A wide variety of surface treatments and coatings are applied to metals to enhance their performance, for example, to improve fatigue resistance, increase wear resistance, or improve corrosion or oxidation resistance.

My last blog post was followed by further discussion that might be helpful to other readers.

My article in the November issue of Industrial Heating generated some interest and may have caused some unintended confusion. Here’s an edited comment received and my edited reply.

Fig. 1a. Example of a welded low-carbon steel etched with 2% nital showing the clear superiority of color etching in revealing the grain structure. The as-cast weld metal is shown

Welding is a very important joining process and has been used extensively for at least the past 60 years. Like most processes, there is a need to control welding and ensure a high quality end result.

Before specimens can be etched to reveal prior-austenitic grain boundaries, they must be prepared to a high quality level.

When I worked for Carpenter Technology Corporation in their research center, we encountered several instances where production-lab chart grain-size ratings were between 4 and 5 for a number of specimens on an order (these orders required tests on 20 specimens from different bars).

A variety of etchants were tried to darken the matrix.

A number of etchants have been reported in the literature to selectively outline, outline and color, or attack specific types of carbides in steels.

The specimens were mounted and prepared metallographically. Each was etched with the reagents described below. They were re-prepared between etchants. The etchants used were as follows.