Effect of carbon content on etching response for identification of plastic deformation zones in carbon steels

A recently developed etching technique to detect localized plastic deformation on a macro scale in 0.10-0.55% carbon steels has been studied. Etching was carried out on samples plastically deformed and then heated to 550 °C for a certain period of time. The plastic deformation zone was clearly seen in the low carbon steel (∼0.15%C), whereas it was mainly opaque in the medium carbon steel (0.25-0.35%C) and could not be seen in the high carbon steel (0.55%C). In the case of low carbon steels, severe chemical reaction occurred in the undeformed ferrite grains, while there was a weak reaction in the deformed ferrite. The plastic zone was detectable from the different surface states. Such a change in etching response in the above carbon steels was found to be related to their microstructural morphology (ferrite/pearlite), and a large amount of ferrite grain in the sample, i.e. the low carbon steels, was more suitable for revealing the plastic zone. This was due to the change in microstructure: (i) many tiny cementite particles (Fe₃C) are precipitated in the deformed ferrite matrix during heating process, and (ii) finer ferrite grains are produced in the plastic zone. Because the main reason for observation of the plastic zone is related to the change in microstructure in the ferrite matrix, the material having a large amount of pearlite (high carbon steel) did not show the plastic deformation zone. Based upon the change of microstructure in ferrite, details of the etching mechanism for identification of the plastic zone are discussed.