Nital etching, setting up a lab

Dear Doc: We do nital etching of the entire workpiece after grinding. There’s constant bickering about whether a part is burned. What can we do?

The Doc replies: Accept it. The bickering will continue. Indefinitely. Why? Because nital etching (where you dip the entire part in acid) is a very subjective test. One operator says it’s not burned; a different operator says it’s burned. And if somebody dims the lights slightly, the first operator changes his mind — now it’s burned.

But there are things you can do to reduce the uncertainty of the test so that the bickering is less often and less severe.

An expensive lab will give you a pretty picture (a). A cheap lab will give you a scratchy picture (b). But sometimes, a scratchy picture is just fine for finding out the depth of rehardening. Image courtesy of J. Badger

Here’s one thing I encourage my customers to do: “Calibrate” your nital etch so you feel more confident in the 50 inconsistent shades of gray that the test produces. The best way to “calibrate” it? Purposely burn a few parts to various degrees. Do your regular nital-bathing test and take photos (in the same room under the same light where they are typically examined). Then send the workpieces out for either (1) residual-stress measurements (via X-ray diffraction) or (2) cutting, mounting, polishing and nital etching (where you can see the depth of the burn).