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New experimental data are reported relative to the development of banded structures in Mn-segregated Dual Phase steels during the austenite-to-ferrite transformation stage of their final heat treatment. Two types of ferrite are detected to grow simultaneously, i.e. allotriomorph and acicular ferrite, with distinct consequences on the topology of the final (ferrite + martensite) microstructure: allotriomorph ferrite sharpens the banded structure of ferrite and martensite between Mn-poor and Mn-enriched regions, respectively, whereas acicular ferrite favours more isotropic microstructures. Owing to an original image treatment procedure, the sharpness of banded structure is quantitatively analyzed for different heat treatment conditions. Results confirm those of previous works, but we propose to rather explain them by considering that a change in a heat treatment parameter results in a variation in the respective amounts of both ferrite types. [ABSTRACT FROM AUTHOR]

In the present work, the transformation characteristics of ductile iron austempered from intercritical austenitization temperature ranges were investigated. For this purpose, an unalloyed ductile cast iron containing 3.50 wt% C, 2.63 wt% Si and 0.318 wt% Mn were intercritically austenitized (partially austenitized) at various temperatures and then rapidly transformed to a salt bath held at the 365 °C for austempering for various times to produce dual matrix structure with different ausferrite volume fractions in ferrite matrix. A microstructure map was created to illustrate the transformation of products quantitatively as a function of austempering time for a particular intercritical and austempering heat treatment temperature and time. It was demonstrated that the total volume fraction of transformed phases was approximately constant for all austempering times after rapidly transforming samples from a particular intercritical temperature to austempering temperature. It was found out that the new ferrite (It is also called epitaxial ferrite) introduced into the intercritically austenitized structure during austempering and its content was dependent on the intercritical austenitizing temperature and austempering time. [ABSTRACT FROM AUTHOR]

The effect of intercritical heat treatments on the tensile properties of iron-based P/M steels was investigated. For this purpose, atomized iron powder (Ancorsteel 1000) was admixed with 0.3 wt.% graphite powder. Tensile test specimens were cold pressed at 700 MPa and sintered at 1120 °C for 30 min under pure argon gas atmosphere. After sintering, ∼20% pearlite volume fraction in a ferrite matrix was obtained. To produce coarse ferrite + martensite microstructures, the sintered specimens were intercritically annealed at 724 and 760 °C and quenched in water. To obtain fine ferrite + martensite microstructures, the sintered specimens were first austenitized at 890 °C and water-quenched to produce a fully martensitic structure. These specimens were then intercritically annealed at 724 and 760 °C and re-quenched. After the intercritical annealing at 724 and 760 °C and quenching, martensite volume fractions were ∼ 18% and 43%, respectively, in both the coarse- and fine-grained specimens. Although the intercritically annealed specimens exhibited higher yield and tensile strength than the as-sintered specimens, their elongation values were lower. Specimens with a fine ferrite + martensite microstructure showed high yield and tensile strength and ductility in comparison to specimens with a coarse ferrite + martensite microstructure. The strength values of specimens increased with increasing martensite volume fraction. [ABSTRACT FROM AUTHOR]

The compiled knowledge in literature regarding the isothermal formation of austenite from different initial microstructures (pure and mixed microstructures), has been used in this work to develop a model for non-isothermal austenite formation in low-carbon steels (C < 0.2 wt%) with a mixed initial microstructure consisting of ferrite and pearlite. Likewise, calculations of relative change in length have been made as a function of temperature, and the differences between theoretical and experimental results have been analysed in 0.1C–0.5Mn low-carbon low-manganese steel. Experimental kinetic transformation, critical temperatures as well as the magnitude of the overall contraction due to austenite formation are in good agreement with calculations. [ABSTRACT FROM AUTHOR]