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Austempered ductile irons (ADIs) are used in applications commonly exposed to severe contact conditions, and as a consequence wear damage frequently followed by failure of components. Hence, wear resistance of the material governs the final life time of a component. In the present work, the sliding wear resistance of two ausferritic spheroidal graphite ductile irons ADI1 and ADI2 used commonly in mining and construction equipment was investigated. ADI1 and ADI2 were heat treated to a similar strength, the volume fraction of the carbon-rich austenite in ADI1 and ADI2 was around 30% and 16%, respectively, and they both contained 10 – 13% nodular graphite. The wear tests were performed using a slider-on-flat-surface (SOFS) tribometer. Case-hardened steel plates made of a high strength steel, 22NiCrMo12–F, were used as the counterface. The wear tests were conducted under lubricated sliding contact at normal loads of 50, 100, 200 and 300 N, and at each load level sliding at 100, 200 and 300 m. The friction force between contacting surfaces was continuously monitored during sliding. The lubrication used in the present investigation was a mineral-oil-based paste commonly used in applications where high frictional heating is generated. Wear mechanisms of the tested specimens were investigated by means of optical and scanning electron microscopy and X-ray diffraction, and the wear damage was quantified using a 3D-profile optical interferometer. The main wear mechanisms, severe plastic deformation and surface delamination, were discussed concerning test conditions and material properties. The ADI1 grade with the higher volume of carbon-rich austenite displayed better resistance to sliding wear at high normal loads. The higher normal loads promoted larger deformation at and beneath the contact surface and initiated austenite transformation into hard martensite. Thus, it was concluded that the increase of wear resistance in ADI1 was due to the formation of marteniste. On the other hand, the ADI2 grade with higher silicon content showed lower wear resistance at high normal loads. This was associated with cracking of the proeutectoid ferrite presented in ADI2.

In this research, different ductile irons and austempered ductile irons were successfully developed using several alloying contents of nickel, copper and microalloying with niobium. Additionally, special nanocarbon powder was added to the molten iron to enhance the nucleation tendency of spheroidal graphite and compensate for the possible negative effect of Nb addition on the nodule morphology. Metallographic analysis showed that increasing the niobium content in the alloy to 0.1 wt % raises the number of graphite eutectic cells and refines the final structure of the graphite. Moreover, the nodule count of graphite slightly increased, but it concurrently decreased the nodularity when the Nb amount reached 0.1 wt %. SEM micrographs illustrated that nano- to microsized niobium carbides (NbC) particles were dispersed in the matrix of the Nb microalloyed ductile irons. Both optical and SEM micrographs clearly showed that alloying of ductile irons with nickel, copper and microalloying with niobium had a significant effect on defining the final pearlite structure. Coarse, fine, broken and spheroidized pearlite structures were simultaneously observed in all investigated alloys. Dilatometry studies demonstrated that the nano NbC particles acted as nucleation sites for graphite and ferrite needles. Therefore, Nb addition accelerated the formation of ausferrite during the austempering stage. Finally, alloying with Cu, Ni and microalloying with Nb led to developing novel grades of ADI with excellent strength/ductility property combination.

The tribological characteristics of spheroidal graphite cast irons with and without nanosized additives are investigated. The tests are performed as in cast iron condition as well after austempering. The spheroidal graphite irons are undergone to austempering in the bainite field, including heating at 900 °С for an hour, after that isothermal retention at 280 °С, 2 h and at 380 °C, 2 h. The lower bainitic and upper bainitic structures are formed during the process. Nanosized additives of titanium carbonitride and titanium nitride TiCN+TiN influence on the graphite phase characteristics and on the microstructure of the cast and austempered spheroidal graphite irons. The changes in the micro structure the irons with nanoadditives lead to an abrasive wear resistance increase. The formation of the strain induced martensite from the retained austenite in the friction contact area during wear is determined in the austempered irons. This is the reason for the wear resistance increase of the irons. The experimental testing of the wear is carried out by cinematic scheme tapper-discunder friction on the fixed abrasive. The microstructure of the patterns is observed by optical and quantitative metallography, X-Ray analysis, SEM and EDX analysis. The hardness testing is performed by Brinnel and Vickers methods.

An experimental characterization of the austempered ductile iron ISO 17804/JS/1050-6/S was performed carrying out tensile tests under different strain rates, temperatures and stress triaxiality levels. Then, composing a yield function surface, a hardening relation, and a damage criterion, a constitutive model was developed to describe the salient features of the observed macroscopic response. In particular, the Mohr-Coulomb yield function was selected to account for the pressure effect observed on the yield surface. A new hardening relation was proposed in order to account for both strain rate and temperature effects. The Bonora’s damage model, developed in the framework of the continuum damage mechanics, was adopted to capture the failure condition under different stress triaxiality levels. The damage model was appropriately modified to account for the effect of strain rate and temperature on the failure strain.

Dislocation-density-related equations were proved to be promising tools to correlate tensile plastic behaviour and microstructure in ductile irons (DIs), revealing distinct relationships between equation parameters and microstructure features in austempered ductile irons (ADI) and isothermed ductile irons (IDI). These equations resulted to be successful also in the characterization of the austempering process through the plastic behaviour analysis of tensile tests of an ADI 1050 that was quenched at different times during austempering. The equation parameters could indeed be correlated to the time window for the best ausferrite, and could predict the precipitation of the deleterious epsilon' carbides for long austempering times. In the present work the results of the tensile plastic behaviour analysis of different DIs through dislocation-density-related equations are reported. The aim of the analysis was to test the capability of these equations to assess the microstructure quality of DIs and support their industrial production.

The use of ductile irons for structural applications is increasing thanks to their interesting mechanical properties, which can be combined with the casting technology to manufacture components having complex geometries. Mechanical properties can be adjusted by varying the chemical composition and the heat treatment parameters (temperatures and holding times). In this paper the fatigue behaviour of some ductile irons in the as-cast as well as in the austempered conditions was investigated. Push-pull, strain-controlled fatigue tests were carried out on ferritic, pearlitic, isothermed and austempered ductile irons covering a range of tensile strengths from about 400 MPa to 1300 MPa. Material parameters of the strain-life were fitted on experimental results in order to assure compatibility. Despite the wide range of tensile strengths, the fatigue behaviour of the different ductile irons could be synthesized on the basis of the plastic strain hysteresis energy.

In this study, the effect of microstructure on the mechanical properties of four types of ductile cast irons with different morphologies was investigated using circumferentially notched cylindrical specimens with different notch root radii. These cast irons were also austempered using the same austempering heat treatment to make a comparison with the as-cast samples. Characterization of the specimens has been carried out by means of microstructure, hardness, tensile properties, notch tensile strength, notch sensitivity, fracture toughness, and fractography. A mixture of ferrite and pearlite in the microstructure of cast irons gives rise to a material of the highest tensile strength, notch tensile strength, and fracture toughness properties with the intermediate ductility and notch sensitivity. A higher pearlite in the matrix of cast irons gives very important mechanical properties such as hardness and strength, but brittleness of the matrix andnotch sensitivity are greatly increased. Austempering significantly increased the mechanical properties and also reduced the difference between the mechanical properties of the cast irons. Austempered ferritic ductile irons exhibited the highest notch tensile strength and fracture toughness, and the high tensile strength and the intermediate ductility properties with the lowest notch sensitivity, whereas austempered pearlitic ductile irons had the lowest tensile strength, ductility, notch tensile strength, fracture toughness, and the intermediate notch sensitivity properties. The mechanical properties of the as-cast and austempered ductile irons have increased almost linearly with increase in the notch root radius.

In this paper the fatigue characterization of an austempered ductile iron (ADI) is presented. The aim of the work is to provide design engineers involved in fatigue assessments with an engineering tool suitable to deal with notches of different severity. Classically, U-notches are divided into blunt notches and sharp notches. The former are characterized by large notch tip radii such that the high cycle fatigue strength is controlled by the elastic peak stress, i.e. by the elastic stress concentration factor. The latter are characterized by reduced notch tip radii such that the effective stress which controls the high cycle fatigue strength is significantly lower than the elastic peak stress and their behaviour become similar to that of a crack having the same length. Blunt notches are assessed according to the classical Notch Mechanics principles, while sharp notches are treated with the Linear Elastic Fracture Mechanics approach. After presenting the classical Frost diagram which highlights the different fatigue behaviour of sharp and blunt notches, fatigue test results generated from notches of different severity are presented as well as a synthesis in a diagram able to account for short cracks/notches, long cracks, sharp notches and blunt notches.

In this paper, ductile irons with different microstructures were machined in a group of experiments using standard testing procedure and the effect of microstructure on the machinability was investigated. Bars (diameter 90mm × 320mm) of ferritic/pearlitic, tempered martensitic, and lower ausferritic microstructures were produced by various heat treatments and then tested to evaluate their machinability. Five groups of ductile iron specimens with various microstructures were tested following ISO 3685: 1993 (E) standard. Specimens were subjected to dry cutting; cutting force, flank wear width, and surface roughness were measured to compare the machining performances. In addition, further turning tests were conducted on the specimens of austempered microstructure at various feed rates (while keeping the cutting speed and depth of cut constant), and both surface roughness and cutting force values were recorded. The influence of feed rate on the machinability of austempered ductile iron was investigated and a correlation between feed rate and machinability, in terms of cutting force and surface roughness, was established. Not only the tangential cutting force component but also the feed force component was taken into consideration in these tests.

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. In the present work a series of test were conducted in order to evaluate the machinability ADI bars that were austempered at various temperatures and times and the machinability is investigated by adopting tool life, tool wear rate, cutting forces, and surface finish produced on a job as general criteria. Machinability tests were carried out according to ISO 3685: 1993 (E) standard “Tool Life Testing with Single Point Turning Tools” on eight different ADI structures, austempered at 250, 300, 350 and 400 °C for 1 and 2 h. The cutting tools used in the tests were coated carbide inserts, ISO SNMG 120408 (K10), clamped on tool holders CSBNR 2525 M12. No cutting fluid was used during the turning operations. Cutting forces, flank wear and surface roughness values were measured throughout the tool life and the machining performance of ADI having different structures were compared. In the machinability tests structures austempered at 300 °C for 1 h and 2 h were observed to produce unexpected results. That is to say, structures having less hardness values seemed to wear the tool faster than the harder structures. In order to investigate the grounds of this case, some more tests on these structures were conducted.