Corrosion fatigue of austempered ductile iron

Corrosion fatigue (CF) behavior has been investigated for an austempered ductile iron (ADI) by conducting systematic fatigue tests at 20 Hz, including both high-cycle fatigue (HCF, S-N curves) and fatigue crack growth (FCG, da/dN-K curves), in air, lubrication oil and several aqueous environments. Results showed the HCF resistance of ADI was dramatically reduced by the given aqueous media, in particular, to a greater extent with a decrease in pH value. However, the given room-temperature aqueous solutions did not exert significantly detrimental effects on the Stage II crack growth compared with an atmospheric environment but an increase in solution temperature caused enhanced Stage II crack growth. Among the given variables of the bulk environment, pH had the greatest influence on HCF response while temperature had the most influence on the FCG of long cracks. In addition, SAE 10W40 lubrication oil provided an inert environment to remove the corrosive effect and enhance the CF resistance of ADI. The overall comparisons indicated the environmental effects would generate more influence on Stage I cracking than on Stage II cracking for the given ADI.     

Influence of heat treatment on fatigue crack growth of austempered ductile iron

Fatigue crack growth (FCG) behavior has been investigated for two different grades of austempered ductile irons (ADIs). These ADIs were produced from an alloyed ductile iron (DI) and heat treated respectively at two austempering temperatures, 300 and 360°C, to generate two different ausferrite microstructures. FCG tests using compact tension (CT) specimens were conducted under load control with three load ratios, R = 0.1, 0.5 and 0.7. The fatigue crack growth rates (FCGRs) of the given ADIs were compared with those of the as-cast DI with a bull's eye microstructure to examine the influence of austempering treatment on the FCG behavior of DI. The FCG behavior for the given materials was found to be dependent on the matrix structure with a demonstration that the as-cast DI had a better FCG resistance than did the ADIs at low K regime and vice versa at high K regime. As for the comparison made between the two ADIs, the one austempered at 360°C exhibited a lower FCG rate as a result of its coarse ausferrite microstructure, higher volume fraction of retained austenite, and greater toughness. The ADIs also demonstrated a load ratio dependence of intrinsic FCGR; that is, the enhancement of the FCGR with an increase in R value could not be rationalized by the crack closure effects.     

Fatigue crack propagation damaging micromechanisms in ductile cast irons

Ductile iron discovery in 1948 gave a new lease on life to the cast iron family. In fact, these cast irons are characterized both by a high castability and by high toughness values, combining cast irons and steel good properties. Ductile cast irons are also characterized by high fatigue crack propagation resistance, although this property is still not widely investigated.In the present work, three different ferritic–pearlitic ductile cast irons, characterized by different ferrite/pearlite volume fractions, and an austempered ductile cast iron were considered. Their fatigue crack propagation resistance was investigated in air by means of fatigue crack propagation tests according to ASTM E647 standard, considering three different stress ratios (R = K_min/K_max = 0.1; 0.5; 0.75). Crack paths were investigated by means of a crack path profile analysis performed with an optical microscope. Crack surfaces were extensively analysed by means of a scanning electron microscope both considering a traditional procedure and performing a quantitative analysis of 3D reconstructed surfaces, mainly focusing graphite nodules debonding.     

Fatigue crack nuclei in austempered ductile cast iron

ABSTRACT Short fatigue crack nuclei in austempered ductile cast iron have been studied using optical microscopy, scanning electron microscopy, atomic force microscopy and X‐ray microtomography and by electron backscatter diffraction analysis. Fatigue cracks nucleate at graphite nodules and shrinkage microporosity. The crack nuclei are arrested and retarded by barriers in the microstructure, by either blocking of slip at boundaries or owing to the requirement for tilt and twist of the stage I crystallographic crack at grain boundaries. These observations indicate that both the size of the defects, such as graphite nodules and microporosity, and the size of the prior austenite grains control the largest crack nucleus that can develop, and hence determine the component fatigue limit.     

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.     

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.     

Short fatigue cracks in austempered ductile cast iron (ADI)

The growth of short fatigue cracks was investigated in an austempered ductile cast iron (wt% 3.6C, 2.5Si, 0.6Mn, 0.15Mo, 0.3Cu), austenitized at 870 °C and then austempered at 375 °C for 2 h. At stress amplitudes close to the fatigue limit endurance limit of 10⁷ cycles, subcritical crack nuclei initiated at graphite nodules. The crack nucleus decelerated and arrested after propagating a short distance. The position of an arrested crack tip was characterized using an electron backscatter diffraction technique, demonstrating that short fatigue cracks in austempered ductile cast iron (ADI) can be arrested by boundaries such as those between ausferrite sheaves or packets and prior austenite grains. Refinement of the prior austenite grain size decreased the size of subcritical crack nuclei. It is proposed that the arrest and retardation of short crack nuclei are controlled by the austenite grain size and graphite nodule size. This determines the fatigue endurance limit.     

Low-cycle fatigue of austempered ductile irons

The effects of austempering temperature and isothermal transformation time on the low-cycle fatigue (LCF) behaviour in ductile irons have been studied. The fracture surfaces were observed by a scanning electron microscope in order to understand the fracture mechanism of LCF. From the results, it can be concluded that the best LCF behaviour is for the irons austenitized at 950 °C and there is very good cyclic stability at the lower strain amplitude irrespective of the austempering condition. However, there is a little cyclic softening at higher strain amplitudes for all the austempering conditions. Under a larger strain amplitude, the best LCF behaviour is for the specimen that has undergone austempering at a higher temperature, but under a smaller strain amplitude, the best LCF behaviour is for the specimen austempered at 350 °C.     

Effect of microstructure on the fatigue strength of an austempered ductile iron

Rotating bending fatigue tests were carried out on austempered ductile iron containing 1.5 wt% nickel and 0.3 wt% molybdenum. The ductile iron was austenitized at 900 or 1050 °C and then austempered at 280 or 400 °C for different lengths of time to obtain different microstructures. The fatigue strength was correlated with the amount of retained austenite and its carbon content, which were both determined by X-ray diffraction technique. While the tensile strength decreased with increasing retained austenite content, the fatigue strength was found to increase. Carbide precipitation was found to be detrimental to fatigue strength. Lower austenitizing temperature resulted in better fatigue strength.     

Influence of microstructure on high-cycle fatigue behavior of austempered ductile cast iron

An investigation was carried out to examine the influence of austempering heat treatments and the resultant microstructure of austempered ductile cast iron, on the fatigue crack growth rate, fatigue threshold, and high-cycle fatigue strength of the material. Two different approaches were used to study the fatigue behavior of this relatively new material, that is, a traditional S-N curve approach for determination of fatigue strength and a fracture mechanics-based approach for determination of the fatigue threshold. Compact tension and cylindrical specimens prepared from alloyed nodular ductile cast iron were given three different austempering heat treatments to produce three different microstructures. The fatigue threshold and high-cycle fatigue behavior of these specimens were studied in room temperature ambient atmosphere. The results of the present investigation demonstrate that the fatigue threshold of the material increases with increase in volume fraction of carbon-saturated austenite. The fatigue strength of the material, on the other hand, was found to increase with decrease in austenitic grain size. The crack growth process in the material was a combination of ductile striations and microvoid coalescence, and crack propagation by connecting the graphite nodules along its path.     

Effect of thickness on ductile fatigue crack propagation in polycarbonate

Resistance to slow fatigue crack propagation in polycarbonate is investigated with respect to specimen thickness. In the thickness range considered (0.33–3.22 mm), microscopic analysis reveals that a zone of yielded material constitutes the resistance to crack propagation. As the sheet thickness is increased, the amount of yielded material per unit crack surface is reduced. Consequently, faster crack growth rates are observed with increasing specimen thickness. Crack layer theory is applied to analyse crack propagation kinetics and stability. Accordingly, a specific enthalpy of damage (yielding) is found constant for the thickness range considered and is equal to 60 J g^–1 . It is further noticed that as the thickness increases, resistance to crack initiation increases.     

Boundary element analysis of ductile crack growth using the traction-free fundamental solution

A new boundary element formulation in two-dimensional rate-independent plasticity is given. This new formulation uses a so-called traction-free fundamental solution so that the resulting boundary integral equation converges in the normal sense, and more important, a formal differentiation of the boundary integral equation leads to a valid integral representation for the in-plane stress component on the boundary. No finite difference approximation is needed to construct the stress recovery routine. The new boundary element method is then used to solve the problem of quasi-static ductile crack growth. Numerical simulations based on a set of experimental data have been carried out to evaluate a new path-independent integral,T* _M . TheT* _M ,-integral is a modified version of Atluri'sT*-integral. This modified version has an advantage of having a less singular domain integral near the crack flank so that it is numericaly preferable toT*.     

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.     

Fracture toughness of austempered chilled ductile iron

Abstract

The structure and properties of ductile iron are highly dependent on the solidification mechanism, and chills are used to promote directional solidification to obtain sound castings. A series of fracture toughness experiments was carried out, involving austempered chilled ductile iron containing 3·42%C, 1·8%Si, and other alloying elements. By using copper chills of different thicknesses, the fracture toughness with varying chill rate was examined. Fracture toughness tests were carried out using three point bend specimens, each with a chevron notch, according to ASTM Standard E399 : 1990. It was found that the fracture toughness of austempered chilled ductile iron is highly dependent on the location in the casting from where the test specimens are taken and also on the nickel and molybdenum contents of the material. Chill thickness, however, also affects the fracture toughness of the material.     

Metallography of bainitic transformation in austempered ductile iron

Abstract

An unalloyed nodular cast iron has been used to investigate the development of microstructure on heat treating in the bainite temperature region. Specimens were austenitised at 900°C for 1·5 h, then austempered for 1, 2, or 3 h at 250,300, and 350°C, respectively, and examined by light, transmission electron, and scanning electron microscopy. Experimental results indicate a microstructure consisting of a stable, highly enriched, retained austenite with one of two lower bainitic ferrite morphologies. One of these morphologies is carbide free acicular ferrite for specimens austempered at 350°C for 1 h and the other is bainitic ferrite in which carbide is distributed within the ferrite produced by different heat treatment conditions. Austempering at 350°C for 2 h and at 300°C for 1 and 2 h resulted in the formation of transition carbides in bainitic ferrite platelets. The η carbide was formed at 350°C for 2 h by precipitation from a bainitic ferrite supersaturated with carbon. By contrast, ? carbide was associated with austempering at 300°C for 1 and 2 h and precipitates either on the austenite twin/bainitic ferrite boundaries or within the bainitic ferrite. The fracture mode of tensile and impact specimens in the austempered condition was fully ductile compared with as cast specimens, which had mixed fracture characteristics.

MST/1646     

On the mechanism of fatigue crack propagation in ductile metallic materials

An overview of our research performed during the last 15 years is presented to improve the understanding of fatigue crack propagation mechanisms. The focus is devoted to ductile metals and the material separation process at low and intermedial crack propagation rates. The effect of environment, short cracks, small‐scale yielding as well as large‐scale yielding are considered. It will be shown that the dominant intrinsic propagation mechanism in ductile metallic materials is the formation of new surface due to blunting and the re‐sharpening during unloading. This process is affected by the environment, however, not by the length of the crack and it is independent of large‐ or small‐scale yielding.     

The relationship between fatigue strength and microstructure in an austempered Cu-Ni-Mn-Mo alloyed ductile iron

Rotating bending fatigue measurements are reported for an austempered ductile iron containing 3.5 wt% C, 2.6 wt% Si, 0.48 wt% Cu, 0.96 wt% Ni, 0.27 wt% Mo, and 0.25 wt% Mn. The iron was austenitized at 870, 900 and 950°C and then austempered at 370 and 400°C for times between 30 and 240 min to obtain various austempered microstructures. The correlation between fatigue strength and austempered microstructure represented by the parameter XγCγ, where Xγ is the amount of high C austenite and Cγ its C content is examined. It is shown that fatigue strength increases as XγCγ increases. The highest fatigue strength is obtained with an ausferrite structure; the presence of martensite and/or carbide in the structure reduces the fatigue strength. Lower austenitizing temperatures increase the fatigue strength. This revised version was published online in November 2006 with corrections to the Cover Date.     

Formation of strain-induced martensite in austempered ductile iron

The present work has been taken up to study the influence of microstructure on the formation of martensite in austempered ductile iron. Ductile iron containing 1.5 wt.% nickel and 0.3 wt.% molybdenum was subjected to two types of austempering treatments. In the first, called as conventional austempering, the samples were austempered for 2 h at 300, 350 or 400 °C. In the second treatment, called as stepped austempering, the samples were initially austempered at 300 °C for 10, 20, 30, 45 or 60 min. These were subsequently austempered for 2 h at 400 °C. Tensile tests revealed considerable variation in the strain-hardening behaviour of the samples with different heat treatments. In the case of samples subjected to conventional austempering, it was found that strain-hardening exponent increased with increasing austempering temperature. In the case of samples subjected to stepped austempering, increased strain hardening was observed in samples subjected to short periods of first step austempering. Study of the microstructures revealed that increased strain hardening was associated with the formation of strain-induced martensite. There was a greater propensity for the formation of strain-induced martensite in the samples containing more of blocky austenite. Retained austenite in the form of fine films between sheaths of ferrite was relatively more stable. Studies revealed that the morphology, size and carbon content of the retained austenite were important parameters controlling their tendency to transform to martensite.     

Fracture behavior and mechanism in austempered ductile iron

The fracture behavior of copper-alloyed austempered ductile iron (ADI) was studied using metallography and fractography of selected samples. Three different grades of ADI were developed by austenitization at 900 °C for 60 min, followed by austempering for 60 min at either 270, 330, or 380 °C. The variation in austempered microstructure was determined by scanning electron microscopy of metallographically prepared samples, and structural parameters such as volume fraction of austenite, carbon content, and bainitic needle width were determined from the X-ray diffraction of powdered samples. The effect of austempering temperature on these structural parameters and on hardness, 0.2% proof stress, ultimate tensile strength (UTS), percent elongation, and impact strength was also studied. The fracture behavior under tensile and impact loading was determined by examination of the fractured surfaces and transverse cross sections near the fracture surface. The hardness, 0.2% proof stress, and UTS decrease and the impact energy increases as the austempering temperature is increased, and the morphology of the bainitic structure changes from lower to upper.     

Brittle and ductile crack propagation using automatic finite element crack box technique

In this paper, a numerical automatic crack box technique (CBT) is developed to perform fine fracture mechanics calculations in various structures without complete re-meshing. This technique aims to simulate the fatigue crack growth under mixed mode loading in 2D medium and shell structures calculated with the ABAQUS code, for elastic and for elastic-plastic materials. Using this method, series of numerical calculations by FEM of the mixed mode crack growth are carried out and compared with experimental tests such as a special cracked specimen subjected to different mixed mode loads. The crack growth paths are determined by using different elastic and elastic-plastic crack extension criteria. It is shown that the proposed technique is an efficient tool to simulate the crack extension angle in elastic and elastic-plastic materials. Nevertheless further experiments are needed to confirm conclusions deduced from elastic-plastic calculations.Using this technique, several phenomena influencing the crack extension are analyzed: the overload during fatigue, the fracture toughness of the material in relation with its critical J integral and its behaviour law.