Corrosion behavior of boro-tempered ductile iron

In this study, the effect of boro-tempering heat treatment on the microstructure and corrosion behavior of unalloyed ductile iron was investigated. The corrosion characteristics of ductile iron have been determined by current-potential curves. To determine the corrosion rates, the anodic and cathodic Tafel regions extrapolating to corrosion potentials were used. The inhibitor efficiency was calculated from i corr values. Optical microscope and X-ray diffraction (XRD) were used to examine the microstructure of polished and etched specimens. Thicknesses of the boride layers formed on samples were measured by an optical micrometer attached to the optical microscope. Results show that boro-tempering heat treatment can be successfully applied to ductile iron. The corrosion potential has shifted to more positive values in the boronized samples. The boride layer has behaved like an anodic inhibitor. The boronizing time has affected the corrosion rate. The increase in boronizing time has made the coating thicker, which has increased the corrosion resistance of the material. The best inhibition and the lowest corrosion rate have been performed on the sample which was boronized for 5 hours after cooling in furnace. The tempering at higher temperatures leads to an increase in the corrosion resistance of the materials tested here

Homogeneous formation of epsilon carbides within the austenite during the isothermal transformation of a ductile iron at 410 °C

The transformation of a ductile iron at 410 °C for different times, after austenitization for 30 minutes at 900 °C, is analyzed in detail. Upper bainite and a high volume fraction of austenite are formed for intermediate annealing times. A certain amount of martensite is observed after quenching not only for short transformation times but also for intermediate times. The formation of the martensite on cooling after intermediate transformation times is due to the decrease in carbon concentration of the retained austenite because of the homogeneous precipitation of epsilon carbides within. This homogeneous precipitation of epsilon carbide inside austenite is unambiguously observed. The epsilon carbide, pre-precipitated in austenite, which transforms to martensite on cooling, continues growing in the martensite after transformation. For long times of austempering at 410 °C, some complex large carbides or silicocarbides are formed, probably from the epsilon carbide, which result in the total decomposition of austenite.Peer reviewe