*4.2. Microstructure and Mechanical Properties*

The annealing treatment, as shown in Figure 5, produced a microstructure consisting of pearlite and ferrite, as expected. The material is characterized by an equiaxed grain structure with eutectoidic islands randomly oriented, while quenching and tempering allowed us to obtain tempered martensite (Figure 7). The microstructure obtained after these two treatments agree with the ones reported in literature.

The austempering treatment produced a completely different microstructure; in fact, the observation carried out on the samples with optical and scanning electron microscopes (Figure 8) revealed the presence of a multiphase microstructure. This microstructure consists of ferrite, martensite, and retained austenite.

During soaking at IBT temperatures a significant fraction of austenite stabilized due to the carbon in FCC iron. After the heat treatment, retained austenite showed two morphologies: i) films between martensite laths, and ii) blocks according to [9,33,35–38,43,44]. The authors also observed that the austenite carbon content was not homogeneous, in particular, it was higher in the first type of retained austenite compared to the blocky austenite.

No upper or lower bainite was present in the microstructure, as expected. As a matter of fact, carbon partitioning, which takes place during the treating cycles, especially during annealing and the first cooling phase, may modify bainite start temperature (Bs). In detail, the increase in the carbon content in austenite can reduce Bs, as predicted in some models, as those reported by Lee [45]

Concerning the mechanical properties, the hardness values agreed with the microstructure present in the samples. The austempering treatment produced and increased hardness, due to the high volume fraction of un-tempered martensite, and the results are in accordance with the behavior reported in literature [46]. The Q&T sample was characterized by lower hardness, due to martensite tempering which led to martensite transformation and internal stress reduction.

Hardness and microstructural constituents can also explain tensile test results. The samples subjected to annealing treatment exhibited coarser microstructure and lower hardness and tensile strength compared to the other samples but were characterized by the highest ductility.

Considering the austempering treatment, the samples with the highest hardness values and the highest tensile strength, with a predominantly brittle behavior, were obtained. The brittle phenomenon was also confirmed by the fractographs, in which were not visible the features typical of a ductile fracture. This behavior is certainly determined by the microstructure, which mainly consisted of martensite, while the just mentioned plastic deformation was attributed to the ferrite volume fraction.
