Macro Etch testing of Welds

In this video, you will see a simple method for macro etch testing of welds.

Qualifying a welding procedure usually requires a macro etch test but I find there is a lot of value in doing my own macro etch testing to prove out settings.

Check out this mini series on TIG welding

Macro etch testing is what is known as destructive testing so you cant do it on real parts unless you are willing to scrap a part.

It basically involves cutting thru the weld, and then polishing the cross section to remove saw blade marks or grinding scratches.

Polishing requires gradually using finer abrasives until a smooth surface is achieved.

There are abrasives specifically designed for macro etch polishing but I have found that a flap disc followed by 2 or 3 scotch brite discs gets the surface smooth enough to reveal the weld nugget.

It is true that the finer the polish, the more detail the etchant will reveal but a macro etch test is usually used specifically for determining depth of penetration and not microstructure details so usually the red scotch brite disc is fine enough.

If you dont have any of these pads, a 240 grit sanding disc will usually work ok for the fine polish.

My down and dirty macro etch test procedure looks something like this:

• Cut cross section using either bandsaw, or grinding wheel
• Smooth cross section with 80 grit flap disc using light pressure
• Brown scotch brite disc
• Red scotch brite disc
• blue scotch brite disc

Using a Q tip or acid brush like you might use to apply flux for soldering pipe, I wet the tip with the etchant and swap the weld area several times until the weld nugget is plainly revealed.

Word of Caution on handling etchant solutions:

These are either acid based or alkaline and can be dangerous if not handled properly. 

Read the caution labels

Wear the necessary PPE like safety glasses, faceshield, and gloves and avoid any contact with skin.

Etchants for carbon steel:

5% nital etchant is the standard but not that easy to get.

Naval jelly rust remover works ok but it takes a long time and requires a really smooth polish.

PCB etchant containing ferric chloride ( just go to amazon and search for "PCB etchant containing ferric chloride" and you should find it easily)

Etchants for Aluminum:

I mostly use Easy off oven cleaner for macro etching aluminum (active ingredient is sodium hydroxide) 

Some aluminum cleaners like Dynaflux work just fine too.

Remember that the macro etch test only tests one cross section at a time and is not a comprehensive test.

But it can be used as a very good learning and teaching tool and that is why I use the macro etch test in so many of my welding videos.