Simultaneous prediction of austemperability and processing window for austempered ductile iron

Abstract

A model is developed for simultaneous prediction of the processing window and austemperability of austempered ductile iron (ADI). The processing window represents a frame of time and temperature in which ADI satisfies optimum mechanical properties defined by ASTM A897M:1990. Austemperability is the maximum section size of ductile iron that can be austempered without formation of pearlite during the austempering process. The outcome of the model presents the processing window and austemperability as a three dimensional diagram (processing - austemperability window). The processing window boundaries are estimated according to a model for prediction of the time for ausferritic reaction in ADI. The austemperability of ductile iron is predicted according to the estimated pearlite curve of the TTT diagram and a mathematical model that simulates conduction of heat in a solid cylinder. The heat transfer model is calibrated for a ductile iron of composition (wt-%) 3.63C, 2.4Si, 0.39Mn, 0.4Mo, 0.25Cu, 0.04Ni, 0.04Mg. The model for the processing - austemperability window is validated for a ductile iron of composition (wt-%) 3.41C, 2.46Si, 0.36Mn, 0.18Mo, 0.25Cu, 0.036Mg at 285, 380, and 400 ° C austempering temperatures. Results show that the material satisfies ASTM A897M:1990 standard for the chosen experimental points within the processing - austemperability window without formation of pearlite in the microstructure.     

Transformation kinetics of unalloyed and high Mn austempered ductile iron

Abstract

The kinetics of bainitic transformation was studied in unalloyed and 1%Mn alloyed ductile irons. The samples were subjected to different austempering heat treatment cycles in dilatometry equipment. The results showed the effect of Mn on delaying the start and end of bainitic transformation, supposedly because of Mn segregation to the primary austenite grain boundaries. Decreasing austempering temperature led to precipitation of carbides inside the lower bainitic ferrite plates and therefore, more dilatation during the bainitic reaction. The dilatometric results were also used to calculate n and k in the Johnson-Mehl-Avrami equation and to derive the time-temperature-transformation diagrams. X-ray results were used to calculate the quantitative amount of precipitated carbides within lower bainitic ferrite and the dilatation during austempering.     

Application of bainite transformation model to estimation of processing window boundaries for Mn-Mo-Cu austempered ductile iron

Abstract

The processing window enables the heat treater to produce an austempered ductile iron that satisfies the ASTM standard A897M:1990. The aim of the present study is to determine whether a bainite transformation model developed for steels is applicable for calculating the extent of the processing window for an austempered ductile iron ADI of composition wt- Fe-3.55C-2.72Si-0.25Mn-0.25Mo-0.25Cu-0.04Mg. The bainitic model is calibrated for the start of the processing window, according to an autocatalysis factor that describes the extent to which the formation of one plate of ferrite activates others. The time interval between the beginning and the end of the processing window is used for estimation of the end of the processing window. Experimental data sourced from the literature for the ADI are used to calibrate the model, and it is shown that the bainitic model is applicable for calculation of the processing window. Further upgrading of the model is required for its application to ADIS of various compositions.     

Microstructure and impact behaviour of ASTM A105/AISI 304L friction weldments

Abstract

Instrumented impact testing has been used to investigate the influence of the microstructure of the heat affected zone (HAZ) on the impact fracture behaviour of ASTM A105/AISI 304L friction weldments. The friction layer in the HAZ has been found to consist of two different parts. In the A105 side, a mechanically mixed layer made of bainite containing 304L 'protrusions' is present. The hardness of the bainite was found to increase as the friction pressure was decreased. In the 304L side, the friction layer was made of a thick shear band, formed by thermoplastic instability during welding. The impact fracture toughness was found to depend on both the crack nucleation and propagation stages, whose characteristics were related to the dynamic fracture toughness of the friction layers.     

Fractographic analysis of austempered ductile iron

This investigation involves a systematic study of the fracture surfaces of two grades of austempered ductile iron (ADI) broken under quasi‐static, dynamic and cyclic loading conditions. The study used electron microscopy, optical microscopy and image post‐processing. The results show that the predominating fracture mechanism in ADI upon impact loading changes from quasi‐cleavage to ductile (with little areas of cleavage facets) as the testing temperature increases. Noticeably, even at the lower temperatures tested, the fracture surface of ADI shows clear signs of ductile fracture mechanisms. In particular, graphite nodule cavities suffer marked plastic deformation. Fracture after bending tests at room temperature was characterized by a mix of quasi‐cleavage facets, deformation of the contour of nodular cavities and microvoid coalescence. In the case of fatigue fracture at room temperature, the fracture surfaces show a flat appearance which has notorious differences with those reported for other loading conditions, but the typical fatigue striations were not found. The particular features identified on the fatigue fracture surfaces can be used to identify fatigue failures. It was also shown that the determination of the direction of main crack propagation by using the experimental methodology proposed earlier by the authors is applicable to ADI fractured by impact and quasi‐static loads. The results provide information potentially useful to fractographic analyses of ADI, particularly in samples that fail in service under unknown conditions.     

Fracture toughness of austempered chilled ductile iron

The structure and properties of ductile iron are highly dependent on the solidification mechanism and chills are used to promote directional solidification to get sound castings. A series of fracture toughness experiments were carried out involving austempered chilled ductile iron containing 3.42% C, 1.8% Si and other alloying elements. By using copper chills of different thickness, the fracture toughness of varying the chill rate was also examined. The fracture toughness tests were carried out using three-point bend specimens, each with a chevron notch, as per ASTM E 399 1990 standards. It was found that austempered chilled ductile iron is highly dependent on the location on the casting from where the test samples are taken and also on the Ni and Mo content of the material. Chill thickness, however, also affects the fracture toughness of the material.     

Multiphase matrix structure of unalloyed austempered ductile iron

A multiple phase structure characterised by a mixture of lenticular prior martensite (PM), fine needle bainitic ferrite and film retained austenite (RA) of an unalloyed ductile iron is developed. The designed austempering consists of initial rapid quenching to 210, 200 and 180°C respectively and finally austempering at 220°C for 240 min. The optimum mechanical properties, with a tensile strength of 1330?MPa, an elongation of 3.1% and 422HB, can be achieved by controlling the volume fraction of PM to 12.3% and the RA content to 18.1%. This is mainly attributed to PM that can accelerate the subsequent bainitic transformation and promote refinement of multiphase colonies.     

Production of Si55 Al20 Fe10 Ni5 Cr5 Zr5 bulk amorphous alloy by hot pressing

Abstract

The temperature dependence of the relative density was examined for a Si₅₅ Al₂₀ Fe₁₀ Ni₅ Cr₅ Zr₅ alloy which was produced by hot pressing of the amorphous powder during heating up to various temperatures under a critical pressure of 1·5 GPa through a heating/pressing cycle. The density of the compacts increased with increasing temperature and reached a maximum near the crystallisation temperature of 698 K. The relative density of the compacts reached 98·3% at the critical condition of 1·5 GPa and 687 K. The hardness value of the bulk specimen was 940 HV(10 gf, 0·098 N), corresponding to that of the amorphous ribbon. Exposure to higher temperatures resulted in the precipitation of the crystalline phase. The present results indicate that Si based alloys can be produced in a compact form with a low fraction of voids by hot pressing the amorphous powder.     

Processing parameters for the mechanical working of 9 Cr–1 Mo steel: processing maps approach

Abstract

Processing and instability maps using a dynamic materials model have been developed for 9Cr–1Mo steel in the temperature range 850 to 1200°C and strain rate range 0.001–100 s^–1 with a view to optimising its hot workability. The efficiency of power dissipation increased with increase in temperature and decrease in strain rate. The 9Cr–1Mo material exhibited two dynamic recrystallisation domains, one with a peak efficiency of 37% occurring at 950°C and 0.001 s^–1 and the other with a peak efficiency of 35% occurring at 1200°C and 0.1 s^–1. These results are in good agreement with those found in industry.     

Role of pre-formed martensite on transformation of austempered ductile iron

A commercial ductile iron is treated by a novel austempering process to obtain a good combination of strength and ductility. The samples are austenitised at 890°C for 10 min, then quenched into patented quenching liquid, and austempered in an electric furnace at 220°C for 5, 10, 30, 60, 240 and 600 min, respectively, finally air cooled. The bending test and the tensile test are conducted and microstructural features are analysed on the austempered ductile iron. The optimum mechanical property is achieved at 220°C for 240 min. Main reason for high strength and ductility is the formation of a fine structure consisting of multiple phases of pre-formed martensite and lath bainitic ferrite with film retained austenite.     

Effect of austempering temperature on microstructure of ausferrite in austempered ductile iron

The effect of austempering temperature on the microstructure of ausferrite in austempered ductile iron was investigated. The results show that the grain sizes of retained austenite and acicular bainitic ferrite both become larger with the increase of austempering temperature. As the austempering temperature is 240°C, the crystallographic relationship between ferrite and austenite in ausferrite follows Greninger-Troiano relation. However, Nishiyama–Wassermann relation and Greninger-Troiano relation both appear in ausferrite austempered at 300°C. At this temperature, the point-to-point misorientations of individual ferrite needle austempered at 300°C are less than 1°, being less than those at 240°C. This means the ferrite needles at 300°C contain fewer defects. However, some poles of ferrite needles obviously deviate from their ideal positions, which mainly comes from some ends of ferrite needles.     

Experimental and numerical assessment of fracture toughness of dual-phase austempered ductile iron

The effects of the microstructure topology on the fracture toughness of dual-phase austempered ductile iron are studied in this paper by means of finite element modelling and experimental testing. To this end, specimens with matrix microstructures ranging from fully ferrite to fully ausferrite were studied and the preferential zones and phases for crack propagation were identified in every case. The effectiveness of the ausferrite phase as a reinforcement of the ferritic matrix via the encapsulation of the brittle and weak last-to-freeze (LTF) zones was confirmed. The toughening mechanism is consequence of the increment in the crack path longitude as it avoids the encapsulated LTF zones. Besides, the presence of small pools of allotriomorphic ferrite increase the crack propagation resistance of the ausferrite-ferrite matrices.     

Development and investigation of austempered ductile iron (ADI) for thick-walled gear components

Forschung im Ingenieurwesen - Austempered ductile irons (ADI) are attractive for thick-walled and highly stressed gear components due to the possibility to design complex structures requiring...     

Morphology and constitution of the phases in as-welded microstructure of austempered ductile iron

Abstract

It was found by optical and electron microscopic examination of the microstructure of as-weld austempered ductile iron that the weld matrix is composed of austenite and bainite, the volume fractions of which were determined. In addition, the carbon content of austenite was measured and therefore the average carbon content of the matrix was calculated. In the matrix of the weld metal two types of bainite, bainite ferrite and lower bainite, were found. According to the morphology and distribution of the bainite plates, the nucleation and growth modes of bainite was inferred.     

Fracture mechanics based design of a railway wheel made of austempered ductile iron

In the present contribution numerical stress analyses are presented at static and cyclic loads for the fracture mechanical assessment of railway wheels made of austempered ductile iron (ADI) with graphite nodules. The emphasis is placed on the safe dimensioning of the ADI wheel against fracture and fatigue crack growth. Therefore a linear elastic fracture mechanical analysis is carried out assuming hypothetical crack-like defects at exposed places. The assessment rests on fracture mechanical strength parameters of the ADI material, determined experimentally under static and cyclic loading.     

Solid particle erosion behavior of carbidic austempered ductile iron modified by nanoscale ceria particles

In this study, the solid-particle erosion behavior of carbidic austempered ductile iron (CADI) containing different amounts of ceria nanoparticles was investigated. Attempt was made to correlate the erosion resistance of the material to its microstructure and mechanical properties. It was demonstrated that the erosion resistance of CADI was improved by adding ceria nanoparticles. The maximum erosion rate of unmodified CADI occurred at the impact angle of 60–75°, which slightly shifted to lower angles when ceria nanoparticles were added, corresponding to more ductile characteristic. During erosion tests, the retained austenite in CADI partially transformed to martensite although the transformation was suppressed by the addition of ceria nanoparticles. The ceria-modified CADI showed a considerably increased erosion resistance at various impact angles.     

Influence of nodule count on fatigue properties of ferritic thin wall ductile iron

Abstract

The present work focuses on the study of the influence of nodule count on the fatigue resistance of ductile iron. Fatigue tests were carried out on specimens taken from thin wall ductile iron plates of 2 and 4 mm thickness and standard Y blocks of 12·7 mm thickness, showing nodule counts ranging between 1800 and 300 nod mm^?2. All samples were ferritised before testing to obtain a homogeneous ferritic matrix. The results showed a large dispersion of fatigue strength values. Nevertheless, careful examination of the fracture surfaces showed the presence of very small casting defects on many test samples. When only the results measured on sound test samples were accounted for, a significant increase in fatigue strength was found as the nodule count increases. Casting defects, particularly microshrinkage, revealed by scanning electron microscopy on the fracture surfaces, were responsible for the premature failure, due to shortening of the crack initiation stage. The fatigue lives measured experimentally were compared with estimations based on the fracture mechanics theory.     

Effect of microcracking on the micromechanics of fatigue crack growth in austempered ductile iron

The effect of microcracking on the mechanics of fatigue crack growth in austempered ductile iron is studied in this paper. The mechanism of fatigue crack growth is modelled using the boundary element method, customized for the accurate evaluation of the interaction effects between cracks and microcracks emanating from graphite nodules. The effects of nodule size and distribution and crack closure are considered, with deviation bounds of computed results estimated through weight-function analyses. A continuum approach is employed as a means of quantifying the shielding effect of microcracking on the dominant propagating crack, due to the reduction of stiffness of the material in the neighbourhood of the crack tip. Although the results obtained may not yield actual numbers for real cases, they are in accordance with experimental observations and demonstrate how the main factors affect the crack growth of the macrocrack.     

The influence of depth of cut on the machinability of an alloyed austempered ductile iron

The volume fraction of high carbon austenite present in the microstructure of austempered ductile iron (ADI) is one of the important factors that influence the mechanical and physical properties of the alloy. Formation of martensite by TRIP (transformation induced plasticity) mechanism during the machining operation in which a large amount of stress is applied to the microstructure results in a decrease in machinability of austempered ductile iron which has affected the expansion of ADI in industry. In this article, the effect of depth of cut as a machining variable is assessed in an alloyed austempered ductile iron containing Cu, Ni and Mo. The measurements of mechanical properties including impact energy, tensile strength, hardness and microhardness along the cross-section of samples are reported for samples austenitized at 870 °C followed by austempering at 375, 340 and 300 °C. Results indicate that contrary to the behavior of many alloys, in austempered ductile iron, reducing the depth of cut will not improve the machinability. In the case of studied composition, cutting with depths of 0.5 and 0.1 mm had the best and worst results, respectively.     

Effects of processing parameters on direct laser sintering of multicomponent Cu based metal powder

Abstract

Direct laser sintering of a multicomponent Cu based metal powder was successfully processed through the mechanism of liquid phase sintering with partial melting of the powder. The effects of processing parameters such as laser power, scan speed, scan line spacing and layer thickness on the densification and microstructural evolution of the laser sintered powder were investigated. It was found that with increasing laser power or decreasing scan speed, the density of the sintered parts increased and the microstructures became denser. However, the combination of higher laser powers (>400 W) and higher scan speeds (≥0·06 ms^?1) gave rise to 'balling' effect. A successive transition from discontinuous scan tracks to coherently joined ones occurs with decreasing scan line spacing. Lowering the thickness of the powder layer promises an improvement in bonding coherence between sintered layers. A single factor termed 'energy density by volume' is defined to evaluate the combined effect of various processing parameters on the density of laser sintered powder. With increasing the energy density by volume up to ～0·16 kJ mm^?3, the densification rate is relatively high. However, with intensifying the energy density over ～0·23 kJ mm^?3, the mechanism of particle bonding may change into full melting/solidification, leading to a decrease in the sintered density.