Best etchant for 3-series Stainless 5

[deco0404]

Hi guys,

I have been doing some research on etchants for these materials and can't really decide what is the best to use. We are looking at grain structure ( micro )and weld integrity (macro), but as we are just getting into this we may also need to be looking for sigma phase as well (i know it is rare in these materials, but we do get requests for it anyway!)

I know electrolytic oxalic works, and have come across a modified murakami's which I believe is quite useful.

Any suggestions would be very welcome

Declan

 

[TVP]

Glyceregia is a very good general purpose etchant. Excellent info is available from Buehler, Struers, and Cartech on their websites:

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

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

http://www.cartech.com/techarticles.aspx?id=1450

 

[JoeyV87]

Marbles Reagent is great for general structure.

Always use Nital (or Picral if pearliteis present) when you can get away with it.

For Sigma, electrolytic 40% NaOH will bring it up a lovely blue colour.

I'd disagree (sorry!) with Glyceregia being good, it's very messy and glycerol will slowly dissolve through standard latex gloves. It is, however, a necessary evil for high nickel alloy steels and monel.

Your best bet, if you have it available, is to look up the specific grade of steel in ASM Handbook 9 and see what they use to etch it.

 

[EdStainless]

electrolytic oxalic is always where I go first. good general structure details.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[Guest102023]

I have used electrolytic NaOH (20~40%) at room temperature, with some success on furnace tube grades. Sigma phase appears as yellow-brown.
There are specific etchants that probably work better, but involve more noxious ingredients and/or heating, which I did not have proper facilities to use safely.

 

[CoryPad]

I disagree with the suggestions to use nital or picral. Austenitic stainless steels have no pearlite, and they are essentially immune to nitric acid. High concentration nitric acid is used to passivate austenitic stainless steels, so low concentration nitric (i.e. nital) does absolutely nothing.

Electrolytic oxalic acid is great if you have the electrolytic equipment, otherwise, aqua regia and glyceregia both work.

 

[deco0404]

Thanks for your input guys.......

I know for a fact that Nital doesn't work. Vilella's is not great. I hate using glyceria with a passion, so will try the oxalic. I have the gear, but as I havn't used it in a very long time, can someone tell me what mixture to use. Is it oxalic and water or ethanol, what is the concentration and what voltage ( 6v i seem to remember )and what amperage, and an estimated time. I have done a lot of searches, but it seems oxalic is used extensively in bee-keeping and as such the searches are quite difficult

regards

Declan

 

[unclesyd]

Our plant is 90% 3XX SS of all types and electrolytic Oxalic was used the majority of the time with few exceptions.

If you use Oxalic Acid make sure it's saturated, The 10% normally called out.

The use of Oxalic takes a little finesse on the operators part.

 

[JoeyV87]

Brimstoner - slightly concerned at your comment. With electrolytic NaOH, Sigma is definitely blue. Austenite is "orange" and ferrite shows as a dirty yellow colour.

Other than that, oxalic is the most commonly used for good reason. I've found that concentration really doesn't matter, it's mainly the voltage you use (I find ~7V best, but I wouldn't vouch for the accuracy of my power pack).

 

[Guest102023]

I have used oxalic a lot on 300 SS, but don't like its aggressiveness and the way it ditches the grain boundaries. Also, if it includes a weld, the base metal will barely be touched compared to the weld zone (it eats the ferrite phase rapidly). Oxalic is not very subtle!

Don't be too concerned about getting the right voltage; I was told by a metallography expert that it is not too important. I used a $25 multi-voltage power source from Radio Shack.

There are nitric acid etches (used hot IIRC) that are different from Nital (which won't do much for SS), but I have not tried that. I think I have used aqua regia also, it is simple to mix - try a diluted form for your first attempt, just in case it is too fast.

The text by van der Voort has been my constant companion for many years ...