Many of the properties of austempered ductile iron depend on the austenite which is retained following the bainite reaction. A neural network model within a Bayesian framework has been created using published data to model the retained austenite content. The model allows the quantity of retained austenite to be estimated as a function of the chemical composition and heat treatment parameters. The computer programs associated with the work have been made freely available ( http //www.msm.cam.ac.uk/map/mapmain.html )

Estimation of the amount of retained austenite in austempered ductile irons using neural networks

The present work focuses in the study of the effect that a new two-step austempering heat treatment process, developed by Putatunda, has on the mechanical properties with emphasis on the response to the abrasive wear. A ductile cast iron melt was prepared in an industrial facility obtaining both, Y-blocks (ASTM A 897) to be used for the laboratory tests samples, and bucket tips to perform field tests by using a wheel loader. The results show that the two-step austempering process promotes an increase in the amount of retained austenite which in turn improves most of the mechanical properties, such as ultimate stress, yield stress, hardness and impact toughness. Two different abrasion tests were carried out in order to evaluate the material response to different tribosystems. The dry sand-rubber wheel abrasion test (ASTM G 65) was used at the lab, where the results show better wear resistance for the two-stepped ADI in comparison with conventional ADI. On the other hand, the bucket tips tested in a more severe environment, showed an opposite tendency.

Abrasion resistance of ductile iron austempered by the two-step process

To the design engineer, hardness often means an easily measured quantity which indicates something about the strength and heat treatment of a metal. Austempered ductile iron (ADI) is an alloyed and...

Prediction of the Vickers hardness in austempered ductile irons using neural networks

An investigation was carried out to examine the influence of step-down austempering process on the mechanical properties of austempered ductile cast iron (ADI). Compact tension and cylindrical tensile specimens were prepared from nodular cast iron as per ASTM standards and were subjected to two different austempering heat treatments; that is, conventional and step-down austempering in the upper bainitic temperature range. The specimens were austempered at four different temperatures; that is, 343, 357, 371, and 385°C. The influence of these austempering processes on the microstructure and the mechanical properties of the material was examined at room temperature and ambient atmosphere. The influence of these austempering processes on the plane strain fracture toughness of the material was also investigated. The present test results indicate no significant difference in the mechanical properties and fracture toughness in ADI processed by conventional as well as step-down austempering process at the same au...

Comparison of the Mechanical Properties of Austempered Ductile Cast Iron (ADI) Processed by Conventional and Step-Down Austempering Process

The present work analyzes the effect of boron addition to an Austempered Ductile Iron, in amounts from zero to 120 ppm. It has been found that boron has a strong effect on the equivalent carbon content, resulting in an increase on the precipitated graphite volume and a decrease in the dissolved carbon content in the matrix. This in turn, increases the ferrite volume fraction in the as-cast conditions from 0.24 in the base alloy to 0.78 for the iron with 120 ppm of boron. Furthermore, a decrease in the nodularity from 100% in the base alloy to 83% with 120 ppm of boron has been observed. During austempering, the transformation to ausferrite was faster and lower volumes of martensite and unstable austenite were detected when boron increased; this promoted lower hardness values, 239 HV for the base iron and 189 HV for the 120 ppm boron alloy. The increase in hardness and strength, typical for the start of bainite formation, were not observed in the boron added irons, but just in the base alloy. Under this basis, it is assumed that at least the addition of 60 ppm of boron extended the optimal processing window. The higher values of strength and ductility were obtained for the alloy with 60 ppm of boron; these results are discussed in terms of the graphitizing effect of boron in these irons and the reduced amount of carbon dissolved in austenite.

Effects of boron addition and austempering time on microstructure, hardness and tensile properties of ductile irons

In the austempering of ductile cast iron, it is necessary to alloy the metal to prevent pearlite transformation. The addition of alloying elements delays the pearlite transformation and at the same time, causes a similar delay in the beginning of the stage I reaction. In general, alloying addition levels greater than a specified amount are not desirable in the conventional austempering process due to the decrease in toughness and ductility. In this experiment, 1% manganese alloyed ductile iron was used in the successive-stage, High-Low Austempering Temperature Process (HLAT Process) in different time combinations of high and low temperature austempering to improve the toughness of high manganese ductile cast iron. The impact strength provided by the HLAT Process is a function of high temperature austempering time as well as low temperature austempering time. The impact strength at the best process combination is much higher than conventional high manganese ADI and well comparable to low alloy aust...

Effects of Successive-stage Austempering on the Structure and Impact Strength of High-Mn Ductile Iron

Manganese, which may be used in ductile cast iron as a potent hardenability promoter, segregates in the intercellular region. This segregation becomes more severe as a consequence of poor inoculation, low cooling rate, or increasing of nominal Mn content in the alloy. In severely Mn-segregated regions, Mn eutectic carbide may be formed, which has a deteriorating effect on the mechanical properties of casting. In this study, a 1 Pct Mn ductile iron was used to investigate the chemical composition and crystal structure of the Mn eutectic carbide by electron microscopy (scanning and transmission), X-ray, and an electron probe microanalyzer (EPMA). Transmission electron microscope (TEM) and X-ray studies show that the crystal structure of carbide is orthorhombic with lattice parameters ofa = 14.825,b = 11.415, andc = 8.880 (A). The concentrations of Mn, Si, and Cr in carbide, analyzed by scanning electron microscope-energy-dispersive X-ray spectrometer (SEM-EDX) and transmission electron microscope-energy-dispersive X-ray spectrometer (TEM-EDX), were 5.0 to 7.0, 0.5 to 2.8 and 1.5 to 2.2 (wt Pct), respectively. The ratio of Fe plus other metal atoms to C was calculated from EPMA experiments to be 2.5-2.9. It was shown that by diminishing Mn segregation, precipitated eutectic carbide can be reduced. It is expected that this can be achieved by reducing nominal content of Mn or by increasing nodule count.

Chemical composition and structural identification of eutectic carbide in 1 pct Mn ductile iron

Addition of Mn for preventing pearlite formation in austempered ductile iron causes intercellular segregation which may lead to precipitation of stable eutectic carbide. SEM-EDX and TEM-EDX were us...

T. Ohide

Papers

Effects of Successive-stage Austempering on the Structure and Impact Strength of High-Mn Ductile Iron

Chemical composition and structural identification of eutectic carbide in 1 pct Mn ductile iron

Eutectic Carbide Formation in High Mn Ductile Iron