Summary for etching for metallography Discussion
Etching for metallography is proving difficult because the samples develop heavy smut that hides the grain structure after an HCl-based wet-bench process. Ian Williams explains that the company cuts, linishes, and acid-etches high-alloy carbon steel bars to check for defects like tree ringing and inclusions, but the current 34% HCl diluted 50/50 at 76 °C, followed by a 65 °C water rinse, still leaves residue despite many trials with time and temperature. The answer from Ian Williams says the more customary approach for microscopic grain-structure work is Nital (nitric acid and alcohol), which produces little smut in the short etch times normally used and can be removed with plain water and a wet finger. The implication is that the current hydrochloric-acid method may be the wrong etchant for the inspection goal, rather than a post-cleaning problem.
May 13, 2015
Q. Thanks to everyone on this site. I have read previous Questions and Answers and gleaned a lot of information that I require.
We manufacture a variety of high alloy carbon steel bars, from which we have to cut samples for testing. These samples need to be linished then acid etched, to visually inspect the surface for defects such as Tree Ringing or inclusions.
We have recently introduced an automated Wet Bench type machine for etching the samples, which has one process bath for both etching and rinsing. We use HCl @ 34% diluted 50/50 @ 76 °C, and water at 65 °C for the rinse. Unfortunately, after extensive trials changing times and temperatures, we are still getting the smut residue on the surface, as in previous entries on this site. Certain alloys are worse than others.
My question is:
Is there a cleaning product or manual process, which will enable us to remove this smut post treatment, that will not effect or scratch the etched surface. It requires only a visual inspection, but we have to etch to reveal the grain structure of the steel. This is proving very difficult to see, with the smut on the surface. Whether pass or fail, the sample is then photographed and scrapped.
Ian Williams
steel – Sheffield, Yorkshire, UK
| May 18, 2015 A. It’s more customary to etch steel with Nital (nitric acid and alcohol) for microscopic examination of grain structure. Nital produces little smut in the few seconds necessary, and it’s easily removed with plain water and a wet finger . |
Jeffrey Holmes, CEF
Spartanburg, South Carolina