Alloyed Ductile Iron Research Articles

Precipitation and dissolution of carbides in low alloy ductile iron plates of varied thickness

The present work has investigated the feasibility of achieving carbide free microstructures in low alloy thin wall ductile iron (TWDI) after dissolution heat treatments. Studies were conducted using low alloy and unalloyed hypereutectic irons, obtained from thin and regular thickness blocks, cast using sand moulds. The amount of carbides present in the microstructure was measured before and after annealing heat treatments carried out at temperatures ranging between 870 and 930°C. As cast specimens contained up to 30 vol.-% of carbides. The results show that annealing heat treatments dissolved more than 90% of as cast carbides present in thinner wall specimens, while dissolution was below 30% for 13 and 25 mm Y block specimens. Chemical microanalysis demonstrated that the content of carbide stabilising elements in the carbides increased as the cooling rate during solidification diminished. Therefore, carbides present in irons solidified at a slower cooling rate are more difficult to dissolve by annealing.

Austempering and Austempered Ductile Iron Microstructure in Copper Alloyed Ductile Iron

The variation in the austempered microstructure, the volume fraction of retained austenite, Xλ, the average carbon content of retained austenite, Cλ, their product XλCλ and the size of bainitic ferrite needles with austempering temperature for 0.6% Cu alloyed ductile iron have been investigated for three austempering temperatures of 270, 330, and 380 °C for 60 min at each temperature after austenitization at 850 °C for 120 min. The austempering temperature not only affects the morphology of bainitic ferrite but also that of retained austenite. There is an increase in the amount of retained austenite, its carbon content, and size of bainitic ferrite needles with the rise in austempering temperature. The influence of austempering time on the structure has been studied on the samples austempered at 330 °C. The increase in the austempering time increases the amount of retained austenite and its carbon content, which ultimately reaches a plateau.

Abrasion and impact properties of partially chilled ductile iron

This paper reports the results of laboratory tests carried out to evaluate the abrasion wear resistance and impact properties of partially chilled ductile iron (PCDI). The versatility of the casting process allows the use of coolers (commonly referred as chills) at specified locations in the mold, in order to increase the solidification rate and in consequence, obtain a high carbide phase formation. Three different composition heats of low alloy ductile iron were studied, varying the silicon content and/or equivalent carbon. Besides abrasion and impact properties, an exhaustive microstructural analysis was carried out, evaluating the carbide content, nodule count and matrix phases. The impact toughness was low due to the high carbide content. It was observed that under the current experimental conditions (ASTM G 65 standard), very low wear rates relative to SAE 1010 were obtained for those samples with high carbide content and pearlitic matrix.

Dry sliding wear of low alloyed austempered ductile iron

A multiple low alloyed ductile iron with 0.8 wt-%Ni and 0.25 wt-%Mo was austempered in single and two step processes at 300 and 400°C for 120 min. Specimens were used to study the effect of austempering conditions on the wear behaviour of this material. Sliding wear tests were carried out using a pin on disc apparatus, the tes tmaterials rubbing under dry atmospheric conditions against a surface of hardened steel (55 HRC) at speeds of 0.6, 0.7 and 1.0 m s-1 and normal loads of 15.82 and 22.84 N. Test durations were 30, 60, 90 and 120 min. Scanning electron microscopy was used to examine the worn surfaces of test specimens. It was found that two step austempered specimens exhibited wear resistance that was higher than that of specimens austempered at 400°C, and almost as high as that of specimens austempered at 300°C. These two step austempered specimens, moreover, gave the highest impact energy and showed the best combination of mechanical properties. During two step austempering, the first stage reaction (formation of ausferrite) was completed in the intercellular area before the undesired second stage reaction (precipitation of carbides) had started in the eutectic cells. The two step treatment resulted in a duplex structure: upper and lower bainitic ferrite without formation of carbides. This structure was responsible for the improvement of mechanical properties and the good wear resistance. The results show that a well balanced choice of smaller additions of multiple alloying elements can reduce the negative effects of segregation and resulting structural inhomogeneity. MST/5472

Effect of microstructure on properties of ADI and low alloyed ductile iron

Microstructure, tensile, impact, hardness, fractography and wear characteristics were investigated for: (1) Austempered ductile iron (ADI); and (2) low alloyed ductile iron. Comparison has been made between the properties of these two types and that of conventional ductile iron. Detailed analysis, of the fracture mode for the 3 types of ductile iron, which failed under tensile and impact testing, were presented using the SEM. The wear properties were determined using pin-on-ring machine, under dry sliding conditions. The variation of mass loss and coefficient of friction with sliding distance, at different loads and speeds were presented and discussed. The wear mechanisms were investigated by means of subsurface observations. Microhardness test was used to study the change in the matrix strength with distance from the worn surface due to plastic deformation.

Dual matrix structure (DMS) ductile cast iron: The effect of heat treating variables on the mechanical properties

Ductile cast iron with Dual Matrix Structure (DMS) is a new type of cast iron, about which there is very little published information. In this paper the effect of processing type and partial austenitization time on the mechanical properties of Ni-Mo low alloyed ductile cast iron are investigated. In the process of partial austenitization, increasing the heating time above 400 seconds, results in an increase in both the ultimate tensile strength and yield strength. However, impact strength is reduced by an increase in heating time of up to 600 seconds. In the process of incomplete austenitization, better mechanical properties are obtained at higher austenitization temperatures. In the processes of incomplete austenitization, and incomplete ferritization the cooling method after austenitization (i.e., quenching and tempering or austempering) has a pronounced effect on the ductility whereas it has a small effect on the process of partial austenitization. Comparison of the properties of DMS as cast, normalised, quenched- tempered and austempered ductile cast iron show a better ductility for DMS ductile cast iron.

EFFECT OF NICKEL CONTENT ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AUSTEMPERED DUCTILE CAST IRON

Alloyed ductile Cast iron samples were produced using induction coreless furnace. The designed alloys have the following composition (3.4-3.8 % C , 2.3-2.7 % Si , 0.15-0.54 % Mn , 0.02 % S , 0.02 % p , 0.4-0.5 % Mo and variable Ni content from 0.5-4 % ) The austempering heat treatment was performed using Salt bath furnaces, at a temperature of 900° c for 30 min. and the isothermal transformation was carried out at temp. of 220° C and 400° C . Austempering transformation time was varied from 10 min. to 24 hrs. to investigate its effect on the final structure and mechanical properties .The results showed that, the best ductility and impact toughness for Austempered alloyed ductile Cast iron at 400° C was obtained by holding time equal to 60 min., while the optimum holding time at 220° C was 2 hrs. More over the alloy containing 0.5 % Mo & 2-4 % Ni provokes increase in hardenability and mechanical properties .

The Processing Window for Austempering Ductile Iron

A PC-controlled vacuum heat treating system was used to austemper an alloyed ductile iron with 0.77%Cu and 0.5%Ni at 400°C after austenitizing at 850°C. During the progress of isothermal transformation, the resistivity of austempered ductile iron vs. time was recorded and then analyzed to determine the processing window. X-ray diffraction, heat tinting technique, and microstructural examination were conducted to examine the suitability of this processing window in terms of microstructural changes. The data of mechanical properties of the ductile iron from the same casting batch and same austempering were extracted from a concurrent work to discuss the suitability of this processing window. Both the evaluation of microstructures and mechanical properties confirmed the suitability of this processing window. A slow rate transformation during the time interval of processing window was observed which was associated with the continuous decomposition of blocky austenite and resulted in a slight variation of the mechanical properties.

On the Characterization of Nodule Size Distributions of Commercial Ductile Iron Alloys

On the Characterization of Nodule Size Distributions of Commercial Ductile Iron Alloys

Role of austenite in promoting ductility in an austempered ductile iron

Measurements of the austempering kinetics and mechanical properties of an alloy ductile iron after single and stepped austempering treatments following austenitising at 920°C are presented. The kinetic and tensile data are analysed to show how the thermal and mechanical stability of the austenite phase vary with austempering treatment. Both the thermal and mechanical instability of the austenite phase are shown to limit the ductility achieved in single austempering treatments at 375 and 400°C to such an extent that the Charpy impact energy fails to satisfy the ASTM standard A897M: 1990. Stepped austempering treatment improves both the thermal and mechanical stability of the austenite phase and increases the impact energy to such an extent that the ASTM standard is satisfied over a wide range of austempering conditions. A well defined relationship is shown to exist between impact energy and the austenite carbon content for different austempering treatments after austenitising at 920°C.

Stepped heat treatment for austempering of high manganese alloyed ductile iron

AbstractA stepped heat treatment is proposed for overcoming the difficulty of obtaining ductility in an austempered alloyed ductile iron. The method is illustratedfor an iron containing 0·67%Mn, 0·25%Mo, and 0·25%Cu, using an austenitising temperature of 920°C, afirst step austempering temperature of 400°C for 120 min, and a second step austempering temperature of 285°C. The change in the microstructure and phase characteristics with time during the second austempering step are described. Related changes in the mechanical properties compared with a single austempering treatment at 400°C are an increase in the ultimate tensile strength from 770 to 970 MN m−2, an increase in elongation from 2·5 to 7·5%, and an increase in the unnotched Charpy impact energy from 40 to 150 J.MST/3119

Modeling the evolution of equiaxed miorostructures in castings

With the goal of expanding the scope of solidification modeling, research initiatives have ranged from simply predicting the path of the isotherms to exploring the more complex problem of predicting microstructural evolution. This article presents the mathematical and physical principles involved in this type of modeling and provides examples for a number of Al-Si, Al-Cu, gray iron and ductile iron alloys. Clearly micro- and macrostructural modeling represents significant progress in the field of solidification and the subsequent cooling of castings.

Transformation Induced Plasticity in Austempered Low Alloy Ductile Iron

Transformation Induced Plasticity in Austempered Low Alloy Ductile Iron

Theory for the Stress Corrosion Cracking of Ductile Iron Alloys Based on a Model for the Anodic Dissolution Reactions of Pure Iron

It is generally agreed that iron and low-alloyed steels are susceptible to anodic stress corrosion cracking (SCC) only in the presence of certain anions or molecules in the electrolyte solution. This type of attack is further limited within rather narrow ranges of potential and pH, and in published anodic polarisation diagrams for steels susceptible to see some special features are often seen together with considerable deviations between slow and fast potential sweep recordings.The above features can be interpreted on the basis of a recently proposed model for the kinetics of the anodic dissolution of iron. According to this model SCC will be limited to a potential region comprising the upper end of a transition between two dissolution reactions. Exposure of new metal surface due to yielding results in attack on this surface by the faster of the two reactions over a transition period, the duration of which will depend on the nature of anions or molecules present in the electrolyte solution as well as on pH and potential. This fast process is accelerated by defect sites, and it may, depending on all co-operative conditions, proceed much faster than the steady-state corrosion rate prior to yield.

The Corrosion Resistance of Ductile Iron In Sea Water and Petroleum Tanker Services★

Abstract Laboratory data are given which compare the corrosion resistance of gray iron, steel and ductile iron in sea water and various acid solutions of different concentrations. Data also are presented to show the performance of ductile iron when used for product lines on petroleum tankers. Of particular interest is information presented for a low alloyed ductile iron which has given complete satisfaction under highly aggressive conditions in petroleum tanker service. 6.2.2