - Kuala Lumpur, Malaysia

Since you say "turbine" you really need to follow the OEM's procedures as some allow different chemicals than others do.
Nitric acid with a bit of ammonium bifluoride works very well, but some companies do not allow it. Do not use HCl, muriatic acid,which no one allows.
A better way is to use a vacuum furnace or a furnace with a slight reducing atmosphere.
Your blue is simply oxidation of some of the metals in the alloy and surface contamination. The parts need to be clean and dry before going into the oven.

James Watts
- FL

I am assuming that these are jet engine compressor blades you probably want something which will not render them useless.
GE standard practice manual has a section 70-24-00 which deals with preparation and use of various etches for engine components.
This will cover an etch solution for inconel 718.

Ciaron Murphy
Aerospace - South Wales, UK