Alejandro Daniel Basso

Influence of austenitising and austempering temperatures on microstructure and properties of dual phase ADI

Abstract The present work describes studies about the influence of processing variables on the microstructure and properties of dual phase austempered ductile iron (ADI). The upper and lower critical temperatures of conventional ductile iron melt were determined. Heat treatments involving austenitising within the intercritical interval, followed by austempering, allowed microstructures to be obtained composed of different combinations of free ferrite and ausferrite. Mechanical and fracture toughness tests performed on samples with mixed structures showed interesting combinations of strength and toughness, in comparison with fully ferritic and fully ausferritic matrices, particularly when austempering was carried out at 350°C. The results of the critical crack size, expressed by the relationship (K IC/σYS)2, which indicates the relative toughness of the material, showed the best values for ferritic matrices with ∼20% ausferrite. This effect is attributed to the location of the ausferrite in the last to freeze regions (the weakest areas in the matrix) where it acted as a reinforcing phase.

Wear Behavior of Carbidic Ductile Iron with Different Matrices and Carbide Distribution

Similar to other mechanical properties, wear resistance is entirely dependent on a materials microstructure, which, in turn, is related to the chemical composition and solidification rate, which controls the type of phase, size, amount, and dispersion. Depending on the tribosystem, the abrasive wear resistance of ductile iron (DI) may be improved by heat treatment as well as by reinforcing the matrix with hard particles such as carbides, typically obtained by alloying with elements such as chromium. The solidification rate mainly depends on wall thickness and mold characteristics. In DI, the solidification rate affects microstructural characteristics, such as nodule size, nodule count, carbide size and distribution, and matrix refinement, also including the last to freeze (LTF) amount, size, and distribution. This study evaluates the influence of the wall thickness (12.5, 25, 50, and 75 mm) on the abrasion resistance and impact toughness of DI with different matrices reinforced with carbides. Carbidic structures were obtained by alloying the melt with Cr, and the different types of matrices such as pearlitic, martensitic, and ausferritic (CADI) were obtained under as-cast conditions or by heat treatment. The results reflect the influence of cooling rate on the microstructural characteristics and its relationship with the mechanical properties, particularly the abrasive wear resistance. It was demonstrated that, under the present experimental conditions, the highest carbide content and matrix hardness, obtained from the 12.5-mm-thick part with a martensitic matrix, resulted in the highest abrasion resistance.

Macro and microstructural characterisation of high Si cast steels – Study of microsegregation patterns

Abstract In this study the as-cast macro and microstructures of medium C – high Si cast steels of three different levels of alloying are characterised. The application of a colour-etching reagent sensitive to Si segregation effectively revealed the solidification macrostructure, showing that the patterns of macrostructure and microsegregation are governed by the initial precipitation of δ-ferrite dendrites. A study of microsegregation carried out using advanced EDS techniques showed that, for the studied chemical compositions, Si, Mn, Cr, Ni, Mo and Al tend to concentrate at the last liquid to solidify. Accordingly, effective partition coefficients of values below unity were calculated for all alloying elements tested. It was verified that the minimum local Si contents measured on the steels investigated were greater than 1.7%, value above the minimum value (1.5%) necessary to obtain carbide-free bainite after austempering.

Assessment of the austemperability of high-silicon cast steels through Jominy hardenability tests

ABSTRACT The austemperability of seven high silicon cast steels with different alloy contents was characteri sed. The maximum round bar diameter that can be fully austempered changed from about 10 mm for an unalloyed steel to more than 70 mm for a low-alloy steel. The austemperability was calculated by applying a procedure based on a standard Jominy test and the characterisation of the microstructure along the Jominy sample. The method, which was validated experimentally, creates a relatively simple procedure to measure austemperability. Processing factors such as the ability of the salt bath to extract heat and the austempering temperature are accounted for the method. The metallographic study revealed the influence of microsegregation on hardenability, which is particularly important for cast steels.

Study of dimensional change of high-silicon ductile iron with ADI and Dual-Phase-ADI microstructures starting from different as-cast structures

Abstract In this work the percent dimensional change (DC%) and its dispersion in high silicon Ductile Iron with ADI and Dual-Phase-ADI structures obtained through heat treatment starting from different as-cast microstructures, was studied. Specimens were obtained from three different melts with Si content ranging from 3.1 to 4.2%. Dual-Phase-ADI structures were obtained using special heat treatment, starting from as-cast structures. Regarding dimensional change, DC% in ADI and Dual-Phase-ADI decrease as Si% and the proportion of ferrite phase in as-cast specimens increased. On the other hand, DC% increased with austenitizing temperature and ausferrite content in the final microstructure. A fuzzy model to predict DC% in ADI parts was adapted to include DC% prediction in Dual-Phase-ADI. The fuzzy model allowed to predict the DC% for ADI and Dual-Phase-ADI parts with an root mean square <0.05%.

Study of austempering kinetics of high silicon bainitic cast steels

ABSTRACT This study investigates the effect of chemical composition and microsegregation on the bainitic transformation of high Si cast steels at two different austempering temperatures. The advance of the bainite precipitation as a function of time and temperature was monitored through measurements of the amount of retained austenite by X-ray diffraction, the evolution of hardness and microstructural characterization. The results showed that the bainitic transformation starts preferentially at zones with lower contents of alloying elements but later covers the entire matrix. The austempering heat treatments resulted in an apparently homogeneous bainitic microstructure throughout the entire matrix after a short period of time. The results encourage the continuation of the development of high Si steels suitable to produce carbide-free austempered cast parts.

Influence of the Chemical Composition on the Ausferritic Transformation in Carbide-Free Bainitic Cast Steel

This work focuses on the study of the solid state transformations that take place during the austempering of high silicon carbide-free bainitic cast steels with different chemical composition. In order to get this objective three cast steel melts with different chemical compositions were produced, evaluating the influence of Cr, Mn, Ni, Al and Co. For each of these steels, samples were subjected to an austempering heat treatment at 340 oC varying the austempering time from 5 sec to 120 sec. The results show that small regions of free ferrite were obtained during continuous cooling from the austenitising to the austempering temperatures in unalloyed high silicon cast steels. At short austempering time (5 sec), the presence of a small fraction of ausferrite was observed. Austempering for 60 sec showed a larger amount of ausferrite. However, the ausferritic reaction is incomplete, and some martensite also was present, mainly located in last to freeze (LTF) zones. After an austempering of 120 sec, a fully ausferritic matrix was obtained. The addition of Cr and Mo avoided the initial precipitation of free ferrite, and lowered the isothermal transformation kinetics. On the other hand the use of Al and Co increase the presence of ferrite formed during continuous cooling and accelerates the kinetics of the ausferritic reaction.