Steel structure: discontinuity macroanalysis

To detect defects, breaking the continuity of the metal, flocks, cast and rolled structure structures prepare transverse macrosections - templates. These templates are subjected to deep and surface etching.

The sample surface is ground and etched with special reagents..

deep etching

The pickling operation is carried out in a fume hood.. Macrosections are removed from the reagent with tongs or a rubber-gloved hand..

For deep etching carbon, manganese, chromic, chromium molybdenum and chromium vanadium steels are used 50 %-hydrochloric acid solution at a temperature 60-70 ºС during 15-25 minutes. For other alloyed structural and tool steels, the duration of trawling is increased to 25-35 minutes.

Etching ferritic and austenitic steels, corrosion resistant, apply a solution, consisting of 1000 ml hydrochloric acid, 100 ml of nitric acid, 200 ml of potassium dichromate on 1000 ml of water. Etching is carried out at a temperature 60-70 ºС during 30-40 minutes.

After etching, the macrosection acquires a relief surface with clearly visible axes of dendrites. (cast steel), segregation zone and cracks.

surface etching

Hein's reagent is usually used for surface etching., containing on 1000 ml of water 53 g ammonium chloride NH4Cl and 85 g copper chloride CuCl2.

When immersing a macrosection on 30-60 seconds into the reagent an exchange reaction occurs. As a result of this reaction, iron displaces copper from an aqueous solution., and it settles on the surface of the section. On the plots, insufficiently protected by copper due to pores, cracks and non-metallic inclusions, etching occurs. Then the macrosection is removed from the solution, the layer of settled copper is removed with a cotton swab under running water and the macrosection is wiped dry, to prevent its rapid oxidation in air.

This reagent well reveals the nature of segregation and banding of deformed steel., worse - cast metal structure and cracks, especially those caused by flakes. They are revealed by deep etching.

Source: Metallurgy and heat treatment of steel