Abstract Austempered Ductile Iron Research Articles

Nondestructive microstructural characterization of austempered ductile iron

Abstract Austempered ductile iron (ADI) has been preferred in a wide range of applications due its unique combination of high strength, good ductility, wear resistance and fracture toughness together with lower cost and lower density compared to steels. Magnetic Barkhausen noise (MBN) measurement offers a better alternative to traditional characterization techniques by being fast and non-destructive. A simple linear regression using only one single independent variable cannot correlate the MBN with the microstructure of ADI, since its microstructure is multi-component. Multiple linear regression analysis (MLRA) was used to build a model that uses the characteristic features of microstructural constituents as input parameters to predict the MBN. For that purpose, Cu-Ni-Mo alloyed ductile iron samples austempered between 325 and 400 °C and for 45–180 min duration were used. The results show that MBN is most sensitive to the size and shape of acicular ferrite and retained austenite. Moreover, MBN is almost insensitive to the size, morphology and volume fraction of graphite particles. This indicates that retained austenite pins the domain walls more effectively than the graphite particles. Considering the results MLRA, MBN technique can be used to characterize the ausferritic microstructure of ADI.

Process Optimization for an Energy Efficient Heat Treatment of ADI*

Abstract Austempered ductile iron (ADI) materials show a good combination of mechanical properties, with high tensile strength levels (800-1600 MPa) and an elongation at fracture of up to 10 %. Due to its high fracture toughness as well as high fatigue resistance, ADI is a very attractive material group for applications under cyclic loads. However, the material properties are very sensitive to the applied process conditions during heat treatment. The basis for a substantial increase in the application of ADI materials in the industry requires the establishment of a predictable and robust process design. Here, an integrated process simulation of the heat treatment process coupled to required information about the as-cast quality of the component would aid in understanding process dependencies and defining a robust process window. For the austenitization step, the simulation can be used to determine the needed treatment times and temperatures. In the process simulation software MAGMA5, a new fast and efficient algorithm for heat exchange between thermally coupled diffusely radiating interfaces has been implemented and can be used for heat treatment simulation. The accuracy and computational performance of this new radiation model for the heat treatment of ADI is illustrated here for a real industrial suspension part. The steering knuckle is heated in a typical heat treatment box. Surface-to-surface radiation between the parts and with the surrounding furnace results in inhomogeneous temperature distributions. Integrated capabilities to perform virtual experiments and automatic optimization are used to determine optimal process conditions to meet both the quality requirements and the minimum required treatment time during austenitization. ▪

INFLUENCE OF AUSTEMPERING TIME AND AUSTEMPERING TEMPERATURE IN MICROSTRUCTURE AND MECHANICAL PROPERTIES IN AUSTEMPERED DUCTILE IRON

Austempered Ductile Iron (ADI) has excellent mechanical properties related to its microstructure ausferrite, and with the cycle of austempering heat treatment, many mechanical properties can be obtained from the same alloy, simply changing the time and temperature. To evaluate the influence of austempering time and temperature on the ADI, analyzed the modifications in the microstructures and mechanical properties of the samples of ductile iron, subjected to austempering heat treatment with austenitizing time and temperature of 910 °C and 90 minutes and during the austempering bath, the samples were submitted to different temperatures, 300, 320, 340, 360 e 380 °C, and for four different times for each temperature, 75, 110, 145 and 180 minutes. For the microstructural analysis, the microscopic techniques were used: optical and scanning electron and mechanical properties were obtained by mechanical testing of hardness and impact. The results show that there is a relationship between austempering temperature with microstructure and mechanical properties. The highest retained austenite and energy absorbed were 25.73% and 130 J, respectively, for the austempered sample at 380 °C and 180 minutes and the highest hardness value was 458 HB for the austempered sample at 300 °C and 75 minutes.

Analytical strength assessments of austempered ductile iron components

Abstract Austempered ductile iron (ADI) is a heat-treated ductile iron with improved tensile strength and toughness. Therefore, ADI is an interesting material for the construction of technical parts. However, for practical use, a strength assessment is necessary. An often-used option for an analytical strength assessment is the FKM-guideline. This guideline includes the strength assessment for steel, cast iron and aluminum materials, but it has not yet been especially investigated for ADI. In this work the use of the guideline for ADI is investigated. On the basis of experimental data, the accuracy of the guideline is examined and the limitations are discussed. Furthermore, the parameters that are used for the calculation in the guideline can be adjusted for the improvement of the accuracy of the strength assessment. In this way in particular, the forecast for shear stresses and the high strength materials can be improved.

Machinability study of manganese alloyed austempered ductile iron

Austempered ductile iron (ADI) is a heat-treated ductile cast iron with ausferrite microstructure. ADI has high strength and hardness with good ductility and toughness. However, because of the high strength and hardness of ADI, there is a concern regarding its machinability. In the present study, machinability of ADI is assessed using tool life and the surface roughness. Machining tests are conducted using Taguchi experimental design. The effects of machining parameters, heat treatment parameters, on manganese alloyed ADI in the dry turning operation are studied and it was found that austempering temperature and cutting speed have the major contributions on tool life, whereas feed rate has the major contribution on surface roughness of the material within the range of values under study. Metallographic study has revealed coarser structure at higher austempering temperature of 420� which resulted in better tool life. 0.64 wt% of manganese in the ADI has resulted in lower surface roughness value.

Impact of Molybdenum on Heat-Treatment and Microstructure of ADI

Austempered Ductile Iron (ADI) is characterized by high tensile strength with acceptable ductility. Steel, as a large competitor to ADI, also meets the tensile and yield strength. Nevertheless, the main advantages of ADI compared to steel are the lower density (7.2 g/cm3 to 7.85 7.87 g/cm3) for weight reduction and lower manufacturing costs because of less energy consumption during the production. One of the main problems of producing ADI is the quenching process during heat treatment of thick-walled castings. The inner part of a massive casting – in contrast to the outer part – cools down more slowly, resulting in a heterogeneous microstructure with parts of pearlite and ferrite embedded in austenite before reaching the isothermal transformation temperature. Molybdenum is, besides nickel, copper and manganese, one of the possible alloying elements that postpone the transition point of ferrite and/or pearlite. To investigate the influence of molybdenum in thick-walled castings experiments with different molybdenum contents were performed. In dependence on the molybdenum content, different austenisation and ausferritisation temperatures and times are examined in order to investigate the transformation points, fraction and morphology of different phases. The mechanism of molybdenum in ADI has been investigated by means of dilatometer tests, microstructure analysis and mechanical tests.

Virtual Optimization of Process and Material Properties for ADI*

Abstract Austempered Ductile Iron (ADI) materials show a good combination of mechanical properties, with high tensile strength levels (800–1600 MPa) and an elongation at fracture of up to 10 %. For that reason ADI is a very attractive material group for applications under cyclic loads. However, the material properties are very sensitive to the applied process conditions during heat treatment. In order to optimize the microstructure in an industrial heat treatment process for ADI castings virtually, several aspects concerning microstructure formation as well as industrial heat treatment process conditions have to be considered in simulation. Coupling process simulation with virtual Design of Experiments offers a new methodology to assess and optimize heat treatment processes.

Mechanical Characterization of Austempered Ductile Iron Obtained by Two Step Austempering Process

Austempered ductile iron (ADI) is known to have a good combination of mechanical properties due its unique ausferrite microstructure. The strength of ADI is mainly a function of the austempering temperature and the stability of ausferrite matrix. To increase the stability of the ausferritic matrix, two stage austempering processes was developed. During this investigation, in the Ist step, ductile iron specimens were austenitized at 900 °C for 60 min followed by quenching to 250 °C in salt bath. In the IInd step, after quenching at 250 °C, the salt bath was gradually heated to 350 °C, 400 °C and 450 °C respectively where specimen were soaked for 120 min. The tensile strength and impact strength were evaluated according to ASTM standards. The results were compared with that obtained by conventional austempering process by quenching directly into salt bath at 400 °C for 120 min. Both tensile and impact strength were found to have improved by two step austempering process. During Ist stage of austempering, martensite was observed while during IInd stage of austempering microstructures revealed acicular ferrite and carbon stabilized austenite. The fractographic examination revealed mixed type of fracture mode and intergranular fracture was seen under SEM. It was further observed that the tensile strength decreased whereas the impact strength increased with IInd stage of austempering temperature.

Prediction of Cutting Force of Austempered Ductile Iron Based on BP Neural Network

Austempered Ductile Iron (ADI) is a new generation of cast iron materials developed in recent decades; but its excellent comprehensive mechanical properties make it become hard-machining material. The main objective of this paper is to develop the prediction model of cutting force of ADI based on the BP neural network. The main factors that affect the prediction of cutting force were analyzed with experimental data. The structure of BP neural network and the reasonable training function were determined when experimental data was used to develop the prediction model of cutting force. Prediction error was between ± 4% when the prediction model was used to predict cutting force. The approach to the prediction of cutting force based on BP neural network is valuable for application.

A Case Study on Effect of Feed Rate on Machinability of Austempered Ductile Iron

Austempered Ductile Iron (ADI) is a type of nodular, ductile cast iron subjected to heat treatments - austenitising and austempering. Whilst machining is conducted prior to heat treatment and offers no significant difficulty, machining post heat treatment is demanding and often avoided. Phase transformation of retained austenite to martensite leading to poor machinability characteristics is a common problem experienced during machining. This case study explains the effect of feed rate on machinability of ADI using cutting force analysis and tool failure analysis. The experimental design consists of conducting drilling trials on grade 1200 and 1400 at constant depth of cut, 25mm; constant speed, 45m/min; no coolant and variable feed rates from 0.2 to 0.35 mm/rev (increment of 0.025mm/rev). Metallography and X-ray diffraction technique was carried out in order to identify and quantify the microstructural phases before and after drilling. The results from the trial infer that the best way to machine ADI efficiently without tool failure is using low feeds and high speeds and without coolant.

Evaluation of water embrittlement on ‘dual phase’ ADI

Austempered ductile iron (ADI) suffers an embrittlement phenomenon when loaded in contact with water and other liquids. This phenomenon causes noticeable drops in elongation, ultimate tensile strength and fracture toughness of ADI of different strength grades. This paper studies the susceptibility to embrittlement of a new kind of austempered ductile iron called dual phase ADI (DPADI), which shows a matrix composed by the typical ausferrite present in ADI mixed with allotriomorphic ferrite. The new material is obtained by means of specific heat treatments that involve a partial austenitising stage. The susceptibility of DPADI to this type of embrittlement was evaluated by carrying out tensile tests in dry and wet conditions. Fracture surfaces were observed using scanning electron microscopy. The results showed a gradual decrease of the degree of embrittlement caused by contact with water as the ferrite content in the matrix increases.

Characteristics of the Transformations Occurring within the Intercritical Interval of Ductile Iron

Austempered Ductile Iron (ADI) and fully ferritic ductile iron are frequently used to produce safety parts. A new kind of ductile iron (DI), usually referred to as “dual phase ADI” is currently under development. The matrix of this new material is composed of ausferrite (regular ADI microstructure) and free (or allotriomorphic) ferrite. This combined microstructure is obtained by subjecting DI to heat treatments comprising incomplete austenitization stages at temperatures within the intercritical interval (three phase α-γ-Gr field of the Fe-C-Si stable diagram) followed by an austempering step in a salt bath in order to transform the austenite into ausferrite. This work describes some alternatives to control morphologies and amounts of phases present in the final microstructure. A revision of the results obtained on this matter by the authors as well as by other researchers is also provided. A ductile iron melt was produced in a metal casting laboratory. Samples were heat treated following different thermal cycles to obtain a wide range of duplex microstructures, containing different amounts and morphologies of microconstituents. The results account for the fact that the transformations occurring within the α-γ-Gr field are strongly affected by the chemical composition, previous matrix, and holding times. The role played by the microsegregation and availability of sites for heterogeneous nucleation is also worth noticing. Starting from ferritic matrices and holding the samples at constant temperature within the intercritical interval, austenite nucleation and growth take place mainly in the last to freeze zones. On the other hand, when starting from fully recrystallised austenitic matrices, allotriomorphic ferrite can precipitate during the isothermal holding within the intercritical interval at the austenite grains boundaries forming a continuous net. The kinetics of the γ  α transformation is notoriously slower than the α  γ one. The influence that part size exerts on the final microstructure is also analyzed.

Austempering study of unalloyed and alloyed ductile irons

Austempered ductile iron (ADI) proved to be an advanced material as it possesses remarkable combination of high strength, ductility and toughness with good wear resistance and machinability. These properties can be achieved upon adequate heat treatment which yields optimum microstructure for a given chemical composition. In the present paper, an investigation has been conducted on unalloyed ADI and alloyed with 0·47%Cu and 1·6%Cu + 1·5%Ni, austempered at 350°C for the time up to the 6 h. The microstructure and fracture mode developed throughout these treatments have been studied by means of light, transmission and scanning electron microscopy, as well as X-ray diffraction. It was shown that the strength, elongation and impact energy strongly depend on amount of ausferritic ferrite and stable, high carbon enriched retained, reacted austenite. Based on the results, optimal processing windows for alloyed ductile irons used, have been established. In addition, for alloyed ADIs with maximum amount of reacted, retained austenite, transition temperature was also obtained.

Assessment of Machining Characteristics of Austempered Ductile Iron

Austempered Ductile Iron (ADI) is a modified Spheroidal Graphite Iron (SGI) produced by applying a two-stage heat treatment cycle of austenitising and austempering. The microstructure of ADI also known as “ausferrite” consists of ferrite, austenite and graphite nodules. Machining ADI using conventional techniques is often problematic due to the microstructural phase transformation from austenite to martensite. Machining trials consisted of drilling ADI-Grades900, 1050, 1200 and 1400 using inserted (TiAlN PVD coated) type drills. The cutting parameters selected were; cutting speeds [m/min] of 30 and 40; penetration rates [mm/rev] of 0.1 and 0.2; to a constant depth of 20mm. The machining characteristics of ADI are evaluated through surface texture analysis and microhardness analysis. These results indicate that microhardness is modified during machining and surface texture is improved using a cutting fluid.

Environmentally assisted embrittlement of ADI by contact with liquids

Austempered ductile iron (ADI) suffers an uncommon embrittlement process when it is stressed above its yield strength in contact with water and other liquids. The advances in the understanding and characteristics of this environmentally assisted embrittlement process, since it was first reported, are summarised. The extent of embrittlement for different ADI strength grades is characterised. The influence of various liquid environments is discussed. The features of the fracture surfaces are also analysed. Recent results of testing under applied potential are presented. An explanation of the fracture mechanism is proposed, based on the recent identification of cracks which developed in the last to freeze portions as a result of plastic deformation.

Modelling microstructural evolution and mechanical properties of austempered ductile iron

Austempered ductile iron (ADI) is finding an ever increasing worldwide market in the automotive and other sectors. It offers a range of mechanical properties superior to those of other cast irons, and shows excellent economic competitiveness with steels and aluminium alloys. The aim of the present research is to develop a generic model that will enable the producers of ADI to optimise their product in terms of microstructure and mechanical properties, hence minimising the need for expensive and exhaustive experimental trials and reducing alloy development lead times.

Properties of ductile cast iron nodularised with multiple calcium–magnesium based master alloy

Austempered ductile iron (ADI) is gradually replacing many fabricated and forged steel components in engineering applications. One of its advantages is the combination of good castability, machinability, and mechanical properties with significant savings in cost and weight compared with equivalent steel components. A problem in the production of ADI is the use of expensive and dangerously reactive magnesium as a graphite nodulariser. There is a need to find cheaper, safer, and equally effective substitutes. Results of an investigation of the effectiveness of a multiple calcium–magnesium based master alloy nodulariser and the properties of the ductile iron and ADI produced are reported. Up to 96% nodularity could be obtained using a special Ca–CaC2–Mg master alloy, compared with 98% using magnesium alone. The mechanical properties were also comparable.

Machinability of alloyed austempered ductile iron

Austempered ductile iron (ADI) has found enormous applications in recent years due to its high strength and hardness, coupled with substantial ductility and toughness. The high strength and hardness of ADI have caused many researchers and engineers to doubt the machinability of this material. Many investigations have adopted tool life, tool wear rate, cutting forces, and surface finish produced on a job as general criteria for evaluating the machinability of ADI. In the present investigation, an attempt has been made to evaluate the machinability of ADI alloyed with nickel by calculating the machinability index based on material removal rate and unit power consumed at various cutting speeds and feeds. The results thus obtained are presented in this paper.

Effect of Retained Austenite on Properties of Austempered Ductile Iron

Austempered ductile iron (ADI) exhibits a favourable combination of strength and toughness, and has been used as a substitute for quench-tempered or carburise-quenched steel. A characteristic featu...

AUSTEMPERED DUCTILE IRON

Abstract AUSTEMPERED DUCTILE IRON has high strength with excellent toughness and ductility. Its strength is twice that of conventional ductile irons, comparable to heat-treated wrought steels. Composition is balanced to provide foundry compatibility, minimal segregation and good response to austempering. This datasheet provides information on composition, physical properties, elasticity, and tensile properties as well as fracture toughness. It also includes information on heat treating and machining. Filing Code: CI-53. Producer or source: AMAX.